understanding neutron radiography reading i–ndt h book-nrt

Understanding Neutron RadiographyReading I–NDT-HBook-NRT My ASNT Level III, Pre-Exam Preparatory Self Study Notes 21 June 2015

Charlie Chong/ Fion Zhang

Military Applications


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 Shanghai21th June 2015

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


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Reading ISECTION 12-Neutron Radiography


PART 0INTRODUCTIONNeutron radiography is a valuable nondestructive testing technique whichideally complements conventional radiography. The first publications coveringneutron radiography found it convenient to compare a neutron radiograph,side-by-side, with an X-ray image, to point out the benefits of the neutronimage. This sometimes gave the impression that the two techniques werecompetitive; happily, this has turned out not to be the case. Neutronradiography is now a widely accepted, specialized testing technique.

Sometimes an object can be most thoroughly analyzed radiographically withboth neutrons and X (or gamma) rays. This Section of the NDT Handbook is auseful reference to practicing neutron radiographers and can serve as anintroduction for students or conventional radiographers as well.


The text includes helpful discussions on: 1. neutron sources; 2. moderation;3. collimation;4. techniques for neutron radiography;5. neutron imaging methods; and 6. reference material concerning regulatory control, 7. neutron radiography standards and cross sections.


A discussion of applications is also included, with some well- llustratedexamples. All of the authors for this Section deserve the gratitude. of the technical community, the Handbook Editor and the Handbook Coordinator. Special thanks go to Harry Berger, Industrial Quality, Inc., for his service as primary author and contact with the publications staff.

The authors in turn extend their thanks to John P. Barton (N-Ray Engineering) for his helpful review of the chapter, to Roger A. Morris (Los Alamos NationalLaboratory) for permission to use the table of thermal neutron cross sectionsand attenuation coefficients, and to the American Society for Testing andMaterials for permission to reprint thetable of thermal neutron cross sectionsand attenuation coefficients (Table 9), and the Figure on halfvalue layers (Fig.10).


PART 1PRINCIPLES OF NEUTRON RADIOGRAPHY


1.1 DevelopmentRadiography with thermal neutrons can be traced to the mid-1930s, shortlyafter the discovery of the neutron. Research work in this field has carriedthrough to the present time, with a significant increase in developmentalactivity since 1960. Commercial interest in thermal neutron radiographybegan in the mid-1960s.

1.2 PrinciplesNeutron radiography extends the ability to image the internal structure of aspecimen, beyond what can be accomplished with photon (X and gamma)radiation. Similarities, as well as obvious differences, exist when neutronradiography is compared to photon radiographic techniques. Similaritiesinclude the ability to produce a visual record of changes in density, thicknessand composition of a specimen. Indeed, the neutron radiograph can look verymuch like a photon radiograph.


Advantageslt is the differences between the techniques which provide the advantages ofneutron radiographyover photon radiography.

The major difference is the way in which neutrons are removed from the inspection beam by the specimen. Neutrons interact only with the nuclei of the atoms in the specimen; the neutrons may be scattered or absorbed by the atomic nuclei.

Because the neutron interactions involve nuclei rather than the numerousorbital electrons, marked differences between the transmission of neutrons and the transmission of photons through a specimen may take place.

Keypoints:Neutrons interact only with the nuclei of the atoms in the specimen


Neutron TransmissionMathematically the relationship for neutron transmission looks much like thatfor photons, but the variation of the action site (electron orbits or nucleus)produces large differences in the amount of transmitted beam.

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 linearattenuation coefficient for photons; t is the thickness of specimen in the beampath;

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).


For photons:

I = Ioe –μx t Eq.1For Neutron


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).


Neutron TransmissionFor photons:I = Ioe –μx t Eq.1

For NeutronI = Ioe –Nσt = Ioe –μn t Eq.2

t

Io I

μx for γ & X ray

μn for Neutron Ioe –μnt

Ioe –μxt


Figure 1 provides a comparison of the change in attenuation with increasingatomic number for X rays (125 keV) and thermal (0.025 eV) neutrons(1:5000000) . Such comparisons indicate some of the advantages of using neutrons for radiography. One advantage is found in the imaging of certain low atomic number materials in some high atomic number matrices. Photon radiography works best for the opposite circumstances. Neutrons can image a high cross section element in a low cross section matrix (an element's cross section is its total probability per atom for scattering or absorbing a unit of applied energy), such as cadmium in tin or in lead. Even changes in the isotopic composition of some elements can be imaged because cross sections of the isotopes may be different.


Keywords:an element's cross section is its total probability per atom for scattering or absorbing a unit of applied energy.


FIGURE 1. Mass Attenuation Coefficients for the Elements (as a Function of Atomic Number) for Thermal Neutrons (Black Dots) and X-rays (Solid Line, X-ray Energy about 125 kVJ.)


Neutrons can image a high cross section element in a low cross section matrix (an element's cross section is its total probability per atom for scattering or absorbing a unit of applied energy),such as cadmium in tin or in lead.


Neutron RadiographyThere is a marked difference between the photon radiography and neutron radiography; In neutron radiography, the neutrons interact only with the nuclei of the atom in the specimen not the numerous orbiting electrons.

for Photon:

for NeutronWhere μx = linear attenuation coefficient for photon

Where μn = linear attenuation for neutronN= number of atom /cm3

σ = neutron cross section of the material


Mass-attenuation coefficient (cm2 g-1) for the elements as a function of atomic number for both X-rays (solid line) and thermal neutrons (circles).


gadolinium

samarium/europium

cadmiumhydrogen/boron

beryllium/ water

fixed energy of 150KV


Radioactive ObjectsAn additional advantage of neutron radiography is its ability to radiographspecimens that are intense sources of photons (radioactive specimens). Theneutrons transmitted through a radioactive specimen will strike a metaldetection foil such as indium, dysprosium or gold, rather than a converterscreen with film. Atomic nuclei in the metal screen absorb neutrons toproduce short-lived radioactive isotopes. After removal from the neutronbeam, the decay of radioisotopes in the screen exposes a film, giving anautoradiograph of the specimen.


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.


lmagingNeutron radiographs usually are recorded on conventional X-ray film . Although neutrons have little direct effect on film, many techniques have beendevised to convert neutrons into radiations that will expose a film or produce light for a real-time imaging system. These converter screens are similar to the intensifying screens used in photon radiography.

DisadvantagesDisadvantages of neutron radiography include (1) the high cost of the sources,(2) the relatively large size of the most practical neutron source assemblies,and (3) the personnel protection and safeguard problems associated with neutrons.

These disadvantages combine to yield a major limitation of the technique; noreally portable, inexpensive system is available. Nevertheless, equipment toproduce neutron radiographs is available; in special circumstances, theunique information provided by neutron radiography outweighs thedisadvantages.


ApplicationsSuch critical areas as the inspection of: aerospace components, explosives, adhesive components, nuclear control materials and nuclear fuel are examples of applications

making use of neutron radiography's advantages.

Neutron radiography is also useful for the detection of corrosion (particularly in aircraft structures) and for locating areas of water entrapment and hydrogen embrittlement.


PART 2EQUIPMENT AND PROCEDURES


2.1 Neutron SourcesThe two main constituents of the nucleus are the proton and the neutron. Theforce between any pair of these particles is (1) strong, (2) attractive, and (3) of very short range; stable nuclei must have an external source of energy supplied for a separation of particles to occur. Any nucleus can be disrupted ifadequate energy is supplied, but several light materials as well as several heavy materials can be made to yield free neutrons by supplying onlymoderate amounts of energy to their nuclei. A very limited number of materials emit neutrons during the spontaneous disruption of their nuclei. Thegeneral types of reactions used for neutron production are outlined below.


■ FissionWhen a neutron enters a nucleus, the new (compound) nucleus gains anenergy equal to the sum of the binding energy and the kinetic energy of theneutron. For some heavy isotopes, the addition of the binding energy of aneutron is sufficient to cause instability leading to nuclear fission. There canbe, on the average, more than one free neutron produced per neutronabsorbed, in assemblies containing fissionable materials. This net gain in freeneutrons makes a nuclear chain reaction possible and is the basis for themore prolific neutron sources in general use. Isotopes most commonly usedin fission reactors are U-235 and Pu-239.


U-235 and Pu-239.


Positive-Ion BombardmentNeutrons can be readily obtained from the action of energetic positive ionssuch as protons or alpha particles on light materials such as deuterium 2

1H,tritium 3

1H, beryllium Be, lithiumLi or boron B. The source of positive ions can be either:■ a radioactive isotope or ■ a particle accelerator.


Alpha Particles


PhotoneutronsThe necessary energy for neutron emission can be supplied to the nuclei in the form of intermediate to high energy photons. Target materials which are of special interest for photoneutron production are Be-9 and possibly H-2 for low energy photons (1.6 to ≈ 9 Me V) and uranium for high energy photons(E ≈ 9 MeV or higher).


Spontaneous FissionA few materials fission spontaneously, that is, without the input of neutronsfrom outside the material. A practical spontaneous fission source for neutronradiography is Cf-252.

The WikiRadioisotopes Which Undergo Spontaneous Fission. Certain isotopes undergo spontaneous fission with emission of neutrons. The most commonly used spontaneous fission source is the radioactive isotope californium-252. Cf-252 and all other spontaneous fission neutron sources are produced by irradiating uranium or another transuranic element in a nuclear reactor, where neutrons are absorbed in the starting material and its subsequent reaction products, transmuting the starting material into the SF isotope. Cf-252 neutron sources are typically 1/4" to 1/2" in diameter and 1" to 2" in length. When purchased new a typical Cf-252 neutron source emits between 1×107

to 1×109 neutrons per second but, with a half life of 2.6 years, this neutron output rate drops to half of this original value in 2.6 years. The price of a typical Cf-252 neutron source is from $15,000 to $20,000.

