Understanding neutron radiography reading iii level1 nrt

Understanding Neutron Radiography Reading III-Level1Exercise My ASNT Level III, Pre-Exam Preparatory Self Study Notes 3 July 2015

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Nuclear Source-Reactors

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Neutron Source-Cyclotron

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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 3th July 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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â– http://minerals.usgs.gov/minerals/pubs/commodity/ Charlie Chong/ Fion Zhang

Neutron Cross Section of the elements

â– http://periodictable.com/Properties/A/NeutronCrossSection.html Charlie Chong/ Fion Zhang

Screen Types-1 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. 7. Boron used for neutron shields.

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Screen Types-2 1.

2.

Transfer screen-indium or dysprosium, In, Dy. 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 (cellulose nitrate film) 6. Neutron Accelerator Target material: Beryllium, Be. 7. Beam filter, Beryllium thermalized thermal neutron further and pass only cold neutron.

8. Cadmium remove thermal & cold neutrons and pass epithermal neutrons.

9. Fast neutron direct radiography used Tantalum or transfer radiography with Holmium.

10. Gadolinium Gd, conversion screens emit- (1) gamma rays and (2) conversion electronn 11. Dysprosium (16166Dy) conversion screens emit: (1) high-energy betas β, (2) low-energy gammas γ, and (3) internal-conversion electrons e.

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IVONA TTS Capable.

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http://www.naturalreaders.com/

Reading III Content  Reading One: ASNTHBVol4Chapter16  Reading Two: ASNTNRTMQ123  Reading Three:  Reading Four:

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Reading-1 ASNTHBVol4Chapter16

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PART 1. Applications of Neutron Radiography Neutron radiation is similar to X-radiation. The radiation can originate from an effective point source or can be collimated to shine through an object in a coherent beam. The pattern of penetrating radiation can then be studied to reveal clues about the internals of the object. The information conveyed can be very different from that obtainable with X-rays. Whereas X-rays are attenuated by dense metals more than by hydrocarbons, neutrons are attenuated more by hydrocarbons than by most metals. The difference can mean much more than the reversal of a positive image to a negative image.

Neutrons, for example, can reveal details within high density surroundings that cannot be revealed by other means. A typical application for neutron radiography is shown in the images of a pyrotechnic device (Fig. 1), where the small explosive charge is encased in metal. Other applications include inspection of explosive cords used in pilot ejector mechanisms; inspection of gaskets, seals and O-rings inside metallic valves; confirmation that coolant channels in jet engine turbine blades are free of blockage; studies of coking in jet engine fuel nozzles; and screening of aircraft panels to detect low level moisture or early stage corrosion in aluminum honeycomb (Fig. 2).

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FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit.

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FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit.

(b)

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FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit. (c)

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FIGURE 1. Electric bridge wire squid: (a) drawing and (b) neutron radiograph of part as aid to interpretation; (c) helium-3 gaseous penetrant applied to serviceable unit; (d) penetrant applied to dysfunctional unit. (d)

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FIGURE 2. Comparison of neutron radiographs of moisture globules in aluminum honeycomb panel, later dried: (a) before processing; (b) after processing. (a)

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FIGURE 2. Comparison of neutron radiographs of moisture globules in aluminum honeycomb panel, later dried: (a) before processing; (b) after processing. (b)

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User’s Guide Unlike many other forms of nondestructive testing, neutron radiography is not a do-it-yourself technique. There have been neutron radiography service centers in the United States since 1968. To try out neutron radiography on an object of interest, it is simply necessary to locate the services currently available and, if agreed, mail your item to them. Typically, the neutron radiograph and your item will be mailed back within a day or two. The cost could be less than 1 or 2 h of an engineer’s time. If assistance is required to interpret the findings, this too may be requested on a service basis, as may referrals to more specialized neutron radiographic techniques. The providers of neutron radiography services use equipment and expertise that is highly specialized. Even though one or more neutron radiography service centers have been operating successfully for over 30 years, there has been no inhouse neutron radiography available at any general service, commercial nondestructive testing center.

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The interested user is therefore advised to seek a supplier of neutron radiographic services using leads such as society directories or the published literature. Because neutrons are fundamentally different from X-rays, any object that is a candidate for inspection by X- adiography could also be a candidate for neutron radiography. If X-rays cannot give sufficient information, then trials with neutron techniques may be prudent. The most frequently successful complement to X-radiography is static radiography with thermal neutrons. This approach is reviewed next.

Then more specialized neutron radiology techniques are reviewed, such as neutron (1) computed tomography, (2) dynamic neutron imaging, (3) high frame rate neutron imaging, (4) neutron induced autoradiography and (5) neutron gaging.

For each of the neutron radiology techniques different neutron energies may be selected. The user should be aware that many of the specialized services are only available at one or two centers worldwide. It is therefore important to shop in the global market and to take advantage of the excellent communications existing between neutron radiography centers in various countries.

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PART 2. Static Radiography with Thermal Neutrons 2.1 Neutron Energy Thermal energy neutrons are those that have collided repeatedly with a moderator material, typically graphite or water, such that they reach an equilibrium energy with the thermal energy of the moderator nuclei. The attenuation coefficients for thermal neutrons differ from material to material in a way that is different from X-rays as shown in Table 1. As a consequence, a high degree of contrast between the elements in an object is possible. In addition, thermal neutrons are relatively easy to obtain and easy to detect.

Keywords: Thermal Neutron: they reach an equilibrium energy with the thermal energy of the moderator nuclei.

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TABLE 1. Comparison of X-ray and thermal neutron attenuation.

a. Other materials relatively transparent to thermal neutrons include gold, silver, platinum, titanium, silicon, tin and zinc. b. Other materials relatively opaque to thermal neutrons include hydrogenous oils, plastics, rubbers, explosives and light elements boron and lithium.

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2.2 Neutron Collimation Because the source of thermal neutrons is a dispersed moderator volume, rather than a point source, it is necessary to use a collimator between the source and the object.

In preference to a single tube parallel sided collimator or a multiple slit collimator, the most frequently used design uses divergent beam geometry.

The collimator may be used to extract a beam in any one of a variety of different geometries including horizontal or vertical, radial or tangential to the source.

A collimator that is tangential to the source can provide a thermal neutron beam relatively free of fast neutron and gamma ray contamination.

An incidental consequence of the divergent collimator principal is that even very large objects can be radiographed using an array of side-by-side films (Fig. 3).

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FIGURE 3. Radiographs of full size motorcycle: (a) neutron radiograph; (b) xradiograph.

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The source of thermal neutrons is a dispersed moderator volume, rather than a point source

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ASMV17 Neutron Radiography

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ASMV17 Neutron Radiography

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

Ug =D∙ t/L I = Ф/16∙(L/D)2 I = Ioe –μnt μn = N’σ N’ = nuclei/cm2 N’ = ρN/A N = Avogadro's number μn = N’σ = [ρN/A]∙σ

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ASMV17 Neutron Radiography

For photons:

I = Ioe –μx t

Eq.1

For Neutron

I = Ioe –Nσt = Ioe –μn t

Eq.2

Where:  I is the transmitted beam;  Io is the incident beam;  μx is the linear attenuation coefficient for photons;  t is the thickness of specimen in the beam path;  N is the number of atoms per cubic centimeter;  σ is the neutron cross section of the particular material or isotope (a probability or effective area); and,  μn is the linear attenuation coefficient for neutrons (μn = Nσ).

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5.1 Neutron cross sections Neutron cross sections are defined in Part 1 of this Section. Values for thermal neutrons for many materials (elements) are given in Table 9 (see Bibliography item 8 for a more extensive compilation). Generally, neutron cross sections decrease with increasing neutron energy; exceptions include resonances, as mentioned earlier. Cross section values can be used to calculate the attenuation coefficients and the neutron transmission as shown in eqs. 1 and 2. For compound inspection materials, the method for calculating the linear attenuation coeffici ent is shown following Table 9.

If the material under inspection contains only one element, then the linear attenuation coefficient is: μ = ρ∙Nσ/ A

Eq.7 (where ρ∙N/A is the number of nuclei/cm2)

Where: μ -is the linear attenuation coefficient of specific neutron (cm-1 ) ; ρ is the material density (g/cm3); N is Avogadro's number (6.023 X 1023 atoms/gram-molecular weight) ; σ is the total cross section in barns (cm2 ) ; and A is the gram atomic weight of material.

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2.3 Neutron Imaging ■Collimation Ratio The collimation ratio is the ratio L¡D-1 of the collimator length L to aperture diameter D. This ratio helps to predict image sharpness. ■Imaging Processes For static thermal neutron radiography of nonradioactive objects, two important imaging processes are (1) the gadolinium converter with single emulsion X-ray film and (2) the neutron sensitive storage phosphor (neutron imaging plate).

For static neutron radiography of radioactive objects, additional imaging processes are (1) dysprosium foil activation transfer to film, (2) indium foil activation transfer to film and (3) track etch imaging using a boron converter and cellulose nitrate film.

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The established direct imaging technique uses thin gadolinium layer vapor deposited on a solid converter screen, which is held flat against a single emulsion film inside a vacuum cassette of thin aluminum construction. An exposure of 109 neutrons per square centimeter (109 n/cm2) can give a high resolution, high contrast radiograph if careful dust free film darkroom procedures are used.

Neutron sensitive imaging plates consist of a thin phosphor layer containing a mixture of storage phosphor, neutron converter and organic binder. Following the neutron exposure stage is the information readout phase, in which the plate is scanned by a thin laser beam stimulating the emission of a pattern of light.

Merits of this neutron imaging technique include five decades of linearity (?) , wide dynamic range, direct availability of digital data for processing converter efficiencies of 30 to 40 percent, and spatial resolution acceptable for some applications.

