understanding neutron radiography reading iii-level1-nrt

Understanding Neutron RadiographyReading III-Level1ExerciseMy ASNT Level III, Pre-Exam Preparatory Self Study Notes 3 July 2015

Charlie Chong/ Fion Zhang http://homework55.com/apphysicsb/ap5-28-08/

Nuclear Source-Reactors

Charlie Chong/ Fion Zhang


Neutron Source-Cyclotron

The Magical Book of Neutron Radiography



ASNT Certification GuideNDT Level III / PdM Level IIINR - Neutron Radiographic TestingLength: 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


Electron emission radiography Micro-radiography Laminography (tomography) Control of diffraction effects Panoramic exposures Gaging Real time imaging Image analysis techniques

3. Techniques/Calibrations Blocking and filtering Multifilm technique Enlargement and projection Stereoradiography Triangulation methods Autoradiography Flash Radiography In-motion radiography Fluoroscopy


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 NumberNDT Handbook, Third Edition: Volume 4,Radiographic Testing 144ASM Handbook Vol. 17, NDE and QC 105

Fion Zhang at Shanghai3th July 2015

http://meilishouxihu.blog.163.com/


Greek Alphabet


Charlie Chong/ Fion Zhang http://greekhouseoffonts.com/


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."


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


Neutron Cross Section of the elements

http://periodictable.com/Properties/A/NeutronCrossSection.html


Screen Types-11. 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 material5. Scintillator screen: Zinc sulfide, Lithium carbonate, plastid scintillator6. Neutron Accelerator Target material: Beryllium, Be.7. Boron used for neutron shields.


Screen Types-21. 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 material5. 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 electronn11. Dysprosium (16166Dy) conversion screens emit: (1) high-energy betas ,

(2) low-energy gammas , and (3) internal-conversion electrons e.


IVONA TTS Capable.

http://www.naturalreaders.com/


Reading IIIContent Reading One: ASNTHBVol4Chapter16 Reading Two: ASNTNRTMQ123 Reading Three: Reading Four:


Reading-1ASNTHBVol4Chapter16


PART 1. Applications of Neutron RadiographyNeutron radiation is similar to X-radiation. The radiation can originate from aneffective point source or can be collimated to shine through an object in acoherent beam. The pattern of penetrating radiation can then be studied toreveal clues about the internals of the object. The information conveyed canbe very different from that obtainable with X-rays. Whereas X-rays areattenuated by dense metals more than by hydrocarbons, neutrons areattenuated more by hydrocarbons than by most metals. The difference canmean much more than the reversal of a positive image to a negative image.Neutrons, for example, can reveal details within high density surroundingsthat cannot be revealed by other means. A typical application for neutronradiography is shown in the images of a pyrotechnic device (Fig. 1), wherethe small explosive charge is encased in metal. Other applications includeinspection of explosive cords used in pilot ejector mechanisms; inspection ofgaskets, seals and O-rings inside metallic valves; confirmation that coolantchannels in jet engine turbine blades are free of blockage; studies of coking injet engine fuel nozzles; and screening of aircraft panels to detect low levelmoisture or early stage corrosion in aluminum honeycomb (Fig. 2).


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



(b)



(c)



(d)


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

(a)


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

(b)


Users GuideUnlike many other forms of nondestructive testing, neutron radiography is nota do-it-yourself technique. There have been neutron radiography servicecenters in the United States since 1968. To try out neutron radiography on anobject of interest, it is simply necessary to locate the services currentlyavailable and, if agreed, mail your item to them. Typically, the neutronradiograph and your item will be mailed back within a day or two. The costcould be less than 1 or 2 h of an engineers time. If assistance is required tointerpret the findings, this too may be requested on a service basis, as mayreferrals to more specialized neutron radiographic techniques. The providersof neutron radiography services use equipment and expertise that is highlyspecialized. Even though one or more neutron radiography service centershave been operating successfully for over 30 years, there has been no in-house neutron radiography available at any general service, commercialnondestructive testing center.


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 asneutron (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 toshop in the global market and to take advantage of the excellentcommunications existing between neutron radiography centers in variouscountries.