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


Radioactive Decay

http://chemwiki.ucdavis.edu/Physical_Chemistry/Nuclear_Chemistry/Nuclear_Reactions


2.2 Neutron EnergiesAs with all penetrating radiation, many different energies are available for usein radiography. Neutron energy ranges which are potentially useful forradiography include thermal, epithermal or resonance, cold and fast. Table 1describes these various energy ranges and discusses their usefulness.


Thermal Neutrons“Produced by slowing down of fast neutrons until the average energy of the neutron is equal to that of the medium. Thermal neutrons provide good discriminatory capability between different materials; sources are readily available. (0.01 eV to 0.3 eV)”

Most neutron radiography has been done with thermal neutrons. The reason is that useful and interesting attenuation characteristics are found in the thermal region. In addition, imaging of thermal neutron beams is relatively straightforward and efficient. A source for thermal- neutron radiography must include moderator material to slow down, or thermalize, the source's fast neutrons. Fortunately, moderator materials can be incorporated into mostsource assemblies. The energy distribution in a beam extracted from a weakly absorbing slowing down medium is such that thermal-neutron radiography can be done, if the intensity of the beam is sufficient. Actually, such beams contain a wide range of neutron energies, but the imaging process usually provides excellent discrimination against higher energy neutrons.


Thermal Neutron


There are applications in which thermal neutrons do not penetrate theinspection object sufficiently for meaningful neutron radiography. Epithermalneutrons are sometimes used to satisfy test requirements for theseapplications.

Epithermal Neutron: “Produced at energies greater than thermal. e.g. fission energies. and surrounded by a moderator. Neutrons are slowed down until they have energies in thermal equilibrium with the moderator molecules. At any location where thermal equilibrium has not been achieved thedistribution of neutron velocities will contain velocities that exceed that permitted by a Maxwellian distribution of the moderator temperature. Such neutrons are referred to as epithermal (0.3 eV to 104 eV)”


The energy range for neutrons in this category can vary, but a commonlyused interval is 0.3 to 10 keV. The neutron beam may be monoenergetic; a multi-energy beam may be used if proper care is exercised in image detection.The most extensive use of epithermal neutron radiography has been the inspection of highly enriched nuclear reactor fuel specimens. A comparisonthat demonstrates the improved neutron transmission for epithermal neutrons through enriched reactor fuel is shown in Fig. 2. Similarly improved transmission can be obtained with other materials. For example, epithermal neutrons can be used for inspecting thickness of hydrogenous materialsgreater than can be inspected with thermal neutrons. Very high sensitivities to a particular material or isotope can be obtained if the neutron radiography is accomplished at the energy of a resonance, by using a detector with a resonance reaction or a time-of-flight separation of energies (exposing the detector only to those neutrons which travel from the source in a specific elapsed time).


FIGURE 2. Neutron Radiographs of Highly Radioactive, Enriched, Mixed Oxide Reactor Fuei(Pellets about 6 mm Diameter), after Irradiation to about 10 Percent Burnup; (a) Thermal Neutron Transfer Radiograph; (b) lndlum Transfer Radiograph, Taken with lndlum Resonance Neutrons and cadmium Filter, Shows Greater Transmission Through the Fuel.


Cold Neutrons“Materials possess high cross-sections at these energies. which decrease the transparency of most materials but also increase the efficiency of detection. Aparticular advantage is the reduced scatter in materials at energies below theBragg cutoff. (Less than 0.01 eV)”

Very low energy neutrons can offer advantages for some specializedinspections; the penetrating ability of neutrons can be greatly enhanced forsome radiographic specimens by taking advantage of the reduced scatter atneutron energies below the Bragg cutoff (the point where an energy'swavelength, compared to the specimen's atomic spacing, becomessufficiently long to prohibit diffraction). Specifically, iron becomes moretransparent at a neutron energy of about 0.005 eV because of reducedscatter. In fact, the use of cold neutrons allows radiographic inspection of ironspecimens in the thickness range of 10 to 15 cm. Another application for coldneutrons involves taking advantage of the high absorption cross sections inmany materials. This may allow the imaging of small concentrations ofmaterials, too small to be imaged well with thermal neutrons. The efficiency ofdetectors also increases in the cold energy region.


Fast Neutrons“Fast neutrons provide good penetration Good point sources of fast neutrons are available. At the lower energy end of the spectrum fast neutron radiography may be able to perform many inspections performed with thermal neutrons. but with a panoramic technique. Poor material discrimination occurs. however. because the cross-sections tend to be small and similar. (103 eV to 20 MeV)”

The primary advantages of fast neutrons for radiography are their excellent penetrating qualities and their point emission. Interference from (1) scattering and (2) limited detector response combine to restrict the practical applications of fast neutron radiography. Also, the similarity of attenuation cross sections for most materials for fast neutrons places a significant restriction on contrasting ability. Most neutrons are born in the fast region; this means that moderator materials are to be avoided (or at least restricted) if radiography is to be done with high-energy neutrons. The availability of point neutron sources has been a significant factor in attracting investigation into radiography with fast neutrons.


2.3 ModerationIn general, liberated neutrons have considerably more kinetic energy than theatoms or molecules of the host material; this energy may be dissipatedthrough numerous collisions with nuclei in the host material. Thetransformation of fast neutrons to slow neutrons is achieved by a moderatingmaterial (moderator). Its presence produces a slowing down of the fastneutrons by elastic scattering collisions (between the moderator nuclei andthe neutrons) until the average kinetic energy of the neutrons is the same asthat of the moderator nuclei.

Keywords:■ elastic scattering collisions■ until the average kinetic energy of the neutrons is the same as that of the

moderator nuclei.


ModeratorsA complete description of the suitability of a substance as a moderatorrequires information on its scattering cross section, the average loss inneutron energy per elastic collision, the number of scattering centers per unitvolume, and the absorption cross section. Two parameters which provide ameasure of the efficiency of a material as a moderator are the slowing downpower and the moderating ratio.

Keywords:■ scattering cross section■ absorption cross section■ slowing down power■ moderating ratio


■ The slowing down power represents the average decrease in the logarithm of the neutron energy per unit of path length.

■ The moderating ratio is the ratio of the slowing down power to themacroscopic absorption cross section (Σ, or the number of atoms times theatomic cross section value).

The moderating ratio = slowing down powermacro. absorption cross section

The slowing power and the moderating ratio for several moderators are listed in Table 2. Water and other hydrogenous materials have relatively small moderating ratios because of the large absorption cross section for hydrogen; however a large slowing-down power makes water, oil and plastic materials attractive for small, compact neutron systems.


TABLE 2. Slowing Down Properties of Moderators

*The absorption cross section in heavy water is greatly affected by light water contamination.

60~ 6,000-20.000*

135175

1.30.180.160.06

Water and other hydrogenous mtlsHeavy WaterBerylliumGraphite

Moderating RatioSlow Down PowerModerator


Thermalization FactorsThermalization factors have been determined experimentally for manyneutron sources moderated in water. The thermalization factor is the ratio ofthe total 4π (all directions) fast neutron yield from the source in neutrons persecond (n/s) divided by the peak thermal neutron flux in the surroundingmoderator, in neutrons per square centimeter seconds (n/cm2∙s). Table 3gives experimental values for several sources in a water moderator. Neutronsources that are physically small (allowing efficient moderation to take place)and which yield relatively low energy neutrons (that are easy to slow down)have favorably low thermalization factors. A schematic arrangement forsource moderation of a thermal neutron beam is shown in Fig. 3.


TABLE 3. Some Thermallzatlon Factors Measured In a Water Moderator

* Rounded-up figures are quoted to draw attention to the variability ofthermalization factors in practice due to thermal neutron absorption in thetarget holder or source. Sb-Be and large Am-Be sources are likely to sufferparticularly in this respect. Care was taken to minimize flux distortions inthese measurements apart from the beryllium-target reactions where a heavystainless-steel target holder was used in anticipation of high-flux work later.


FIGURE 3. Moderator Surrounding a Fast Neutron Source (left). A StraightCollimator Is Shown with the Neutron Beam Moving Left-to-Right Toward theObject with Detector.


2.4 Neutron CollimationBeam extraction and neutron collimation form an important part of neutronradiography's source technique. Early facilities used collimators designed torender the beam parallel (single tube or multiple tube), but nearly all of thenewer facilities use divergent collimators.

Collimator DesignsMany types of collimators have been proposed and used. The principle of thepoint source, the parallel-wall collimator, and the divergent collimator areillustrated in Fig. 4. The divergent collimator is widely used. Although thedivergent collimator appears similar to point-source geometry, it is generallyused to extract a beam from a relatively large moderator assembly. Therefore,structures (walls) are required to limit the uncollimated background radiationreaching the imaging plane. Limiting the background radiation is generally asimportant as geometric collimation for obtaining good quality neutronradiographs.


FIGURE 4. Neutron Collimators: (a) Point Source for Fast-NeutronRadiography; (b) Parallel-Wall Collimator; (c) Divergent Collimator.


Divergent CollimatorsThe primary advantage of the divergent collimator is that a uniform beam canbe projected easily over a large inspection area. Other significant advantagesinclude the fact that a uniform flux is not required over a large volume at theinlet end and the fact that the inlet imposes minimal perturbation to the sourceassembly. Some distortion occurs at the edges of the beam coverage from adivergent collimator because the neutron paths are radial rather than parallel.However, the beam distortion generally does not present problems except ina few special applications.


L/D RatioThe important geometric factors for a neutron collimator are the total length (L)from inlet aperture to detector and the effective dimension of the inlet of thecollimator (D) . This information is usually expressed as the LID ratio. Theseparameters determine the angular divergence of the beam and, to a largeextent, the neutron intensity at the inspection plane. An efficient collimatorrequires that scattering from structural components and penetration ofcollimator walls by uncollimated background radiations be minimized.