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For neutron radiography of highly radioactive objects, dysprosium and indium foil activation transfer to film and track etch imaging each offer complete discrimination against gamma ray fogging. Examples of nuclear fuel neutron radiography are shown in Fig. 4. Dysprosium transfer can be combined with a cadmium indium foil sandwich for dual energy radiography. Alternative track etch techniques have been developed to yield more precise dimensional measurements.

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FIGURE 4. Neutron radiographs of nuclear fuel: (a) longitudinal cracks in pellets; (b) missing chips in compacted fuels; (c) inclusions of plutonium in pellets; (d) accumulation of plutonium in central void; (e) deformed cladding; (f) hydrides in cladding. (a)

(b)

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FIGURE 4. Neutron radiographs of nuclear fuel: (a) longitudinal cracks in pellets; (b) missing chips in compacted fuels; (c) inclusions of plutonium in pellets; (d) accumulation of plutonium in central void; (e) deformed cladding; (f) hydrides in cladding. (c)

(d)

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FIGURE 4. Neutron radiographs of nuclear fuel: (a) longitudinal cracks in pellets; (b) missing chips in compacted fuels; (c) inclusions of plutonium in pellets; (d) accumulation of plutonium in central void; (e) deformed cladding; (f) hydrides in cladding. (e)

(f)

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â– Image Quality Indicators For any nondestructive system, the best measure of quality is to compare the image of the test object with an image of a similar object that contains a known artificial discontinuity, a defect standard, or reference standard.

However, neutron radiography has the same problems as other nondestructive testing methods: the quantity of reference standards required is too large to obtain and maintain. In lieu of a reference standard, neutron radiographers have chosen to fabricate a resolution indicator that emulates the worst case scenario with gaps placed between and holes placed beneath different plastic thicknesses.

For defining the neutron beam characteristics a beam purity indicator has been devised to accompany the sensitivity indicator. The image quality indicator system of ASTM International has become the primary or alternate system for most manufacturing specifications on an international basis. The no umbra device, a device to measure resolution, is described in ASTM E 803-91 and can be used to determine the collimation ratio L¡D–1 of the neutron radiography facility.

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ASTM E803 - 91(2013) Standard Test Method for Determining the L/D Ratio of Neutron Radiography Beams

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2.4 Nuclear Reactor Systems A nuclear reactor system operated for over 30 years solely to provide a commercial neutron radiographic service is illustrated in Fig. 5. The reactor core, positioned underground in a tank of water, is only about 0.38 m (15 in.) in diameter and operates at 250 kW power. The tangential beam tube is orientated vertically with air displaced by helium. Parts for neutron radiography can therefore be supported on horizontal trays. Usually the neutron imaging uses a gadolinium converter with fine grain radiographic film and the exposure time at a selected collimation is typically about 2 min.

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FIGURE 5. Representative neutron radiographic service center for nonnuclear applications.

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Tangential Beam Tube

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http://www-llb.cea.fr/spectros/spectro/2t1.html

Another reactor that has provided neutron radiography services since 1968 is illustrated in Fig. 6. It is above ground and the fuel of the 100 kW core is arranged in an annulus with a moderator region in the center.

Two horizontal beams are extracted from the central moderator, one for direct film neutron radiography of nonradioactive objects, the other for dysprosium activation transfer neutron radiography of radioactive nuclear fuel.

Another service for static neutron radiography of radioactive nuclear fuel has been provided by a 250 kW nuclear reactor installed in a hot cell complex (Fig. 7). Also several university reactors in the United States have been equipped for neutron radiography. Worldwide, over fifty nuclear reactors have contributed to development of this field.

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FIGURE 6. Representative neutron radiographic service center for nuclear and nonnuclear applications.

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FIGURE 7. Hot cell fuel inspection system.

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Hot cell Shielded nuclear radiation containment chambers are commonly referred to as hot cells. The word "hot" refers to radioactivity. Hot cells are used in both the nuclear-energy and the nuclear-medicines industries. They are required to protect individuals from radioactive isotopes by providing a safe containment box in which they can control and manipulate the equipment required.

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Hot cell Shielded nuclear radiation containment chambers are commonly referred to as hot cells. The word "hot" refers to radioactivity. Hot cells are used in both the nuclear-energy and the nuclear-medicines industries. They are required to protect individuals from radioactive isotopes by providing a safe containment box in which they can control and manipulate the equipment required.

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

Hot cells at the Argonne National Laboratory. Each cell is equipped with a viewing window and two remote manipulators.

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

Applications: Hot cells are used to inspect spent nuclear fuel rods and to work with other items which are high-energy gamma ray emitters. For instance, the processing of medical isotopes, having been irradiated in a nuclear reactor or particle accelerator, would be carried out in a hot cell. Hot cells are of nuclear proliferation concern, as they can be used to carry out the chemical steps used to extract plutonium from reactor fuel. The cutting of the used fuel, the dissolving of the fuel and the first extraction cycle of a nuclear reprocessing PUREX process (highly active cycle) would need to be done in a hot cell. The second cycle of the PUREX process (medium active cycle) could be done in gloveboxes.

Hot cells are commonly used in the nuclear medicines industry: - for the production of radiopharmaceuticals, according to GMP guidelines (industry) - for the manipulation and dispense of radiopharmaceuticals (hospitals) The user must never be subject to shine paths that are emitted from the radioactive isotopes and therefore there generally is heavy shielding around the containment boxes, which can be made out of stainless steel 316 or other materials such as PVC or Corian. This shielding can be ensured by the use of lead (common) or materials such as concrete (very large walls are therefore required) or even tungsten. The amount of radioactivity present in the hot cell, the energy of the gamma photons emitted by the radioisotopes, and the number of neutrons that are formed by the material will prescribe how thick the shielding must be. For instance a 1 kilocurie (37 TBq) source of cobalt-60 will require thicker shielding than a 1 kilocurie (37 TBq) source of iridium-192 to give the same dose rate at the outer surface of the hot cell.

Also if some actinide materials such as californium or spent nuclear fuel are used within the hot cell then a layer of water or polyethylene may be needed to lower the neutron dose rate.

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

Viewing windows: In order to view what is in the hot cell, cameras can be used (but these require replacing on a regular basis) or most commonly, lead glass is used. There are several densities for lead glass, but the most common is 5.2 g/cm3. A rough calculation for lead equivalence would be to multiply the Pb thickness by 2.5 (e.g. 10 mm Pb would require a 25 mm thick lead glass window). Older hot cells used ZnBr2 solution in a glass tank to shield against high-energy gamma rays. This shielded the radiation without darkening the glass (as happens to leaded glass with exposure). This solution also "self-repairs" any damage caused by radiation interaction, but leads to optical distortion due to the difference in optical indices of the solution and glass.

Manipulators: Telemanipulators or tongs are used for the remote handling of equipment inside hot cells. These are incredibly valuable as they do not require the user to place his/her arms inside the containment box and be subject to heavy finger/hand doses. These need to be used in conjunction with a shielded sphere which can be made by most lead engineering companies.

Gloves: Lead loaded gloves are often used in conjunction with tongs as they offer better dexterity and can be used in low radiation environments (such as hot cells used in hospital nuclear medicine labs). Some companies have developed tungsten loaded gloves which offer greater dexterity than lead loaded gloves, with better shielding than their counterparts. Gloves must be regularly replaced as the chemicals used for the cleaning/ sterilisation process of the containments cause considerable wear and tear.

Clean rooms: Hot cells are generally placed in clean rooms with an air classification ranging from D to B (C is the most common). It is extremely rare to find a hot cell which is placed in a class A or unclassified clean room.

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

Hot Cell

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http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

Hot Cell Charlie Chong/ Fion Zhang

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Hot Cell

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http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

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http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

Hot Cell

Hot Cell

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http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

Hot Cell

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http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

Hot Cell

Hot Cell

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http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

2.5 Accelerator Based Systems An initial user of neutron radiography need not, in general, be concerned with accelerator source options unless there is an established need either for an in-house system or for a transportable system. Almost all neutron radiography service providers use a nuclear reactor source. One exception has been the powerful spallation type accelerator in Switzerland; the accelerator is a multipurpose facility comparable in complexity and cost to a research reactor.

An in-house system that was operated successfully for over 15 years at the United States Department of Energy’s Pantex Plant used a van de graaff accelerator. The operation of this machine, which accelerates over 200 ΟA of deuterons at 3 MeV into a beryllium target, is illustrated in Fig. 8.

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FIGURE 8. Cross section showing van de graaff principle.

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The system provided a peak thermal neutron flux of about 109 neutrons per square centimeter second (109 n∙cm-1∙s-1), two orders of magnitude less than the reactor systems described above but sufficient for low throughput work using 2 h exposure times and a relatively low beam collimation ratio.

Cyclotrons and radio frequency quadrupole accelerators are other candidates for a potential custom designed in-house neutron radiographic system.

Neutron radiographic performance data have been reported for designs with a variety of sizes, neutron yields and costs. For transportable systems much of the development work has used sealed tube acceleration of deuterium tritium mixtures. This can consist of a source head that is maneuverable with long high tension cable linking it to the high voltage power supply and control unit as illustrated (Fig. 9). The particular type shown yields a peak thermal neutron flux of about 108 neutrons per square centimeter second with a tube operation half life of about 200 h.

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FIGURE 9. Components of mobile deuterium tritium neutron radiographic system: (a) deuterium tritium source head, typically on 6 m (20 ft) cables; (b) cooling unit (left) and power supply; (c) control unit.

