Understanding neutron radiography reading ii tnr of materials

Understanding Neutron Radiography Reading II

My ASNT Level III, Pre-Exam Preparatory Self Study Notes 27 June 2015

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Nuclear Applications

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Nuclear Applications

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The Magical Book of Neutron Radiography

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ASNT Certification Guide NDT Level III / PdM Level III NR - Neutron Radiographic Testing Length: 4 hours Questions: 135 1. Principles/Theory • Nature of penetrating radiation • Interaction between penetrating radiation and matter • Neutron radiography imaging • Radiometry 2. Equipment/Materials • Sources of neutrons • Radiation detectors • Non-imaging devices

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3. Techniques/Calibrations

• Electron emission radiography

• Blocking and filtering

• Micro-radiography

• Multifilm technique

• Laminography (tomography)

• Enlargement and projection

• Control of diffraction effects

• Stereoradiography

• Panoramic exposures

• Triangulation methods

• Gaging

• Autoradiography

• Real time imaging

• Flash Radiography

• Image analysis techniques

• In-motion radiography • Fluoroscopy

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4. Interpretation/Evaluation • Image-object relationships • Material considerations • Codes, standards, and specifications 5. Procedures • Imaging considerations • Film processing • Viewing of radiographs • Judging radiographic quality 6. Safety and Health • Exposure hazards • Methods of controlling radiation exposure • Operation and emergency procedures Reference Catalog Number NDT Handbook, Third Edition: Volume 4, Radiographic Testing 144 ASM Handbook Vol. 17, NDE and QC 105 Charlie Chong/ Fion Zhang

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Fion Zhang at Shanghai 27th June 2015

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Greek Alphabet

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Why Neutron Radiography? "finding lead in a paraffin block (or a needle in a haystack) would work for x rays while looking for paraffin in a lead block or a straw in a needle-stack would work for neutrons."

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Why Neutron Radiography? "finding lead in a paraffin block (or a needle in a haystack) would work for x rays while looking for paraffin in a lead block or a straw in a needle-stack would work for neutrons."

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Why Neutron Radiography? "finding lead in a paraffin block (or a needle in a haystack) would work for x rays while looking for paraffin in a lead block or a straw in a needle-stack would work for neutrons."

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Neutron Cross Section of the elements

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IVONA TTS Capable.

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Reading II Content  Reading One: E748  Reading Two: ASMHB17-NRT  Reading Three: E1316  Reading Four: Neutrons provide unique penetrating radiation

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Reading-1 E748

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1. Scope 1.1 Purpose - Practices to be employed for the radiographic examination of materials and components with thermal neutrons are outlined herein. They are intended as a guide for the production of neutron radiographs that possess consistent quality characteristics, as well as aiding the user to consider the applicability of thermal neutron radiology (radiology, radiographic, and related terms are defined in Terminology E 1316). Statements concerning preferred practice are provided without a discussion of the technical background for the preference. The necessary technical background can be found in Refs (1-16).

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1.2 Limitations - Acceptance standards have not been established for any material or production process (see Section 5 on Basis of Application).

Adherence to the practices will, however, produce reproducible results that could serve as standards. Neutron radiography, whether performed by means of a reactor, an accelerator, subcritical assembly, or radioactive source, will be consistent in sensitivity and resolution only if the consistency of all details of the technique, such as neutron source, collimation, geometry, film, etc., is maintained through the practices. These practices are limited to the use of photographic or radiographic film in combination with conversion screens for image recording; other imaging systems are available. Emphasis is placed on the use of nuclear reactor neutron sources.

1.3 Interpretation and Acceptance Standards - Interpretation and acceptance standards are not covered by these practices. Designation of accept-reject standards is recognized to be within the cognizance 莤厚 of product specifications.

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1.4 Safety Practices - General practices for personnel protection against neutron and associated radiation peculiar to the neutron radiologic process are discussed in Section 17. For further information on this important aspect of neutron radiology, refer to current documents of the National Committee on Radiation Protection and Measurement, the Code of Federal Regulations, the U.S. Nuclear Regulatory Commission, the U.S. Department of Energy, the National Institute of Standards and Technology, and to applicable state and local codes.

1.5 Other Aspects of the Neutron Radiographic Process - For many important aspects of neutron radiography such as technique, files, viewing of radiographs, storage of radiographs, film processing, and record keeping, refer to Guide E 94. (See Section 2.)

1.6 The values stated in either SI or inch-pound units are to be regarded as the standard.

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1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. (For more specific safety information see 1.4.)

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2. Referenced Documents 2.1 ASTM Standards:  E 94 Guide for Radiographic Testing  E 543 Practice for Evaluating Agencies that Perform Nondestructive Testing E 545 Method for Determining Image Quality in Direct Thermal Neutron Radiographic Examination  E 803 Method for Determining the L/D Ratio of Neutron Radiography Beams E 1316 Terminology for Nondestructive Examinations  E 1496 Test Method for Neutron Radiographic Dimensional Measurements

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2.2 ASNT Standard:  SNT-TC-1A Recommended Practice for Personnel Qualification and Certification 2.3 ANSI Standard:  ANSI/ASNT- P- 89 Standard for Qualification and Certification of Nondestructive Testing Personnel 2.4 Military Standard:  MIL-STD-410 Nondestructive Testing Personnel Qualification and Certification

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3. Terminology 3.1 Definitions - For definitions of terms used in these practices, see Terminology E 1316, Section H.

4. Significance and Use 4.1 These practices include types of materials to be examined, neutron radiographic examination techniques, neutron production and collimation methods, radiographic film, and converter screen selection. Within the present state of the neutron radiologic art, these practices are generally applicable to specific material combinations, processes, and techniques.

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5. Basis of Application 5.1 Personnel Qualification - Nondestructive testing (NDT) personnel shall be qualified in accordance with a nationally recognized NDT personnel qualification practice or standard such as ANSI/ASNT-CP-189, SNT-TC-1A, MIL-STD-410, or a similar document. The practice or standard used and its applicable revision shall be specified in the contractual agreement between the using parties.

5.2 Qualification of Nondestructive Agencies - If specified in the contractual agreement, NDT agencies shall be qualified and evaluated as described in Practice E 543. The applicable edition of Practice E 543 shall be specified in the contractual agreement.

5.3 Procedures and Techniques - The procedures and techniques to be used shall be as described in these practices unless otherwise specified. Specific techniques may be specified in the contractual agreement.

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5.4 Extent of Examination - The extent of examination shall be in accordance with Section 16 unless otherwise specified.

5.5 Reporting Criteria/Acceptance Criteria - Reporting criteria for the examination results shall be in accordance with 1.3 unless otherwise specified. Acceptance criteria (for example, for reference radiographs) shall be specified in the contractual agreement.

5.6 Reexamination of Repaired/Reworked Items - Reexamination of repaired/reworked items is not addressed in these practices and, if required, shall be specified in the contractual agreement.

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6. Neutron Radiography 6.1 The Method - Neutron radiography is basically similar to X radiography in that both techniques employ radiation beam intensity modulation by an object to image macroscopic object details. X rays or gamma rays are replaced by neutrons as the penetrating radiation in a through-transmission examination.

Since the absorption characteristics of matter for X rays and neutrons differ drastically, the two techniques in general serve to complement one another.

6.2 Facilities - The basic neutron radiography facility consists of a source of fast neutrons, a moderator, a gamma filter, a collimator, a conversion screen, a film image recorder or other imaging system, a cassette, and adequate biological shielding and interlock systems. A schematic diagram of a representative neutron radiography facility is illustrated in Fig. 1.

6.3 Thermalization - The process of slowing down neutrons by permitting the neutrons to come to thermal equilibrium with their surroundings; see definition of thermal neutrons in Terminology E 1316, Section H.

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FIG. 1 Typical Neutron Radiography Facility with Divergent Collimator

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7. Neutron Sources 7.1 General - The thermal neutron beam may be obtained from: ■ ■ ■ ■

a nuclear reactor, a subcritical assembly, a radioactive neutron source, or an accelerator.

Neutron radiography has been achieved successfully with all four sources. In all cases the initial neutrons generated possess high energies and must be reduced in energy (moderated) to be useful for thermal neutron radiography.

This may be achieved by surrounding the source with light materials such as: ■ ■ ■ ■ ■ ■

water, oil, plastic, paraffin, beryllium, or graphite.

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The preferred moderator will be dependent on the constraints dictated by the energy of the primary neutrons, which will in turn be dictated by neutron beam parameters such as thermal neutron yield requirements, cadmium ratio, and beam gamma ray contamination. The characteristics of a particular system for a given application are left for the seller and the buyer of the service to decide.

Characteristics and capabilities of each type of source are referenced in the References section. A general comparison of sources is shown in Table 1.

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TABLE 1 Comparison of Thermal Neutron Sources

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7.2 Nuclear Reactors - Nuclear reactors are the preferred thermal neutron source in general, since high neutron fluxes are available and exposures can be made in a relatively short time span. The high neutron intensity makes it possible to provide a tightly collimated beam; therefore, high-resolution radiographs can be produced.

Ug = Dt/L

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7.3 Subcritical Assembly - A subcritical assembly is achieved by the addition of sufficient fissionable material surrounding a moderated source of neutrons, usually a radioisotope source. Although the total thermal neutron yield is smaller than that of a nuclear reactor, such a system offers the attractions of adequate image quality in a reasonable exposure time, relative ease of licensing, adequate neutron yield for most industrial applications, and the possibility of transportable operation.

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Subcritical Assembly Critical mass A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties (specifically, the nuclear fission crosssection), its density, its shape, its enrichment, its purity, its temperature, and its surroundings. The concept is important in nuclear weapon design.

Explanation of criticality When a nuclear chain reaction in a mass of fissile material is self-sustaining, the mass is said to be in a critical state in which there is no increase or decrease in power, temperature, or neutron population.

A numerical measure of a critical mass is dependent on the effective neutron multiplication factor k, the average number of neutrons released per fission event that go on to cause another fission event rather than being absorbed or leaving the material. When k = 1, the mass is critical, and the chain reaction is barely self-sustaining.

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https://en.wikipedia.org/wiki/Critical_mass

A subcritical mass is a mass of fissile material that does not have the ability to sustain a fission chain reaction. A population of neutrons introduced to a subcritical assembly will exponentially decrease. In this case, k < 1. A steady rate of spontaneous fissions causes a proportionally steady level of neutron activity. The constant of proportionality increases as k increases.

A supercritical mass is one where there is an increasing rate of fission. The material may settle into equilibrium (i.e. become critical again) at an elevated temperature/power level or destroy itself, by which equilibrium is reached. In the case of supercriticality, k > 1

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https://en.wikipedia.org/wiki/Critical_mass

7.4 Accelerator Sources - Accelerators used for thermal neutron radiography have generally been of the low-voltage type which utilize the 3H(d,n)4He reaction, high-energy X-ray machines in which the (x,n) reaction is applied and Van de Graaff and other high-energy accelerators which employ reactions such as 9Be(d,n) 10B. In all cases, the targets are surrounded by a moderator to reduce the neutrons to thermal energies. The total neutron yields of such machines can be on the order of 1012·n·s-1; the thermal neutron flux of such sources before collimation can be on the order of 109n·cm-2·s-1, for example, the yield from a Van de Graaff accelerator.

Total flux Ф 1012·n·s-1 D

I L I = Ф/16(L/D)

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Accelerator Sources-Linear Accelerator

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Accelerator Sources-Cyclotron

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7.5 Isotopic Sources - Many isotopic sources have been employed for neutron radiologic applications. Those that have been most widely utilized are outlined in Table 2. Radioactive sources offer the best possibility for portable operation. However, because of the relatively low neutron yield, the exposure times are usually long for a given image quality. The isotopic source252Cf offers a number of advantages for thermal neutron radiology, namely, low neutron energy and small physical size, both of which lead to efficient neutron moderation, and the possibility for high total neutron yields.

TABLE 2 Radioactive Sources Employed for Thermal Neutron Radiography

A: These comments compare sources in the table.

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8. Imaging Methods and Conversion Screens 8.1 General - Neutrons are nonionizing particulate radiation that have little direct effect on radiographic film. To obtain a neutron radiographic image on film, a conversion screen is normally employed; upon neutron capture, screens emit prompt and delayed decay products in the form of nuclear radiation or light. In all cases the screen should be placed in intimate contact with the radiographic film in order to obtain sharp images.