PART 2. Static Radiography with Thermal Neutrons2.1 Neutron EnergyThermal energy neutrons are those that have collided repeatedly with amoderator material, typically graphite or water, such that they reach anequilibrium energy with the thermal energy of the moderator nuclei. Theattenuation coefficients for thermal neutrons differ from material to material ina way that is different from X-rays as shown in Table 1. As a consequence, ahigh degree of contrast between the elements in an object is possible. Inaddition, 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.


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 hydrogenousoils, plastics, rubbers, explosives and light elements boron and lithium.


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

In preference to a single tube parallel sided collimator or a multiple slitcollimator, 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 neutronbeam 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).


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


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

ASMV17 Neutron Radiography

Charlie Chong/ Fion Zhang ASMV17 Neutron Radiography


Parallel & Divergent Collimator -Fig. 2 Thermalization and collimation of beam in neutron radiography. Neutron collimators can be of theparallel-wall (a) or divergent (b) type. The transformation of fast neutrons to slow neutrons is achieved bymoderator materials such as paraffin, water, graphite, heavy water, or beryllium. Boron is a typically usedneutron-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 geometricfactor that determines the angular divergence of the beam and the neutron intensity at the inspection plane

ASMV17 Neutron Radiography

Ug =D t/LI = /16(L/D)2I = Ioe nt

n = NN = nuclei/cm2

N = N/AN = Avogadro's number

n = N = [N/A]


For photons:

I = Ioe x t Eq.1For Neutron

I = Ioe Nt = Ioe n t Eq.2Where: I is the transmitted beam; Io is the incident beam; x 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; is the neutron cross section of the particular material or isotope

(a probability or effective area); and, n is the linear attenuation coefficient for neutrons (n = N).


5.1 Neutron cross sectionsNeutron cross sections are defined in Part 1 of this Section. Values for thermal neutrons for many materials (elements) are given in Table 9 (seeBibliography item 8 for a more extensive compilation). Generally, neutron cross sections decrease with increasing neutron energy; exceptions includeresonances, as mentioned earlier. Cross section values can be used to calculate the attenuation coefficients and the neutron transmission as shownin 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:

= N/ A Eq.7 (where N/A is the number of nuclei/cm2)Where: -is the linear attenuation coefficient of specific neutron (cm-1 ) ; is the material density (g/cm3); N is Avogadro's number (6.023 X 1023 atoms/gram-molecular weight) ; is the total cross section in barns (cm2 ) ; and A is the gram atomic weight of material.


2.3 Neutron Imaging Collimation RatioThe collimation ratio is the ratio LD-1 of the collimator length L to aperturediameter D. This ratio helps to predict image sharpness.

Imaging ProcessesFor static thermal neutron radiography of nonradioactive objects, twoimportant imaging processes are (1) the gadolinium converter with singleemulsion X-ray film and (2) the neutron sensitive storage phosphor (neutronimaging 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.


The established direct imaging technique uses thin gadolinium layer vapor deposited on a solid converter screen, which is held flat against a singleemulsion 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 oflight.

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


For neutron radiography of highly radioactive objects, dysprosium and indium foil activation transfer to film and track etch imaging each offer completediscrimination 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 tracketch techniques have been developed to yield more precise dimensionalmeasurements.


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

(a)

(b)



(c)

(d)



(e)

(f)


Image Quality IndicatorsFor any nondestructive system, the best measure of quality is to compare theimage of the test object with an image of a similar object that contains aknown artificial discontinuity, a defect standard, or reference standard.However, neutron radiography has the same problems as othernondestructive testing methods: the quantity of reference standards requiredis too large to obtain and maintain. In lieu of a reference standard, neutronradiographers have chosen to fabricate a resolution indicator that emulatesthe worst case scenario with gaps placed between and holes placed beneathdifferent 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 LD1 of the neutron radiography facility.


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


2.4 Nuclear Reactor SystemsA nuclear reactor system operated for over 30 years solely to provide acommercial neutron radiographic service is illustrated in Fig. 5. The reactorcore, 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 isorientated vertically with air displaced by helium. Parts for neutronradiography can therefore be supported on horizontal trays. Usually theneutron imaging uses a gadolinium converter with fine grain radiographic filmand the exposure time at a selected collimation is typically about 2 min.


