Understanding neutron radiography post exam reading viii part 2c of 2a

Understanding Neutron Radiography Reading VIII Part 2(c)of 2 16th August 2016 Post Exam Reading

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Tokamak

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Tokamak

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Wendelstein Fusion Reactor

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Wendelstein Fusion Reactor

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Wendelstein Fusion Reactor

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Wendelstein Fusion Reactor

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Wendelstein Fusion Reactor

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Wendelstein Fusion Reactor

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Twisty Plasma Fusion

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Twisty Fusion Reactor

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The Magical Book of Neutron Radiography

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数字签名者：Fion Zhang DN：cn=Fion Zhang, o=Technical, ou=Academic, email=fion_zhang @qq.com, c=CN 日期：2016.08.24 01:26:56 +08'00' Charlie Chong/ Fion Zhang

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

Fion Zhang at Copenhagen Harbor 23th August 2016

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SME- Subject Matter Expert http://cn.bing.com/videos/search?q=Walter+Lewin&FORM=HDRSC3 https://www.youtube.com/channel/UCiEHVhv0SBMpP75JbzJShqw

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Gamma- Radiography TABLE 1. Characteristics of three isotope sources commonly used for radiography.

Source

T½

Energy

HVL Pb

HVL Fe

Specific Activity

Dose rate*

Co60

5.3 year

1.17, 1.33 MeV

12.5mm

22.1mm

50 Cig-1

1.37011

Cs137

30 years

0.66 MeV

6.4mm

17.2mm

25 Cig-1

0.38184

Ir192

75 days

0.14 ~ 1.2 MeV (Aver. 0.34 MeV)

4.8mm

?

350 Cig-1

0.59163

Th232

Dose rate* Rem/hr at one meter per curie

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0.068376

八千里路云和月

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闭门练功

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Fusion

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Special Screens Neutron Sensitive Screens Real time radiography may be performed using neutron beams when the fluorescent screen is a good neutron absorber. Elements with high thermal neutron cross sections, such as lithium-6, boron-10 and gadolinium are used in neutron sensitive screens.

Plastic scintillation materials can be used for radioscopy with fast neutrons.

The characteristics of screen composition and construction are more important in neutron imaging than in X-ray imaging because the intensity of available neutron sources is generally lower than for X-ray sources. It is important that the screen absorb a sufficient quantity of neutrons to obtain an acceptable light yield for adequate contrast.

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Neutron radioscopic imaging is accomplished with portable accelerator neutron sources and neutron reactors with beam ports suited for radioscopic imaging. The effective focal spot of neutron sources is typically large. For most neutron sources, whether accelerator type or reactor type, the focal spot is defined by the collimator opening at the neutron source. To transport a reasonable number of neutrons down the beam tube, these collimator openings are necessarily larger than the focal spots possible with X-ray machines and linear accelerators.

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Neutron radiography has been used to detect fluids in process pipes that normally carry gases. This study determined that it was possible to detect fluids in small diameter pipe, 6.0 to 14.3 mm (0.25 to 0.563 in.) outside diameter with a ratio of outside diameter to inside diameter of 3. Fluid thicknesses from 0.08 to 0.89 mm (0.003 to 0.035 in.) in thickness were detectable. Although these studies were conducted with a reactor using transfer techniques, it was felt that in-plant tests could be performed with the neutron isotope source californium-252. Better detectability was obtained by angling the beam (in the tests angling the pipe) at a sufficient angle to the neutron beam to effectively increase the thickness of liquid to make it more attenuating to the radiation. Liquid level gages using radiation transmission principles have several applications in determining liquid heights and fluid density values. In simple applications, a radiation gage can be installed to determine when the liquid in a tank has reached a certain level.

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With suitable calibration, computed tomography can have higher sensitivity to smaller changes in density or thickness than in conventional radiography.

Medical systems routinely claim the ability to achieve sensitivities within 0.5 percent of thickness or material density. Computed tomography has been used with X-ray, gamma ray and neutron beams in examining fuel and other nuclear materials. A few of the applications are discussed next.