8.2 Direct Method - In the direct method, a film is placed on the source side of the conversion screen (front film) and exposed to the neutron beam together with the conversion screen. Electron emission upon neutron capture is the mechanism by which the film is exposed in the case of gadolinium conversion screens.

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The screen is generally one of the following types: 1.

2.

3.

a free-standing gadolinium metal screen accessible to film on both sides; a sapphire coated, vapordeposited gadolinium screen on a substrate such as aluminum; or a light-emitting fluorescent screen such as gadolinium oxysulfide or 6LiF/ZnS. Exposure of an additional film (without object) is often useful to resolve artifacts that may appear in radiographs.

Such artifacts could result from screen marks, excess pressure, light leaks, development, or nonuniform film. In the case of light-emitting conversion screens, it is recommended that the spectral response of the light emission be matched as closely as possible to that of the film used for optimum results.

The direct method should be employed whenever high-resolution radiographs are required, and high beam contamination of low-energy gamma rays or highly radioactive objects do not preclude its use.

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8.3 Indirect Method - This method makes use of conversion screens that can be made temporarily radioactive by neutron capture. The conversion screen is exposed alone to the neutronimaging beam; the film is not present.

Candidate conversion materials include (1) rhodium, (2) gold, (3) indium, and (4) dysprosium.

Indium and dysprosium are recommended with dysprosium yielding the greater speed and emitting less energetic gamma radiation.

It is recommended that the conversion screens be activated in the neutron beam for a maximum of three half-lives (3 x T½) . Further neutron irradiation will result in a negligible amount of additional induced activity. After irradiation, the conversion screens should be placed in intimate contact with a radiographic film in a vacuum cassette, or other light-tight assembly in which good contact can be maintained between the radiographic film and radioactive screen.

X- ay intensification screens may be used to increase the speed of the autoradiographic process if desired.

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For the indirect type of exposure, the material from which the cassette is fabricated is immaterial as there are no neutrons to be scattered in the exposure process. In this case, as in the activation process, there is little to be gained for conversion screen-film exposures extending beyond three halflives. It is recommended that this method be employed whenever the neutron beam is highly contaminated with gamma rays, which in turn cause film fogging and reduced contrast sensitivity, or when highly radioactive objects are to be radiographed. In short, this method is beam gamma-insensitive.

8.4 Other Imaging Systems - The scope of these practices is limited to film imaging (see 1.2). However, other imaging systems such as track-etch or radioscopic systems are available.

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Track-etch Ion tracks are damage-trails created by swift heavy ions penetrating through solids, which may be sufficiently-contiguous for chemical etching in a variety of crystalline, glassy, and/or polymeric solids.[1][2] They are associated with cylindrical damage-regions several nanometers in diameter[3][4] and can be studied by Rutherford backscattering spectrometry (RBS), transmission electron microscopy (TEM), small-angle neutron scattering (SANS), smallangle X-ray scattering (SAXS) or gas permeation.

"Fresh" (latent or unetched) Californium-252 fission tracks[1] in a chromite (FeCr2O4) grain from the Allende meteorite, showing up in a weak-beam darkfield TEM image which lights up the strain-fields around the 40Ă&#x2026;-diameter trackdamage cores. This work confirmed chromite's ability to record nuclear particle tracks in spite of its relatively low resistivity.

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Track-etch

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More Reading on Radioscopy ■ http://www.ndt.net/article/wcndt00/papers/idn284/idn284.htm ■ http://www.nationalboard.org/Index.aspx?pageID=164&ID=199

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9. Neutron Collimators 9.1 General - Neutron sources for thermal neutron radiology generally involve a sizeable moderator region in which the neutron motion is highly multidirectional. Collimators are required to produce a beam and thereby produce adequate image resolution capability in a neutron radiology facility. It should be noted that in the definitions of collimator parameters, it is assumed that the object under examination is placed as close to the imaging system as possible to decrease both magnification and image unsharpness due to the finite neutron source size. Several types of collimators are available. These include the widely used divergent type, multichannel, pinhole, and straight collimators. The image spatial resolution properties of the beams are generally set in part by the diameter or longest dimension of the collimator entrance port (D) and the distance between that aperture and the imaging system (L). An exception is the multichannel collimator in which D is the diameter of a channel and L is the length of the collimator. It should be noted that the detection system used in conjunction with a multichannel collimator will register the collimator pattern.

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Registry can be eliminated by empirically adjusting the distance between the collimator and the imaging system until the pattern disappears. Ratios of L/D as low as 10 are not unusual for low neutron yield sources, while higher resolution capability systems often will display L/ D values of several hundred or more. Method E 803 details the method of measuring the L/D ratio for neutron radiography systems. The actual spatial resolution or image unsharpness in a particular radiologic examination will depend, of course, on factors additional to the beam characteristics. These include the object size, the geometry of the system, and scatter conditions. For the typical calculation of geometric unsharpness, the size of the X-radiologic source, F, would be replaced by the size of the effective thermal neutron radiologic source (D) as discussed in Guide E 94.

Keywords: radiologic source

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9.2 Divergent Collimator - The divergent collimator is a tapered reentrant port into the point of highest thermal neutron flux in the moderator. The walls of the collimator are lined with a thermal neutron absorbing material to permit only unscattered neutrons from the source to reach the object and the image plane. This type of collimator is preferred when larger objects will be radiographed in a single exposure. It is recommended that the divergent collimator be lined with a neutron absorber which produces neutron capture decay products that will not result in background fogging of the film, such as 6Li carbonate. A typical divergent collimating system is illustrated in the schematic diagram of Fig. 1.

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9.3 Multichannel Collimator - The multichannel collimator is an array of tubular collimators stacked within a larger collimator envelope. It is recommended as a means of achieving a high degree of collimation within a short collimation length. When this type of collimator is employed, a suitable collimator to detector distance should be maintained to avoid registry of the collimator pattern on the radiologic image.

9.4 Straight Collimator - A straight-tube reentrant port can also be used instead of the tapered assembly described in 9.2. Although such collimators were widely used in early neutron radiologic work, the need to examine larger objects and to achieve higher resolution has fostered the use of divergent collimators. a straight collimator when it is employed in conjunction with a pinhole iris. The pinhole is generally fabricated from a neutron-opaque material such as Cd, Gd, or 10B. The resolution attainable will be dependent on the pinhole diameter D. A schematic diagram of this system is illustrated in Fig. 2.

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FIG. 2 Pinhole Collimator

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Parallel & Divergent Collimator Fig. 2 Thermalization and collimation of beam in neutron radiography. Neutron collimators can be of the parallel-wall (a) or divergent (b) type. The transformation of fast neutrons to slow neutrons is achieved by moderator materials such as paraffin, water, graphite, heavy water, or beryllium. Boron is a typically used neutron-absorbing layer. The L/D ratio, where L is the total length from the inlet aperture to the detector (conversion screen) and D is the effective dimension of the inlet of the collimator, is a significant geometric factor that determines the angular divergence of the beam and the neutron intensity at the inspection plane

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ASMV17 Neutron Radiography

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ASMV17 Neutron Radiography

10. Beam Filters 10.1 Thermal Neutron Radiography - In general, filters may not be necessary.

However, it may be desirable to employ Pb or Bi filters in the neutron beam to minimize beam gamma-ray contamination. Whenever Bi gamma-ray filters are employed in a high neutron flux environment, the filter should be encased in a sealed aluminum can to contain alpha particle contamination due to the 210Po produced by the neutron capture reaction in 209Bi. Gamma rays can cause film fogging and reduced contrast sensitivity. In particular, some scintillator converter screens exhibit sensitivity to beam gamma-ray contamination. This effect can be minimized by careful selection of the screen/film combination.

Keywords: gamma-ray contamination

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11. Masking 11.1 General - In general, masking is not often used in thermal neutron radiology. Where it is desirable to reduce scatter or to reduce unusual contrasts, the choice of masking materials should be made carefully.

Materials that scatter readily, such as those containing hydrogen or materials that emit radiation that may be readily detected, for example, as indium, dysprosium, or cadmium, should be avoided or used with exceptional care.

Lithium-containing materials may be useful for masking purposes.

Background fogging may result from the 470 keV gamma ray from boron.

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Fig. 1 Mass attenuation coefficients for the elements as a function of atomic number for thermal (4.0 × 10-21 J, or 0.025 eV) neutrons and x-rays (energy 125 kV). The mass attenuation coefficient is the ratio of the linear attenuation coefficient, μ, to the density, ρ, of the absorbing material.

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ASMV17 Neutron Radiography

12. Effect of Materials Surrounding Object and Cassette 12.1 Backscatter - As in the case of X radiography, effects of back-scattered radiation, for example, from walls, etc., can be reduced by masking the radiation beam to the smallest practical exposure area. Effects of backscatter can be determined by placing a neutron-absorbing marker of a material such as gadolinium and a gamma-absorbing marker of a material such as lead on the back of the exposure cassette. If problems with backscatter are shown, one should minimize in the exposure area materials that scatter or emit radiation as discussed in Section 11. Backscatter can be minimized by placing a neutron absorber such as gadolinium behind the cassette.

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13. Cassettes 13.1 Material of Construction - The cassette frame and back may be fabricated of aluminum or magnesium as employed in standard X-ray film cassettes. Aluminum or magnesium entrance window X-ray cassettes can be used directly for neutron radiography. Special vacuum cassettes designed specifically for neutron radiography are preferred to conventional X-ray cassettes. Plastic window X-ray cassettes should not be used. The plastic entrance face may be replaced with thin, 0.25 to 1.7-mm thick 1100 reactor grade, or 6061T6 aluminum, or magnesium to eliminate image resolution degradation due to scattering; use of hydrogenous materials in the construction of a cassette can lead to image degradation and the use of these materials should be considered carefully.

13.2 Vacuum Cassettes - Whenever possible, vacuum cassettes should be employed to hold the converter foil or scintillator screen in intimate contact with the film both in the direct and indirect exposure methods. Cassettes of the type that maintain vacuum during the exposure or that must be pumped continuously during the exposure are equally applicable. Vacuum storage minimizes atmospheric corrosion of converters such as dysprosium and substantially increases their useful life.

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14. Thermal Neutron Radiographic Image Quality 14.1 Image Quality Indicators - Image quality indicators for thermal neutron radiography are described in Method E 545. The devices and methods described therein permit: (1) the measurement of beam composition, including relative thermal neutron to higher energy neutron composition and relative gamma-ray content; and (2) devices for indicating the sensitivity of detail visible on the neutron radiograph.

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15. Contrast Agents 15.1 Improved Contrast - Contrast agents are useful in thermal neutron radiology for demonstrating improved contrast of a tagged material or component. For thermal neutron radiography even simple liquids such as water or oil can serve as effective contrast agents. Additional useful marker materials can be chosen from neutron-attenuating materials such as boron, cadmium, and gadolinium. Of course, the deleterious effect of the contrast agent employed upon the test object should be considered.

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16. Types of Materials To Be Examined with Thermal Neutron Radiography 16.1 General - This section provides a categorization of applications according to the characteristics of the object being examined. The following paragraphs provide a general list of four separate categories for which thermal neutron radiographic examination is particularly useful. Additional details concerning neutron attenuation are discussed in Appendix X1.

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16.2 Detection of Similar Density Materials - Thermal neutron radiography can offer advantages in cases of objects of similar-density materials, that can represent problems for X-radiography. Some brazing materials, such as cadmium and silver, for example, are readily shown by thermal neutron radiography. Contrast agents can help show materials such as ceramic residues in investment-cast turbine blades. Inspection of castings for voids or uniformity and of cladding materials can often be accomplished with thermal neutron radiography. Material migration in solid-state electroniccomponents, electrolyte migration in batteries, diffusion between light and heavy water, and movement of moisture through concrete are examples in which thermal neutron radiography has proveduseful.

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16.3 The Detection of Low-Density Components and Materials in HighDensityContainments - This recommended category includes the examination of metal-jacketed explosive devices, location andmeasurement of hydrogen in cladding materials and weldments, and of moisture in assemblies, location of fluids and lubricants in metal containmentsystems, examination of adhesive bonds in metal parts including honeycomb, location of liquid metals in metal parts, location of corrosion products in aluminum airframe components, examination of boron-filament composites, studies of fluid migration in sealed metal systems, and the determination of poison distribution in nuclear reactor fuel rods or control plates.