FIGURE 5. Representative neutron radiographic service center fornonnuclear applications.


Tangential Beam Tube

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


Another reactor that has provided neutron radiography services since 1968 isillustrated in Fig. 6. It is above ground and the fuel of the 100 kW core isarranged 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.


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


FIGURE 7. Hot cell fuel inspection system.


Hot cellShielded 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.


Hot cellShielded 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.

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.



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.



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 Pbwould 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/ sterilisationprocess 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.



Hot Cell

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


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Charlie Chong/ Fion Zhang http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

Hot Cell

Charlie C

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Hot C

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http://ww

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Charlie Chong/ Fion Zhang http://wwojnar.com/2012/10/research-nuclear-reactor-maria/

Hot Cell


2.5 Accelerator Based SystemsAn initial user of neutron radiography need not, in general, be concerned withaccelerator source options unless there is an established need either for anin-house system or for a transportable system. Almost all neutron radiographyservice providers use a nuclear reactor source. One exception has been thepowerful spallation type accelerator in Switzerland; the accelerator is amultipurpose facility comparable in complexity and cost to a research reactor.An in-house system that was operated successfully for over 15 years at theUnited States Department of Energys Pantex Plant used a van de graaffaccelerator. The operation of this machine, which accelerates over 200 A ofdeuterons at 3 MeV into a beryllium target, is illustrated in Fig. 8.


FIGURE 8. Cross section showing van de graaff principle.


The system provided a peak thermal neutron flux of about 109 neutrons per square centimeter second (109 ncm-1s-1), two orders of magnitude less thanthe 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 tritiummixtures. This can consist of a source head that is maneuverable with longhigh tension cable linking it to the high voltage power supply and control unitas illustrated (Fig. 9). The particular type shown yields a peak thermal neutronflux of about 108 neutrons per square centimeter second with a tube operationhalf life of about 200 h.


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.


2.6 High Intensity Californium-252 SystemsOf the many radioactive neutron sources, such as polonium-210, berylliumand americium-244 beryllium, one has dominated interest for neutronradiography: californium-252. This transplutonic isotope is produced as abyproduct of basic research programs. In the United States, somegovernment centers have been able to obtain the source on a low cost loanbasis from the Department of Energy. The isotope yields neutrons byspontaneous fission at a rate of 2 109 neutrons per second per milligramand has a half life of 2.5 years. A high yield source of up to 50 mg can besmaller than a tube of lipstick (Fig. 10). An in-house stationary system hasoperated at the United States Department of Energys installation at Pantexwith a total source strength of 150 mg californium-252. It provided sets of ninefilms, 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; andexposure time of under 24 h.


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


A maneuverable source system has operated at McClellan Air Force Base with a total source strength of 50 mg californium- 52. It provided singleneutron 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 7 108 neutrons per square centimeter second when loaded with 40 mg californium-252.


FIGURE 11. Elevation of subcritical multiplier system.


Low Cost In-House SystemThere is evidence that an extremely low intensity californium-252 neutronsource could provide a convenient, low cost in-house system. A source sizeof only 100 g can provide useful quality neutron radiographs by using highlyefficient imaging systems that need only 105 neutrons per square centimeterexposure. This is 10 000 times less than the exposure used typically withgadolinium and single emulsion film. The small source size would mean aninexpensive source and also inexpensive shielding, handling and interlockrequirements. Therefore, a nondestructive testing center with a variety of X-ray, ultrasonic and other inspection capabilities could easily incorporate asmall californium-252 based neutron radiographic capability using anunderground storage geometry in an existing radiographic bay. Becauseneutron radiography yields unique information, such an inexpensive in-housecapability could be an important complement to an otherwise full servicenondestructive testing center.