Experiments were performed on nuclear fuel rod assemblies to determine debris formation and relocation (DFR) in simulated accidents to study the effects of overheated conditions on the assemblies. Light water reactor (LWR) fuel rod assemblies are placed in the core of a research reactor and where the fuel is fission heated. The assembly is then exposed to superheated steam to create conditions that might exist in a core damage accident.

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Similar tomographic work was done on irradiated fuel elements at another facility. However, these fuel elements have up to 2.78 Sv·s–1 (106 R·h–1) gamma activity so that isotope or X-ray exposures are out of the question as the large gamma intensity would expose the film or saturate any other detector. This work called for neutron radiographic tomography with transfer foils. In neutron radiography, transfer foils are used in high intensity gamma beams. Neutrons are absorbed in the transfer foil, in this case 0.13 mm (0.024 in.) thick indium. Neutron radiography of large nuclear fuel bundles requires neutron beam energies of sufficient energy to penetrate the full array of fuel pins in the bundle. In most neutron radiography, the images are formed by the capture of thermal or lower energy neutrons. However, in this case, the reactor needed to produce epithermal neutrons. These neutrons provide sufficient penetration of the fuel bundles and are imaged by the 1.46 eV resonance of indium.

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Because the gamma radiation from the fuel bundles is quite large, positioning adjustments to the specimen and rotation for the various radiographs must be accomplished remotely. Also the indium foils must be placed remotely for each exposure. A drawing (Fig. 30) shows one port of the reactor for neutron radiography and other features. Exposures of the indium foil in the reactor required 18 min with a collimator length-to- diameter ratio of 125. The activated transfer screens were placed against a medium grain radiographic film in a vacuum cassette (so intimate contact results) immediately following exposure. The transfer screen must be left in contact with the film for at least four half lives (indium half life = 54 min) but normally these exposures were left overnight. The resultant beta particle emission from the indium screen exposes the film, which in turn is processed in an automatic processor. Thirtysix films were exposed for each fuel bundle. Figure 31 shows a reconstruction of the neutron tomographic image of one cross section in a 91-pin fuel bundle using the 36 neutron radiographs to form the image.

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FIGURE 30. Neutron radiographic facility used for neutron tomography of hot fuel assemblies.33

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Neutron radiographic facility

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FIGURE 31. Tomographic reconstruction of 91-pin fuel bundle using 36 neutron radiographs.33

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Neutron Tomographic

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Neutron tomography of irradiated fuel bundles is an expensive form of nondestructive testing and it would probably not find general application for examination of routine fuel bundles. However, there are safety tests being conducted on fuel bundles in which there is a very large capital investment and where the expected results have warranted the cost of tomography. In these instances, the irradiation effects on the fuel under the deliberately induced severe operating conditions have caused very gross displacement of the internal components. These features would obviously be lost or disturbed during disassembly of the bundle. Neutron tomography thus becomes a valuable and effective way to study the relationship of these internal components before disassembly.

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Neutron Radiography of Special Aerospace Components Typical aerospace items that have been neutron radiographed include jet engine turbine blades, adhesively bonded honeycomb and laminates, ordnance or pyrotechnic devices, flight controls and metal assemblies containing nonmetal O rings or seals. Inspection of such parts is possible because neutron radiographic testing can detect certain low density materials through heavy metal sections. Attenuation of X-radiation is determined largely by the electron density of the material being examined, so that thicker and/or denser materials appear more opaque. Neutrons undergo two main types of reactions with atomic nuclei: absorption (capture) and scattering. The mass attenuation coefficient for thermal neutrons is thus a function of both the scattering and capture probabilities for each element; the density of a particular material or component is a poor predictor whether it will be relatively transparent or opaque to the passage of neutrons.

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High attenuation coefficients for thermal neutrons are exhibited by hydrogen and boron.

Hydrogen has the highest scattering coefficient whereas boron, cadmium, samarium and gadolinium have unusually high neutron capture probabilities.

For this reason, hydrogenous or boron containing materials being inspected by neutron radiography can be seen or delineated from other elements in many cases where X-radiography is inadequate. It is thus possible to expose a specimen (for example, a hydrogenous or borated explosive or a fuel sealed in a metallic container) to a beam of thermal neutrons and to project an image having excellent resolution and contrast, thereby distinguishing between the charge material and its container while revealing any imperfections in the specimen. Similarly, neutron radiography permits detection of hydrides, which can cause hydrogen embrittlement in welds and can be used for nondestructive testing of ordnance (explosive) devices to determine the relative density of the charge material and/or the presence of voids or cracks.