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16.4 The Examination of Highly Radioactive Objects - The technique of indirect neutron imaging is insensitive to gamma radiation in the imaging beam or from a radioactive object that could produce fogging of the film with the resulting loss in contrast sensitivity. This category of recommended examinations includes the inspection of irradiated reactor fuel capsules and plates for cracking and swelling, the determination of highly enriched nuclear fuel distribution in assemblies, and the inspection of weld and braze joints in irradiated subassemblies.

16.5 Differentiation Between Isotopes of the Same Element - Neutron attenuation is a function of the particular isotope rather than the element involved. There are certain isotopes that have either very high or very low attenuation and, therefore, are subject to detection by thermal neutron radiology. For example, it is possible to differentiate between isotopes such as 1H and 2H or 235U and 238U.

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17. Activation of Objects and Exposure Materials 17.1 Objects - Certain objects placed in the neutron beam may be activated, depending upon the: ■ incident neutron energy (Mev), ■ intensity (n/cm2) and ■ exposure time (s), and ■ the material activation cross section (cm-2) and ■ half-life (T½).

Therefore, objects under examination may become radioactive. In extreme cases this could produce film fogging, thereby reducing contrast. Safety is a strong consideration; radiation monitoring of objects should be performed after each exposure. Objects that exhibit a radiation level too high for handling should be set aside to allow the radiation to decay to acceptable levels. In practice, since neutron exposure times are normally short, a short decay period will usually be satisfactory.

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17.2 Cassettes - Radiographic cassettes containing materials such as aluminum and steel can become activated, particularly on multiple exposures.

Monitoring of radiation to determine safe handling levels can alleviate safety problems and minimize film fogging. Activated cassettes, screens, and objects should be kept away from unexposed or unprocessed film. Converted X-radiography cassettes are virtually worthless for high-resolution industrial neutron radiography. Vacuum cassettes should be employed whenever possible to maintain the film and converter foil in intimate contact during the exposure. This holds for both the direct and indirect methods.

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17.3 Conversion Screens - Conversion screens used for direct exposure methods are usually chosen for low activation properties. Conversion screen materials such as gadolinium, boron, or lithium seldom cause problems.

(Gd, B, Li) However, conversion screens for the indirect exposure method are chosen for high-activation potential. Therefore, exposed and activated screens such as indium, dysprosium, rhodium, or gold should be handled with care. Screens should be handled with gloves or tongs and should be moved in a shield.

High-radiation exposures to the fingers are a potential hazard.

(Dy, Rh, Au) A cassette will shield much of the beta radiation emitted by the commonly used indirect exposure converter screens. Conversion screens should normally be allowed at least a three half-life decay period ( 3 x T1/2 )before reuse to prevent double exposures.

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18. Keywords 18.1 neutron attenuation; neutron collimator; neutron radiography; neutron sources

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APPENDIXES (Nonmandatory Information)

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X1. Attenuation Of Neutrons By Matter X1.1 A major advantage of using neutrons for radiography is that radiologic observation of certain material combinations is easily accomplished with slow neutrons where, because of attenuation differences, problems will arise with X rays. For example, the high attenuation of slow neutrons by elements such as hydrogen, lithium, boron, cadmium, and several rare earths means that these materials can readily be shadowed with neutrons even when they are combined in an assembly with some high atomic weight material such as steel, lead, bismuth, or depleted uranium. Although the heavy material would make X radiography difficult, neutron radiography should yield a successful inspection. Further, the differences in slow neutron attenuation often found between neighboring materials in the periodic table offer an advantage for neutron radiologic discrimination between materials that have similar X-ray attenuation characteristics.

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X1.2 This advantage is illustrated in Fig. X1.1 in which the mass attenuation coefficients μ/r are plotted as a function of atomic number of the attenuating element for both X rays (about 120 kVp energy) and slow neutrons. There are many apparent attenuation differences. The coefficient μ/r is normally used in attenuation calculations in the exponential relationship: I/Io = e –(μ/ρ)ρx

(X1.1)

where: I/Io = ratio of emergent radiation intensity to the intensity incident on a material, μ = linear attenuation coefficient, r = density, and x = thickness. μ = σtotal , total cross section area cm2 x Number of nuclei in cm2 Number of nuclei in 1 gram of material = N/gram atomic weight (A), Number of gram of material in 1 cm2 = density, ρ Number of nuclei in 1 cm2 = ρN/A μ = (ρN/A)∙σtotal Charlie Chong/ Fion Zhang

X1.3 For neutrons, it is more convenient to have the relationship between attenuation coefficient and cross section, as follows: μ = P∙σtotal = p∙(σabs + σscatt) where: P = number of nuclei per cm3 of attenuating material, = total cross section (cm2), equal to the sum of absorption σabs and σtotal scattering σscatt cross sections, and μ = the linear attenuation coefficient (cm-1).

A tabular listing of linear attenuation coefficients is shown in Table X1.1 and a comparative plot is given in Fig. X1.2; these values should be considered only as general guides. The data presented in Fig. X1.3 give half-value-layer thicknesses for thermal neutrons for many materials.

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X1.4 In radiologic situations, radiation that is transmitted through the object being examined is recorded so that those areas in which radiation has been removed, either by absorption or by scattering, may be observed. (Eq X1.1) and (Eq X1.2) are valuable in assessing the relative change in transmitted radiation intensity for several materials and thicknesses within an object of interest.

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FIG. X1.1 Approximate Mass Attenuation Coefficients μ/ρ as a Function of Atomic Number

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FIG. X1.2 Calculated Thermal Neutron and 100 and 500 KEV X-Ray Linear Attenuation Coefficients (Îź) as a Function of Atomic Number (A)

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FIG. X1.3 Half-Value Layers of Selected Materials for Thermal Neutrons

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FIG. X1.3 Half-Value Layers of Selected Materials for Thermal Neutrons

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TABLE X1.1 Thermal Neutron Linear Attenuation Coefficients Using Average Scattering and Thermal Absorption Cross Sections for the Naturally Occurring Elements

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Neutron Cross Section of the elements

â&#x2013; http://periodictable.com/Properties/A/NeutronCrossSection.html Charlie Chong/ Fion Zhang

X2. Calculation Of The Linear Attenuation Coefficient Of A Compound ■ Element’s μ X2.1 If the material under examination contains only one element, then the linear attenuation coefficient is as follows: μ = (ρN/A)∙σ

(X2.1)

where: μ = linear attenuation coefficient, cm-1, ρ= material density, gm·cm-3, N = Avogadro’s number = 6.023 x 1023 atoms·g-mol-1, σ = total cross section, cm2, and A = gram atomic weight of material. (ρN/A) = numbers of nuclei in 1 cm-3 of material.

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■ Compound's μ X2.2 If, on the other hand, the material under examination contains several elements, or is in the form of a compound, then the linear attenuation coefficient is as follows: μ = (ρN/M)∙(ѵ1σ1 + ѵ2σ2 +..... + ѵiσi)

(X2.2)

where: μ = linear attenuation coefficient of the compound, cm−1, ρ = compound density, g·cm−3, N = Avogadro’s number = 6.023 x 1023 atoms·g-mol−1, M = gram molecular weight of the compound, σi = total cross section of the ith atom, cm2. (ρN/M) = numbers of nuclei in 1 cm-3 of material (compound).

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X2.3 As an example, consider the calculation of the linear attenuation coefficient, μ, for the compound polyethylene CH2: μ = (ρN/M)∙(1σC + 2σH) ρ = 0.91 g·cm−3, N = 6.023 x 1023 atoms·g-mol−1, M = 2(1.0079) + 14.011 = 14.0268 σH = (20.49 + 0.333）10-24 cm2 = 20.823 x 10-24 cm2 σC =（4.74 + 0.0035）10-24 cm2 = 4.744 x 10-24 cm2 μ = 1.8126 cm-2

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â&#x2013; http://minerals.usgs.gov/minerals/pubs/commodity/ Charlie Chong/ Fion Zhang

TABLE X1.1 Thermal Neutron Linear Attenuation Coefficients Using Average Scattering and Thermal Absorption Cross Sections for the Naturally Occurring Elements

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References

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End Of Reading

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Reading-2 ASMHB-17 NRT

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1.0 Principles of Neutron Radiography Neutron radiography is similar to conventional radiography in that both techniques employ radiation beam intensitymodulation by an object to image macroscopic object details. X-rays or -rays are replaced by neutrons as the penetrating radiation in a through-transmission inspection. The absorption characteristics of matter for x-rays and neutrons differ drastically; the two techniques in general serve to complement one another. Neutrons are subatomic particles that are characterized by relatively large mass and a neutral electric charge. The attenuation of neutrons differs from the attenuation of x-rays in that the processes of attenuation are nuclear rather than ones that depend on interaction with the electron shells surrounding the nucleus. Neutrons are produced by nuclear reactors, accelerators, and certain radioactive isotopes, all of which emit neutrons of relatively high energy (fast neutrons). Because most neutron radiography is performed with neutrons of lower energy (thermal neutrons), the sources are usually surrounded by a moderator, which is a material that reduces the kinetic energy of the neutrons.

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2.0 Neutron Versus Conventional Radiography.

Neutron radiography is not accomplished by direct imaging on film, because neutrons do not expose x-ray emulsions efficiently. â&#x2013; In one form of neutron radiography, the beam of neutrons impinges on a conversion screen or detector made of a material such as dysprosium or indium, which absorbs the neutrons and becomes radioactive, decaying with a short half-life. In this method, the conversion screen alone is exposed in the neutron beam, then immediately placed in contact with film to expose it by autoradiography. â&#x2013; In another common form of imaging, a conversion screen that immediately emits secondary radiation is used with film directly in the neutron beam. Neutron radiography differs from conventional radiography in that the attenuation of neutrons as they pass through the testpiece is more related to the specific isotope present than to density or atomic number.

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X-rays are attenuated more by elements of high atomic number than by elements of low atomic number, and this effect varies relatively smoothly with atomic number. Thus, x-rays are generally attenuated more by materials of high density than by materials of low density. For thermal neutrons, attenuation generally tends to decrease with increasing atomic number, although the trend is not a smooth relationship. In addition, certain light elements (hydrogen, lithium, and boron), certain medium-to-heavy elements (especially cadmium, samarium, europium, gadolinium, and dysprosium), and certain specific isotopes have an exceptionally high capability of attenuating thermal neutrons (Fig. 1). This means that neutron radiography can detect these highly attenuating elements or isotopes when they are present in a structure of lower attenuation.

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Fig. 1 Mass attenuation coefficients for the elements as a function of atomic number for thermal (4.0 Ă&#x2014; 1021 J, or 0.025 eV) neutrons and x-rays (energy 125 kV). The mass attenuation coefficient is the ratio of the linear attenuation coefficient, , to the density, , of the absorbing material.

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Thermal (slow) neutrons permit the radiographic visualization of low atomic number elements even when they are present in assemblies with high atomic number elements such as iron, lead, or uranium. Although the presence of the heavy metals would make detection of the light elements virtually impossible with x-rays, the attenuation characteristics of the elements for slow neutrons are different, which makes detection of light elements feasible.

Practical applications of neutron radiography include the inspection of metaljacketed explosives, rubber O-ring assemblies, investment cast turbine blades to detect residual ceramic core, and the detection of corrosion in metallic assemblies.

Using neutrons, it is possible to detect radiographically certain isotopes: for example, certain isotopes of hydrogen, cadmium, or uranium. Some neutron image detection methods are insensitive to background Îł-rays or x-rays and can be used to inspect radioactive materials such as reactor fuel elements. In the nuclear field, these capabilities have been used to image highly radioactive materials and to show radiographic differences between different isotopes in reactor fuel and control materials. The characteristics of neutron radiography complement those of conventional radiography; one radiation provides a capability lacking or difficult for the other.

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3.0 Neutron Sources The excellent discrimination capabilities of neutrons generally refer to neutrons of low energy, that is, thermal neutrons. The characteristics of neutron radiography corresponding to various ranges of neutron energy are summarized in Table 1. Although any of these energy ranges can be used for radiography, this article emphasizes the thermal-neutron range, which is the most widely used for inspection. In thermal-neutron radiography, an object (testpiece) is placed in a thermal-neutron beam in front of an image detector.