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2.6 High Intensity Californium-252 Systems Of the many radioactive neutron sources, such as polonium-210, beryllium and americium-244 beryllium, one has dominated interest for neutron radiography: californium-252. This transplutonic isotope is produced as a byproduct of basic research programs. In the United States, some government centers have been able to obtain the source on a low cost loan basis from the Department of Energy. The isotope yields neutrons by spontaneous fission at a rate of 2 × 109 neutrons per second per milligram and has a half life of 2.5 years. A high yield source of up to 50 mg can be smaller than a tube of lipstick (Fig. 10). An in-house stationary system has operated at the United States Department of Energy’s installation at Pantex with a total source strength of 150 mg californium-252. It provided sets of nine films, each 350 × 425 mm (14 × 17 in.), approaching reactor quality by using gadolinium with a very fine grain X-ray film; a collimator ratio of 65; and exposure time of under 24 h.

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FIGURE 10. Californium-252 sources compared in size to postage stamp.

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A maneuverable source system has operated at McClellan Air Force Base with a total source strength of 50 mg californium- 52. It provided single neutron radiographs using a fast scintillator screen; high speed, light sensitive film; a collimator ratio of 30; and an exposure time of 12 min. This system was designed for the specific application of scanning intact aircraft to detect hidden problems at an early stage, such as moisture or corrosion in aluminum honeycomb.26 Another example of a high yield californium-252 system design uses a subcritical multiplier to amplify the central neutron flux. This design (Fig. 11) produces a peak central flux of 7 Ă— 108 neutrons per square centimeter second when loaded with 40 mg californium-252.

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FIGURE 11. Elevation of subcritical multiplier system.

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â– Low Cost In-House System There is evidence that an extremely low intensity californium-252 neutron source could provide a convenient, low cost in-house system. A source size of only 100 Îźg can provide useful quality neutron radiographs by using highly efficient imaging systems that need only 105 neutrons per square centimeter exposure. This is 10 000 times less than the exposure used typically with gadolinium and single emulsion film. The small source size would mean an inexpensive source and also inexpensive shielding, handling and interlock requirements. Therefore, a nondestructive testing center with a variety of Xray, ultrasonic and other inspection capabilities could easily incorporate a small californium-252 based neutron radiographic capability using an underground storage geometry in an existing radiographic bay. Because neutron radiography yields unique information, such an inexpensive in-house capability could be an important complement to an otherwise full service nondestructive testing center.

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Californium-252 Neutron Source

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http://www.orau.org/ptp/collection/Sources/cf-252.htm

Californium-252 Neutron Source Neutron Fluence Particle fluence is defined as the number of particles traversing a unit area in a certain point in space in a unit period of time. Most frequently, it is measured in n∙cm-2.

In particular, neutron fluence in high-energy physics applications is of interest in the context of the radiation environment around the interaction regions of colliders; it serves as a measure for potential radiation damage for the detector systems to be used there. It is common practice to express charged and neutral particle contributions to radiation in terms of dose ( Radiation Measures and Units) and 1 MeV neutron equivalent fluence ( also NIEL Scaling), respectively.

The 1 MeV equivalent MeV equivalent neutron fluence is the fluence of 1 MeV neutrons producing the same damage in a detector material as induced by an arbitrary particle fluence with a specific energy distribution. The choice of this particular normalization is partly due to historical reasons, as the standard energy to scale to was considered first in damage studies in the MeV range, in neutron physics; however, there is also a physical background: the neutron spectra expected in detectors at future hadron colliders typically have a probability density peaking in this energy region.

See also ASTM E261 - 10 Charlie Chong/ Fion Zhang

http://ikpe1101.ikp.kfa-juelich.de/briefbook_part_detectors/node123.html

PART 3. Special Techniques of Neutron Radiography 3.1 Dynamic Neutron Radioscopy Services that provide different types of dynamic neutron radioscopy have been developed at numerous nuclear reactor centers worldwide. They cover frame rates that range from 30 frames per second (real time motion display similar to television) to 1000 frames per second range (a high frame rate) or to 10 000 frames per second (a very high frame rate). An example of a real time dynamic neutron radioscopic application is illustrated in Figure 12. A beam from a 28 MW reactor was used to study the flow characteristics of lubricant inside an operating jet engine. Other applications have included studies of absorption and compression refrigerator designs, studies of automotive parts in motion and a large range of two-phase flow studies. For high throughput dynamic neutron imaging one reactor center has been equipped with three separate beams, each with its neutron imaging system and digital image interpretation system.

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FIGURE 12. Frames from real time studies of operating aircraft engine: (a) first view; (b) second view.

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Other reactor centers have developed techniques for simultaneous neutron and gamma ray dynamic imaging using a pair of scintillator screens in conjunction with a low light level television camera and video processing. The development of dynamic neutron radioscopic services with a high frame rate of 1000 frames per second has capitalized on the availability of very high intensity steady state neutron beams (with a flux of 108 neutrons per square centimeter second) and very high frame rate video cameras used with rapid response neutron sensitive scintillator screens. A very high frame rate capability, up to 10 000 frames per second, uses the ability of certain reactors to be pulsed, giving a high neutron yield for a time duration of a few milliseconds. The event to be studied, such as the burn cycle of a pyrotechnic event 爆破效果, is synchronized to the neutron pulse time.

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3.2 Subthermal Neutron Radiology (Cold) The neutron attenuation coefficient of a particular material can change significantly as the neutron energy is changed. The pattern of this variation also changes abruptly from one element to another. Therefore, selection of different energy neutrons provides possibilities for quite different neutron radiology penetration and contrast. Neutron radiology service reactors have developed neutron beams of selected subthermal or cold neutrons using three techniques: (1) beam filtration by polycrystal beryllium, which passes only long wavelength, low energy neutrons below 0.005 eV, (2) a refrigerated moderator volume and (3) selection of longer wavelength, low energy neutrons by multiple internal reflection in a gently curved guide tube.

Keypoints: Beam filter, Beryllium pass only cold neutron.

Cadmium remove thermal & cold neutrons and pass epithermal neutrons.

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The effect of this energy selection is typically to increase the transparency of certain materials while simultaneously increasing the contrast or detectability of hydrogenous materials (see Table 2 and Fig. 13). Just as thermal neutron radiography gives different information to X-radiography, so subthermal or cold neutron radiography gives information different from that of regular thermal neutron techniques. An example is given in Fig. 14. It is possible, using a guide tube, to select only very cold neutrons (that is, energies below 0.001 eV) and this can provide high sensitivity for very thin hydrogenous specimens.

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TABLE 2. Relative neutron attenuation coefficients.

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FIGURE 13. Attenuation of materials for thermal and cold neutrons.

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FIGURE 14. Neutron radiographs of explosive bridge wire igniter: (a) thermal neutron image; (b) cold neutron image.

Explosive charge Charlie Chong/ Fion Zhang

3.3 Epithermal and Fast Neutron Radiology A reactor beam, although consisting primarily of thermal neutrons, will contain a proportion of both subthermal and epithermal (high energy) neutrons. With a filter such as cadmium, the thermal and subthermal neutrons can be removed and only the epithermal part of the neutron energy spectrum will be transmitted. For the inspection of enriched nuclear fuel the higher penetration of epithermal neutrons provides a valuable difference from thermal or subthermal neutron radiography. Indium has a high resonance capture cross section at about 1.4 eV epithermal energy. Cadmium wrapped indium foil activation transfer imaging techniques have been used for this application.

Another epithermal neutron technique uses an indium foil filter in the incident beam to remove neutrons close to the specific resonance energy. This beam is passed through the object and an indium detector is used on the far side.

Keypoints: Beam filter, Beryllium pass only cold neutron.

Cadmium remove thermal & cold neutrons and pass epithermal neutrons.

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The technique can provide high sensitivity to small quantities of hydrogen in the object because hydrogen can change the energy of an incident neutron more than heavier elements. The term fast neutron radiography refers normally to those neutron energies yielded by an unmoderated accelerator source or radioactive source. Fast neutron radiography provides high penetration but little contrast between elements. The accelerator can provide a point source. Tantalum is one of several detector materials for direct exposure and scintillator screens can be used. Alternatively, foil activation transfer with holmium has been demonstrated.

Keypoints: Beam filter, Beryllium pass only cold neutron.

Cadmium remove thermal & cold neutrons and pass epithermal neutrons.

Fast neutron direct radiography: Tantalum Fast neutron transfer radiography; Holmium

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3.4 Neutron Computed Tomography Computed axial tomography has been developed for neutron radiography and can provide detailed cross sectional slices of the object to be analyzed.

Although the principle is similar to that of X-ray computed tomography, the information conveyed by neutrons can be unique. In a typical facility the object is rotated in the neutron beam and data are stored for upward of 200 angles. Detectors used have included a scintillator screen 6LiF-ZnS (Ag), viewed by a cooled charge coupled device camera and alternatively a storage phosphor image plate loaded with Gd2O3 combined with an automatic laser beam scanner. Using a high intensity neutron radiography beam of over 108 neutrons per square centimeter second, computed tomography of two-phase flow volumes has been processed as a time averaged three-dimensional analysis.

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3.5 Neutron Gaging and Neutron Probe Techniques Neutron gaging is the measurement of attenuation of a collimated small diameter beam of radiation as it is transmitted by a specimen. A neutron radiology service center equipped with a nuclear reactor has demonstrated that the imaging techniques can be complemented by the more quantitative techniques of gaging. The gaging technique can inspect items of greater thickness than can be inspected with neutron radiography. It has been used for static gaging of discrete assemblies and for continuous scanning of long objects for acceptable uniformity. There are also a variety of neutron probe techniques in which radiation, typically gamma, is observed as a result of neutron radiation incident on the object. For example the associated particle sealed tube neutron generator enables the flight time of the incident neutron to be used in conjunction with gamma ray spectroscopy to indicate the chemical composition within an object. This technique has been developed for identification of hidden explosives, drugs or nuclear materials. Another example of a neutron probe is neutron interferometry to detect phase shifts of the neutron wave properties. This neutron phase topography has been proposed for very high sensitivity material testing.