Californium-252 Neutron Source

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


Californium-252 NeutronSourceNeutron FluenceParticle 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 ncm-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 MeVneutron 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

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


PART 3. Special Techniques of NeutronRadiography3.1 Dynamic Neutron RadioscopyServices that provide different types of dynamic neutron radioscopy havebeen developed at numerous nuclear reactor centers worldwide. They coverframe rates that range from 30 frames per second (real time motion displaysimilar to television) to 1000 frames per second range (a high frame rate) orto 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. Abeam from a 28 MW reactor was used to study the flow characteristics oflubricant inside an operating jet engine. Other applications have includedstudies of absorption and compression refrigerator designs, studies ofautomotive parts in motion and a large range of two-phase flow studies. Forhigh throughput dynamic neutron imaging one reactor center has beenequipped with three separate beams, each with its neutron imaging systemand digital image interpretation system.


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


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 rateof 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 fewmilliseconds. The event to be studied, such as the burn cycle of a pyrotechnicevent, is synchronized to the neutron pulse time.


3.2 Subthermal Neutron Radiology (Cold)The neutron attenuation coefficient of a particular material can changesignificantly as the neutron energy is changed. The pattern of this variationalso changes abruptly from one element to another. Therefore, selection ofdifferent energy neutrons provides possibilities for quite different neutronradiology penetration and contrast. Neutron radiology service reactors havedeveloped neutron beams of selected subthermal or cold neutrons usingthree techniques: (1) beam filtration by polycrystal beryllium, which passesonly long wavelength, low energy neutrons below 0.005 eV, (2) a refrigeratedmoderator volume and (3) selection of longer wavelength, low energyneutrons 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.


The effect of this energy selection is typically to increase the transparency ofcertain materials while simultaneously increasing the contrast or detectabilityof hydrogenous materials (see Table 2 and Fig. 13). Just as thermal neutronradiography gives different information to X-radiography, so subthermal orcold neutron radiography gives information different from that of regularthermal 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 below0.001 eV) and this can provide high sensitivity for very thin hydrogenousspecimens.


TABLE 2. Relative neutron attenuation coefficients.


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


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

Explosive charge


3.3 Epithermal and Fast Neutron RadiologyA reactor beam, although consisting primarily of thermal neutrons, will containa proportion of both subthermal and epithermal (high energy) neutrons. Witha filter such as cadmium, the thermal and subthermal neutrons can beremoved and only the epithermal part of the neutron energy spectrum will betransmitted. 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 incidentbeam 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.


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 acceleratorsource 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


3.4 Neutron Computed TomographyComputed axial tomography has been developed for neutron radiography andcan provide detailed cross sectional slices of the object to be analyzed.Although the principle is similar to that of X-ray computed tomography, theinformation conveyed by neutrons can be unique. In a typical facility theobject is rotated in the neutron beam and data are stored for upward of 200angles. Detectors used have included a scintillator screen 6LiF-ZnS (Ag),viewed by a cooled charge coupled device camera and alternatively a storagephosphor image plate loaded with Gd2O3 combined with an automatic laserbeam scanner. Using a high intensity neutron radiography beam of over 108neutrons per square centimeter second, computed tomography of two-phaseflow volumes has been processed as a time averaged three-dimensionalanalysis.


3.5 Neutron Gaging and Neutron Probe TechniquesNeutron gaging is the measurement of attenuation of a collimated smalldiameter beam of radiation as it is transmitted by a specimen. A neutronradiology service center equipped with a nuclear reactor has demonstratedthat the imaging techniques can be complemented by the more quantitativetechniques 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 ofneutron 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 beenproposed for very high sensitivity material testing.


Neutron Induced AutoradiographyBy exposing a painting to thermal or cold neutrons and later imaging theradioactivity induced in the various paint components, a technique has beendeveloped sensitive to many elements including manganese, potassium,copper, sodium, arsenic, phosphorus, gold, iron, mercury, antimony andcobalt. The neutron exposures were originally performed in a moderator block(thermal column), close to a reactor core. However, beams similar to thoseused for transmission neutron radiography have been used for this neutroninduced 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.