The neutron beam may be obtained from a nuclear reactor, a radioactive source, or an accelerator. Several characteristics of these sources are summarized in Table 2. For thermal-neutron radiography, fast neutrons emitted by these sources must first be moderated and then collimated (Fig. 2).

The radiographic intensities listed in Table 2 typically do not exceed 10-5 times the total fast-neutron yield of the source.

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Part of this loss is incurred in moderating the neutrons, and the remainder in bringing a collimated beam out of a large-volume moderator. Collimation is necessary for thermal-neutron radiography because there are no useful point sources of low-energy neutrons. Good collimation in thermal-neutron radiography is comparable to small focal-spot size in conventional radiography; the images of thick objects will be sharper with good collimation.

On the other hand, it should be noted that available neutron intensity decreases with increasing collimation.

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Table 1 Characteristics of neutron radiography at various neutron-energy ranges

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Table 2 Properties and characteristics of thermal-neutron sources

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Fig. 2 Thermalization and collimation of beam in neutron radiography.

Neutron collimators can be of the parallel-wall (a) or divergent (b) type. The transformation of fast neutrons to slow neutrons is achieved by moderator materials such as paraffin, water, graphite, heavy water, or beryllium. Boron is a typically used neutron-absorbing layer. The L/D ratio, where L is the total length from the inlet aperture to the detector (conversion screen) and D is the effective dimension of the inlet of the collimator, is a significant geometric factor that determines the angular divergence of the beam and the neutron intensity at the inspection plane.

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3.1 Nuclear Reactors.

Many types of reactors have been used for thermal-neutron radiography. The high neutron flux generally available provides high-quality radiographs and short exposure times. Although truck-mounted reactors are technically feasible, a reactor normally must be considered a fixed-site installation, and testpieces must be taken to the reactor for inspection. Investment costs are generally high, but small medium-cost reactors can provide good results.

When costs are compared on the basis of available neutron flux (typically, 1012 n/cm2 路 s flux is often available at collimator entrance, and 106 to 107 n/cm2 路 s flux is available at the film plane), reactor sources can be less costly than other sources.

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3.2 Accelerators. The accelerators most often used for thermal-neutron radiography are: • The low-voltage type employing the reaction 3H + 2H → 4He + 1n , a(d,T) generator, where n, d, and T represent the neutron, deuteron (the nucleus of a deuterium atom, D or 2H, that consists of one neutron and one proton), and tritium (3H), respectively • High-energy x-ray machines, in which (x,n) reactions are used, where x represents x-ray radiation • Van de Graaff accelerators • More recently, high-energy linear accelerators and cyclotrons to generate neutrons by charged-particle reactions on beryllium or lithium targets

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■ Low-Voltage Accelerators. A (d,T) generator provides fast-neutron yields in the range of 1010 to 1012 n/s. Target lives in sealed neutron tubes are reasonable (100 to 1000 h, depending on yield), and the sealed-tube system presents a source similar to that of certain types of x-ray machines.

■ High- nergy X-Ray Machines. An (x,n) neutron source is a high-energy x-ray source such as a linear accelerator that can be converted for the production of neutrons by adding a suitable secondary target: for example, beryllium. X- ays having energies above an energy threshold level cause the secondary target to emit neutrons; in beryllium, the threshold x-ray energy for neutron production is 2.67 × 10-13 J (1.66 MeV). Useful neutron radiography has been performed with an 8.8 × 10-13 J (5.5 MeV) linear accelerator having an x-ray output of 0.17 C/kg · min (650 R/min) at 1 m (3 ft). Changeover time from neutron emission to x-ray emission for this source was only 1 h. Beam intensities for neutron radiography with this source were about 5 × 104 n/cm2 · s. with reasonable beam collimation.

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■ Van de Graaff Accelerators.

Much higher beam intensities have been obtained by the acceleration of deuterons onto a beryllium target in a 3.2 × 10-13 J (2.0 MeV) Van de Graaff generator. An intensity of 1.2 × 106 n/cm2 · s was achieved (with medium collimation), and it is estimated that an acceleration voltage of 4.8 × 10-13 J (3.0 MeV) would improve beam intensity by a factor of approximately six. The principle of the Van de Graaff machine is illustrated in Fig. 3. A rotating belt transports the charge from a supply to a high-voltage terminal. An ion source within the terminal is fed deuterium gas from a reservoir frequently located within the terminal. A radio-frequency system ionizes the gas, and positive ions are extracted into the accelerator tube. The terminal voltage of about 3 MV is distributed by a resistor chain over about 80 gaps forming the accelerator tube, all of which is enclosed in a pressure vessel filled with insulating gas (N2 and CO2 at 2.0 MPa, or 290 psi).

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Fig. 3 Cross section showing Van de Graaff principle as it is applied to neutron radiography. Source: Ref 6

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The particle beam is extracted along flight tubes. In a typical neutron reaction, the beam bombards a water-cooled beryllium target in the center of the water moderator tank, which also serves as a partial shield. The higher-energy accelerators indicated above can provide neutron yields of 1013 n/s and moderated, well-collimated beam intensities of the order of 106 n/cm2 · s.

A few 4.8 × 10-13 J (3.0 MeV) Van de Graaff generators have recently been placed in service for thermal-neutron radiography. In one such Van de Graaff system designed for neutron radiography, deuterons (4.8 × 10-13 J, or 3 MeV; 280 A) are accelerated onto a disk-shaped, water-cooled beryllium metal target. Neutrons in the range of 3.2 to 9.6 × 10-13 J (2 to 6 MeV) are emitted preferentially in the forward direction and are moderated in water. The 4 (solid angle) yield of 5 × 1011 n/s produces a peak thermal neutron flux of 2 × 109 n/cm2 · s. At a collimator ratio of 36:1, the typical exposure time for highquality film (3 × 109 n/cm2) is about 2 h.

Keywords: Neutron Yield Collimated beam intensity/ Neutron flux Charlie Chong/ Fion Zhang

The accelerator tank for the 4.8 Ă&#x2014; 10-13 J (3 MeV) machine measures 5.2 m (17 ft) in length and 1.5 m (5 ft) in diameter. The weight is 6100 kg (13,500 lb).

The dimensions of the water tank are approximately 1 m (3 ft) on each side.

Neutron beams can be extracted through three horizontal beam collimators.

Unlike reactors, subcritical multipliers, or (d, T) accelerators, the Van de Graaff accelerators utilize no radioactive source material and sometimes require less stringent license processes. Other acceleration machines or reactions can be used for thermal-neutron radiography. However, those described above have been most widely used.

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3.3 Radioactive Sources.

There are many possible radioactive sources. The characteristics of several radioisotopes that are commonly used are summarized in Table 3.

Table 3 Properties and characteristics of several radioisotopes used for thermal-neutron radiography

(γ, n) (α, n) (α, n)

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γ

γ γ

Table 3 Properties and characteristics of several radioisotopes used for thermal-neutron radiography

(α, n) (α, n)

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γ

■ Radioisotopes offer the best prospect for a portable neutron-radiographic facility, but it should be recognized that the thermal-neutron intensity is only about 10-5 of the total fast-neutron yield from the source. Consequently, neutron radiography using a radioisotope as a neutron source normally requires long exposure times and fast films. For example, a typical 3.7 × 1011 Bq (10 Ci) source would provide a total fast-neutron yield of the order of 107 n/s. The radiographic intensity would be about 102 n/cm2 · s, and a typical exposure time using a fast film/converter-screen combination would be about 1 h.

Californium-252, usually purchased in the form shown in Fig. 4, has been the most frequently used radioactive source for neutron radiography.

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Fig. 4 Cross section of doubly encapsulated 252Cf source. Source: Ref 6

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3.4 Subcritical Assembly.

Another type of source that has received some attention is a subcritical assembly. This type of source is similar to a reactor, except that the neutron flux is less and the design is such that criticality cannot be achieved. A subcritical assembly offers some of the same neutron multiplication features as a reactor. It is somewhat easier to operate, and safety precautions are less stringent, because it is not capable of producing a self-sustaining neutron chain reaction.

Interesting reading: https://en.wikipedia.org/wiki/Critical_mass

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4.0 Attenuation of Neutron Beams Unlike electrons and electromagnetic radiation, which interact with orbital electrons surrounding an atomic nucleus, neutrons interact only with atomic nuclei. Usually, neutrons are deflected by interaction with the nuclei, but occasionally a neutron is absorbed into a nucleus. When a neutron collides with the nucleus of an atom and is merely deflected, the neutron imparts some of its kinetic energy to the atom. Both the neutron and the atom move off in different directions from the original direction of motion of the neutron.

This process, known as scattering, reduces the kinetic energy of the neutron and the probability that the neutron will pass through the object (testpiece) in a direction that will permit it to be detected by a device placed behind the object. True absorption of neutrons occurs when they are captured by nuclei.

The capture of a neutron transforms the nucleus to the next-higher isotope of the target nucleus and sometimes produces an unstable nucleus that then undergoes radioactive decay.

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The probability that a collision between a neutron and a nucleus will result in capture is known as the capture cross section and is expressed as an effective area per atom. (The capture cross section is usually measured in barns, 1 barn equaling 10-24 cm2 or 1.6 × 10-5 in.2.) The capture cross section varies with neutron energy, atomic number, and mass number. (the probability of neutron/ nuclei collision that results in capture) For thermal neutrons (energy of about 4.0 × 10-21 J, or 0.025 eV), the average capture cross section varies randomly with atomic number, being high for certain elements and relatively low for other elements. The cross section actually varies by isotope rather than element. However, radiographers usually consider an average cross section for an element. For intermediate neutrons (energies of 8.0 × 10-20 to 1.6 × 10-15 J, or 0.5 eV to 10 keV) and for fast neutrons (energies exceeding 1.6 × 10-15 J, or 10 keV), the capture cross section is normally smaller than that for thermal neutrons, and there is much less variation with atomic number.

For fast neutrons, most elements are similarly absorbing, and scattering is the dominant process of attenuation.

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In relation to other types of penetrating radiation, many materials interact less with neutrons. Therefore, neutrons can sometimes be used to inspect greater thicknesses than can be conveniently inspected with electromagnetic radiation. The combined effect of scattering and capture can be expressed as a mass-absorption coefficient; this coefficient is used to determine the exposure factor for the neutron radiography of a given object (testpiece). For a given material, attenuation varies exponentially with thickness, and the basic law of radiation absorption (discussed in the article "Radiographic Inspection" in this Volume) applies to neutron attenuation as well as to the attenuation of electromagnetic radiation. μ = (ρN/A)∙σ I = Io e -μt

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5.0 Neutron Detection Methods Detection methods for neutron radiography generally use photographic or xray films. In the so-called direct-exposure method, film is exposed directly to the neutron beam, with a conversion screen or intensifying screen providing the secondary radiation that actually exposes the film (Fig. 5a).

Alternatively, film can be used to record an autoradiographic image from a radioactive image-carrying screen in a technique called the transfer method (Fig. 5b).

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Fig. 5 Schematics of neutron radiography with film using the direct-exposure method (a) and the transfer method (b). The cassette is a light-tight device for holding film or conversion screens and film in close contact during exposure.

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5.1 Direct-Exposure Method.

Conversion screens of thin gadolinium foil or a scintillator have been most widely used in the direct-exposure method. When bombarded with a beam of neutrons, some of the gadolinium atoms absorb some of the neutrons and then promptly emit γ-rays. The γ-rays in turn produce internal conversion electrons that actually expose the film; these are directly related in intensity to the intensity of the neutron beam. Scintillators, on the other hand, are fluorescent materials often made of zinc sulfide crystals that also contain a specific isotope, such as 6Li3 or 10B5. In a neutron beam, these isotopes react with neutrons as follows: + 1n0 → 3H1 + α (4He2) 10B + 1n →7Li + α (4He ) 5 0 3 2 6Li

3

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The α particles emitted as a result of these reactions cause the zinc sulfide to fluoresce, which in turn exposes the film. Gadolinium oxysulfide, a scintillator originally developed for conventional radiography, is now widely used for neutron radiography. Scintillators provide useful images with total exposures as low as 5 × 105 n/cm2. The high speed and favorable relative neutron/gamma response of scintillators make them attractive for use with nonreactor neutron sources. For high-intensity sources, gadolinium screens are widely used. Gadolinium screens provide greater uniformity and image sharpness (high contrast resolution of 10 μm, or 400 μin., has been reported), but an exposure about 30 or more times that of a scintillator is required, even with fast films. Excessive background radiation should be kept to a minimum because it can have a detrimental effect on image quality.