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â– Neutron Induced Autoradiography By exposing a painting to thermal or cold neutrons and later imaging the radioactivity induced in the various paint components, a technique has been developed sensitive to many elements including manganese, potassium, copper, sodium, arsenic, phosphorus, gold, iron, mercury, antimony and cobalt. The neutron exposures were originally performed in a moderator block (thermal column), close to a reactor core. However, beams similar to those used for transmission neutron radiography have been used for this neutron induced autoradiography of paintings. Typically, a series of autoradiographs is taken using a range of neutron exposure times and different decay times before imaging. This, combined with a range of scintillator screen and film sensitivities, can provide extensive information about successive layers of each painting.

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3.6 Closing Industry standards have been published on neutron radiographic testing.

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End Of Reading 1

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Reading-2 ASNTNRTMQA123 Level-I

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Level 1 Questions Neutron Radiographic Testing Method

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Level 1 Answers Neutron Radiographic Testing Method

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Level 1 Answers Neutron Radiographic Testing Method

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Q1. Neutron penetration is greatest in which of the following materials? a. hydrogenous material b. water c. lead d. boron carbide Q2. In general, by increasing the neutron energy from a neutron radiographic source: a. greater neutron penetration is achieved b. greater neutron radiographic contrast can be obtained c. radiographic exposure time can be reduced d. resolution can be increased Q3. The time required for one-half of the atoms in a particular sample of radioactive material to disintegrate is called: a. the inverse square law b. a curie c. a half-life d. the exposure time Charlie Chong/ Fion Zhang

Q4. Generally, the attenuation of neutrons by a given material is: a. reported to the Atomic Energy Commission b. greater for fast neutrons than thermal neutrons c. an indication of the quality of the X-radiographic technique d. appreciably greater for thermal and epithermal neutrons than for fast neutrons Q5. The mass absorption coefficients for thermal neutrons when plotted against regularly increasing atomic numbers of periodic elements presents a: a. blurred picture b. regularly increasing picture c. random picture d. dark picture

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Q6. Many of the absorption differences between neutrons and X-rays indicate clearly that the two techniques: a. cause radiation problems b. complement each other c. increase exposure speed d. fog radiographic film Q7. The neutron cross section is the term normally used to denote: a. the danger in handling radioactive material b. the absorbing power of a material for neutrons c. the atomic number of neutron reactor material d. radiation detection equipment Q8. The sharpness of the outline in the image of the radiograph is a measure of: a. subject contrast b. radiographic definition c. radiographic contrast d. film contrast Charlie Chong/ Fion Zhang

Q9. The highest quality direct neutron radiographs obtainable today use: a. imaging screens using lithium-zinc sulfide as the imaging materials b. high-speed radiographic films c. dysprosium as an imaging screen d. gadolinium as an imaging screen (?)

Q10. When doing neutron radiography on radioactive materials, the materials are best handled: a. directly by personnel equipped with special protective clothing b. by remote handling equipment c. directly by personnel with special protective clothing except when radiographs are being made d. by the same methods used for nonradioactive materials

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Q11. Gadolinium conversion screens are usually mounted in rigid holders called: (direct radiography?) a. film racks b. cassettes c. emulsifiers d. diaphragms Q12. The best high-intensity source of thermal neutrons is: a. a Cf-252 source b. an accelerator c. a nuclear reactor d. a Cf-252 source plus a multiplier Q13. Scattered radiation caused by any material, such as a wall or floor, on the film side of the specimen is referred to as: a. primary scattering b. undercut c. reflected scattering d. back-scattered radiation Charlie Chong/ Fion Zhang

Q14. What has the highest thermal neutron absorption cross section? a. gold b. Indium c. gadolinium d. dysprosium Q15. Conversion screens are used in neutron radiography: a. to convert neutron energy into ionizing radiation b. to increase the exposure time c. both a and b are reasons for using conversion screens d. neither a nor b is a reason for using conversion screens Q16. A curie is the equivalent of: a. 0.001 mCi b. 1000 mCi c. 1000 MCi d. 100 MCi

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The neutrons transmitted through a radioactive specimen will strike a metal detection foil such as indium, dysprosium or gold, rather than a converter screen with film.

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Q17. Short wavelength electromagnetic radiation produced during the disintegration of nuclei of radioactive substances is called: a. X-radiation b. gamma radiation c. scatter radiation d. back-scattered radiation Q18. A photographic record produced by the passage of neutrons through a specimen onto a film is called: a. a fluoroscopic image b. a radiograph c. an isotopic reproduction d. none of the above Q19. Possible reactions that can occur when a fast neutron strikes a nucleus are: a. scattering and radiative capture b. microshrinkage and static charges caused by friction c. sudden temperature change and film contrast d. uniform thickness and filtered radiation Charlie Chong/ Fion Zhang

Q20. For inspection of radioactive objects or those that emit gamma radiation when bombarded with neutrons, a preferable detection method is the: a. direct exposure method b. transfer method c. isotopic reproduction method d. electrostatic-belt generator method

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Q21. Materials that are exposed to thermal neutron beams: a. must not be handled for at least 3 minutes after exposure has ceased b. must be stored in a lead-lined room c. may be radioactive after exposure to neutrons has ceased d. should be monitored by means of a neutron counter Q22. Hydrogenous material has a: a. high macroscopic scattering cross section (?) b. high absorption cross section c. high microscopic absorption cross section d. low microscopic scattering cross section Q23. The penetrating ability of a thermal neutron beam is governed by: a. attenuation characteristics of the material being penetrated b. time c. source-to-film distance I=Ioe-Îźnt d. all of the above

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Q24. A graph showing the relationship between film optical density and exposure is called: a. a bar chart b. a characteristic curve c. an exposure chart d. a logarithmic chart Q25. The three main steps in processing a radiograph are: a. developing, frilling, and fixation b. developing, fixation, and washing c. exposure, developing, and fixation d. developing, reticulation, and fixation Q26. Radiographic contrast in a neutron radiograph is least affected by: a. developer temperature b. radiographic exposure time c. radiographic beam collimation d. radiographic film fog

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Ug?

Q27. Higher resolution can be achieved in direct neutron radiography by: a. placing lead intensifying screen between a gadolinium screen and film b. increasing the L/D ratio of the collimation system c. increasing the exposure time d. increasing the distance between the object and the film cassette Q28. The main reason for using neutron radiography in place of Xradiography is: a. lower cost b. higher resolution in all cases c. the ability to image objects and materials not possible with X -rays d. simpler radiographic procedure required than X -radiography Q29. The best material for mounting specimens for neutron radiographic inspection is: a. cardboard b. plastic c. steel d. aluminum Charlie Chong/ Fion Zhang

Q30. Which of the following materials is best for making identification labels when using the neutron radiographic process? a. aluminum b. brass c. cadmium or gadolinium d. lead

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Q31. As a check on the adequacy of the neutron radiographic technique, it is customary to place a standard test piece on the source side of the cassette.

This standard test piece is called: l a. a reference plate w il b. a lead screen c. a penetrameter d. an image quality detector Q32. A densitometer is: a. a meter used to measure neutron intensity b. an instrument used to measure film density c. a meter used to measure the density of a material d. a meter used to measure gamma content Q33. The ability to detect a small discontinuity or flaw is called: a. radiographic contrast b. radiographic sensitivity. c. radiographic density d. radiographic resolution Charlie Chong/ Fion Zhang

Q34. Movement, geometry, and screen contact are three factors that affect radiographic: a. contrast b. unsharpness c. reticulation d. density Q35. The difference between the densities of two areas of a radiographic film is called: a. radiographic contrast b. subject contrast c. film contrast d. definition

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Q36. The selection of the proper type of film to be used for neutron adiographic examination of a particular part depends on the: a. thickness of the part b. material of the specimen c. neutron energy d. none of the above (all the above?)

Q37. When radiographing a part that contains a large crack, the crack will appear on the radiograph as: a. a dark, intermittent, or continuous line b. a light irregular line c. either a dark or light line d. a fogged area on the radiograph Q38. Radiographic sensitivity, in the context of defining the minimum detectable flaw, depends on: a. the graininess of the film b. the unsharpness of the flaw image in the film c. the contrast of the flaw image on the film d. all of the above Charlie Chong/ Fion Zhang

Q39. An Image Quality Indicator is used to measure the: a. size of discontinuities in a part b. density of the film c. amount of film contrast d. quality of the radiographic technique Q40. Unwanted inclusions in a part will appear on a radiograph as: a. a dark spot b. a light spot c. a generalized gray area of varying contrast d. either a dark or a light spot or area depending on the relative absorption ratio of the part material and the inclusion material Q41. A sheet of cadmium with an opening cut in the shape of the part to be radiographed may be used to decrease the effect of scattered neutrons, which undercuts the specimens. Such a device is called a: a. mask b. filter c. back-scatter absorber d. lead-foil screen Charlie Chong/ Fion Zhang

Q42. The accidental movement of the specimen or film during exposure or the use of a source-film distance that is too small will: a. produce a radiograph with poor contrast . b. make it impossible to detect large discontinuities c. result in unsharpness of the radiograph d. result in a fogged radiograph Q43. Dysprosium (16166Dy) conversion screens emit: a. low-energy betas and gammas b. high-energy betas β, low-energy gammas γ, and internal-conversion electrons e (more reading!) c. beta particles only d. low-energy gamma rays only Q44. Materials in common usage for moderation of fast neutron sources include: a. aluminum, magnesium, and tin b. water, plastic, paraffin, and graphite c. neon, argon, and xenon d. tungsten, cesium, antimony, and columbium Charlie Chong/ Fion Zhang

TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials

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TABLE 7.4. The characteristics of some possible neutron radiography converter materials