As in X- radiography, the geometric unsharpness can be calculated for neutron radiography, using the values L and D. The value D is, in effect, thefocal spot size and the value L is the source-to-film distance.

Therefore, the geometric unsharpness, Ug, is as follows:

Ug = D(t/L-t) Eq.3

where t is the object thickness or the separation of the object plane of interest from the detector. In cases where t is small compared to L, as would be the case in many practical situations,

Ug = t/(L/D) = tD/L Eq.4


Geometric Unsharpness (photon radiography)

Ug = f∙b/a


Geometric Unsharpness (neutron radiography)

Ug = f∙b/a

f

a

b

a’


Neutron IntensityThe values L and D are also related to the neutron intensity needed forradiography. For a collimator with dimensions L and D and an entrance portat a point in the moderator where thermal neutron flux is Ф, the intensity atthe collimator's exit can be approximated by the following relationship (for around aperature of diameter D):

I ≈ Ф Eq.516(L/D)2

IФ


for my ASNT exam

I ≈ Ф16(L/D)2


BackscatterIt is good practice to contain all radiation that passes through the imagingplane in a beam catcher. Radiation backscatter can seriously degrade thequality of any radiograph; a well designed beam catcher will keep backscatterfrom reaching the imaging device.

Keywords:beam catcher

ghost ca

tcher


2.5 Types of Neutron SourcesMany sources have been used for neutron radiography and gaging. These

include (1) reactor, (2)accelerator,(3) radioactive and (4)subcritical assembly sources.

A summary of the general characteristics of these various sources is given inTable 4. The limits of available intensity, as well as the other observations inthe Table, have been generalized to give an indication of what may normallybe expected from each type of source.



TABLE 4. Average Characteristics of Thermal-Sources


TABLE 4. Average Characteristics of Thermal-Sources

*Neutrons per square centimeter per second. n/cm2•s**These classifications are relative

Stable operation.medium investment cost.possibly portable.

On-off operation. medium cost. possibly mobile.

Stable operation,medium to high investmentcost, mobility difficult

Stable operation,medium to high investmentcost. mobility difficult

Long

Average

Average

Short

Poor to Medium

Medium

Good

Excellent

101 to 104

103 to 106

104 to 106

105 to 108

Radioisotope

Accelerator

Subcritical Assembly

Nuclear reactor

CharacteristicsExposureTime

Resolution**TypicalRadiographic

Intensity*

Type of Source


Nuclear ReactorThe majority of practical neutron radiography has been done using a nuclearreactor as the source. The main reasons for this are listed below.1. Reactors are prolific sources of neutrons, even when operating at low to

medium power levels.2. Most reactor beams are rich in thermal neutrons.3. Most of the early research in, and the applications of, neutron radiography

were associated with the reactor community.4. Neutron radiography can be essentially a byproduct of many reactor

operations.5. Organizations that first offered neutron radiography as a commercial

service used reactors as their neutron sources.6. Most present applications have not required portability from the neutron

source.7. Experience with reactors has shown long, relatively trouble-free operation.

Question: The reactor described here; as a operating neutron source or asthe top of the chain in neutron source production?


Some desirable characteristics of a reactor for radiography are low cost perneutron; high available thermal neutron flux for beam extraction; somecapability for beam tailoring; small physical size; and low power output. Thetwo most formidable problems associated with procuring a special purposereactor are overall cost, plus the need to satisfy the requirements andregulations imposed by various regulatory agencies. The capital cost of areactor facility is high compared to most other pieces of nondestructive testequipment. Even a small reactor would require several trained people tooperate it and perform the radiography.


AcceleratorsNumerous nuclear reactions can be used to produce neutrons fromaccelerated charged particles. Free neutrons can be produced by positive-ionbombardment of selected materials with accelerating potentials in the 100keV to many MeV energy range. Some specific reactions for positive-ionbombardment are H-3 (d, n) He-4, H-2 (d, n) He-3, Li-7 (p, n) Be-7 and Be-9(d, n) B-10.


Fast atom bombardment (FAB) is an ionization technique used in mass spectrometry in which a beam of high energy atoms strikes a surface to create ions. It was developed by Michael Barber at the University of Manchester. When a beam of high energy ions is used instead of atoms, the method is known as liquid secondary ion mass spectrometry.] The material to be analyzed is mixed with a non-volatile chemical protection environment called a matrix and is bombarded under vacuum with a high energy (4000 to 10,000 electron volts) beam of atoms. The atoms are typically from an inert gas such as argon or xenon. Common matrices include glycerol, thioglycerol, 3-nitrobenzyl alcohol (3-NBA), 18-crown-6 ether, 2-nitrophenyloctyl ether, sulfolane, diethanolamine, and triethanolamine. This technique is similar to secondary ion mass spectrometry and plasma desorption mass spectrometry.


(d.T) SourceA reaction that has received much attention for neutron radiographyapplications is H -3 ( d,n) He-4, often abbreviated as a (d,T) or deuteron-ritium source. The source is attractive because a relatively high yield can beachieved from low bombarding energies (about 150 keV). Operation at yieldsof about 1011 n/s can be expected from such machines; higher yields havebeen achieved. However, target/ tube lifetimes are limited at high neutronoutput, and the emitted neutrons are very energetic (≈14 MeV). A practicalthermalization factor for 14 Me V neutrons is typically 1000.

2D + 3T → n + 4He En = 14.1 MeV

Light Ion AcceleratorsTraditional particle accelerators with hydrogen (H), deuterium (D), or tritium (T) ion sources may be used to produce neutrons using targets of deuterium, tritium, lithium, beryllium, and other low-Z materials. Typically these accelerators operate with energies in the > 1 MeV range,



(d.T) Source d.T 聚变反应中子源Neutron generators are neutron source devices which contain compact linear accelerators and that produce neutrons place in these devices byaccelerating either deuterium, tritium, or a mixture by fusing isotopes of hydrogen together. The fusion reactions take of these two isotopes into a metal hydride target which also contains deuterium, tritium or a mixture of these isotopes.

Fusion of deuterium atoms (D + D) results in the formation of a He-3 ion and a neutron with a kinetic energy of approximately 2.5 MeV.

Fusion of a deuterium and a tritium atom (D + T) results in the formation of a He-4 ion and a neutron with a kinetic energy of approximately 14.1 MeV. Neutron generators have applications in medicine, security, and materials analysis.

Thousands of such small, relatively inexpensive systems have been built over the past five decades.

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


Nuclear physicist at the Idaho National Laboratory sets up an experiment using an electronic neutron generator.



Neutron generator theory and operationSmall neutron generators using the deuterium (D, hydrogen-2, 2H) tritium (T, hydrogen-3, 3H) fusion reactions are the most common accelerator based (as opposed to isotopic) neutron sources. In these systems, neutrons are produced by creating ions of deuterium, tritium, or deuterium and tritium and accelerating these into a hydride target loaded with deuterium, tritium, or deuterium and tritium. The DT reaction is used more than the DD reaction because the yield of the DT reaction is 50–100 times higher than that of the DD reaction.

2D + 3T → n + 4He En = 14.1 MeV

2D + 2D → n + 3He En = 2.5 MeV

Neutrons produced from the DT reaction are emitted isotropically (uniformly in all directions) from the target while neutrons from the DD reaction are slightly peaked in the forward (along the axis of the ion beam) direction. In both cases, the associated He nuclei are emitted in the opposite direction of the neutron.



The gas pressure in the ion source region of the neutron tubes generally ranges between 0.1 – 0.01 mm Hg. The mean free path of electrons must be shorter than the discharge space to achieve ionization (lower limit for pressure) while the pressure must be kept low enough to avoid formation of discharges at the high extraction voltages applied between the electrodes. The pressure in the accelerating region has however to be much lower, as the mean free path of electrons must be longer to prevent formation of a discharge between the high voltage electrodes.

The ion accelerator usually consists of several electrodes with cylindrical symmetry, acting as electric lenses. The ion beam can be focused to a small spot of the target that way. The accelerators usually have several stages, with voltage between the stages not exceeding 200 kV to prevent field emission.

In comparison with radionuclide neutron sources, neutron tubes can produce much higher neutron fluxes and monochromatic neutron energy spectrums can be obtained. The neutron production rate can also be controlled.



High Energy MachinesSeveral compact accelerators are commercially available. These are capableof accelerating positive ions to energies between 1 and 30 MeV; examplesinclude Van de Graaff accelerators and cyclotrons. Many of these machinesare versatile in that the accelerated particle can be a proton, a deuteron oranother positive ion. However, sophisticated equipment is required to supplyaccelerating potentials of this magnitude, and the capital investment for suchequipment is substantial. Good radiographic results have been reported for15-30 minute exposures using a Van de Graaff accelerator operating at 3 MeV, 280 μA with the Be-9 (d,n) B-10 reaction.


(X.n) SourcesElectron accelerators can be used as neutron sources by irradiating asuitable neutron-yielding material with bremsstrahlung radiation (photon) , produced when energetic electrons strike a target of high atomic number. Any nuclide (nuclear species) can be made to undergo photodisintegration with photons of adequate energy; for radiographic purposes, beryllium and uranium appear to be the most useful materials. Beryllium has a low threshold energy for photoneutron production (≈1.66 MeV) and is normally used with photon energies up to 10 Me V.

Radioisotopes Which Decay With High Energy Photons Co-located With Beryllium or Deuterium Gamma radiation with an energy exceeding the neutron binding energy of a nucleus can eject a neutron. Two examples and their decay products:

94Be + >1.7 Mev photon → 1 neutron + 2 42He

2H (deuterium) + >2.26 MeV photon → 1 neutron + 1H



(X.n) Sources

X- radiation in bremsstrahlung radiation range?