Keywords: Zinc Sulfide-Scintillator Gadolinium oxysulfide-Scintillator Gadolinium screens-Non Scintillator

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5.2 In the transfer method, a thin sheet of metal called a transfer screen, which is usually made of indium or dysprosium, is exposed to the neutron beam transmitted through the specimen. Neutron capture by the isotope 115In49 or 164Dy66 (Dysprosium) induces radioactivity, indium having a half-life of 54 min and dysprosium a half-life of 2.35 h. The intensity of radioactive emission from each area of the transfer screen is directly related to the intensity of the portion of the transmitted neutron beam that induced radioactivity in that area. The radiograph to be interpreted is made by placing the radioactive transfer screen in contact with a sheet of film. The particle β and γ-ray emissions from the transfer screen expose the film, with film density in various portions of the developed image being proportionally related to the intensity of radioactive emission.

Keywords: Transfer screen-indium or dysprosium, In, Dy. Thermal neutron filter using Cadmium for epithermal neutron radiography, Cd. Converter screen uses gadolinium which emit beta particles, Gd.

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The transfer method is especially valuable for inspecting a radioactive specimen. Although the radiation emitted by the specimen (especially Îł-rays) causes heavy film fogging during conventional radiography or direct-exposure neutron radiography, the same radiation will not induce radioactivity in a transfer screen. Therefore, a clear image of the specimen can be obtained even when there is a high level of background radiation. In comparing the two primary detection methods, the direct-exposure method offers high speed, unlimited image integration time, and the best spatial resolution.

The transfer method offers insensitivity to the Îł-rays emitted by the specimen and greater contrast because of lower amounts of scattered and secondary radiation.

Keypoints: Direct Method: Best spatial resolution Transfer Method; Greater contrast

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The direct-exposure method offers:  high speed,  unlimited image integration time, and  the best spatial resolution.(?)

The transfer method offers:  insensitivity to the γ-rays emitted by the specimen and  greater contrast because of lower amounts of scattered and secondary radiation.

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5.3 Real-time imaging, in which light from a scintillator is observed by a television camera, can also be used for neutron radiography. Because of low brightness, most real-time neutron radiographic images are enhanced by an image-intensifier tube, which may be separate or integral with the scintillator screen. This method can be used for such applications as the study of fluid flow in a closed system or the study of metal flow in a mold during casting. The lubricants moving in an operating engine have been observed with the real-time neutron imaging method.

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Real Time Radiography Set-Up

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http://crisasantos.com.br/com/neutron-radiography

6.0 Applications

Various applications concerning the inspection of ordnance 军备物资, explosive, aerospace, and nuclear components. The presence, absence, or correct placement of explosives, adhesives, O- ings, plastic components, and similar materials can be verified. Nuclear fuel and control materials can be inspected to determine the distribution of isotopes and to detect foreign or imperfect material. Ceramic residual core in investment cast turbine blades can be detected. Observations of corrosion in metal assemblies are possible because of the excellent neutron sensitivity to the hydrogenous corrosion product. Hydride deposition in metals and diffusion of boron in heat treated boron-fiber composites can be observed. The following examples illustrate the application of neutron radiography to the inspection of radioactive materials and several assemblies of metallic and nonmetallic components.

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Example 1: Thermal-Neutron Radiography Used to Determine Size of Highly Radioactive Nuclear Fuel Elements.

Highly radioactive nuclear fuel elements required size measurements to determine the extent of dimensional changes that may have occurred during irradiation. Generally, inspection is done in a hot cell, but because hot-cell inspection is a relatively long, tedious, and costly procedure, neutron radiography was selected. The fuel elements to be inspected consisted of 6.4 mm ( ¼ in.) diam cylindrical pellets of UO2-PuO2; the plutonium content was 20%, and the uranium was enriched in 235U. The pellets had been irradiated to 10% burnup, which resulted in a level of radioactivity of 3 × 10-2 Ci/kg∙h (10 KR/h) at 0.3 m (1 ft). Five elements were selected for inspection. A neutron radiograph was taken by activating 0.25 mm (0.010 in.) thick dysprosium foil with a transmitted beam of thermal neutrons.

An autoradiograph of the activated-dysprosium transfer screen on a mediumpeed x-ray film yielded the result shown in the positive print in Fig. 6.

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Both 235U and plutonium have high attenuation coefficients for thermal neutrons. The high contrast of the fuel pellets made it possible to measure pellet diameter directly from the neutron radiographs. These measurements were both repeatable and statistically significant within 0.013 mm (0.0005 in.).

Later, radiographic measurements were compared with physical measurements made in a hot cell. The two sets of values corresponded within 0.038 mm (0.0015 in.).

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Fig. 6 Positive print of a thermal-neutron radiograph of five irradiated nuclear fuel elements, taken to determine if dimensional changes occurred during irradiation. Radiograph was made using a dysprosium transfer-screen method. Dark squares in middle element are voids.

Rods voids.

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Example 2: Indium-Resonance Technique for Determining Internal Details of Highly Radioactive Nuclear Fuel Elements.

The five nuclear fuel elements inspected for dimensional changes in Example 1 were further inspected for internal details. This was necessary because the thermal-neutron inspection procedure did not reveal any internal details; it only shadowed the pellets, as shown by the positive print in Fig. 6. To inspect for internal details, an indium-resonance technique, which utilizes epithermal neutrons, was used.

In this technique, a collimated neutron beam was filtered by 0.5 mm (0.02 in.) of cadmium to remove most of the thermal neutrons. Filtering produced a neutron beam with a nominal average energy somewhat above thermal. The epithermal neutron beam passed through the fuel elements and activated a sheet of indium foil. Neutrons with an energy of about 2.34Ă&#x2014;10-19 J (1.46 eV), which is the resonance-absorption energy for indium (not the object!) , were primarily involved in activation.

Comments: Indium-Resonance Technique is a type of transfer technique.

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The positive print of a radiograph made with epithermal neutrons shown in Fig. 7 reveals considerable internal details, in contrast to the lack of internal details in Fig. 6. With epithermal neutrons, there was less attenuation by the fuel elements than with thermal neutrons. Therefore, internal details that were not revealed by thermal neutron radiography: such as cracking or chipping of fuel pellets, and dimensional features of the central void in the fuel pellets (including changes in size and accumulation of fission products)- were revealed with the indium-resonance technique.

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Fig. 7 Positive print of a neutron radiograph of the same five nuclear fuel elements shown in Fig. 6. Radiograph was made with epithermal neutrons and an indium-resonance technique, and it reveals internal details not shown in the thermal-neutron radiograph in Fig. 6

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Example 3: Use of Conventional and Neutron Radiography to Inspect an Explosive Device for Correct Assembly.

Small explosive devices assembled from both metallic and nonmetallic components required inspection to ensure correct assembly. The explosive and the components made of paper, plastic, or other low atomic number materials, which are less transparent to thermal neutrons than to x-rays, could be readily observed with thermal-neutron radiography. Metallic components were inspected by conventional x-ray radiography. A positive print of a thermal-neutron, direct-exposure radiograph of a 50 mm (2 in.) long explosive device is shown in Fig. 8(a). The radiograph was made on Industrex R film (Eastman Kodak), using a gadolinium-foil screen. Total exposure was 3 Ă&#x2014; 109 n/cm2, which was achieved with an exposure time of 4 to 5 min.

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At the top in Fig. 8(a), just inside the stainless steel cap, can be seen a line image that corresponds to a moisture absorbent made of chemically treated paper. Below the paper is a mottled image, which is the explosive charge.

Below the explosive charge are plastic components and, at the very bottom, epoxy. A conventional radiograph of the same device is shown in Fig. 8(b).

The metallic components, which were poorly delineated in the thermalneutron radiograph, are more clearly seen in Fig. 8(b). Together, the two radiographs verified that both metallic and nonmetallic components were correctly assembled.

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Fig. 8 Comparison of positive prints of a thermal-neutron radiograph (a) and a conventional radiograph (b) of a 50 mm (2 in.) long explosive device. Neutron radiograph reveals details of paper, explosive compound, and plastic components not revealed by x-rays.

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Example 4: Use of Neutron Radiography to Detect Corrosion in Aircraft Components.

Aluminum honeycomb components are extensively used for aircraft construction. The aluminum material is subject to corrosion if exposed to water or humid environments. Thermal-neutron radiography is an excellent method of detecting hidden corrosion in these assemblies. The corrosion products are typically hydroxides or water-containing oxides; these corrosion products contain hydrogen, a material that strongly attenuates thermal neutrons. The aluminum metal, on the other hand, is essentially transparent to the neutrons. Therefore, a thermal-neutron radiograph of a corroded aluminum honeycomb assembly shows the corrosion product and other attenuating components such as adhesives and sealants. Figure 9 depicts a thermal-neutron radiograph of an aluminum honeycomb assembly showing the beginnings of corrosion.

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The white line image the middle of the radiograph represents the adhesive coupling together two core sections. The faint white smears in the upper half of the image and the double dot in the lower left area are images of corrosion as disclosed by the thermal-neutron radiograph. Developmental work has shown that thermal-neutron imaging techniques are capable of detecting the corrosion product buildup represented by an aluminum metal loss of 25 Îźm (1000 Îźin.). The neutron method, therefore, is a very sensitive technique for the detection of corrosion. adhesive coupling

corrosion

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Fig. 9 Thermal-neutron radiograph of aluminum honeycomb aircraft component showing early evidence of hydrogen corrosion. See text for discussion. Courtesy of D. Froom, U.S. Air Force, McClellan Air Force Base

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Example 5: Use of Neutron Radiography to Detect Corrosion in Adhesive - Bonded Aluminum Honeycomb Structures.

Aluminum corrosion of aircraft surfaces has plagued both military and civilian aircraft. Identification of this corrosion has been difficult, at best, usually being detected after the corrosion has caused the part to fail. Of the nondestructive testing methods used to detect aluminum corrosion, thermal neutron radiography has proved the most sensitive method to date. The detection of aluminum corrosion is based on the attenuation properties of hydrogen associated with the corrosion products rather than aluminum and aluminum oxide with their low attenuation coefficients. Depending on the environment, the corrosion products include aluminum trihydrates, monohydrates, and various other aluminum salts. Because the linear attenuation coefficient for aluminum is similar to that of water and about 28 times greater than that for aluminum, a 0.13 mm (0.005 in.) corrosion layer should be detectable under optimum conditions.

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The sensitivity standard plate for aluminum corrosion fabricated by the Aeronautical Research Laboratories (Australia) contains corrosion products varying from 0.13 to 0.61 mm (0.005 to 0.024 in.) thick (Fig. 10). Aluminum corrosion of honeycomb structures is complicated by the bonding adhesives that may appear similar in a neutron radiograph (Fig. 11a) (see the article "Adhesive-Bonded Joints" in this Volume). Tilting of the honeycomb structure will alleviate this problem by allowing adhesive found along the bond lines to be distinguished from the randomly distributed corrosion products (Fig. 11b).

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Fig. 10 Standard plate for aluminum corrosion detection contains 0.13 to 0.61 mm (0.005 to 0.024 in.) thick corrosion products. Courtesy of R. Tsukimura, Aerotest Operations Inc.

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Fig. 11 Effect of bonding adhesives on the quality of neutron radiographs obtained when checking for aluminum corrosion in honeycomb structures.

Radiograph taken (a) normal to specimen surface and (b) tilted at any angle other than 90째 to specimen surface. Courtesy of R. Tsukimura, Aerotest Operations Inc.

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Example 6: Use of Neutron Radiography to Verify Welding of Dissimilar Materials (Titanium and Niobium).

Exotic metal welded joints are a product of the extremely cold environment of space and man's desire to explore the vast emptiness of space. For space vehicles, attitude control rockets provide the fine touch for proper vehicle alignment.

For one application, a titanium-niobium welded joint was required between the light-weight propellant tank and the nozzle section. Attempts to verify weld integrity using conventional radiography were not productive. Thermal neutron radiography provided the image required to ensure quality welds.