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Practical.NR Table 7.4

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Practical.NR Table 7.4

Internal-conversion Electrons

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Q45. In the converter screen technique, the neutron image is produced by alpha, beta, or gamma radiation and it is thereby: a. used to measure neutron beam divergence b. externally cooled during the process c. photographically more detectable than the unconverted neutron image d. an important factor for determining Young's modulus of the material Q46. Converter screen material characterized by lithium, boron, and gadolinium has little tendency to become radioactive but does: a. protect the radiographic film from excessive pressure b. recharge the focal point size of the neutron source c. filter and collimate the excess neutrons d. emit radiation immediately upon the absorption of a neutron Q47. Gadolinium is frequently employed as a neutron absorber because of its: a. extremely low cost b. high neutron absorption for a given thickness c. ability to absorb gamma rays d. ability to diffract alpha particles

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TABLE 7.4. The characteristics of some possible neutron radiography converter materials

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Practical.NR Table 7.4

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Practical.NR Table 7.4

Q48. An excellent radiograph is obtained under given exposure conditions with a thermal neutron flux of 2 x 106 n/cm2∙s for 10 minutes. If other conditions are not changed, what exposure time would be required if the neutron flux was lowered to 1 x 106 n/cm2∙s? a. 5 minutes b. 10 minutes c. 20 minutes d. 30 minutes Q49. Neutron converter screens should be inspected for flaws or dirt: a. daily b. each time they are used c. occasionally d. when flaws are detected on the radiograph

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Q50. The primary advantage of using a Cf-252 source for neutron radiography is its: a. portability b. low cost per unit neutron flux compared to other neutron radiographic sources c. high resolution d. long useful life without source quality degradation

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More Reading- before going further!

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Reading- 2+ Booster

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7. IMAGE RECORDERS The neutron radiographic image recording on film is based upon two different principles: 1) Silver Halide Based- A chemical process, caused by photons γ or electrons β?e? in a photographic or Xray film: the image generation occurs through a photon or electron triggered conversion of dispersed minute grains of 0.1 to 3 μm silverhalide crystals to metallic silver in a gelatine coating on triacetate or polyester film base. During the film development and fixation process, only the metallic silver is retained (black) and fixed on the film, providing a high contrast image.

2) Trek etch Based- A physical process, caused by alpha particles α (in thermal NR) or recoil protons γ (in fast NR) in a nitrocellulose film: the image generation is based on traces (defects) in the nitrocellulose film produced by the alphas α or protons γ. The alphaparticles originate from a neutron/alpha reaction in a converter layer of boron or/and lithium in contact with the film. The traces in the film are visualized and fixed by etching in an alkaline solution.

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Practical.NR Chapter 7

The alpha particles Îą originate from a neutron/alpha reaction in a converter layer of boron or/and lithium in contact with the film. The traces in the film are visualized and fixed by etching in an alkaline solution.

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Practical.NR Chapter 7

7.1. PHOTOGRAPHIC FILM 7.1.1. Converters.

The range of image recorders which have been used in radiography with neutron beams is now extensive and varied, but all have one thing in common: a neutron converter or intensifying screen, the purpose of which is to absorb incoming neutrons and in consequence emit more directly detectable radiation such as charged particles or light. A large number of materials meet the basic requirements of an intensifying screen: high thermal neutron absorption coupled with efficient emission of effective secondary radiation.

However, for thermal neutron work a limited number are generally in use: boron, dysprosium, gadolinium, indium and lithium (Table 7.1 and chapter 8).

A wide range of methods for displaying and recording the distribution of the secondary radiation produced by a screen are employed. These include: photosensitive film (Xray film), by far the most popular; sheets of etchable plastic (nitrocellulose film described in 7.2); electronic image intensifiers, and arrays of photomultiplier tubes.

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Since most of the accepted screen materials can be used in one form or another with each readout technique, the whole provides a wide range from which to choose for the radiographic problem in hand. Even when expense excludes the most sophisticated ones. A rational choice of image recorder can only be made with a knowledge of the basic performance characteristics of each screen readout system; signal buildup with exposure, neutron registration efficiency, spatial resolution, and where relevant, half-life of the secondary radiation. Data on the detectors and converters are given in chapter 8.

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TABLE 7.1. Nuclear properties of neutron converters and intensifying screens

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7.1.2. Optical film density.

Film density is defined by the equation D = log (Io/I), where D is the density, Io is the light intensity incident on a particular area of a processed film, and I is the light intensity transmitted. Note, that the quantity (Io/I) in the formula above is the reciprocal of (I/Io, the fraction of the incident light transmitted by the processed film, or the transmittance of the film. The tabulation below illustrates some relations between transmittance (I/Io), per cent transmittance (I/Io) x 100 and film density, D. This table shows that an increase in density of 0.3 reduces the light transmitted to one half of its former value; a change of 1.0 in density indicates a change in light transmission by a factor of 10. In general, since density is a logarithm function, a certain increase in density will always correspond to the same per centage decrease in transmittance. The form of the mathematical definition on film density means, in effect, that there are no units of density. In this respect density is similar to a number of other physical quantities, for example pH, specific gravity and atomic weight.

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TABLE 7.2. Transmittance and film density

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7.1.3. Characteristic curve of a film.

The most common, as well as the most convenient and most instructive method of representing the response of a film to light or converter radiation is by means of the characteristic curve (Fig. 7.1). This curve sometimes is referred to as the sensitometric curve or the H and D curve, after Hurter and Driffield, who were the first to use it in 1890. It expresses the relationship between the logarithm of the exposure and the resulting film density.

Characteristic curves are obtained by giving a film a series of known exposures, and then plotting density against logarithm of exposure or neutron fluence. It should be emphasized here that the shape of the curve does not depend on the radiographed subject or its scattering properties.

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However, the shape of the curves for films exposed to light, as in radiography with fluorescent screens of photography, do not depend upon the type of film, the color of the exposing light and the processing conditions used. All that these films know is that they are being exposed to various intensities of light, and their characteristic curves show graphically how they respond to these intensities. The characteristic curve of a film exposed to X-rays or gammarays depends only on the film type and the processing conditions, not upon the quality of the radiation nor the scattering characteristics of the subject.

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Figure 7.1. Characteristic curve of a typical X-ray film

Log relative exposure Charlie Chong/ Fion Zhang

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7.1.4. Film contrast.

In discussing the relationship between the characteristic curve and contrast, a few definitions must be established. Radiographic contrast between two areas of the radiograph is the difference between the densities of those two areas. Fundamentally, the images of two regions of slightly differing X-ray absorption can be differentiated in the finished radiograph only because of the radiographic contrast between them.

Radiographic contrast depends upon both (1) subject contrast and (2) film contrast.

Subject contrast is the ratio of neutron absorption by two selected portions of a subject. Subject contrast depends upon the nature of the subject, the neutron energy and type of converter screen used. But it is independent of the other exposure variables such as time, the characteristics of processing of the film used.

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Film contrast refers to the slope (steepness) of the characteristic curve of the film. It depends upon the: (1) type of film, (2) the processing it receives, and (3) the film density.

It is this latter quantity, film contrast, with which this section is concerned.

Since the shape of the characteristic curve is independent of the major radiographic variables, film contrast can be considered quite independently of subject contrast, although, as pointed out above, both contribute equally to the radiographic contrast that enables one area to be distinguished from another when the finished radiograph is viewed on the illuminator. As can be seen in Fig. 7.1, the slope, or steepness of the characteristic curve at first increases with increasing film density (the toe); then, in the middle range of densities becomes fairly straight; and finally, at higher densities the slope decreases as density increases (the shoulder).

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Figure 7.1. Characteristic curve of a typical X-ray film

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The shoulders of the curves for industrial X-ray films, and of the directexposure type of medical X-ray films, come at densities far above those that can be viewed on available illuminators. Changes in the slope of the characteristic curve have a definite relationship to the visibility of details in the radiograph. For example, two slightly different thicknesses in the subject will transmit slightly different exposure to the film. The exposures will have a certain ration, i.e., will have a certain log exposure difference between them.

The difference in densities corresponding to the two exposures will depend upon just where on the characteristic curve they fall; the steeper the curve, the greater will be the density difference. This means that a certain log exposure interval in the middle of the curve of Fig. 7.1 will correspond to a greater density difference than the same interval at either end.

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The slope of a curve at a particular point is expressed as the slope of a straight line drawn tangentially to the curve at that point. When applied to the characteristic curve of a photographic or radiographic material, the slope of such a straight line is called the gradient of the film material at the particular density.

In Fig. 7.2, the tangents to the curve have been drawn at two points, and the corresponding gradients (ratio a.a'/b.b') have been evaluated. Note, that the gradient is less than 1.0 in the toe and much greater than 1.0 in the central portion of the characteristic curve. Now consider two slightly different thicknesses in a subject, and assume that the thinner section transmits 20% more radiation than the thicker. The difference in logarithm of relative exposure (log E) is 0.08 and is independent of the exposure time. If this subject is radiographed with an exposure that puts the developed densities on the toe of the chacteristic curve where the gradient is 0.5, the Xray intensity difference of 20% is represented by a density difference of 0.04 (see Fig. 7.3), corresponding to a difference in light transmission of 10%.

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Figure 7.2. Characteristic curve of a screen-type X-ray film. Gradients have been evaluated at two points on the curve

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Figure 7.3. Characteristic curve of a screen-type medical X-ray film. Tlie density difference for a 20% difference in exposure have been evaluated for the two values of gradient illustrated in Figure 7.2.

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If the exposure is such that the densities fall on that part of the curve where the gradient is 3.4, the 20% intensity difference results in a density difference of 0.31 (or a difference in light transmission of 104%). In general then, if the gradient of the characteristic curve is greater than 1.0, the intensity ratios, or subject contrasts, of the radiation emerging from the subject are exaggerated in the brightness ratios of the radiograph, and the higher the gradient, the greater is the degree of exaggertion. Thus, at densities for which the gradient is greater than 1.0, the film acts as a contrast amplifier. Similarly, if the gradient is less than 1.0, the subject contrasts are less apparent in the radiographic reproduction.