The threshold energy for photoneutron production in uranium is ≈9 MeV; the yield rises quite rapidly as photon energy increases. The yield from a uranium target is considerably higher than the yield from a beryllium target for energies in excess of 15 MeV. The higher energy machines with uraniumtargets could produce high flux intensities (> 1011 n/cm2∙s) and offer potential for many practical applications. Although a linear accelerator has the disadvantage of high X-ray background, it has the advantage of serving a dual role; it can be used for neutron radiography and for high-energy X-radiography. Changeover time from one radiation to the other could he as low as a few hours.


High Energy Bremsstrahlung Photoneutron/ photofission SystemsNeutrons (so called photoneutrons) are produced when photons above the nuclear binding energy of a substance are incident on that substance, causing it to undergo giant dipole resonance after which it either emits a neutron or undergoes fission. The number of neutrons released by each fission event is dependent on the substance.

Typically photons begin to produce neutrons on interaction with normal matter at energies of about 7 to 40 MeV, which means that megavoltage photon radiotherapy facilities may produce neutrons as well, and require special shielding for them. In addition, electrons of energy over about 50 MeV may induce giant dipole resonance in nuclides by a mechanism which is the inverse of internal conversion, and thus produce neutrons by a mechanism similar to that of photoneutrons.

Keywords:giant dipole resonance



Bremsstrahlung , from bremsen "to brake" and Strahlung "radiation", i.e. "braking radiation" or "deceleration radiation") is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typically an electron by an atomic nucleus. The moving particle loses kinetic energy, which is converted into a photon, thus satisfying the law of conservation of energy. The term is also used to refer to the process of producing the radiation. Bremsstrahlung has a continuous spectrum, which becomes more intense and whose peak intensity shifts toward higher frequencies as the change of the energy of the accelerated particles increases.

Strictly speaking, braking radiation is any radiation due to the acceleration of a charged particle, which includes synchrotron radiation, cyclotron radiation, and the emission of electrons and positrons during beta decay. However, the term is frequently used in the more narrow sense of radiation from electrons (from whatever source) slowing in matter.

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


Bremsstrahlung emitted from plasma is sometimes referred to as free/free radiation. This refers to the fact that the radiation in this case is created by charged particles that are free both before and after the deflection (acceleration) that caused the emission.

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


Characteristic x-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels. The characteristic x-ray emission which is shown as two sharp peaks in the illustration at left occur when vacancies are produced in the n=1 or K-shell of the atom and electrons drop down from above to fill the gap. The x-rays produced by transitions from the n=2 to n=1 levels are called K-alpha x-rays, and those for the n=3→1 transition are called K-beta x-rays.

http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/xrayc.html


Transitions to the n=2 or L-shell are designated as L x-rays (n=3→2 is L-alpha, n=4→2 is L-beta, etc. ). The continuous distribution of x-rays which forms the base for the two sharp peaks at left is called "bremsstrahlung" radiation. X-ray production typically involves bombarding a metal target in an x-ray tube with high speed electrons which have been accelerated by tens to hundreds of kilovolts of potential. The bombarding electrons can eject electrons from the inner shells of the atoms of the metal target. Those vacancies will be quickly filled by electrons dropping down from higher levels, emitting x-rays with sharply defined frequencies associated with the difference between the atomic energy levels of the target atoms.



The frequencies of the characteristic x-rays can be predicted from the Bohr model. Moseley measured the frequencies of the characteristic x-rays from a large fraction of the elements of the periodic table and produced a plot of them which is now called a "Moseley plot".

Characteristic x-rays are used for the investigation of crystal structure by x-ray diffraction. Crystal lattice dimensions may be determined with the use of Bragg's law in a Bragg spectrometer.



Bremsstrahlung X-Rays



"Bremsstrahlung" means "braking radiation" and is retained from the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. It is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higherfrequencies when the energy of the bombarding electrons is increased. The curves above are from the 1918 data of Ulrey, who bombarded tungsten targets with electrons of four different energies.

The bombarding electrons can also eject electrons from the inner shells of the atoms of the metal target, and the quick filling of those vacancies by electrons dropping down from higher levels gives rise to sharply defined characteristic x-rays.



Bohr’s Model


Electron Orbital


Plasma SourcesRadiography with fast neutrons from an electromagnetic plasma acceleratorhas also proven to be feasible. Up to 1010 neutrons can be produced in 100nanoseconds (ns) from a machine powered by a capacitor bank of only 30kilojoules (kJ). Up to 1012 neutrons can be produced from a large machinewith a capacitor bank of 400 kJ. Such a device would allow radiographicrecording of fast events without blur due to object motion.

Plasma Focus and Plasma Pinch DevicesThe plasma focus neutron source (see Plasma focus, not to be confused with the so-called Farnsworth-Hirsch fusor) produces controlled nuclear fusion by creating a dense plasma within which ionized deuterium and/or tritium gas is heated to temperatures sufficient for creating fusion.


Radioactive SourcesMany isotopic neutron sources make use of either the (α,n) or the (γ,n)reaction for neutron production. These sources have been used for manyyears for a variety of applications and they have the desirable features ofbeing reliable and at least semi-portable. However, the thermal-neutronintensities that can be achieved from such isotopic source assemblies tend tobe low, especially when compared to an operating nuclear reactor.Spontaneous fission of transplutonium elements is another neutronproducingreaction that has received considerable interest for neutron radiographicapplications. Some pertinent data for several isotopic sources are tabulated inTable 5.


TABLE 5. Some Radloaalve Sources for Neutron Radiography and Gaging.

Easily shielded gamma output long half-life. high cost

0.821.28 x 10104.3138 daysα, nPo-210-Be

Short half-life and high gamma background, available in high intensity sources, low neutron energy is an advantage for thermalization.

1.74.5 x 1042.7 X 1090.02460 days γ, nSb-124-Be

Gamma rayenergy (Mev)

Gamma dose*(rad/hr at 1m)

Neutron Yield n/s∙g

Average NeutronEnergy (MeV)

T½Reaction Source



Easily shielded gamma output, long half-life. high cost

0.062.51 x 107≈ 4458 yearsα, nAm-241-Be

High cost long half-life

0.10.44.7 X 107≈ 489 years α, nPu-238-Be





T½Reaction Source



High yield source. but half life is short

0.040.31.4 x 1010≈ 4163 daysα, nCm-242-Be

Increased radiation yield over Am-241-Be for relatively little more cost but with a short half-life

0.040.06

Low1.2 X 109(80% Am,20% Cm)

≈ 4163 daysα, nAm-241-Cm-242-Be





T½Reaction Source



Very high yield source, present cost high, projected future cost makes it attractive,small size and low energy are advantages for moderation

0.040.1

2.93 x 10122.32.65 years

Spontaneous fission

Cf-252

Long half-life, low gamma background are attractive. Source can also be used asspontaneous fission source. with about half the neutron yield. Because Cm-244 is produced in nuclear fuel, this radioisotope could be widely available as a byproduct material

0.040.22.4 x 108≈ 418.1 years

α, nCm-244-Be





T½Reaction Source


■ (γ,n) SourcesThe (γ,n) reaction will probably not find much use for general neutronradiographic applications. The main reason for this is the high gamma-raybackground associated with such sources. The antimony-berylliumcombination has had some application in inspection of irradiated reactor fuel;in this application, the gamma radiation from the neutron source is not aproblem because the high radiation level of the specimen itself requires ahotcell (highly shielded) operation.


■ (α,n) SourcesSources that make use of the (α,n) reaction provide marginal neutron intensity for most practical neutron radiography. These sources can be used if long exposure times are suitable. Also, (α,n) reactions produce adequate intensity for many gaging applications where high-efficiency neutron detectors can be used.


■ Cf-252On the basis of technical performance, spontaneous fission from Cf-252 is themost attractive isotopic source for neutron radiography. Its neutron intensity islimited by economic rather than technical considerations, and the gammabackground is low enough to allow direct-exposure radiography. Cf-252 hasbeen studied widely and several investigators report the production of usefulradiographs when using this material as the neutron source. The quality of theradiographs has generally been below the quality obtained from reactors butthis can be improved if long exposure times are acceptable. The source is well suited for applications which require moderate resolution, an in-house system, or a source which can be moved.


Radioisotopes Which Undergo Spontaneous Fission Certain isotopes undergo spontaneous fission with emission of neutrons. The most commonly used spontaneous fission source is the radioactive isotope californium-252.

Cf-252 and all other spontaneous fission neutron sources are produced by irradiating uranium or another transuranic element in a nuclear reactor, where neutrons are absorbed in the starting material and its subsequent reaction products, transmuting the starting material into the SF isotope.

Cf-252 neutron sources are typically 1/4" to 1/2" in diameter and 1" to 2" in length. When purchased new a typical Cf-252 neutron source emits between 1×107 to 1×109 neutrons per second but, with a half life of 2.6 years, this neutron output rate drops to half of this original value in 2.6 years. The price of a typical Cf-252 neutron source is from $15,000 to $20,000.



Subcritical Flux BoostersBy defintion, it is not possible to maintain a chain reaction in a subcriticalreactor assembly. However, a subcritical reactor gives a neutron amplificationof (1/1-keff), where keff is the effective multiplication constant for the system.Thus, an isotopic or accelerator source, driving a subcritical assembly, couldsupply more neutrons for radiography than could be obtained from the samesource in a purely moderating medium. A subcritical assembly used as a fluxbooster for neutron radiography should be relatively inexpensive, small andmoveable, and constructed so that there is little chance of achieving anunwanted criticality. Work on subcriticals indicates that the flux advantage inthe assemblies, compared to the neutron source surrounded by a puremoderator, is small unless keff of the system is close to unity. Subcriticalsystems have provided an increase in available flux for neutron radiographyof about 30 times.