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Explosive and Pyrotechnic Devices One of the important applications of neutron radiography is the quality control of certain critical explosive devices, such as pilot seat ejection cartridges, detonating cords and pressure cartridges for aerospace applications. These devices normally contain explosives or propellants of low atomic numbers. In most instances, they also have metal housings. Therefore, it is nearly impossible to inspect them effectively with postassembly X-ray examination.

However, because hydrogenous materials have a high neutron scatter rate, a neutron radiograph will image gaps, cracks, low density areas or other discontinuities that could prevent normal operation. A neutron radiograph will image certain materials of low atomic number whereas the X-radiograph does not. The powder train inside an explosive device is well defined in the neutron radiograph, for example, but is not shown in the X-radiograph. Hence, neutron radiography has played a major role in yielding a no failure reliability for ordnance devices in aerospace programs.

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Explosive and Pyrotechnic Devices

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Explosive and Pyrotechnic Devices

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Adhesive Bonded Composite Structures Thermal neutrons are highly attenuated by boron and hydrogen atoms.

Therefore, when an adhesive bonded laminate or honeycomb specimen is neutron radiographed, the hydrocarbon adhesive becomes very apparent because of its high neutron scatter. This condition is reversed for Xadiography where metal components are high attenuators. A high density boron fiber composite would be completely opaque to a thermal neutron beam, as shown in Fig. 29. In Fig. 29, specimens have aluminum honeycomb cores with adhesive bonded skins of graphite epoxy, boron epoxy and fiberglass.

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Adhesive Bonded Composite Structures

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Adhesive Bonded Composite Structures

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FIGURE 29. Attenuation in adhesive bonded, aluminum honeycomb core, fiber matrix facing sheet, composite specimens: (a) low kilovoltage X-rays; (b) thermal neutrons.

Legend 1. graphite 2. boron 3. fiber glass

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Neutron Tomography Neutron radiography has already proven itself as a reliable method for the detection of hydrogenous substances, such as moisture and corrosion in aircraft components. The advantage of neutrons is that low atomic number nuclei, in particular hydrogen, have higher interaction cross sections than the surrounding material. Figure 37 shows a neutron radiograph of an aerospace valve, in which the O rings are clearly visible as light horizontal bands. A neutron tomography system generally consists of an intense neutron source, object turntable, a scintillator screen, a mirror, a cooled charge coupled device camera, and computer imaging and processing support. With such an arrangement, the actual distribution of materials across a given path can be determined.

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FIGURE 34. Simplified illustration of computed tomography for aerospace structures.

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FIGURE 35. Computed tomography system.

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FIGURE 36. Digital images of turbine blade: (a) fluoroscopic; (b) tomographic.

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As aerospace and nondestructive test engineers have become more familiar with the capabilities of neutron radiography and tomography, the techniques have been used more. Neutron tomography systems have been used to determine the hydrogen content in aircraft compressor blades and the exact placement and shape of O rings in critical components of spacecraft (Fig. 38).

An Air Force Research Laboratory program was initiated to develop and evaluate advanced radiographic and radioscopic systems. To ensure broad applicability to many aerospace inspection needs, a diverse team was assembled consisting of commercial companies and government contractors.

The program evaluates detectors that are proven in other industries and that are fast and easy to use. To reduce reliance on more costly film techniques and enhance inspection productivity, these detectors include large area, flat panel, X-ray detectors modified from medical applications, recently available dental sensors and photographic industry digital charge coupled devices modified for radiography.

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FIGURE 37. Neutron radiograph of aerospace valve.

O rings are visible as light bands.

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FIGURE 38. Neutron tomographic image of aerospace valve, showing bend in O ring.