This defect standard weld shows the porosity at the seam and the similar thermal neutron attenuation for both titanium (Ti) and niobium (formerly known as columbium, Cb) (Fig. 12a). For comparison, the x-ray radiograph image is also shown (Fig. 12b).

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Fig. 12 Comparison of thermal neutron (a) and x-ray (b) radiographs of a titanium-niobium welded joint. Courtesy of R. Tsukimura, Aerotest Operations Inc.

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Example 7: Use of Neutron Radiography to Detect Core Material Still Remaining in the Interior Cooling Passages of Air-Cooled Turbine Blades.

Investment casting of turbine blades using the lost wax process results in relatively clean castings. As the demand for higher-powered turbine engines has increased, the interior cooling passages for air-cooled turbine blades have become more and more complex. Concurrently, the removal of the core material has become increasingly more difficult. Incomplete removal of the core results in restricted flow through the cooling passages and possible failure of the overheated blade.

Previously, visual inspection was the nondestructive inspection method of choice for residual core detection. However, current designs preclude the use of borescopes and other visual means for the interior passages. Xadiography has proved rather ineffective in detecting residual core material.

Thermal neutron radiography is the nondestructive testing method of choice, especially when gadolinium oxide (Gd2O3) is used to dope the core material (1 to 3% by weight) (Fig. 13) prior to casting the blade.

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When concerns about the possible detrimental effects of Gd2O3 during the casting process prevents its use in the core material, a procedure was developed to tag the residual core material after the core removal process.

The castings are dipped in a gadolinium solution [Gd(NO3)2 in solution] to impregnate any residual core, which is then imaged and subsequently detected by neutron radiography. The blades shown in Fig. 14 have been tagged. The neutron radiograph shows any residual core material greater than 0.38 mm (0.015 in.) in diameter. Figure 15 is a schematic of typical core fragments in investment cast turbine blades detected by thermal neutron radiography. Image clarity of gadolinium tagged or doped cores is much greater than that of normal cores.

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Fig. 13 Residual core material in a gas-cooled aircraft-engine turbine blade as detected by thermal neutron radiography. The excess core material, tagged with 1.5% Gd2O3, is shown circled in the second photo from the right.

Courtesy of R. Tsukimura, Aerotest Operations Inc.

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Fig. 14 Thermal neutron radiograph of 12 turbine blades tagged with Gd2O3 solution.

One of the 12 blades (located in the top row and second from the left) contains residual core material in its upper right-hand corner cooling passage. Courtesy of R. Tsukimura, Aerotest Operations Inc.

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Fig. 15 Schematic of turbine blade core standards: gadolinium [Gd(NO3)3 in solution] tagged core, normal core (no gadolinium tagging or doping), and Gd2O3 doped core. Typical core fragments of various thicknesses are shown.

Source: R. Tsukimura, Aerotest Operations Inc.

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Example 8: Use of Neutron Radiography to Verify Position of Explosive Charges and Seating of O-Ring Seals in Explosive Bolt Assemblies.

There are many critical applications of explosive release devices in aircraft, space, and missile systems. Nondestructive testing is an important step in the quality control portion of the production cycle for these units.

Thermal neutron radiography has proved an indispensable tool in the nondestructive testing arsenal, particularly for thick-walled, metal devices, such as explosive bolts (Fig. 16). The inner details of explosive bolts can be imaged only by thermal neutron radiography methods (Fig. 17). This particular type of bolt from a missile system is activated from the bottom by actuating the firing pin onto the primer. The short section of mild detonating cord carries the energy to the output charge, which fractures the bolt and allows the bolt to be severed. In addition to the explosive charges, the internal O-ring seals, including the concentric pair around the firing pin and for the body are readily visible. For safety's sake, determining the presence of the shear pin can also be accomplished through the use of thermal neutron radiography.

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Fig. 16 Schematic showing location of critical components that comprise an explosive bolt. Source: R. Tsukimura, Aerotest Operations Inc.

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Fig. 17 Thermal neutron radiograph showing two sample bolts identical to the workpiece shown schematically in Fig. 16. Courtesy of R. Tsukimura, Aerotest Operations Inc.

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Example 9: Application of Neutron Radiography to Determine Potting Fill Levels in Encapsulated Electronic Filters.

Electronic filters are an integral component of all space and satellite systems.

Because the cost of these satellites is very high and the cost to repair them even more prohibitive, high reliability filters are necessary. A common mode of filter failure is that caused by inadequate potting of the internal components and the subsequent physical breakdown of the filter during periods of high vibration, such as that encountered during vehicle launch. Thermal neutron radiography is the method of choice for determining potting fill levels in encapsulated filters. The potting material attenuates the thermal neutrons and appears as the light density area. Voids in the potting material, the fill level, and the distribution can readily be detected with neutron radiography.

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Applications of Neutron Imaging in Earth Sciences and Biosciences 2D & 3D Neutron radiography & Tomography Non-destructive imaging techniques are extremely powerful diagnostic tools in the study of internal structures without the need to break open the test sample. The requirement of non-destructive testing chiefly has two sources; the specimen may be rare or even unique (e.g. geological and archaeological artefacts), or breaking may defeat the purpose of the investigation (e.g. bonding between surfaces and internal fluid flows).

The analytical information provided by neutron radiography (2-D) and tomography (3D) are entirely complementary to their better-known x-ray and gamma-ray counterparts, but have important advantages; in particular, neutrons can penetrate many materials, including metals, relatively easily, whilst being highly sensitive to hydrogen. This has lead to application in fields as wide-ranging as archaeology, engineering, biomaterials, biology and earth sciences.

Neutrograph at the ILL exploits the most intense neutron beam in world in use for this technique, giving it the highest time resolution. In the case of radiography, fast processes can be resolved below 1 ms, while 3-D tomographic imaging can be carried out in 10 s. Neutrograph is also highly sensitive in distinguishing between low contrasting materials. Currently, the spatial resolution is ~150 microns, but it is foreseen that this will improve in the near future.

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http://see.leeds.ac.uk/ebi/studentship-neutron.htm

Real Time Imaging: Combustion Chamber

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http://see.leeds.ac.uk/ebi/studentship-neutron.htm

Tomography of Antarctic Conifer Fossils 3-D tomographic reconstruction of a late Early Eocene age conifer fossil (~53 million years old), discovered on Seymour Island, West Antarctica reveals a beautifully preserved new flora, Araucariaceae, much resembling Araucaria araucana, the modern-day Monkey Puzzle tree. In this case, neutron tomography has replaced the time-consuming and utterly destructive serial thin-sections method. The 3-D structure has been held in tact with the leaves spiraling around a central woody stem.

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http://see.leeds.ac.uk/ebi/studentship-neutron.htm

Dynamic Radiography of Fluid Flow Through Sandstone In situ 2-D dynamic radiographic imaging of consecutive fluid flows through an initially water-saturated cylindrical sandstone core, Ф 5cm. Internal structures and fault zones are clearly visible, and appear to behave quite differently for different fluid combinations.

nitrogen-water

oil-water

water-nitrogen

water-oil

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http://see.leeds.ac.uk/ebi/studentship-neutron.htm

Neutron graph

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http://crisasantos.com.br/com/neutron-radiography

Neutron graph

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http://crisasantos.com.br/com/neutron-radiography

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Reading-3 E1316

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E1316 Section H: Neutron Radiologic Testing (NRT) Terms The terms defined in Section H are the direct responsibility of Subcommittee E07.05 on the Radiology (Neutron) Method. Additional radiological terms can be found in Section D. activation - the process of causing a substance to become artificially radioactive by subjecting it to bombardment by neutrons or other particles. attenuation coefficient - related to the rate of change in the intensity of a beam of radiation as it passes through matter. (See linear and mass attenuation coefficient.) attenuation cross section - the probability, expressed in barns, that a neutron will be totally absorbed by the atomic nucleus. barn - a unit of area used for expressing the area of nuclear cross sections.

1 barn = 10-24 cm2

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(3)

cadmium ratio - the ratio of the neutron reaction rate measured with a given bare neutron detector to the reaction rate measured with an identical neutron detector enclosed by a particular cadmium cover and exposed in the same neutron field at the same or an equivalent spatial location.

NOTE 27 - In practice, meaningful experimental values can be obtained in an isotropic neutron field by using a cadmium filter approximately 1 mm thick.

Cassette - light-tight device for holding film or conversion screens and film in close contact during exposure. contrast agent - a material added to a component to enhance details by selective absorption of the incident radiation.

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conversion screen - a device that converts the imaged neutron beam to radiation or light that exposes the radiographic film. cross section - the apparent cross-sectional area of the nucleus as calculated on the basis of the probability of occurrence of a reaction by collision with a particle. It does not necessarily coincide with the geometrical cross-sectional area Ď&#x20AC;r 2. It is given in units of area, 1 barn = 10â&#x2C6;&#x2019;24 cm2. direct exposure imaging - in the direct exposure imaging method, the conversion screen and image recorder are simultaneously exposed to the neutron beam. electron volt - the kinetic energy gained by an electron after passing through a potential difference of 1 V.

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facility scattered neutrons - neutrons scattered in the facility that contribute to the film exposure. Îł - effective gamma content. Îł is the percent background film darkening caused by low-energy photon radiation absorbed by pair production in 2 mm of lead.

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Pair Production

hν = E- + E+ = (m0c2 + K-) + (m0c2 + K+) = K- + K+ + 2m0c2

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http://electrons.wikidot.com/pair-production-and-annihilation

gamma ray - electromagnetic radiation having its origin in an atomic nucleus. half-life T½ - the time required for one half a given number of radioactive atoms to undergo decay. half-value layer - the thickness of an absorbing material required to reduce the intensity of a beam of incident radiation to one-half of its original intensity. image quality indicator - a device or combination of devices whose image or images on a neutron radiograph provide visual or quantitative data, or both, concerning the radiographic sensitivity of the particular neutron radiograph.

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indirect exposure - a method in which only a gamma-insensitive conversion screen is exposed to the neutron beam. After exposure, the conversion screen is placed in contact with the image recorder.

L/D ratio - one measure of the resolution capability of a neutron radiographic system. It is the ratio of the distance between the entrance aperture and the image plane (L) to the diameter of the entrance aperture (D).

Ug = Dt/L , I = Ф /[16â&#x2C6;&#x2122;(L/D)2] Linear attenuation coefficient - a measure of the fractional decrease in radiation beam intensity per unit of distance traveled in the material (cm-1). low-energy photon radiation - Gamma- and X-ray photon radiation having energy less than 200 keV (0.2 Mev) (excluding visible and ultraviolet light).

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mass attenuation coefficient - a measure of the fractional decrease in radiation beam intensity per unit of surface density cm2∙gm−1. (cm2∙g-1) Mass attenuation coefficient The mass attenuation coefficient or mass narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter. In addition to visible light, mass attenuation coefficients can be defined for other electromagnetic radiation (such as X-rays), sound, or any other beam that attenuates. The SI unit of mass attenuation coefficient is the square meter per kilogram (m2/kg).

Other common units include cm2/g (the most common unit for X-ray mass attenuation coefficients) and mL∙ g−1ccm−1 (sometimes used in solution chemistry). "Mass extinction coefficient" is an old term for this quantity.

The mass attenuation coefficient can be thought of as a variant of absorption cross section where the effective area is defined per unit mass instead of per particle. https://en.wikipedia.org/wiki/Mass_attenuation_coefficient

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moderator - a material used to slow fast neutrons. Neutrons are slowed down when they collide with atoms of light elements such as hydrogen, deuterium, beryllium, and carbon.

Why the heavier elements were not used as moderator?

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NC - effective thermal neutron content or neutron radiographic contrast. NC is the percent background film exposure due to unscattered thermal neutrons. neutron - a neutral elementary particle having an atomic mass close to 1. In the free state outside of the nucleus, the neutron is unstable having a half-life of approximately 10 min. A neutron undergoes spontaneous beta decay to form a proton, a high energy electron (β particle) and an electron antineutrino. Whilst atomic and mass numbers are conserved in the process the combined mass of the products is slightly less than the original neutron mass. This accounts for the energy released. http://www.atnf.csiro.au/outreach//education/senior/cosmicengine/sun_nuclear.html

http://scienceblogs.com/startswithabang/2013/07/0 5/why-did-the-universe-start-off-with-hydrogenhelium-and-not-much-else/

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neutron radiography - the process of producing a radiograph using neutrons as the penetrating radiation. object scattered neutrons - neutrons scattered by the test objects that contribute to the film exposure.