2.6 Application Considerations of SourcesThe fundamental figure of merit FOM for a radiography source is the maximum thermal neutron flux available for beam extraction. This available source intensity is then apportioned to beam collimation or intensity, depending on test requirements. With a given source assembly, beam intensity (speed of exposure) can be increased only by sacrificing collimation (resolution) and vice versa.

I = Ф/16(L/D)2


Neutron IntensityVery few, if any, practical applications of neutron radiography can tolerate adetector exposure of less than 105 n/cm2 • This means that even with poorcollimation, several seconds would be required to produce a fast-filmradiograph of a thin specimen when the flux inside the source assembly is 108

to 109 n/cm2∙s. Neutron sources with intensities lower than this will require very long exposures for radiographic inspection. High resolution neutronradiography requires an available thermal neutron flux before collimation of 1010 n/cm2∙s or more.

Beam CollimationThe collimator L/D ratio should normally be 10 or greater to give usefulradiographs. Collimator L/D ratios between 50 and 500 are recommended formost applications where thick objects are to be inspected (see unsharpnessequations 3 and 4).

Ug = t/(L/D) = tD/L, I = Ф/[16(L/D)2]


Interference RadiationControlling unwanted interference radiation is a more complex problem forneutron radiography than for X-ray or gamma ray radiography. The usualproblem, which includes scattering from all structural materials (includingback-scattering), must be evaluated and controlled. However, neutronradiography is also subject to interference from neutrons which penetrate thecollimator walls, secondary radiation from neutron capture in structuralmaterials, and from gamma-ray contamination in the primary neutron beam.Direct exposure imaging techniques are normally useful only in cases wherethe neutron radiographic beam has a relatively low component ofelectromagnetic radiation. A high gamma-ray background can greatly reducethe radiographic contrast of low atomic number materials in the inspectionobject. For medium speed X-rayfilms, direct metal screen-film imagedetectors give about the same exposure from 105 n/cm2 as from 1milliroentgen (mR) of Co-60 gamma radiation. The exact ratio of neutron andgamma-ray response depends on the film and screen used.


Beam TailoringA neutron beam extracted directly from a source assembly without altering itsbasic content is called a raw or unfiltered neutron beam. Neutron radiographsare usually made with such beams because they give satisfactory results formost applications and they require the least effort to produce. Speciallytailored neutron beams should be considered for neutron radiography onlywhen unfiltered beams do not give the desired results.

■ Gamma FiltrationGenerally, the gamma rays in a neutron beam do not cause seriousinterference problems in neutron radiographs. However, some neutronsources have inherently high gamma background because of specialmaterials used in the assembly. Because some applications require a verylow gamma background, gamma-ray filtration in the neutron beam should beconsidered. The most commonly used material for gamma filtration is bismuth;lead can also be used. The required thickness of the gamma filter is stronglydependent on the intensity and energy of the photons in the beam.


Foil FiltersSome materials have special characteristics in the behavior of their neutroncross sections; these can cause a dramatic change in neutron beamcharacteristics even with a small thickness of the material between the sourceand the image detector. Such foil filters are sometimes used to selectivelyremove neutrons from the beam in a particular energy interval. The mostcommon use is a threshold material which removes most of the neutronsbelow a certain energy.

Cadmium, gadolinium or samarium filters, for example, are used to remove thermal neutrons when the beam is being used for epithermal neutron radiography.

In principle, several materials could be used in combination to allow a narrow band of neutrons to be transmitted through the filter pack. In practice, a multifilter technique would be justified only for very special applications.


gadolinium

samarium

cadmium


■ Window FiltersFor some elements, an inherent interference cancels much of the potentialscattering on the low energy side of the resonance. This low cross sectionsegment will act as a window for neutrons in that energy range. If a sizablethickness of a material which has this characteristic is placed in the neutronbeam, only neutrons with the energy of the window will pass through. Severalmaterials exhibit this effect; examples include (1) iron, (2) scandium and (3) silicon.


45Sc(p,n)45Ti nuclear reactionNeutron dosimetry is distinctive in that its measurement energy range is very wide, from thermal to a few tens of MeV neutrons. As the sensitivity of conventional neutron detectors largely depends on incident neutron energy, it is necessary to measure this energy dependency precisely using mono-energetic neutrons. Therefore, we are developing mono-energetic neutron calibration fields in the energy region from a few keV to 20MeV. There were no facilities in Japan which could measure the dependence of sensitivity on energy in the keV region, where the sensitivity of detectors varies greatly, so we developed the 8 and 27keV mono-energetic neutron calibration fields as shown in Fig.14-4. These fields are formed by bombarding a scandium target with a proton beam, causing a 45Sc(p,n)45Ti nuclear reaction, which has a resonance structure. Then the mono-energetic neutrons produced by this reaction are used for the calibration of the detectors.

Fig.14-5 shows the relationship of relative neutron yield and the incident proton energy. The 8keV and 27keV neutrons are generated by precisely adjusting the proton energy to correspond to one of the resonance peaks in Fig.14-5. Even if the incident proton energy diverges by only 1keV from 2911keV peak, the 8keV neutrons are not generated. The proton energy must be adjusted within an accuracy of 1keV, but such fine-tuning by controlling the acceleration voltage of the accelerator is difficult. Thus, we developed a target voltage control system which can quickly adjust the proton energy to a resonance peak by applying voltage to the target as seen in Fig.14-4. The magnitude of this voltage can be remotely controlled with a computer in the control room. This system achieves stable and reliable generation of the 8keV and 27keV neutrons.

In this way, fields of 8keV and 27keV mono-energetic neutrons making possible the finest calibration in the world have been developed. These fields will make calibration of neutron detectors in the keVregion possible.

http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/mirai-en/2009/14_2.html


Fig.14-5 The neutron yield strongly depends on the incident proton energy due to the 45Sc nucleus structure. Therefore, it is necessary to fine-tune the incident proton energy to a resonance peak at which 8keV or 27keV neutrons are generated.

http://jolisfukyu.tokai-sc.jaea.go.jp/fukyu/mirai-en/2009/14_2.html


■ Spectral Shift from Changing Moderator TemperatureChanging the temperature of the neutron moderator material can have asignificant effect on the average neutron energy extracted from that material.For example, increased intensity of cold neutrons can be obtained by usingcryogenic moderators rather than moderators at room temperature. Also, theepithermal neutron content in a beam can be enhanced considerably with ahot moderator (greater than 300 °K).


■ Time of FlightIt is possible to use time-of-flight techniques to select a narrow neutronenergy increment for radiography. The technique requires a pulsed neutronsource and an image detector system which can be gated on and off in a veryshort time. This method can be used to analyze materials by observingresonances in the neutron spectra; time-offlight can also be used to make asystem sensitive to a particular material by working at a resonance of thatmaterial.

Keywords:pulsed neutron sourcegated on and off in a very short timeresonance


■ Other TechniquesA neutron crystal spectrometer is capable of producing a diffractedmonoenergetic neutron beam essentially gamma-free and usually wellcollimated. Aluminum, sodium chloride, or beryllium crystals are used forneutrons in the 0.02 to 10 eV energy range. The energy of the neutron beamdepends upon the plane spacings of the crystal and the angle of the crystalwith respect to the main beam. Beams from crystal spectrometers aregenerally limited to small sizes and low intensities.


2.7 Neutron Image DetectorsAll the methods used to detect X-ray images can be used to detect neutronimages: film, radiographic paper, real-time fluoroscopy, and so on. In addition,some imaging methods not useful for X-rays can be used witn neutrons:activation transfer and tracketch methods, for example. Various imagingmethods are useful for different energies of neutrons. Since most work hasbeen accomplished with thermal neutrons, that energy range will beemphasized here.


■ Film MethodsFilm techniques involve two different approaches. In one, the direct exposuremethod, the film is actually present in the neutron beam during the exposure.In the other, the transfer method, the film need not be exposed to theneutrons; the film exposure is made by autoradiography of a radioactive,image-carrying metal screen. The two techniques are illustrated in Fig. 5.

Conversion or intensifying screens are used with both techniques.

For the direct exposure method these screens increase the detector response by the emission of radiation to which the adjacent film is sensitive. Direct sensitivity of film to neutrons is relatively low.

For the transfer method, the screens are chosen from materials that tend to become radioactive upon thermal neutron exposure.

The properties of some useful conversion screen materials for thermal neutron imaging by both techniques are given in Table 6.


FIGURE 5. Diagrams for Neutron Radiography with Film Using (a) DirectExposure Method, and (b) Transfer Exposure Method.

(a)

(b)


TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials

promptprompt45 min42 s2.3 min24 sprompt54 min14 sprompt47 h9.2 hpromptprompt1.25 min140 min2.7 days

9103,83011139359120.0001574241,0002103,00061,000254,0002.20080098.8

6 Li(n,α)3 H10 (n,α) 7U103 Rh(n)104mRh103 Rh(n)104Rh107Ag(n)108Ag109 Ag (n)110 Ag113 Cd((n,γ)114Cd115 In(n)116n115 In(n)116mln149Sm(n,γ) 150smI52 Sm(n)153sm151 Eu(n)152mEu155 Gd (n,γ) I56Gd157 Gd(n.γ)158Gd164 Dy(n)165mDy164 Dy(n)165Dy197 Au(n)198Au

LithiumBoronRhodiumSilverCadmiumlndiumSamariumEuropiumGadoliniumDyprosiumGold

LifeCross Section for Thermal Neutrons (barns)

Useful ReactionsMaterial


TABLE 6. Properties of Some Thermal Neutron Radiography Conversion Materials


■ Transfer MethodIn the transfer method, the film is not present in the image beam; this meansthe film will not be exposed to gamma radiation (from either a radioactivespecimen, reactions between neutrons and objects in the beam path, or fromthose gamma rays present in the beam itself.) The transfer technique,therefore, has been widely used to inspect radioactive materials such asirradiated reactor fuels. These neutron radiographs show none of the gammaradiation fogging that would appear in conventional X-radiographs.