3.6 ClosingIndustry standards have been published on neutron radiographic testing.


End Of Reading 1


Reading-2ASNTNRTMQA123Level-I


Level 1 QuestionsNeutron Radiographic Testing Method


Level 1 AnswersNeutron Radiographic Testing Method


Q1. Neutron penetration is greatest in which of the following materials?a. hydrogenous materialb. waterc. leadd. boron carbide

Q2. In general, by increasing the neutron energy from a neutron radiographic source:a. greater neutron penetration is achievedb. greater neutron radiographic contrast can be obtainedc. radiographic exposure time can be reducedd. 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 lawb. a curiec. a half-lifed. the exposure time


Q4. Generally, the attenuation of neutrons by a given material is:a. reported to the Atomic Energy Commissionb. greater for fast neutrons than thermal neutronsc. an indication of the quality of the X-radiographic techniqued. 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 pictureb. regularly increasing picturec. random pictured. dark picture


Q6. Many of the absorption differences between neutrons and X-rays indicate clearly that the two techniques:a. cause radiation problemsb. complement each otherc. increase exposure speedd. fog radiographic film

Q7. The neutron cross section is the term normally used to denote:a. the danger in handling radioactive materialb. the absorbing power of a material for neutronsc. the atomic number of neutron reactor materiald. radiation detection equipment

Q8. The sharpness of the outline in the image of the radiograph is a measure of:a. subject contrastb. radiographic definitionc. radiographic contrastd. film contrast


Q9. The highest quality direct neutron radiographs obtainable today use:a. imaging screens using lithium-zinc sulfide as the imaging materialsb. high-speed radiographic filmsc. dysprosium as an imaging screend. 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 clothingb. by remote handling equipmentc. directly by personnel with special protective clothing except when radiographs are being maded. by the same methods used for nonradioactive materials


Q11. Gadolinium conversion screens are usually mounted in rigid holders called: (direct radiography?)a. film racksb. cassettesc. emulsifiersd. diaphragms

Q12. The best high-intensity source of thermal neutrons is:a. a Cf-252 sourceb. an acceleratorc. a nuclear reactord. 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 scatteringb. undercutc. reflected scatteringd. back-scattered radiation


Q14. What has the highest thermal neutron absorption cross section?a. goldb. Indiumc. gadoliniumd. dysprosium

Q15. Conversion screens are used in neutron radiography:a. to convert neutron energy into ionizing radiationb. to increase the exposure timec. both a and b are reasons for using conversion screensd. neither a nor b is a reason for using conversion screens

Q16. A curie is the equivalent of:a. 0.001 mCib. 1000 mCic. 1000 MCid. 100 MCi


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.


Q17. Short wavelength electromagnetic radiation produced during the disintegration of nuclei of radioactive substances is called:a. X-radiationb. gamma radiationc. scatter radiationd. back-scattered radiation

Q18. A photographic record produced by the passage of neutrons through a specimen onto a film is called:a. a fluoroscopic imageb. a radiographc. an isotopic reproductiond. none of the above

Q19. Possible reactions that can occur when a fast neutron strikes a nucleus are:a. scattering and radiative captureb. microshrinkage and static charges caused by frictionc. sudden temperature change and film contrastd. uniform thickness and filtered radiation


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 methodb. transfer methodc. isotopic reproduction methodd. electrostatic-belt generator method


Q21. Materials that are exposed to thermal neutron beams:a. must not be handled for at least 3 minutes after exposure has ceasedb. must be stored in a lead-lined roomc. may be radioactive after exposure to neutrons has ceasedd. should be monitored by means of a neutron counter

Q22. Hydrogenous material has a:a. high macroscopic scattering cross section (?)b. high absorption cross sectionc. high microscopic absorption cross sectiond. low microscopic scattering cross section

Q23. The penetrating ability of a thermal neutron beam is governed by:a. attenuation characteristics of the material being penetratedb. timec. source-to-film distance I=Ioe-ntd. all of the above


Q24. A graph showing the relationship between film optical density and exposure is called:a. a bar chartb. a characteristic curvec. an exposure chartd. a logarithmic chart

Q25. The three main steps in processing a radiograph are:a. developing, frilling, and fixationb. developing, fixation, and washingc. exposure, developing, and fixationd. developing, reticulation, and fixation

Q26. Radiographic contrast in a neutron radiograph is least affected by:a. developer temperatureb. radiographic exposure timec. radiographic beam collimationd. radiographic film fog

Ug?