P - effective pair production content. P is the percent background exposure caused by pair production in 2 mm of lead. pair production - the process whereby a gamma photon with energy greater than 1.02 MeV is converted directly into matter in the form of an electronpositron pair. Subsequent annihilation of the positron results in the production of two 0.511 MeV gamma photons. process control radiograph - a radiograph which images a beam purity indicator and sensitivity indicator under identical exposure and processing procedures as the test object radiograph. A process control radiograph may be used to determine image quality parameters in circumstances of large or unusual test object geometry.

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Pair Production

hν = E- + E+ = (m0c2 + K-) + (m0c2 + K+) = K- + K+ + 2m0c2

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http://electrons.wikidot.com/pair-production-and-annihilation

radiograph - a permanent, visible image on a recording medium produced by penetrating radiation passing through the material being tested. radiographic inspection - the use of X rays or nuclear radiation, or both, to detect discontinuities in material, and to present their images on a recording medium. radiography - the process of producing a radiograph using penetrating radiation. radiological examination - the use of penetrating ionizing radiation to display images for the detection of discontinuities or to help ensure integrity of the part.

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radiology - the science and application of X rays, gamma rays, neutrons, and other penetrating radiations. radioscopic inspection - the use of penetrating radiation and radioscopy to detect discontinuities in material. radioscopy - the electronic production of a radiological image that follows very closely the changes with time of the object being imaged. real-time radioscopy - radioscopy that is capable of following the motion of the object without limitation of time.

S - effective scattered neutron content. S is the percent background film darkening caused by scattered neutrons. scattered neutrons - neutrons that have undergone a scattering collision but still contribute to film exposure.

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sensitivity value - the value determined by the smallest standard discontinuity in any given sensitivity indicator observable in the radiographic image. Values are defined by identification of type of indicator, size of defect, and the absorber thickness on which the discontinuity is observed. thermalization - the process of slowing neutron velocities by permitting the neutrons to come to thermal equilibrium with a moderating medium. thermalization factor - the inverse ratio of the thermal neutron flux obtained in a moderator, per source neutron. thermal neutrons - neutrons having energies ranging between 0.005 eV and 0.5 eV; neutrons of these energies are produced by slowing down fast neutrons until they are in equilibrium with the moderating medium at a temperature near 20째C.

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total cross section - the sum of the absorption and scattering cross sections. vacuum cassette - a light-tight device having a flexible entrance window, which when operated under a vacuum, holds the film and conversion screen in intimate contact during exposure.

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Thermalization of Neutrons the last stage of the moderation of neutrons in various mediums, in which the role of chemical bonding and the thermal motion of the atoms of the medium become essential.

When the kinetic energy of neutrons is reduced to values less than 1 electron volt, the neutron velocity becomes comparable to the velocity of thermal motion of atoms and molecules. Energy exchange arises between these species and the neutrons, leading to the establishment of an equilibrium Maxwellian velocity distribution of the neutrons. However, because of several factors, such as the motion and binding of atoms, absorption, and the finite size of the system, the energy spectra of neutrons in moderators differ from the equilibrium spectra. The study of the thermalization of neutrons is required for the calculation and prediction of the behavior of thermal reactors.

Research in this area has been the source of new methods for the study of the physics of solids and liquids.

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http://encyclopedia2.thefreedictionary.com/Neutrons%2c+Thermalization+of

Neutron temperature The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term temperature is used, since hot, thermal and cold neutrons are moderated in a medium with a certain temperature. The neutron energy distribution is then adopted to the Maxwellian distribution known for thermal motion. Qualitatively, the higher the temperature, the higher the kinetic energy is of the free neutron.

Kinetic energy, speed and wavelength of the neutron are related through the De Broglie relation.

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https://en.wikipedia.org/wiki/Neutron_temperature

Maxwellian distribution

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http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html

Neutron energy distribution ranges Neutron energy range names But different ranges with different names are observed in other sources. For example, Epithermal neutrons have energies between 1 eV and 10 keV and smaller nuclear cross sections than thermal neutrons.

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https://en.wikipedia.org/wiki/Neutron_temperature

Thermal Neutrons in thermal equilibrium with their surroundings Most probable energy at 20 degrees (C) - 0.025 eV; Maxwellian distribution of 20 degrees (C) extends to about 0.2 eV. Epithermal Neutrons of energy greater than thermal Greater than 0.2 eV Cadmium Neutrons which are strongly absorbed by cadmium Less than 0.4 eV Epicadmium Neutrons which are not strongly absorbed by cadmium Greater than 0.6 eV

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Slow Neutrons of energy slightly greater than thermal Less than 1 to 10 eV (sometimes up to 1 keV) Resonance In pile neutron physics, usually refers to neutrons which are strongly captured in the resonance of U-238, and of a few commonly used detectors (e.g., Indium, Gold, etc.)

1 eV to 300 eV Intermediate Neutrons that are between slow and fast Few hundred eV to 0.5 MeV Fast Greater than 0.5 MeV

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Ultrafast Relativistic Greater than 20 MeV

Pile Neutrons of all energies present in nuclear reactors 0.001 eV to 15 MeV Fission Neutrons formed during fission 100 keV to 15 MeV (Most probable: 0.8 MeV; Average: 2.0 MeV)

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Ultracold neutrons (UCN) Ultracold neutrons are free neutrons which can be stored in traps made from certain materials.

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https://en.wikipedia.org/wiki/Neutron_temperature

Thermal neutrons A thermal neutron is a free neutron with a kinetic energy of about 0.025 eV (about 4.0×10-21 J or 2.4 MJ/kg, hence a speed of 2.2 km/s), which is the energy corresponding to the most probable velocity at a temperature of 290 K (17 °C or 62 °F), the mode of the Maxwell–Boltzmann distribution for this temperature.

After a number of collisions with nuclei (scattering) in a medium (neutron moderator) at this temperature, neutrons arrive at about this energy level, provided that they are not absorbed.

Thermal neutrons have a different and often much larger effective neutron absorption cross-section for a given nuclide than fast neutrons, and can therefore often be absorbed more easily by an atomic nucleus, creating a heavier, often unstable isotope of the chemical element as a result (neutron activation).

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https://en.wikipedia.org/wiki/Neutron_temperature

Fast neutrons A fast neutron is a free neutron with a kinetic energy level close to 1 MeV (100 TJ/kg), hence a speed of 14,000 km/s, or higher. They are named fast neutrons to distinguish them from lower-energy thermal neutrons, and highenergy neutrons produced in cosmic showers or accelerators.

Fast neutrons are produced by nuclear processes: nuclear fission produces neutrons with a mean energy of 2 MeV (200 TJ/kg, i.e. 20,000 km/s), which qualifies as "fast". However the range of neutrons from fission follows a Maxwellâ&#x20AC;&#x201C;Boltzmann distribution from 0 to about 14 MeV in the center of momentum frame of the disintegration, and the mode of the energy is only 0.75 MeV, meaning that fewer than half of fission neutrons qualify as "fast" even by the 1 MeV criterion. nuclear fusion: deuteriumâ&#x20AC;&#x201C;tritium fusion produces neutrons of 14.1 MeV (1400 TJ/kg, i.e. 52,000 km/s, 17.3% of the speed of light) that can easily fission uranium-238 and other non-fissile actinides.

Fast neutrons can be made into thermal neutrons via a process called moderation. This is done with a neutron moderator. In reactors, typically heavy water, light water, or graphite are used to moderate neutrons.

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https://en.wikipedia.org/wiki/Neutron_temperature

other non-fissile actinides.

â&#x2013; http://minerals.usgs.gov/minerals/pubs/commodity/ Charlie Chong/ Fion Zhang

https://en.wikipedia.org/wiki/Neutron_temperature

other non-fissile actinides.

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https://en.wikipedia.org/wiki/Neutron_temperature

Fast reactor and thermal reactor compared Most fission reactors are thermal reactors that use a neutron moderator to slow down ("thermalize") the neutrons produced by nuclear fission.

Moderation substantially increases the fission cross section for fissile nuclei such as uranium-235 or plutonium-239. In addition, uranium-238 has a much lower capture cross section for thermal neutrons, allowing more neutrons to cause fission of fissile nuclei and propagate the chain reaction, rather than being captured by 238U. The combination of these effects allows light water reactors to use low-enriched uranium. Heavy water reactors and graphitemoderated reactors can even use natural uranium as these moderators have much lower neutron capture cross sections than light water. (moderation without capture?)

An increase in fuel temperature also raises U-238's thermal neutron absorption by Doppler broadening, providing negative feedback to help control the reactor. Also, when the moderator is also a circulating coolant (light water or heavy water), boiling of the coolant will reduce the moderator density and provide negative feedback (a negative void coefficient).

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https://en.wikipedia.org/wiki/Neutron_temperature

Intermediate-energy neutrons have poorer fission/capture ratios than either fast or thermal neutrons for most fuels. An exception is the uranium-233 of the thorium cycle, which has a good fission/capture ratio at all neutron energies.

Fast reactors use unmoderated fast neutrons to sustain the reaction and require the fuel to contain a higher concentration of fissile material relative to fertile material U-238. However, fast neutrons have a better fission/capture ratio for many nuclides, and each fast fission releases a larger number of neutrons, so a fast breeder reactor can potentially "breed" more fissile fuel than it consumes.

Fast reactor control cannot depend solely on Doppler broadening or on negative void coefficient from a moderator. However, thermal expansion of the fuel itself can provide quick negative feedback. Perennially expected to be the wave of the future, fast reactor development has been nearly dormant with only a handful of reactors built in the decades since the Chernobyl accident due to low prices in the uranium market, although there is now a revival with several Asian countries planning to complete larger prototype fast reactors in the next few years.

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https://en.wikipedia.org/wiki/Neutron_temperature

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Reading-4 Neutrons provide unique penetrating radiation

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http://spie.org/x18990.xml

In the short-wavelength world of matter waves -- past the UV portion of the spectrum and beyond x rays -- materials look very different and the rules of imaging are different than with photons. "Neutrons show you things that x rays will never be able to," said Wade Richards of the Univ. of California/Davis research nuclear reactor in Sacramento, CA (Figure 1).

Figure 1. Neutron radiograph of a flower corsage shows the method's ability to image thin biological samples. Photo courtesy of Nray Services.

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http://spie.org/x18990.xml

Neutron basics The ways that neutrons interact with matter are very different from the way x rays interact with matter.  X rays interact with the electron cloud surrounding the nucleus of an atom.  Neutrons interact with the nucleus itself.  In general, x-ray attenuation increases as the atomic number of the target material increases; usually, the attenuation is greater for lower energy x rays.  For Neutron radiography, some light elements (such as hydrogen, boron, and carbon) have high thermal neutron attenuation coefficients, while some heavier elements (such as lead) have relatively small attenuation coefficients.

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http://spie.org/x18990.xml

The methods can be used in a complementary fashion. And while the imaging materials are different, neutron radiography is similar to x-ray radiography: a beam of particles penetrates the target, and the shadow of the device is captured.

Patrick Doty, Senior Scientist at Sandia National Labs. (Albuquerque, NM) said that because the intensity of neutron scattering varies irregularly with atomic number and neutron energies vary over a large range (and thus a wide range of useful wavelengths), neutrons can probe the structure of matter over many scales of length.

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FIG. X1.1 Approximate Mass Attenuation Coefficients μ/ρ as a Function of Atomic Number

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FIG. X1.2 Calculated Thermal Neutron and 100 and 500 KEV X-Ray Linear Attenuation Coefficients (Îź) as a Function of Atomic Number (A)

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Neutrons can be divided into several energy groups: ■ cold, ■ thermal, ■ epithermal, ■ fast, and ■ relativistic.

This article concentrates on thermal and fast neutrons. Thermal neutrons have energies of roughly 0.03 eV or less, whereas fast neutrons are 10 to 15 MeV. Fission reactors produce neutrons with a wide range of energies, but because most reactors also have large volumes of moderator (to slow the neutrons down so they will be more efficient at initiating new fissions), reactors are excellent sources of thermal neutrons.