The most common materials for transfer neutron radiography are

■ indium (54 minute half-life) and ■ dysprosium (140 minute half-life);

thicknesses used are usually 125 to 250 μm (0.005 to 0.010 in.)

Since transfer neutron radiographs really involve two exposures, one in the neutron beam and the other an autoradiograph exposure, these radiographs usually require more total time than the direct exposure approach.

Another disadvantage is that there is a lower limit of neutron intensity in which this type of exposure will work. The activation and decay of the radioactivity each have a half-life. There is very little activity or decay radiation to be gained after about three half-lives, when 87.5 percent of the total activity or decay will have been accomplished. Therefore, even for fast films, 104 n/cm2∙s is about the minimum useful intensity for transfer neutron radiography with a high cross section, reasonable half-life conversion material such as dysprosium.


Transfer Method ExamplesTransfer radiographs are mostly used with high intensity neutron sources,such as reactors, and for inspections involving radioactive material, namelynuclear reactor fuel (see Fig. 2 for examples). For transfer to a medium speedX-ray for several halflives, typical neutron exposures with indium would be 5to 10 minutes and with dysprosium 2 to 5 minutes for a thermal neutronintensity of 107 n/cm2∙s.

■ Spatial resolution capabilities of such neutron radiographs are on the order of 50 μm.

■ The contrast sensitivity is such that one can detect thickness differences of one percent (1%) in 25 mm uranium or steel objects.

This contrast is somewhat better than that normally achieved with directexposure neutron radiographs because the contrast of the transferradiographs is not reduced by secondary gamma radiation.


■ Gadolinium Direct ExposuresThe most widely used detection method for industrial neutron radiography isthe direct exposure technique with a gadolinium conversion screen. A typicalarrangement is a single 25.4 μm (0.001 in.) thick gadolinium metal foil in aback screen configuration. In this case it is desirable to use a single emulsionfilm (as opposed to normal, double emulsion X-ray film). The low-energyinternal conversion electrons emitted from the gadolinium upon neutronbombardment essentially expose only the emulsion facing the gadolinium.The elimination of the second film emulsion, therefore, reduces the detectorresponse to thermal neutrons only slightly while substantially reducing thedetector response to the gamma or X-radiation components of the radiationbeam. Slow speed X-ray films with single emulsions are available fromseveral manufacturers.


■ Exposures, Contrast, ResolutionA typical thermal neutron exposure for a slow, single-emulsion, X-ray film anda single gadolinium conversion screen is about 3 X 109 n/cm2 . This can bereduced to about 108 n/cm2 for a fast X-ray film. In addition, some increase inspeed can be obtained by using double gadolinium screens (a 6 ~-tm frontscreen and a 50 ~-tm thick back screen combination has provided goodresults) or a rhodium-gadolinium converter screen combination with doubleemulsion films. The single gadolinium conversion method has providedexcellent spatial resolution in thermal neutron radiographs. An experimentalspatial resolution value of 10 ~-tm has been reported, in reasonableagreement with theoretical analyses. The contrast capability of the directexposure method is usually somewhat poorer than that of the transfer method,because fogging radiation is often present. A typical thickness sensitivity for25 mm steel or uranium objects is 2 percent.


■ ScintillatorsBecause scintillator-film techniques provide much faster results thangadolinium conversion screens (by factors as large as 100), there has beenmuch interest in that detection method. The scintillators usually involvematerials that undergo a prompt reaction with the neutrons, and anassociated phosphor material. Common scintilla tor constituents are alphaemitters such as boron and lithium combined with a phosphor such asZnS(Ag) or scintillating glasses; now widely used is the rare earth phosphorgadolinium oxysulfide. Combined with a fast lightsensitive film , scintillatorscan provide useful the rmal neutron images with total exposures in the rangeof 105 to 106 n/cm2 • The capabilities of scintillator-filrn neutron radiographyvary appreciably with screen and film characteristics. In general terms, aspatial resolution of 50 to 100 J-tm can be achieved. Contrast for metalspecimens such as steel and uranium is on the order of 4 to 6 percent.


■ Reciprocity-Law FailureScintillator neutron radiographs, like other light detection systems employingfilm , are subject to reciprocity law failure. The same total exposure for ascintillator-film detector at one intensity will not necessarily produce the samefilm density at another intensity, as would be true for a lead screenXradiograph or a gadolinium direct exposure neutron radiograph. The lightexposure system is ratedependent. For neutron scintillators, it has been hownthat the effective exposure for constant film density is:

E = Itp Eq.6

where I is the neutron intensity; t is the exposure time over the range of 1 to103 s; and the exponentp is the Schwartzchild index, which is always otherthan 1, and in this case is equal to 0. 7 4. In an experimental case with a B-10based scintillator and a fast, light sensitive X-ray film (the reciprocity lawfailure is dependent on the film), a neutron exposure at 3 X 105 n/cm2-syielded a film exposure four to five times higher than an identical totalexposure at an intensity value of 3 X 103 n/cm2∙s.


The effect of reciprocity law failure complicates exposure calculations at very short (0.001 s) or very long (more than 10 s) exposures. Workers in other light detection fields have also encountered these reciprocity problems. One solution is to cool the detector to minimize reciprocity law failures at long exposure times. This has been verified for neutron radiography. Substantial improvements were found if detectors were cooled to dry ice temperatureduring the neutron exposure.


■ Neutron-Gamma ResponseOne important property of direct exposure methods is the relative response ofthe detector to neutron and gamma or X-radiation. A transfer neutronradiograph has essentially no response to gamma rays. For direct exposures,however, the film is in the beam and will offer some response to gammaradiation. Depending on what information is sought from such a radiograph,the gamma image will probably reduce the contrast obtained with a trueneutron radiograph. In most cases a gamma-ray component in a neutronradiograph is something to be avoided. Of the direct exposure methods justdiscussed, neutron scintillator techniques employing aZnS phosphor (with analpha emitter) provide the best neutron-gamma response ratio. Typically forthese scintillators, an exposure of 104 n/cm2 will yield about the same filmdensity as an exposure of 1 mR of Co-60 gamma radiation; the exact ratio isdependent on the film and other variables. The metal conversion screenmethods typically have more response to gamma rays by a factor of ten. Theglass scintillators and the rare earth scintillators provide a relative neutron-amma response between these two extremes.


■ Film Response, SummaryA comparison of the two general classes of film detection methods shows thattransfer techniques yield high contrast images with no gamma response.Direct exposure methods, on the other hand, provide much faster results andhave yielded much better spatial resolution (?). Although most of this discussion related to X-ray film, other films and photographic materials, including instant film , X-ray sensitive paper, and light-sensitive film and paper can also be used for these photographic neutron detection methods.


■ Real-Time Neutron Image DetectionFor neutron radiography, as for other forms of radiography, it is sometimesdesirable to observe a dynamic event, or to view many objects passing by adetection station. Real-time radiographic systems provide this capability. Mostof the systems depend on the fact that neutron scintillators yield light whenirradiated with thermal neutrons. The resultant light can be amplified by imageintensifiers and/or detected by television cameras. The television display canprovide real-time viewing of the neutron images at locations removed from theradiation area. Several such systems have been assembled and tested forthermal neutron response. Systems have included scintillator screens viewedwith light detectors such as image intensifiers and TV cameras and integralimage intensifier tubes made for neutron imaging. Scintillating screens ofZnS(Ag) + (Li-6)F have been used. Most neutron real-time systems nowmake use of gadolinium oxysulfide scintillators. At high neutron intensity (105to 107 n/cm2-s), these systems show a high-contrast spatial resolution ofabout 0.25 to 0.5 mm.


Contrast sensitivity for steel and uranium samples is in the 2 to 4 percent range. Object motion of several meters per minute can be followed without objectionable blur. At lower neutron intensities where image statistics are less favorable, these characteristics degrade somewhat. For example, in experimental tests at an intensity about 104 n/cm2-s (a value equivalent toless than 103 n/cm2per television frame), the single frame contrast observed for a 12.7 mm thickness of steel was 16 percent. Electronic integration, (adding successive frames) can be used to minimize statistical fluctuations and improve low intensity real-time images.


■ Other Detectors for Thermal Neutron RadiographyIn addition to film and real-time detectors, many other image detectors havebeen used for thermal neutrons. These include: radiographic paper; instantfilm; xeroradiography; multi-wire spark counters; proportional counters;thermoluminescent detectors; pressurized gas cell methods (similar toionography); point detectors such as scintillation counters; and gas celldetectors scanned across the image. One other detector now used routinelyis the tracketch detector.


Track-Etch DetectorsThe tracks caused by radiation damage in a dielectric (a material which iselectrically insulating, or which can sustain an electric field with a minimumloss of power) can be chemically etched in a preferential manner so thetracks become visible. A collection of many tracks can form a visual imagesimilar to the collection of dots used to make a newspaper picture. Forneutron work, most tracketch applications have involved plastics and neutronTrack-Etch Detectors The tracks caused by radiation damage in a dielectric (amaterial which is electrically insulating, or which can sustain an electric fieldwith a minimum loss of power) can be chemically etched in a preferentialmanner so the tracks become visible. A collection of many tracks can form avisual image similar to the collection of dots used to make a newspaperpicture. For neutron work, most tracketch applications have involved plasticsand neutron


■ Track-Etch ResultsThe track-etch method has been investigated mostly with several cellulosenitrate and polycarbonate plastics. Results reported have shown a spatialresolution of 25 J.lm and a sensitivity to thickness variations (in 25.4 mm thickuranium) of 1 percent. These results were obtained for both polycarbonateplastics with U-235 foils and for cellulose nitrate with a (Li-6)F conversionscreen; exposures were in the 108 to 109 n/cm2 range. The most sensitivematerials required an exposure of only 2 X 107 n/cm2 . The cellulose nitratewas etched for 4 min in 6.5 N sodium hydroxide solution at 55 °C; thepolycarbonate was etched in the same solution at 70 °C for 40 min.