Q27. Higher resolution can be achieved in direct neutron radiography by:a. placing lead intensifying screen between a gadolinium screen and filmb. increasing the L/D ratio of the collimation systemc. increasing the exposure timed. increasing the distance between the object and the film cassette

Q28. The main reason for using neutron radiography in place of X-radiography is:a. lower costb. higher resolution in all casesc. the ability to image objects and materials not possible with X -raysd. simpler radiographic procedure required than X -radiography

Q29. The best material for mounting specimens for neutron radiographic inspection is:a. cardboardb. plasticc. steeld. aluminum


Q30. Which of the following materials is best for making identification labels when using the neutron radiographic process?a. aluminumb. brassc. cadmium or gadoliniumd. lead


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:a. a reference plateb. a lead screenc. a penetrameterd. an image quality detector

Q32. A densitometer is:a. a meter used to measure neutron intensityb. an instrument used to measure film densityc. a meter used to measure the density of a materiald. a meter used to measure gamma content

Q33. The ability to detect a small discontinuity or flaw is called:a. radiographic contrastb. radiographic sensitivity.c. radiographic densityd. radiographic resolution

will


Q34. Movement, geometry, and screen contact are three factors that affect radiographic:a. contrastb. unsharpnessc. reticulationd. density

Q35. The difference between the densities of two areas of a radiographic film is called:a. radiographic contrastb. subject contrastc. film contrastd. definition


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 partb. material of the specimenc. neutron energyd. 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 lineb. a light irregular linec. either a dark or light lined. 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 filmb. the unsharpness of the flaw image in the filmc. the contrast of the flaw image on the filmd. all of the above


Q39. An Image Quality Indicator is used to measure the:a. size of discontinuities in a partb. density of the filmc. amount of film contrastd. quality of the radiographic technique

Q40. Unwanted inclusions in a part will appear on a radiograph as:a. a dark spotb. a light spotc. a generalized gray area of varying contrastd. 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. maskb. filterc. back-scatter absorberd. lead-foil screen

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 discontinuitiesc. result in unsharpness of the radiographd. result in a fogged radiograph

Q43. Dysprosium (16166Dy) conversion screens emit:a. low-energy betas and gammasb. high-energy betas , low-energy gammas , and internal-conversion electrons e (more reading!)c. beta particles onlyd. low-energy gamma rays only

Q44. Materials in common usage for moderation of fast neutron sources include:a. aluminum, magnesium, and tinb. water, plastic, paraffin, and graphitec. neon, argon, and xenond. tungsten, cesium, antimony, and columbium



TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials


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

Practical.NR Table 7.4

Charlie Chong/ Fion Zhang Practical.NR Table 7.4


Internal-conversion Electrons


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 divergenceb. externally cooled during the processc. photographically more detectable than the unconverted neutron imaged. 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 pressureb. recharge the focal point size of the neutron sourcec. filter and collimate the excess neutronsd. emit radiation immediately upon the absorption of a neutron

Q47. Gadolinium is frequently employed as a neutron absorber because of its:a. extremely low costb. high neutron absorption for a given thicknessc. ability to absorb gamma raysd. ability to diffract alpha particles

Charlie Chong/ Fion Zhang 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/cm2s 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/cm2s?a. 5 minutesb. 10 minutesc. 20 minutesd. 30 minutes

Q49. Neutron converter screens should be inspected forflaws or dirt:a. dailyb. each time they are usedc. occasionallyd. when flaws are detected on the radiograph


Q50. The primary advantage of using a Cf-252 source for neutron radiography is its:a. portabilityb. low cost per unit neutron flux compared to other neutron radiographic sourcesc. high resolutiond. long useful life without source quality degradation

Charlie Chong/ Fion Zhang http://wwwndc.jaea.go.jp/CN10/index.html


More Reading- before going further!