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Reversed Geometry, Scanning Beam Technique The National Aeronautics and Space Administration has explored a technique, reversed geometry scanning beam radiographic testing. The technique uses an electronically scanning X-ray source and a discrete detector for radioscopic imaging of a structure. The scanning source system has several advantages. Miniaturization of the discrete X-ray detector enables easy positioning inside a complex structure (such as an aircraft wing) allowing images of each surface of the structure to be obtained separately. Additional advantages include multiple detectors that enable the simultaneous acquisition of data from several different perspectives without moving the structure or the measurement system. This provides a means for locating the position of discontinuities and enhances separation of features at the surface from features inside the structure. Finally, the amount of secondary scattered radiation contributing to the noise in the image is reduced compared to conventional radiography.

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Noise reduction facilitates the acquisition and analysis of quantitative data about the integrated material density along the ray path between the source and the detector. Details of the techniques and application to crack detection in aircraft structures have been published. Results are shown for different crack sizes in a range of thicknesses. Application to honeycomb structures is also being presented. A honeycomb specimen with a fatigue crack in one of the face sheets was imaged with the scanned X-ray system. It was shown that the variation in contrast s a function of incident angle can be used to remove some of the image clutter due to the effects from the underlying honeycomb structure, thereby improving the detectability of the crack. A differential laminography image produced from examining the difference in laminographic images at different depths gave the best image of the crack.

The reversed geometry, scanning beam technique is discussed at greater length elsewhere in this book.

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Gaging Techniques Radiation gaging includes a wide variety of measurement types. Gamma rays, X-rays, beta particles, neutrons and positive ions can all be used for radiation gaging. These radiations interact with the test material in a number of useful ways. Despite the wide diversity of techniques that can be used, one technique, gamma or X-ray attenuation gaging, has found the widest application because of its general applicability to all materials and many component configurations. Gamma and X-ray attenuation techniques are particularly well suited for process monitoring applications such as control of thickness in a rolling mill or monitoring density of a solution in process piping.

As an inspection tool for fabricated components, attenuation gaging can be used to ensure that density, composition and thickness of a wide variety of materials have been kept under control.

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Attenuation gaging can achieve extremely high accuracies for some applications; Or operational parameters can be adjusted for rapid testing on an assembly line basis. Interactions of radiation with matter that are useful for gaging are listed in Table 1, along with an indication of measurement applications.

For convenience, these may be classed in two categories: (1) those involving gamma rays and X-rays and (2) radiations based on interactions of nuclear particles (neutrons, positive ions and beta particles).

Gaging with particles from the atomic nucleus gives the name nucleonic to nucleonic gaging, widely used for the online gaging of low density and thin film materials, such as paper and other materials manufactured in sheets and rolls. Nucleonic gaging is a well established family of quality control techniques with a long history in nondestructive testing.

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TABLE 1. Applications of radiation measurement for gaging of density or thickness.

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Neutron Gaging.

Neutron gaging is a very specialized technique most often used for measuring hydrogen content (usually in the form of water), or less often, the content of other isotopes with high neutron cross sections. Neutron gages can be simple, inexpensive systems for measuring moisture in food, soil or other bulk materials. The basis of the measurement technique can be attenuation of thermal or fast neutrons, moderation of fast neutrons, or scattering of thermal neutrons. For some applications, such as moisture measurements, neutron gaging is the best or only available technique. However, neutron gages are of limited and specialized use. A published review provides an excellent summary of neutron gaging.

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Pulsed Fast Neutron Analysis Several techniques have been advanced for detection of explosive devices by using interactions of specific nuclei with gamma rays or fast neutrons.

Techniques using these interactions identify the device by measuring the densities or relative concentrations of the elemental constituents of explosives. Pulsed fast neutron analysis has been advanced as a technology that can interrogate large (truck sized) containers and conveyances for user specified chemicals or materials such as drugs, explosives and hazardous materials. The test object is subjected to short pulses of fast neutrons that pass through container walls and produce gamma rays as they strike the cargo. Gamma sensors measure the radiation, which permits identification of elements (carbon, oxygen and nitrogen) present in small, specific areas of the test object. The system then assembles the small images into a composite image that shows the contents of the container. Studies have reported that the technique can work. As of the turn of the century, however, the cost of the technology has limited its implementation.