■ Image Detectors for Other Neutron Energies/Cold NeutronsThe same general techniques used for thermal neutron image detection applyto other neutron energies. However, the imaging work done with neutronsoutside the thermal energy region has not been as extensive as that donewith thermal neutrons. For cold neutrons, detector exposure requirements willdecrease somewhat compared to those cited for thermal neutrons. Detectionwork had been reported with film methods, both direct exposure and transfer,and with track-etch methods. Conversion materials were the same as usedfor thermal neutrons.


1) Epithermal NeutronsWork with neutrons in the epithermal or resonance energy region(approximately 0.3 eV to 10 keV energy) has been concentrated mainly onthe lower end of the energy spectrum. Although neutron cross sections tendto decrease as the neutron energy increases above the thermal energy range,there are also large resonances that occur in this energy region. Therefore,detectors such as indium, with a large activation resonance at 1.46 eV, havebeen widely used for these neutrons. Gold, with a resonance at 4.9 eV, isanother potentially useful detector. A common detection method has been tofilter the neutron beam with cadmium or gadolinium (to remove thermalneutrons) and to then use a transfer detection method with a conversionscreen such as indium. This technique has been used to examine fast reactorfuels because its higher energy provides greater neutron transmissionthrough enriched U-235 and plutonium materials (see Fig. 2 for an example).Greater transmission (with less scatter) for radiography ofhydrogenousobjects can be accomplished in a similar manner.


2) Resonance NeutronsAnother approach for neutrons in this energy range involves the use of aneutron pulse and a timeof- flight scheme to permit detection of neutrons at aparticular energy. Neutron pulses have been provided by reactors andaccelerators and detected by a variety of methods. Time-gated detectorssuch as position sensitive proportional counters, scintillators with time-gatedimage intensifiers, and photomultiplier tubes have been used.


3) Fast NeutronsThe other energy range of interest for neutron radiography is fast neutrons,energies above 10 ke V. Neutron cross sections at these energies tend to below so that detectors generally require greater neutron exposures than dothermal neutron radiographs. The detection approaches are similar to thoseused for thermal neutrons, but different conversion materials are normallyused. Direct exposure has been obtained with fluorescent screens made forX-radiography; the screens are combined with light-sensitive X-ray films. TheX-ray phosphor screens work primarily because neutron interactions withhydrogen, in the plastic binder or in the plastic or paper backing, yield protonsthat stimulate light from the phosphor. Fast X-ray phosphor screens and filmprovide neutron radiographs of Me V energy neutrons with total exposures onthe order of 107 to 108 n/cm2 . X-ray film without screens also responds tofast neutrons but requires exposures of 4 X 108 n/cm2 to more than 109n/cm2 .


The film exposures can be reduced slightly by adding conversion screens of paraffin, plastic, tantalum or lead. Fast neutron radiographs made by these techniques display changes in metal specimen thickness in the range of 3 to 6 percent and have demonstrated a spatial resolution better than 1 mm. Transfer detection is also used for fast neutrons. Some useful materials for transfer conversion screens are summarized in Table 7.

It should be noted that the neutron reactions cited have threshold energies.The reaction will not occur for energies lower than the threshold. This limitsthe response of the detector to a portion of the neutrons that may be available,thereby tending to increase the required exposure. On the other hand, sincescattered neutrons are oflower energy, the threshold response can bevaluable in eliminating radiographic detection of scattered neutrons.


As an example, copper screens are often used to detect (d-T) accelerator-produced neutrons having an energy of 14 MeV. Since the threshold for the10-minute half-life reaction is 11 MeV, a copper screen exposure of 30 minutes or less will essentially detect only primary neutrons from the source. A typical exposure for that type of radiograph would require 1010 n/cm2 , for transfer to a fast X-ray film.


TABLE 7. Characteristics of Some Transfer-Detection Materials for Fast Neutron Radiography


■ Detector DiscussionImage detectors for neutrons have been shown to provide a broad capabilityin terms of exposure requirements, spatial resolution and contrast. It must berecognized that the image properties in an actual radiographic system willdepend on several factors in addition to the detector properties. The thicknessof the radiographic object and the geometry of the imaging system will have asignificant influence on the spatial resolution obtained. Similarly, scatteredand secondary radiation that reaches the detector will strongly influence thecontrast. The contrast capability discussed for the various detectors hasusually been described in terms of a percentage thickness change that canbe observed in a metal specimen. This is a common method for describingthe capability of an X-radiographic system. The use of that method fordescribing contrast in neutron radiography, therefore, has merit by permittingXradiographic comparisons. On the other hand, neutron radiography seldomoffers an advantage for inspecting a homogeneous material. Its significantadvantage is in differentiating between materials or isotopes. That should bekept in mind when thinking about neutron contrast, neutron radiographic testpieces, or specifications.


PART 3APPLICATIONS


3.1 GeneralGeneral applications for neutron radiography include inspections of nuclearmaterials, explosive devices, turbine blades, electronic packages andmiscellaneous assemblies including aerospace structures (metallic honeycomb and composite components), valves and other assemblies. Industrial applications generally involve the detection of a particular material in an assembly containing two or more materials. Examples include detection of :

(1) residual ceramic core in an investment-cast turbine blade, (2) corrosion in a metallic assembly, (3) water in honeycomb, (4) explosives in a metallic assembly, or (5) a rubber "0" ring in a valve.

Nuclear applications depend on the capability of neutron radiography to yield good, low background radiographs of highly radioactive material (by such methods as transfer or track-etch), to penetrate fairly heavy assemblies and to discriminate between isotopes (e.g. between U-235 and U-238 or Cd-113 andother cadmium isotopes).


3.2 ExplosivesExplosives or pyrotechnic devices account for a large part of neutronradiographic applications. Small explosive charges in metallic assemblies(such as lead, shaped-charge lines or steel explosive bolts) can be detected,measured and assessed in terms of density, uniformity, foreign material, etc.Many of these pyrotechnic devices are relatively small assemblies containingmetal, explosive and, in some cases, plastic components. An X-radiographshows the metallic parts very well. The neutron radiograph shows the lowatomic number materials, including explosives, plastic and adhesives.Together, the two radiographic methods provide a relatively completeinspection. Information sought often includes: the presence or absence of theexplosive; breaks in the explosive train; density; uniformity; and foreignmaterial. Figure 6 shows an explosive bolt as radiographed by both neutronsand X-rays.


FIGURE 6. Radiographs of Explosive Bolt (about 5 cm In Height): fa) Thermal Neutron Radiograph; fbl Xradlograph.Neutron Radiograph Shows Explosive Material (Salt and Pepper-like Image) Through the Stainless Steel Threaded Region In Upper Part of Bolt; Also Shows Paper (Upper White Line), Plastic (MidRegion) and Epoxy flower part).


3.3 Turbine BladesTurbine blades made by the investment-cast process are inspected byneutrons to be sure there is no residual ceramic core left in the internalcooling passages after the leaching process. The nickel alloy used in mostturbine blades is highly attenuating for X-rays but reasonably transparent toneutrons. A few ceramic materials have reasonable attenuation for neutrons.In many applications of this type, the neutron attenuation of the material to befound (the ceramic) is assured by adding a 1 to 2 percent of gadolinia. Theaddition of this high neutron cross section tracer assures that even smalltraces of residual ceramic can be observed. Figure 7 is a neutron radiographof turbine blades, showing residual ceramic.


FIGURE 7. Thermal Neutron Radiograph of Investment-cast Turbine Blade.Residual Ceramic Core Is Detected (Sharp White Image at top Center).


3.4 Electronic DevicesElectronic devices such as relays are inspected by neutron radiography to detect foreign materials, such as cloth or paper, that might interfere with theoperation. An example of a neutron radiograph of a small relay is shown in Fig. 8. If there were pieces of hydrogenous foreign matter in the case, pieces that might prevent the relays from making good contact, the neutron radiograph would show them.


FIGURE 8. Thermal Neutron Radiographs of Electric Relays.


3.5 AssembliesVarious assemblies and components are usefully inspected by neutronradiography. Rubber "0“ rings in metallic assemblies can be detected to besure the ring is present and properly seated (see Fig. 9); this is often a difficulttask for X-radiography because of the relatively high attenuation of thesurrounding metal. Honeycomb assemblies for aerospace and otherapplications can be inspected with neutrons to show adhesives duringmanufacture or repair, or to show the presence of water during service.Neutron radiographs of composite assemblies show distribution of adhesivesor resins. Metallic assemblies can be inspected to show corrosion. This isused primarily for aerospace applications where the metal may be aluminumand the corrosion is an hydroxide which can be detected by neutrons.


FIGURE 9. Thermal Neutron Radiograph of Assembly Showing Two Rubber '"0'" Rings fArrows) That Have Twisted Out of Grooves.


3.6 Contrast AgentsContrast agents can be used, as in the case of the ceramic core in turbine blades, to show areas that might be difficult to detect otherwise. Similarly, liquid penetrants, sometimes doped with additional neutron attenuatingmaterial such as gadolinium, boron or lithium, can enter surfacediscontinuities and serve as a contrast agent to outline delaminations orcracks.