Reading- 2+ Booster


7. IMAGE RECORDERSThe neutron radiographic image recording on film is based upon two different

principles:

1) Silver Halide Based- A chemical process, caused by photons or electrons ?e? in a photographic or Xray film: the image generation occurs through a photon or electron triggered conversion of dispersedminute grains of 0.1 to 3 m 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 (in thermal NR) or recoil protons (in fast NR) in a nitrocellulose film: the image generation is based on traces (defects) in the nitrocellulose film produced by the alphas or protons . 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.

Practical.NR Chapter 7


The alpha particles 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.



7.1. PHOTOGRAPHIC FILM7.1.1. 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 etchableplastic (nitrocellulose film described in 7.2); electronic image intensifiers, and arrays of photomultiplier tubes.



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 fromwhich to choose for the radiographic problem in hand. Even when expenseexcludes the most sophisticated ones. A rational choice of image recordercan only be made with a knowledge of the basic performance characteristicsof each screen readout system; signal buildup with exposure, neutronregistration efficiency, spatial resolution, and where relevant, half-life of thesecondary radiation. Data on the detectors and converters are given inchapter 8.



TABLE 7.1. Nuclear properties of neutron converters and intensifying screens



7.1.2. Optical film density.Film density is defined by the equation D = log (Io/I), where D is the density, Iois 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 indicatesa change in light transmission by a factor of 10. In general, since density is alogarithm function, a certain increase in density will always correspond to thesame per centage decrease in transmittance. The form of the mathematicaldefinition on film density means, in effect, that there are no units of density. Inthis respect density is similar to a number of other physical quantities, forexample pH, specific gravity and atomic weight.



TABLE 7.2. Transmittance and film density



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 isby 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.



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 gamma-rays depends only on the film type and the processing conditions, not upon the quality of the radiation nor the scattering characteristics of the subject.



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

Log relative exposurePractical.NR Chapter 7


7.1.4. Film contrast. In discussing the relationship between the characteristic curve and contrast,a few definitions must be established. Radiographic contrast between twoareas of the radiograph is the difference between the densities of those twoareas. Fundamentally, the images of two regions of slightly differing X-rayabsorption can be differentiated in the finished radiograph only because ofthe 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.



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 majorradiographic variables, film contrast can be considered quite independently ofsubject contrast, although, as pointed out above, both contribute equally tothe radiographic contrast that enables one area to be distinguished fromanother when the finished radiograph is viewed on the illuminator. As can beseen in Fig. 7.1, the slope, or steepness of the characteristic curve at firstincreases with increasing film density (the toe); then, in the middle range ofdensities becomes fairly straight; and finally, at higher densities the slopedecreases as density increases (the shoulder).



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

Log relative exposurePractical.NR Chapter 7


The shoulders of the curves for industrial X-ray films, and of the direct-exposure type of medical X-ray films, come at densities far above those thatcan be viewed on available illuminators. Changes in the slope of thecharacteristic curve have a definite relationship to the visibility of details in theradiograph. For example, two slightly different thicknesses in the subject willtransmit slightly different exposure to the film. The exposures will have acertain ration, i.e., will have a certain log exposure difference between them.The difference in densities corresponding to the two exposures will dependupon just where on the characteristic curve they fall; the steeper the curve,the greater will be the density difference. This means that a certain logexposure interval in the middle of the curve of Fig. 7.1 will correspond to agreater density difference than the same interval at either end.



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 relativeexposure (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 Xrayintensity 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%.



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



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 forthe two values of gradient illustrated in Figure 7.2.



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.



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.



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 wherethe 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 2.5dependent on the manufacturer.



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

* Single coated film



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 preparationof 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.



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



Figure 7.5. Typical signal-exposure characteristics for selected films used in conjunction withcharged particle (electron) emitting neutron intensifying screens of Gadolinium (25 m) and Dysprosium (100 m)

Note: The exposure scale for Dysprosium is estimated for the transfer technique with a foil exposure 3 half-lives



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.