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7.1.5. Film sensitivity (film speed).

It has been shown that the contrast properties of a film are indicated by the shape of the characteristic curve. Another value, which can be obtained from the characteristic curve, is the speed or the sensitivity of the film to radiation.

It is indicated by the location of the curve along the exposure axis. Speeds of radiographic films are usually given as inversely proportional to the exposure required to reach a certain density. However, in practical applications of X-ray films, it is usually more convenient to deal with relative speed. In this method, speeds are expressed in terms of the speed of one particular film, whose relative speed is arbitrarily assigned a value of 1.00.

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For example, if one film requires half the exposure to reach a certain density as does a second film, and if the slower film is chosen as the standard, the faster one will have a relative speed of 2.00. The choice of the film to which a relative speed of 1.00 be assigned, is purely arbitrary, and may be made on the basis of convenience alone. (See Table 7.3).

In a group of characteristic curves, those for the faster films will lie towards the left of the diagram, in the region of smaller values of logarithm of relative exposure, or, phrased differently, in the region where a smaller exposure is needed to produce a certain film density. Conversely, the curves for the slower films will lie towards the right side of the diagram in the region where the relative exposure, or its logarithm, is larger. From such a diagram, relative exposusres to produce a fixed density can be read, and the relative speeds will be inversely proportional to these exposures (see Fig. 7.8). The speeds of industrial X-ray films are usually determined at a density between 1.5 and 2.5 dependent on the manufacturer.

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TABLE 7.3. Approximate comparison of industrial X-ray films for neutron radiography

* Single coated film

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7.1.6. Use of the characteristic curve. The characteristic curve can be used in the solution of quantitative problems arising in radiography, in the preparation of technique charts and in radiographic research. Ideally characteristic curves made under the actual radiographic conditions should be used in solving practical problems. A range of screen-film signal exposure curves are shown in Figs. 7.4 and 7.5. It must be observed immediately that in neutron radiography exposure prediction to better than -25% should not be expected from such curves, even when great care is taken to variations in beam energy spectra between neutron radiographic facilities, used converter foil thickness, type of film developer, manual or machine processing.

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Figure 7.4. Typical signal-exposure characteristic curves for selected films used in conjunction with light-emitting (NE21 and NE905 - 1.3 mm) and charged particle (electron) emitting (Gadolinium - 25 Îźm) neutron intensifying screens used singly behind the films

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Figure 7.5. Typical signal-exposure characteristics for selected films used in conjunction with

charged particle (electron) emitting neutron intensifying screens of Gadolinium (25 Îźm) and Dysprosium (100 Îźm)

Note: The exposure scale for Dysprosium is estimated for the transfer technique with a foil exposure <1 half-life and a film exposure >3 half-lives Charlie Chong/ Fion Zhang

Practical.NR Chapter 7

Close inspection of the gadolinium foil curves in Fig. 7.4 would show that it is in fact close to exponential in shape. It is close to linear, therefore, when a linear exposure scale is used (Fig. 7.5), as is the response of all films when exposed directly to charged particles. Thus, sensitivity improves as exposure is increased. Linear signal-exposure characteristics are, in fact, quite common in neutron radiography, since they also apply to the track-etch technique and to most electronic readout methods. So the less conventional style of presentation used in Fig. 7.5 has advantages over that of Fig. 7.4.

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Unfortunately, these simple and helpful relations must now be qualified, since only with a constant background signal is the full contrast of the recorder realised. Generally, neutron radiography has to be carried out in the presence of a background of both scattered neutrons and gamma radiation which increases with exposure. Background due to gamma radiation can be readily avoided by using Bi filters, track-etch technique, or an intensifying screen in which the neutron produced reaction has a convenient half-life (Table 7.4). In the latter case the screen alone is exposed to the neutron beam and the stored image transferred to a film by autoradiography elsewhere; this is known as the transfer technique. However, scattered neutron background cannot be easily eliminated and generally increases with object thickness.

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TABLE 7.4. The characteristics of some possible neutron radiography converter materials

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Practical.NR Table 7.4

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To make an informed choice of imaging system for a radiographic task we must, ideally, have a detailed knowledge to the basic characteristics of a wide range of recorders. These characteristics may be indentified as the characteristic curve, neutron registration efficiency, and resolving power. The screen/film recorders used for most neutron radiography are of two basic types: 1) Those employing light-emitting intensifying screens, which have a logarithmic response to neutron exposure and suffer from reciprocity-law failure when exposure times are long.

2) Those employing charged particle-emitting screens, which have a linear response and do not suffer reciprocity failure.

A simple analysis, particularly relevant in low-background conditions, suggests that only when a linear-response recorder is used the sensitivity is improved by increasing exposure. However, consideration of the effect of natural statistics shows, that it is advantageous to increase exposures whenever possible. 7.1.7.

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Practical application. As the choice of an image recorder will depend upon the need to obtain either good radiographic quality or high speed, it is only possible to give general guidance as to their selection. When high quality is required a fine grain film or track-etch material should be used; When speed is the important parameter then fast X-radiographic type films should be used.

The image recorders given in Tables 7.5 to 7.7 and 7.9 are recommended, based upon the practical experiences of radiographers. Detailed data on some of the converter materials listed in Table 7.4 are given in chapter 10 in Tables 10.5 to 10.9.

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TADLE 7.5. Some characlcrislics of thermal neutron intensifying screens

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TABLE 7.7. Effective thermal neutron absorption of lithium fluoride and boron carbide screens

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TADLE 7.8. Average exposure data for selected converter/film combinations

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7.1.8. Sensitometric standards. For X-ray films exposed to X- and gammarays ISO has issued a standard for the determination of speed and average gradientt. No such international standard exists for X-ray films used in neutron radiography. However, AFNOR has published a French standard for industrial radiographic films used for neutron radiography with gadolinium converters.

In this standard the mode of irradiation and the method of determining the sensitivity and average contrast of films used in industrial neutronography is described. The spectrum of energy of the electrons emitted by the converter during irradiation with neutrons is replaced by the spectrum emitted by carbon 14. A planar, non filtered carbon 14 source is used for the irradiation of films.

It is placed in close contact with the emulsion side of the film. The irradiation ought to be such as to produce a characteristic curve including net density between 1.5 and 3.5. The sensitivity S is determined from the formula:

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Practical.NR Chapter 7

where E is the electronic fluence corresponding to net density of Dn = 2.

The average gradient G ought to be determined from the characteristic curve (shown in Fig. 7.6) according to the formula:

where E, and E2 are the exposures for net densities of 1.5. and 3.5.

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Practical.NR Chapter 7

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Practical.NR Chapter 7

7.2. NITROCELLULOSE FILM 7.2.1. Film characteristics. Nitrocellulose film is extensively used in thermal NR, especially in NR of radioactive objects. In fast NR it is applied for imaging of hydrogen containing matter, especially for biology, medicine and industry (thick plastic components).

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Except for the low contrast image, application of nitrocellulose film is in general characterized by the following advantages: 1.

2.

3.

4.

5.

6.

Direct imaging capability without activation process, thereby providing an important reduction in processing time and operator exposure.

Simple handling and processing in day-light.

Film is flexible and can be placed directly on objects.

Linear response to exposure time, no saturation of the converter.

Insensitive to gamma-radiation (e.g. from radioactive objects).

Insensitive to light.

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Practical.NR Chapter 7

In relation to thermal NR, the following additional characteristics apply: 1.

2.

3.

Provision of several images with different contrast from one film when developed (etched) in steps and intermediately transferred to copying film.

Sharp image of objects containing low contrast materials.

Shorter or comparable imaging times to indirect method with X- ay film/Dysprosium foil.

In fast NR the following additional advantages in using nitrocellulose film have been identified: 1.

2.

Higher sensitivity in comparison to activation detectors.

Insensitive to gamma radiation background when compared to multiwire chambers and scintillators.

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At present two types of nitrocellulose film and converters are routinely used

by NRWG members: 1. A combined film, in form of sheets, where the nitrocellulose film is coated on both film faces with in water soluble converters, e.g. Kodak CN 85 B, and 2. as separate film and foils, in form of sheets and on rolls, e.g. Kodak CN 85 nitrocellulose film, and Kodak BN 1 converter foil with natural boron or Kodak BE 10 converter foil with enriched boron. The roll-film is especially suited for automatic NR-film cameras, e.g. For stepwise imaging of long objects. The converter foils are re-usable.

The thickness of commercially available nitrocellulose film is approx. 100 Îźm and of the converters approx. 50 Îźm. The films and foils are produced in standard film sizes.

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7.2.2. Film handling and safety precautions. Nitrocellulose film handling and storage requires in addition to cleanliness also special attention and provisions due to: 1. The inflammablity.

2. The low auto-ignition temperature of approx. 180째C (e.g. Kodak CN85).

3. The self-decomposition, causing release of nitrous gases, damaging the film (leading to incidental spots, sticky surface or change of film color).

Film mountings and handling should avoid damage and processing artefacts caused by: 1. Dust, attracted by static electricity, introduced by relative movements of the plastic foils.

2. Moisture, destroying the converter layer of the combined film. Therefore, nitrocellulose films and converter foils should be passed through a decharging and cleaning device prior to mounting.

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These devices are commercially available. In reactor-based NR-facilities using automatic NR-film cameras, ionization of the ambient air by the gamma radiation neutralizes the static electricity produced during stepwise film displacement.