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Concrete Because concrete is an aggregate that severely scatters acoustic waves, the applicability of nonacoustic test methods, such as radiographic and microwave testing, has long been of interest. The ability of gamma rays to provide images of steel rebars inside concrete (Fig. 19) has been demonstrated although access problems prevent widespread implementation of the technique in highway maintenance programs.

Obstacles include (1) the desirability of testing from two sides, often difficult, (2) safety considerations associated with high energy radiation in public places and (3) expense.

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Research by the United States Department of Transportation has included the evaluation of the following techniques for inspecting concrete, steel and asphalt: (1) X-ray computed tomography for determination of crack propagation, void percentage and distribution in concrete;48 (2) validation of X-ray radiography techniques for inspection of bridge cables; (3) development of codes, standards and specifications for radiography and tomography of concrete; (4) measuring cement hydration using a neutron scattering technique; and (5) determination of chloride concentration and depth profiles in concrete using prompt gamma neutron activation analysis.

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Chloride Contamination Chloride contamination is a major contributor to road deterioration. A portable prompt gamma neutron activation spectroscopy system has been developed to analyze the elemental composition (calcium, silicon, aluminum and others) of reinforced concrete and to measure chloride contamination. The portable system consists of a high purity germanium gamma detector with a 70 percent relative efficiency, a californium-252 neutron source and moderator subsystem and a portable multichannel analyzer system integrated with a laptop computer.

Two types of activation experiments were performed to evaluate the device: (1) a detector calibration and (2) an evaluation of the actual performance of the complete system with the californium-252 source using full scale test slabs containing known amounts of chloride. Both techniques indicate that it is feasible to use this technique to measure the chloride content of reinforced concrete in the field. The chloride level for the corrosion threshold can be measured with a precision of 10 percent for a counting time of roughly 6 min.

The prompt gamma neutron activation technique is competitive with the conventional destructive method.

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FIGURE 19. Radiograph of 13 mm (0.5 in.) reinforcement bars and 16 mm(0.6 in.) steel conduit in 0.46 m (18 in.) thick concrete slab.44

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Roofs and Hidden Apertures.

In the field of building conservation, radiographic testing shares at least two applications with another method, infrared thermography: one is looking for hidden apertures in walls, as in the case of the Capitol described above; another is surveying rooftops for moisture ingress and retention. A neutron moisture gage is feasible because of the opacity of water molecules to neutron radiation.

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Prompt Gamma Neutron Activation Analysis Neutron activation analysis (NAA) is a nuclear process used for determining the concentrations of elements in a vast amount of materials. NAA allows discrete sampling of elements as it disregards the chemical form of a sample, and focuses solely on its nucleus. The method is based on neutron activation and therefore requires a source of neutrons. The sample is bombarded with neutrons, causing the elements to form radioactive isotopes. The radioactive emissions and radioactive decay paths for each element are well known.

Using this information, it is possible to study spectra of the emissions of the radioactive sample, and determine the concentrations of the elements within it.

A particular advantage of this technique is that it does not destroy the sample, and thus has been used for analysis of works of art and historical artifacts.

NAA can also be used to determine the activity of a radioactive sample.

If NAA is conducted directly on irradiated samples it is termed Instrumental Neutron Activation Analysis (INAA). In some cases irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest, this technique is known as Radiochemical Neutron Activation Analysis (RNAA).

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

NAA can perform non-destructive analyses on solids, liquids, suspensions, slurries, and gases with no or minimal preparation. Due to the penetrating nature of incident neutrons and resultant gamma rays, the technique provides a true bulk analysis. As different radioisotopes have different half-lives, counting can be delayed to allow interfering species to decay eliminating interference. Until the introduction of ICP-AES and PIXE, NAA was the standard analytical method for performing multi-element analyses with minimum detection limits in the sub-ppm range.[1] Accuracy of NAA is in the region of 5%, and relative precision is often better than 0.1%.[1] There are two noteworthy drawbacks to the use of NAA; even though the technique is essentially non-destructive, the irradiated sample will remain radioactive for many years after the initial analysis, requiring handling and disposal protocols for low-level to medium-level radioactive material; also, the number of suitable activation nuclear reactors is declining; with a lack of irradiation facilities, the technique has declined in popularity and become more expensive.

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

NAA of 59Co

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

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Good Luck!

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