Unfortunately, these simple and helpful relations must now be qualified, since only with a constant background signal is the full contrast of the recorderrealised. Generally, neutron radiography has to be carried out in the presenceof a background of both scattered neutrons and gamma radiation whichincreases with exposure. Background due to gamma radiation can be readilyavoided by using Bi filters, track-etch technique, or an intensifying screen inwhich the neutron produced reaction has a convenient half-life (Table 7.4). Inthe latter case the screen alone is exposed to the neutron beam and thestored image transferred to a film by autoradiography elsewhere; this isknown as the transfer technique. However, scattered neutron backgroundcannot be easily eliminated and generally increases with object thickness.


Charlie Chong/ Fion Zhang Practical.NR Chapter 7


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 thecharacteristic curve, neutron registration efficiency, and resolving power. Thescreen/film recorders used for most neutron radiography are of two basictypes:

1) Those employing light-emitting intensifying screens, which have alogarithmic response to neutron exposure and suffer from reciprocity-lawfailure 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 isimproved by increasing exposure. However, consideration of the effect ofnatural statistics shows, that it is advantageous to increase exposureswhenever possible. 7.1.7.



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 inTables 10.5 to 10.9.


TADLE 7.5. Some characlcrislics of thermal neutron intensifying screens



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



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


7.1.8. Sensitometric standards. For X-ray films exposed to X- and gamma-rays ISO has issued a standard for the determination of speed and averagegradientt. 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:



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.


Charlie C

hong/ Fion ZhangPractical.N

RC

hapter 7


7.2. NITROCELLULOSE FILM7.2.1. Film characteristics. Nitrocellulose film is extensively used in thermal NR, especially in NR of radioactive objects. In fast NR it is applied for imagingof hydrogen containing matter, especially for biology, medicine and industry(thick plastic components).



Except for the low contrast image, application of nitrocellulose film is in general characterized by the following advantages:1. Direct imaging capability without activation process, thereby providing an

important reduction in processing time and operator exposure.2. Simple handling and processing in day-light.3. Film is flexible and can be placed directly on objects.4. Linear response to exposure time, no saturation of the converter.5. Insensitive to gamma-radiation (e.g. from radioactive objects).6. Insensitive to light.



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

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

2. Sharp image of objects containing low contrast materials.3. 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. Higher sensitivity in comparison to activation detectors.2. Insensitive to gamma radiation background when compared to multiwire

chambers and scintillators.



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 mand of the converters approx. 50 m. The films and foils are produced in standard film sizes.



7.2.2. Film handling and safety precautions. Nitrocellulose film handling and storage requires in addition to cleanliness also special attention andprovisions due to:1. The inflammablity.2. The low auto-ignition temperature of approx. 180C (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.



These devices are commercially available. In reactor-based NR-facilities using automatic NR-film cameras, ionization of the ambient air by the gammaradiation 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. 4C 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 ofinflammable solid matter apply to nitrocellulose film also.



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 ofpaper, 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 self-decomposition once per year if the above mentioned and recommended storage conditions and temperature are maintained, otherwise once per 3 months.



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 5C) 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 -20C) 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.



7.2.3. Exposure techniques and mounting. The exposure technique and film processing form two closely-linked parameters and control the imagegeneration and its quality. As nitrocellulose film is an integrating imagerecorder the exposure time is the main controlling factor in image generationwhen considering a given NR-facility and image recording system. Typicalneutron fluences for a standard image on nitrocellulose film are summarizedin Table 7.10. It is common practice to optimize the exposure data for ananticipated application by proof testing. Generally, under-exposure followedby longer etching will provide a higher contrast image; whereas high- xposurefollowed by short etching will deliver a low contrast, but sharper image. Thebest radiographic image is obtained when the film is in contact with the objector very closely positioned behind it.



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.



TABLE 7.9. Neutron fluences for nitrocellulose film imaging



It is common practice in thermal NR to place behind the film/converter arrangement a cadmium screen to trap the back-scattered neutrons. In fastNR 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 filmstack (multifoil technique): vacuum or pressure cassette to ensure propercontact between all layers.



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.



7.3. COMPARISON BETWEEN SILVER-HALIDE AND NITROCELLULOSE FILM WITH REGARD TO IMAGE RECORDING AND QUALITYThe 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 issensitive 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.



The image generation in silver-halide film in NR is based on ionisation by electrons (beta particles) or gamma-rays which interact alo

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