Felt pads are here sufficient for dust removal. In order to limit the effects arising from self-decomposition, the nitrocellulose film should only be kept for short periods in closed confinements. Appropriate ventilation for removal of the nitrous gases should be applied in all other cases. It is strongly recommende that unused film is stored in a cool and dry environment at approx. 4째C and at a max. Relative humidity of 50%, away from any heat or source of ignition. Because the decomposition process is continuous, it is further proposed to take photographic copies of the exposed and processed nitrocellulose film which can then be destroyed. National and international regulations for transportation, storage, inspection and destruction of inflammable solid matter apply to nitrocellulose film also.

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Following recommendations and regulations should be considered by the users of nitrocellulose film: 1. Due to classification in the category of dangerous materials, the transportation by air or at the ground should be only performed by companies specialized in this field. Shipment as standard mail is prohibited.

2. Storage in small quantities is recommended. Storage should never be performed in metallic containers, but always in containers made of paper, cardboard or wood.

3. The storage boxes or containers should never be sealed in order to allow ventilation of the film. Foldings for closing of boxes or containers should be used instead of adhesive tape.

4. Rooms which are used for storage of more than 2 kg of nitrocellulose film should be fireproof, ventilated by forced ventilation system and be marked as a room containing inflammable material. Check on selfdecomposition once per year if the above mentioned and recommended storage conditions and temperature are maintained, otherwise once per 3 months.

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Practical.NR Chapter 7

Following recommendations and regulations should be considered by the users of nitrocellulose film: 5. If the nitrocellulose film is stored at refrigerator temperature (e.g. 4 to 5째C) its quality and stability will not be affected throughout at least the first two years after packing (e.g. See packing date on the boxes).

Storage in a deep-freezer (e.g. -18 to -20째C) extends this period at least to five years.

6. Decomposed or remainders of processed nitrocellulose films should be stored in waterfilled containers until destruction.

7. Destruction of film by burning has to conform to the legal regulations related to highly inflammable materials. It is recommended to subcontract destruction of the film and related waste materials to specialized companies in the field.

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Practical.NR Chapter 7

7.2.3. Exposure techniques and mounting. The exposure technique and film processing form two closely-linked parameters and control the image generation and its quality. As nitrocellulose film is an integrating image recorder the exposure time is the main controlling factor in image generation when considering a given NR-facility and image recording system. Typical neutron fluences for a standard image on nitrocellulose film are summarized in Table 7.10. It is common practice to optimize the exposure data for an anticipated application by proof testing. Generally, under-exposure followed by longer etching will provide a higher contrast image; whereas high- xposure followed by short etching will deliver a low contrast, but sharper image. The best radiographic image is obtained when the film is in contact with the object or very closely positioned behind it.

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Depending on the application and nitrocellulose film type, the following mountings are employed: 1. For the combined film (e.g. Kodak CN 85 B): a) direct positioning onto the object, b) film wrapped in aluminium foil, to prevent contamination in dry installations, c) positioning in watertight aluminium vacuum cassettes in underwater reactor- ased NRfacilities.

2. For the separate film and foils {Kodak CN 85 and Kodak BN 1 or BE 10): a) vacuum or pressure cassettes to ensure proper contact between film and foils, b) dedicated NR-film cameras for stepwise image taking on roll-film; these cameras are equipped with mechanical systems providing film transport and contact pressure.

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TABLE 7.9. Neutron fluences for nitrocellulose film imaging

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Practical.NR Chapter 7

It is common practice in thermal NR to place behind the film/converter arrangement a cadmium screen to trap the back-scattered neutrons. In fast NR the following mountings are in use: 1) For the nitrocellulose film only (Kodak CN 85): same mountings as mentioned above for the combined film.

2) For the nitrocellulose film with polyethylene foil or for a nitrocellulose film stack (multifoil technique): vacuum or pressure cassette to ensure proper contact between all layers.

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Practical.NR Chapter 7

The multifoil technique in fast NR provides an enhancement of contrast. As shown earlier, nitrocellulose film should be handled with care. Mountings should be free of moisture, static electricity and dust. Void-free contact between film and converter is essential. The contact pressure should be approx. 0.1 MPa (1 bar) as experienced in X-ray radiography technique. High temperature should be avoided due to its low auto-ignition temperature. Prior to processing any non-nitrocellulose film material (e.g. tape, ball point or felt tip pen tracks, finger prints) should be removed in order to avoid contamination of the processing baths. Further details about the properties and use of the nitrocellulose films as applied with the tracketch technique can be found in m.

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7.3. COMPARISON BETWEEN SILVER-HALIDE AND NITROCELLULOSE FILM WITH REGARD TO IMAGE RECORDING AND QUALITY The image generation in nitrocellulose film is based upon the n/alpha reaction or on heavy particles (protons). Each particle creates in the film a defect (track) in function of its energy. The accumulated tracks form the image. The image density and sharpness depend primarily on the neutron fluence. The image quality depends thereafter on the processing (etching), which is sensitive to the temperature and concentration of the etching bath and etching duration. The etching process will increase the size of each track, it will form a conical hole with increased depth. With progressive etching, the tracks will interconnect and lead to erosion of the film surface. Finally the film will become foggy and lose contrast.

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The image generation in silver-halide film in NR is based on ionisation by electrons (beta particles) or gamma-rays which interact along their passage through the emulsion with several silver-halide crystals and triggers their conversion to metallic silver. This results in a less pointlike image generation than with nitrocellulose film. The sharpness of the image is therefore determined by the grain size of the film. Its density depends also on the neutron fluence. The efficiency of the nitrocellulose film depends on the type of converter. Nitrocellulose film with integral converter (Kodak CN 85 B) is approx. twice as fast in terms of exposure time as the system using Dysprosium foil and a fine grain silver-halide film (Kodak SR).

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End Of Booster Reading 2+

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Q51. Neutron beams used in nondestructive testing normally contain: a. alpha particles b. positrons c. gamma rays and neutrons d. X-rays Q52. In neutron radiography, LID refers to the: a. limiting neutron energy divided by the neutron density b. largest neutron flux in the system divided by the beam diameter c. distance from the neutron source to the object divided by the source diameter d. distance from the neutron source to the picture divided by the beam diameter Q53. A type of neutron beam collimator is a: a. mean free path diaphragm b. divergent beam collimator c. polycellular field generator d. neutron beam catcher Charlie Chong/ Fion Zhang

Q54. Commonly used converter screens are: a. gadolinium, dysprosium, and indium b. neodymium, plutonium, and technetium c. gadolinium, lead, and indium d. gold. silver, and cadmium Q55. In order to decrease geometric unsharpness: a. neutrons should proceed from as small a source as other considerations will allow (D) b. neutrons should proceed from as large a source as other considerations will allow (D) c. the film should be as far as possible from the object being radiographed (t!) d. the distance from the source to the material examined should be as small as practical (L) Ug = Dt/(L-t)

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Q56. High-resolution gadolinium conversion screens are produced by: a. flame spray techniques b. being grown in large flat crystals c. vacuum vapor deposition d. large brazing systems

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Q57. In order to increase the neutron beam intensity: a. the LID could be lowered b. the neutron energy must be increased c. the test specimen should be moved further from the film d. a smaller source size could be used Q58. Neutron exposure may be due to: a. the direct beam from the neutron source b. scatter radiation arising from objeCts in the direct beam c. both a and b d. neither a nor b Q59. Gadolinium conversion screens emit: a. gamma rays and conversion electrons b. beta particles only c. alpha particles and positrons d. gamma rays only

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Practical.NR Chapter 7

Q60. Which elements are commonly used in the indirect transfer method? a. gadolinium and cadmium b. rhodium and samarium c. dysprosium and indium d. cadmium and dysprosium

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Q61. Which element is commonly used for direct neutron radiography? a. cadmium b. indium c. dysprosium d. gadolinium Q62. Neutron sensitive scintillators provide: a. high quality radiographs with long exposures b. low quality radiographs with long exposures c. low quality radiographs with short exposures d. none of the above

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Practical.NR Table 7.4

Q63. Lead is: a. a good neutron shield b. easily activated by neutrons c. a poor neutron absorber d. an efficient conversion screen Q64. Neutron energy; exposure time, and film type are three important neutron radiographic parameters that can be controlled. What other parameter can be controlled? a. L/D b. conversion efficiency c. type of conversion screen d. both a and c Q65. The purpose of vacuum cassettes is to: a. eliminate scattered radiation b. block unwanted gamma radiation c. assure intimate film-to-foil contact d. protect parts from the radiation emitted by the conversion screen Charlie Chong/ Fion Zhang

66. In making a californium isotope exposure in an unshielded area, you find the dose rate 1.9 m (6ft) from the source is 1200 mR/h. What would be the dose rate at 7.3 m (24ft)? a. 75 mR/h b. 100 mR/h c. 200 mR/h d. 300 mR/h 67. In developing film by hand technique without agitation: a. the radiograph will not show proper contrast b. it will be impossible to fix the radiograph c. there will be a greater fogging of the film d. there will be a tendency for each area of the film to affect the development of the area immediately below it 68. Film developed by automatic processors: a. will have improved detail of the image b. will have a general increase in the film density c. takes longer to develop than when processing manually d. will create less fog on the film Charlie Chong/ Fion Zhang

69. The emulsion side of a single-coated photographic X-ray-type film used for neutron radiography can be detected in the darkroom using standard safe lights as the: a. printed identifying marks on the emulsion side b. darker of the two sides c. duller and lighter color of the two sides d. printed identifying marks on the non-emulsion side 70. The purpose of film containers is to: a. protect the film from light b. protect the film from scratches c. neither a nor b d. both a and b 71. The two most common causes for excessively high density radiographs are: a. insufficient washing and overdeveloping b. contaminated fixer and insufficient washing c. overexposure and contaminated fixer d. overexposure and overdevelopment Charlie Chong/ Fion Zhang