3.7 MetallurgyMetallurgical studies have been made with neutron radiography to observethe distribution of high neutron cross section alloying agents such ascadmium. Hydriding of materials such as zirconium or titanium can bedetected by neutron radiography. Distribution of other agents such as boronor lithium can also be observed. Often radiographs of thin specimens aretaken on fine grain film and enlarged to show small details.


3.8 Nuclear IndustryNuclear applications depend on different capabilities of neutron radiography.Highly radioactive materials, such as irradiated nuclear fuel, can he inspectedby neutron radiography. Such radiographs can be used to measuredimensions, to show the condition of the fuel , to observe coolant leakage orhydriding and to observe isotopic distributions. New nuclear fuel assemblieshave been neutron radiographed to show the fuel condition and to detect thepresence of foreign material in assemblies. Reactor control assemblies havebeen neutron radiographed to show nonuniform distribution, both before andafter reactor service. Poison elements, which are used to control neutron fluxin a reactor, can be neutron radiographed to show their distribution.


3.9 Application SummaryIn industrial applications, neutrons are used to show a neutron-attenuatingmaterial such as an adhesive, rubber, plastic, fluid, or explosive in anassembly. Neutrons are used in these cases because X-radiographiccapability is limited when viewing light materials behind metal. Theseobservations can be made both in time exposures with film and paper or inreal-time using fluoroscopic, television methods. For example, the flow of oilin an operating engine has been observed dynamically using realtime neutronradiography to determine the time for the lubricant to arrive at the upper partsof the engine, to detect oil blockages, and so on. The nuclear applications areby nature concentrated in nuclear centers around the world and fill the needto examine nuclear fuel and to control material before and after irradiation.


PART 4NEUTRON RADIOGRAPHY STANDARDS,RECOMMENDED PRACTICES AND CONTROL


4.1 Personnel QualificationPersonnel qualification in neutron radiography is discussed by the AmericanSociety for N ondestructive Testing in its Recommended Practice No. SNTTC-1A. This document covers seven NDT disciplines and includesrecommended training, experience and test questions for each of thosedisciplines. The document is a guideline for the certification of NOT personnel.


4.2 System Performance StandardsStandards for neutron radiography system performance are written by the

American Society for Testing and Materials (ASTM) , subcommitteeE07.05 on neutron radiography and gaging. There are three ASTM standards applicable to neutron radiography, as listed below.

1. E545 - Standard Method for Determining image quality in thermal neutron radiographic testing

2. E7 48 - Standard Practices for Thermal neutron radiography of materials.3. E803 - Standard Method for Determining the l/d ratio of neutron

radiography beams.

These documents deal with standardization from two viewpoints. E748provides the basis for good working practices which lead to the production of high quality radiographs. E545 and E803 provide detailed methods formeasuring the quality obtained from good facility design and good workingpractice. In both E545 and E803 the results can be quantified and assigned a numerical figure of merit.


4.3 Regulatory ControlThe federal government and many states exercise control of radiationequipment, personnel and procedures.

■ Licensing RequirementsThe Atomic Energy Act of 1954, as amended, establishes a regulatory framework for ensuring health and safety of the public. This is codifed in Title 10, Chapter I, Code of Federal Regulations (CFR). The Act is quite specific on delineating those sources of radiation which are subject to control by the Nuclear Regulatory Commission (formerly Atomic Energy Commission).


Those sources of radiation subject to NRC regulatory control are as follows:

1. Special nuclear material: uranium enriched in the isotope 233 or 235 or plutonium.

2. Source material: uranium or thorium.3. Production facilities: equipment or devices capable of producing significant

quantities of special nuclear materials.4. Utilization facilities: equipment or devices capable of making use of

significant quantities of special nuclear material.5. Byproduct material: radioactive material (except special nuclear material)

made radioactive by the utilization or production of special nucle~r material.


Section 27 4 of the Atomic Energy Act was enacted in 1959. Among otherthings, this section provided a statutory means by which the NRC canrelinquish to the states a part of its regulatory authority. The NRC retainscontrol over production and utilization facilities and large quantities of specialnuclear material. Certain other areas are reserved to NRC as stated in10CFR 150.15 (a). state

Authority over source material, byproduct material, and small quantities ofspecial nuclear material can be borne by individual states. In addition, thestates have regulatory programs for naturally occurring radioactive materials,accelerator-produced radioactive materials, and all radiation-producingmachines (for example, X-ray machines and particle accelerators). Somestates license all radioactive materials and they register all machines. Table 8shows a summary of the regulatory requirements for the various types ofsources.


■ Personnel ProtectionAll NRC licenses must comply with the requirements of 10CFR Parts 19 and 20. Part 19 contains requirements for the posting of notices to workers,providing radiation safety instructions to workers, notification to NRC of the occurrence of incidents, worker rights during NRC inspections. Part 20contains the standards for protection against radiation. This part includes the standards for permissible doses for individuals, permissible levels of radiationin unrestricted areas, survey requirements, personnel monitoring requirements, requirements for precautionary signs and labels, procedures for picking up, receiving, and opening of packages, storage of licensed materials, maintaining certain records, notification to NRC of the occurrence of incidents,and a requirement for reporting certain personnel monitoring results. Part 34 deals specifically with radiography. It deals with personnel training, use ofsurvey meters, leak tests for radiographic sources and other matte rs relating to radiographic use of radioactive sources.


PART 5NEUTRON CROSS SECTIONS AND ATTENUATION


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:

μ = ρ∙Nσ/ A Eq.7

Where:μ -is the linear attenuation coefficient (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.


For photons:

I = Ioe –μx t Eq.1For Neutron


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).


TABLE 9. Thermal Neutron Linear Attenuation Coefficients Using Average Scattering and 2200 m/s Absorption Cross Sections for the Naturally Occurring Elements


If on the other hand, the material under inspection contains several elements,or is in the form of a compound, then the linear absorption coefficient is:

μ = ρ∙N/M (ѵ1σ1 + ѵ2σ2 + ѵ3σ3 +..... ѵiσi ) Eq. 8

Where:μ - is the linear attenuation coefficients of the compound (cm-1) ; ρ is the compound density (g/cm3 ) ; N is Avogradro's number (6.023 X 1023 atoms/gram-molecular weight) ; M is the gram molecular weight of the compound; ѵi is the number of absorbing atoms of ѵi kind per compound molecule; and, σ; is the total cross section of the ith atom (cm2).


As an example, consider the calculations of the linear attenuation coefficient,p.., for the compound polyethylene (CH2)N :

μ = ρ∙N/M (ѵ1σ1 + ѵ2σ2 + ѵ3σ3 +..... ѵiσi ) Eq. 8

μ = ρ∙N/M (ѵCσC + ѵHσH)

for: ρ = 0.91 g/cm3

N = 6.023 X 1023 atoms/g-molM= 14.0268 gѵC = 1σC = 4.803 X 10-24 cm2

ѵH= 2σH = 38.332 X 10-24 cm2

μ = 0.91 x 6.023 x 1023 x (14.0268)-1 (1x4.803+2x38.332) x 10-24

μ = 3.18329 cm-1


σC

σH


FIGURE 10. Half-Value Layers of Selected Materials for a Thermal Neutron Radiograph Beam.


FIGURE 11. Tenth-Value Layers of Selected Materials for Thermal Neutron Radiography.


5.2 Half-Value LayersAn important concept for radiography is the half value layer (HVL); that is, thethickness of material that will reduce the radiation intensity by a factor of two.A plot of half-value layers for a practical thermal neutron radiographic beam isgiven in Fig. 10. This information can be used to estimate the transmissionand detectability of various materials combined with others.

I = Ioe –μn t

at half value layer

I/Io = ½ = e –μn t½

Ln 0.5 = –μn t½

t ½ = Ln 0.5/ - μn = 0.693/ μn


5.3 Tenth-Value LayersThere will be a thickness of material that is sufficiently thick that little of theneutron beam penetrates. In Fig. 11, thicknesses of material that will transmitonly 10% of an incident thermal neutron radiographic beam are plotted. Thisthickness represents about the limit that should be attempted in normalthermal neutron radiography. Variations in neutron energy should beconsidered for thicknesses greater than those shown in Fig. 11.

Similarly:

t 1/10 = Ln 0.1/ - μn = 2.302/ μn


5.4 CONCLUSIONNeutron radiography is a valuable method for nondestructive testing. Theattenuation differences between X-rays and neutrons make these tworadiographic methods, to a large degree, complementary. Figure 6 in thisSection is an illustration of how the two methods provide a more completeinspection when used together. The neutrons in this example show lightmaterials such as the explosive, plastic and epoxy components, while the X-radiograph shows the metallic components.

Neutrons offer sensitivity to different isotopes and can also be very useful for inspecting highly radioactive material. These two characteristics offeradvantages particularly to the nuclear industry. Other areas of application include aerospace, the military and transportation industries. The neutron radiographic technique is relatively expensive, but it can be used to perform inspections that present problems for other NOT methods. When used for these unique inspections, neutron radiography is a cost-effective nondestructive testing echnique.


End Of Reading


Further Reading:http://large.stanford.edu/courses/2011/ph241/chenw2/

http://www.nuclear.engr.utexas.edu/index.php/netl/services/neutron-radiography


Source of Neutrons


Radioactive Decay Interactive

http://periodictable.com/Isotopes/004.7/index.full.html


Source of Neutrons


Source of Neutrons-Beta Decay


Source of Neutrons-Beta Decay

Decay or delay ?


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Source of Neutrons

236U92 ?


Source of Neutrons-Beryllium Decay


Beryllium Decay


Radioactive Decay


Radioactive Decay-Triple-alpha process


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Neutron Energy & Detectability


Peach – 我爱桃子


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