Beams of fast neutrons can (also) be generated indirectly from particle accelerators (which accelerate charged particles into a target that gives off neutrons). "Fast neutrons are very highly penetrating," Doty said. "You can tailor the energies to look at a material, or you can look at shielded things -you can look through lead shielding." Radioisotopes such as californium and americium-beryllium can generate fast neutrons as well, although this is not generally used for radiography.

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3.3 Radioactive Sources.

There are many possible radioactive sources. The characteristics of several radioisotopes that are commonly used are summarized in Table 3.

Table 3 Properties and characteristics of several radioisotopes used for thermal-neutron radiography

(γ, n) (α, n) (α, n)

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γ

γ γ

Table 3 Properties and characteristics of several radioisotopes used for thermal-neutron radiography

(α, n) γ

(α, n)

252Cf

■ http://minerals.usgs.gov/minerals/pubs/commodity/

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Thermal neutrons from reactors At the McClellan Nuclear Radiation Center (MNRC) located just north of Sacramento, CA, Wade Richards, Tom Majchrowski and others use thermal neutrons from the small nuclear reactor for a variety of applications, including inspecting aircraft parts for corrosion.

The MNRC is a small nuclear reactor: whereas a power-generating reactor produces 2000 to 3000 MW, the McClellan reactor produces 2 MW. In nuclear reactors, neutrons are generated by fission in uranium. One neutron hits a 235U and two neutrons emerge. The neutrons emerge isotropically 各向 同性的, heading in all directions. To use the neutrons for imaging, part of the shielding (10-ft.-thick concrete walls outside the tank of water in which the uranium sits) is removed. Pipes lined with neutron-absorbing and -scattering materials remove the particles that are too far out of line, providing a collimated beam of neutrons to the imaging bays. The reactor provides a flux of about 4 X 106 neutrons/cm2/s at the radiography plane.

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Richards said that in the 2D and 3D computer tomography neutron radiography systems in his facility, a converter screen uses gadolinium (which has a high cross-section for thermal neutrons), which emits beta particles when bombarded with neutrons, and the beta particles are caught by a fluorescing zinc sulfide material. Once the image has been converted to visible light, it can be captured on film or by a video camera sitting outside the neutron-beam axis. (Other converters can also be used, including lithium carbonate and plastic scintillators, depending on the application and neutron energies used.) A major application of neutron radiography at the reactor is for real-time imaging of military aircraft wings as a standard maintenance method to detect corrosion.

Keywords: Converter screen: Gadolinium, Indium, Dysprosium Scintillator screen: (1) Zinc sulfide, (2) Lithium carbonate, (3) plastid scintillator Question: Is Cadmium used as converter screen?

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The McClellan site is the only facility in the world equipped with a robot stage and a video-camera radiography system that allows real-time imaging of objects 34 ft. long by 12 ft. high and as heavy as 5000 pounds.

There is always some concern about how much one can bombard the object before the nucleus becomes radioactive. For aluminum, Richards said there is a 2.5-minute decay time. "Within 10 minutes, it's all gone."

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McClellan Nuclear Radiation Center (MNRC)

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McClellan Nuclear Radiation Center (MNRC)

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McClellan Nuclear Radiation Center (MNRC)

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McClellan Nuclear Radiation Center (MNRC) http://mnrc.ucdavis.edu/

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McClellan Nuclear Radiation Center (MNRC) http://mnrc.ucdavis.edu/

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Thermal neutron radiography Glen MacGillivray at Nray Services (Petawawa, ON, Canada) described the complementary relationship between x-ray and neutron radiography: "When one is looking for a flaw, the contrast between the flaw and the unflawed material is paramount. If the attenuation in the material is too large, then insufficient beam penetrates to allow inspection." For applications that require distinguishing materials that attenuate differently, usually one wants to find the higher attenuator within a bulk material of lower attenuation. "So," MacGillivray said, "finding lead in a paraffin block (or a needle in a haystack) would work for x rays while looking for paraffin in a lead block (or a straw in a needlestack) would work for neutrons."

In addition to imaging corrosion in aircraft wings, neutron radiography's ability to detect hydrogen and carbon yield other applications. Neutron computer tomography has been used to analyze flaws and misalignments in o-rings, which are used for sealing joints in rockets and other heavy machinery.

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The method allows researchers to look through the outer (usually metallic) materials to see how artifacts from the fabrication process sometimes put crimps into the rings when they are in place (work of this sort for NASA has been done at McClellan).

In a similar way, neutron radiography can be used for viewing lubricants or other details of interest within a metal structure (Figure 2), or for looking at explosives contained within metals.

"Thermal neutron radiography is limited to reactor-based sources for highresolution production-rate work," MacGillivray said. Accelerator and isotopic sources of neutrons can be used for imaging, but with sacrifices in speed and/or resolution. Images are obtained using a variety of techniques, including direct to film from gadolinium foil, indirect to film from activated foils of dysprosium (Dy, atomic number 66) or indium, to a CCD device from a scintillator, or to an imaging plate from a scintillator. "Imaging speeds of several thousand frames per second have been obtained," MacGillivray said.

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Figure 2. Neutron radiograph of a handgun shows a wide range of contrasts within a complex mechanical structure.

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MacGillivray said the neutron flux required for imaging depends on the application. "We have successfully used image-plane neutron fluxes ranging from 5 X 104 to 4 X 107 neutrons/cm2/s for film imaging," he said.

MacGillivray will be presenting his invited paper, "Imaging with neutrons: the other penetrating radiation," at the Penetrating Radiation Systems and Applications conference at SPIE's Annual Meeting in July.

In addition to the applications mentioned above, neutron radiography can be used with a contrast agent to look for residual ceramic core material inside jet engine turbine blades. Another application is an indirect method of inspecting nuclear fuel -- a very radioactive material that would, if inspected directly, fog the film (by the gamma ray) .

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Fast neutron radiography Beams of fast neutrons, with energies from 10 to 15 MeV, offer different imaging capabilities than thermal neutrons. These energetic particles can be produced by linear accelerators.

Robert Hamm of AccSys Technology (Pleasanton, CA) said his company generates neutrons from linear accelerators by knocking an electron off a proton or deuteron (usually from hydrogen gas), accelerating the positively charged particle, then bombarding a target (usually beryllium). The target then emits secondary radiation (neutrons), mostly moving in the same direction as the ion beam. (Figure 3). "Fast neutrons penetrate matter very easily," Hamm said.

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Figure 3. In addition to being gathered from nuclear reactors, neutrons can be generated by linear accelerators. This machine generates neutrons by accelerating protons into a beryllium target. The systems can generate from 108 to 1013 neutrons/second. Photo courtesy of AccSys Technology.

Keypoint: Neutron Accelerator Target material: Beryllium, Be.

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Fast neutron radiography can be used in prospecting for diamonds or other minerals, Hamm said. It also shows mineral inclusions in rocks. For this application, users shine two beams at the target, one after the other. One neutron beam is at a resonant energy for the mineral, the other is offfrequency. The difference between the images provides information about the interior of the rock.

Meanwhile, James Hall and others at Lawrence Livermore National Labs (Livermore, CA) produce fast neutrons using a "DD source." This is a system in which a deuterium beam is accelerated to the desired energies and hits deuterium with a pressurized gas cell (at 2 to 3 atm). The interaction creates neutrons mostly going in the same direction as the incoming beam.

Hall uses fast neutrons to penetrate steel, lead, or uranium. "We can look at voids of a few cubic millimeters in size behind up to four inches of uranium," he said, "and we can see it well."

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Hall wants to create a system that will fit into a small laboratory and can image cubic-millimeter-scale voids or other structural defects in heavily shielded thick materials. The system should also be able to acquire tomographic image data sets, allowing users to gain a 3D image of the object.

To detect fast neutrons, Hall's group is using a rigid 4-cm-thick plastic scintillator indirectly viewed by a single commercial CCD camera. A thin mirror made of front-surfaced-aluminized Pyrex glass reflects light from the scintillator to the camera, which is well out of the neutron beam path. The camera's CCD is a cryogenically cooled thinned, back-illuminated 1024- X 1024-pixel CCD with an antireflective coating on its active area. The detector can be used for image integration times as long as an hour.

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Applications include looking at thick objects not well penetrated by thermal neutrons. Hall's group has imaged a 6-in.-thick uranium and polyethylene slab assembly (with features machined into the polyethylene). The group also imaged a set of nine conventional step wedges. The step wedges were fabricated from lead, Lucite速, mock high explosive, aluminum, beryllium, graphite, brass, polyethylene, and stainless steel. All were 0.5-in.-thick pieces with uniform steps ranging from 0.5 in. to 5 in. in width. All of steps in each of the materials within the detector's field of view could be discerned in the final processed image. A series of other imaging experiments, including tomographic imaging, have also been performed.

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Reading-5 Neutrons Radiography mini article

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Neutron Radiography Similar to x-ray radiography, neutron radiography is a very efficient tool to enhance investigations in the field of non-destructive testing (NDT) as well as in many fundamental research applications. Neutron radiography is, however, suitable for a number of tasks impossible for conventional x-ray radiography.

The advantage of neutrons compared to x-rays is the ability to image light elements (i.e. with low atomic numbers) such as hydrogen, water, carbon etc.

In addition, neutrons penetrate heavy elements (i.e. with high atomic numbers) such as lead, titanium etc. allowing the study of materials in complex sample environments, for example water accumulation in hydrogen fuel cells: see Fig. 1.

Because neutrons interact with the nucleus rather than with the electron shell, they can also distinguish between different isotopes of the same element.

This makes neutron radiography an important tool in various research applications and in the field of NDT. The MNRC high neutron intensity beams permit short exposure times, high spatial resolution and high sample throughput.

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Fig. 1. Left: Photograph of a hydrogen fuel cell. Right: False colored neutron radiograph of a fuel cell showing the water content of the cell during operation.

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Methods of neutron radiography The detection of neutrons relies on a conversion into visible light; to achieve this, conversion screens containing either Gd or 6Li and a fluorescence material are commonly used. After the conversion the emitted light can be detected by different media such as: • Film, that is then developed in a dark room and results in a permanent image, • Imaging Plates, that can be re-used after being processed by an image reader. (The technology is very similar to x-ray imaging plates used at medical offices), • Digital cameras (CCD, CMOS), allow to capture the image digitally.

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Differences between neutron and x-ray radiography Neutron radiography is based on the principal that neutrons interact with the nucleus of the atom, rather than the electrons. Therefore neutrons are absorbed in matter very differently from x-rays and gamma rays. This means that, contrary to x-rays, neutrons are attenuated by some light materials, such as hydrogen, boron and lithium, but penetrate many heavy materials such as titanium and lead. This allows for some unique applications of neutron radiography.

The figures 2 below impressively demonstrate how neutron radiography can yield different yet complementary information to x-ray radiography.

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Fig.2. X-ray and Neutron radiographs of a 35 mm film SLR camera. Dark elements in the x-ray radiographs are caused by metal components; comparison shows that they are almost transparent for neutrons. Dark componets in the neutron radiograph are due to plastic components which in turn are almost transparent to x-rays.

X-Ray Radiograph

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Fig.2. X-ray and Neutron radiographs of a 35 mm film SLR camera. Dark elements in the x-ray radiographs are caused by metal components; comparison shows that they are almost transparent for neutrons. Dark componets in the neutron radiograph are due to plastic components which in turn are almost transparent to x-rays.

Neutron Radiograph

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Applications Neutron Radiography has a wide range of uses, including: • Imaging casting to ensure that the mold materials don't carry into the castings as impurities.

• Validating the proper fill of pyrotechnical in actuators • Studying the flow of oil in automobile transmissions • Facilitate Fluid flow analysis • Analyze O-ring placements • Image carbon, gun powder grain structure, plastics, lead, and other heavy metals.

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Screen Types 1. Transfer screen-indium or dysprosium, In, Dy. 2. Thermal neutron filter using Cadmium for epithermal neutron radiography, Cd. 3. Converter screen uses gadolinium which emit beta particles, Gd. 4. the beta particles are caught by a fluorescing zinc sulfide material 5. Scintillator screen: Zinc sulfide, Lithium carbonate, plastid scintillator 6. Neutron Accelerator Target material: Beryllium, Be.

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Peach – 我爱桃子

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Good Luck

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