72. Single-emulsion high-resolution X-ray film is very good for neutron radiography because: a. it has a very thin emulsion b. it is sensitive to low-energy radiation and insensitive to high-energy radiation c. it is faster than other films d. both a and b 73. Which of the following materials is suited for construction of vessels or pails used to mix processing solutions? a. stainless steel b. aluminum c. galvanized iron d. tin 74. Excessive exposure of film to light prior to development of the film will most likely result in: a. a foggy film b. poor definition c. streaks d. a yellow stain Charlie Chong/ Fion Zhang

72. Single-emulsion high-resolution X-ray film is very good for neutron radiography because: a. it has a very thin emulsion b. it is sensitive to low-energy radiation and insensitive to high-energy radiation c. it is faster than other films d. both a and b

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75. White crescent-shaped marks on an exposed X-ray film are most likely caused by: a. crimping film after exposure b. crimping film before exposure c. sudden extreme temperature change while processing d. a warm or exhausted fixer 76. Reticulation resulting in a puckered or netlike film surface is probably caused by: a. crimping film after exposure b. sudden extreme temperature change while processmg c. water or developer on unprocessed film d. excessive object-film distance

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77. Frilling 皱边,脱模 or loosening of the emulsion from the base of the film is most likely caused by: a. water or developer on unprocessed film b. the low temperature of processing solutions c. developer solution contamination d. a warm or exhausted fixer solution 78. When the minute silver grains, on which the X-ray film image is formed, group together in relatively large masses, they produce a visual impression called: a. air bells b. graininess c. reticulation d. frilling

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79. Static marks, which are black tree-like or circular marks on a radiograph, are often caused by: a. the film being bent when inserted in a cassette or holder b. foreign material or dirt embedded in screens c. scratches on lead foil screens d. improper film handling techniques 80. The purpose of agitating an X-ray film during development is to: a. protect the film from excessive pressure b. renew the developer at the surface of the film c. disperse unexposed silver grains on the film surface d. prevent reticulation

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81. When manually processing films, the purpose of tapping the hangers sharply two or three times after the films have been lowered into the developer is to: a. disperse unexposed silver grains on the film surface b. prevent frilling c. dislodge any air bubbles clinging to the emulsion d. all of the above 82. The decrease in activity of the developer solution is compensated by: a. constant agitation b. maintaining processing solutions within the recommended temperature range c. avoiding contamination from the wash bath d. adding replenisher

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83. The purpose of fixation is to; a. remove all the undeveloped silver salts of the emulsion b. leave the developed silver as a permanent image c. harden the gelatin d. all of the above 84. For the best results when manually processing film, solutions should be maintained within the temperature range of: a. 65 °F and 75 °F b. 65 °C and 75 °C c. 75 °F and 85 °F d. 75 °C and 85 °C

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85. Water spots on films can be minimized by: a. the rapid drying of wet film b. immersing wet film for 1 or 2 minutes in a wetting agent solution c. using a fresh fixer solution d. cascading water during the rinse cycle 86. The most suitable films for producing neutron radiography are: a. red sensitive films b. PolaroidTM films c. industrial X-ray films d. medical X-ray films

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87. The normal development time for manually processing X-ray film is: a. 12 to 18 minutes in processing solutions at 75 °F b. 3 to 8 minutes in processing solutions at 75 °F c. 12 to 18 minutes in processing solutions at 68 °C d. 5 to 8 minutes in processing solutions at 68 °F 88. A properly exposed radiograph that is developed in a solution at a temperature of 58 °F will be: a. overdeveloped b. underdeveloped c. fogged d. damaged by frilling

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89. An advantage of the fountain-pen type of ionization chamber used to monitor radiation received by personnel is that: a. it provides a permanent record of accumulated dosage b. it provides an imediate indication of dosage c. it is the most sensitive detector available d. all ofthe above 90. What radiation dose would be dangerous, if not fatal, if applied to the entire body in a short period of time? a. 1.5 to 15 R (rem) b. 25 to 70 R (rem) c. 200 to 800 R (rem) d. all of the above doses would most likely be fatal

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Too much to remember?

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91. The average thermal neutron flux that gives a dose of 100 mrem in 40 hours is: a. 700 n/cm2•s b. 70 n/cm2∙s c. 7 n/cm2∙s d. 0.7 n/cm2∙s 92. When working with a neutron radiography facility, the radiation expected is: a. gamma b. beta c. neutron d. all of the above 93. The intensity of neutron radiation is usually measured m: a. roentgens b. ergs c. neutrons/cm-s d. neutrons/cm Charlie Chong/ Fion Zhang

94. What does the term (R/h) refer to when speaking of intensity? a. radiation limits for humans b. roentgens per hour c. X -rays per hour d. radiation in hydrogen 95. Small amounts of exposure to neutrons or gamma rays: a. may have a cumulative effect that must be considered when monitoring for maximum permissible dose b. will be beneficial since they build up an immunity to radiation poisoning. c. will have no effect on human beings d. will have only a short-term effect on human tissues

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96. Overexposure to neutron or gamma rays may cause damage to human: a. blood and skin b. skin c. internal organs d. all of the above 97. A general rule used-to define an excessive amount of radiation exposure is: a. anything above 0.4 R (rem) per week, although small amounts of radiation (0.4 R [rem] per week or less) are beneficial since they build up immunity to these rays b. any dose over 5 R (rem) per week c. any dose that causes a mid-range reading on a Geiger counter d. any unnecessary exposure to radiation

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98. A primary disadvantage of the fountain-pen type of ionization chamber used to measure the amount received by personnel is the: a. delay necessary before the results of a measurement are known b. inaccuracy of such devices in measuring scatter radiation c. inability of such a device to provide a pennanent record of exposure d. cost of recharging such devices 99. The exposure of personnel to X- and gamma radiation can be determined by means of: a. film badges b. dosimeters c. radiation meters d. all of the above

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100. The intensity of gamma rays is measured in: a. roentgens b. ergs c. roentgens per unit of time d. H & D units 101. Divergent neutron beams: a. do not obey the Inverse Square Law of distance b. obey the Inverse Square Law of distance c. use lead for shielding d. none of the above 102. A radioactive source used for neutron radiography is: a. PU-239 b. Co-60 c. Cs-137 d. Cf-252

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103. The half-life of Cf-252 is: a. 9 years b. 2.6 years c. 6 months d. 47.5 years 104. Cf-252 is: a. a spontaneous fission source b. a fissile source c. both a and b d. neither a nor b 105. A normally desirable feature of a thermal neutron beam for neutron radiography is: a. low gamma radiation intensity b. relatively low, fast neutron intensity c. low, angular divergence (so resolution capabilities for thicker objects can be good) d. all of the above Charlie Chong/ Fion Zhang

http://www.chemicalelements.com/elements/cf.html

Q106. To achieve uniformity in neutron radiographs, it is recommended that: a. manual processing be used b. automatic processing be used c. it does not matter which process to use d. the Shockly process be used Q107. The approximate energy of a thermal neutron is: a. 1 MeV b. 0.026 eV (0.01~0.3ev) c. 12 KeV d. 114eV Q108. The material that slows down neutrons is called: a. a moderator b. an accumulator c. a limitor d. none of the above

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TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials

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109. A good moderating material is: a. water b. iron .c. lead d. all of the above 110. If the temperature decreases, the energy of a thermal neutron will: a. increase b. decrease c. stay the same d. none of the above

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111. A main disadvantage of gadolinium screens is: a. that they are expensive b. that they are magnetic c. that they have poor sensitivity to neutrons d. both a and b 112. The main advantage of a divergent beam collimator is: a. that no dividing slats are used which could possibly cause lines on a radiograph b. that there is a minimal neutron reflection if the sides of the collimator are made of a neutron absorbing material c. that it is relatively simple to manufacture d. all of the above

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Q113. The main disadvantage of a divergent beam collimator is: a. that less resolution is possible than with a parallel beam collimator of the same length b. the large physical size of the collimator to achieve L/D ratios that are necessary for good resolution c. that the small end of the collimator near the effective thermal neutron source is small, therefore minimizing the displacement of the moderator d. all of the above Q114. The definition of a neutron is: a. the uncharged particle having a mass slightly greater than that of the proton b. the uncharged particle having a mass slightly less than that of a proton c. the positive charged particle having a mass slightly less than that of the proton d. none of the above

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115. To an Operational Health Physicist, the abbreviation, RBE means: a. Rapid Biological Energy b. Roentgen Background Embrittlement c. Relative Biological Effectiveness d. both a and b 116. The cross section is expressed in area units, the most common being the one that is equal to 10-24 cm2 This is the: a. femto b. tero c. barn d. watt

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Q117. The transfer exposure method is used because: a. it is not sensitive to gamma radiation b. it has greater radiographic sensitivity than the direct exposure method using gadolinium c. it is faster than the direct exposure method d. the screens used in this method emit only internal conversion electrons of about 70 ke V Q118. Which of the following elements has the largest mass absorption coefficient for thermal neutrons? a. boron b. lead c. gadolinium d. copper

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The neutrons transmitted through a radioactive specimen will strike a metal detection foil such as indium, dysprosium or gold, rather than a converter screen with film.

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Q119. The term “ macroscopic cross section (Σ) " is synonymous with the: a. linear attenuation coefficient (μ) for neutrons b. microscopic cross section (σ) for neutrons c. mass attenuation coefficient (σ/ρ) for neutrons d. cadmium ratio for neutrons Comments: μ = ρ∙N/A∙σ, σ = microscopic cross section, Σ= ρ∙σ = macroscopic cross section MACROSCOPIC SCATTERING CROSS SECTIONS The distinction between macroscopic and microscopic cross-section is that the former is a property of a specific lump of material (with its density), while the latter is an intrinsic property of a type of nuclei. https://en.wikipedia.org/wiki/Nuclear_cross_section

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Peach – 我爱桃子

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Good Luck

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Good Luck

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https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang