ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control

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294

Neutron Radiography

Harold Berger, Industrial Quality, Inc.

Introduction

NEUTRON RADIOGRAPHY is a form of nondestructive inspection that uses a specific type of particulate radiation,

called neutrons, to form a radiographic image of a testpiece. The geometric principles of shadow formation, the variation

of attenuation with testpiece thickness, and many other factors that govern the exposure and processing of a neutron

radiograph are similar to those for radiography using x-rays or -rays. These topics are extensively covered in the article

"Radiographic Inspection" in this Volume and will not be discussed here.

This article will deal mainly with the characteristics that differentiate neutron radiography from x-ray or -ray

radiography, as discussed in Ref 1, 2, 3, 4, 5, 6, 7, 8, 9. Neutron radiography will be described in terms of its advantages

for improved contrast on low atomic number materials, the discrimination between isotopes, or the inspection of

radioactive specimens.

References

H. Berger, Ed., Practical Applications of Neutron Radiography and Gaging,

STP 586, American Society for

Testing and Materials, 1976

Neutron Radiography Issue, At. Energy Rev., Vol 15 (No. 2), 1977, p 123-364

N.D. Tyufyakov and A.S. Shtan, Principles of Neutron Radiography,

Amerind Publishing, 1979 (translated

from the Russian)

P. Von der Hardt and H. Rottger, Ed., Neutron Radiography Handbook, D. Reidel Publishing, 1981

J.P. Barton and P. Von der Hardt, Ed., Neutron Radiography, D. Reidel Publishing, 1983

L.E. Bryant and P. McIntire, Ed., Radiography and Radiation Testing, in Nondestructive Testing Handbook,

Vol 3, American Society for Nondestructive Testing, 1985

"Standard Practices for Thermal Neutron Radiography of Materials," E 748, Annual Book of ASTM

Standards, American Society for Testing and Materials

J.P. Barton, G. Farny, J.L. Person, and H. Rottger, Ed., Neutron Radiography, D. Reidel Publishing, 1987

H. Berger, Some Recent Developments in X-Ray and Neutron Imaging Methods, in Nondestructive Testing,

Vol 1, J.M. Farley and R.W. Nichols, Ed., Pergamon Press, 1988, p 155-162

Neutron Radiography

Harold Berger, Industrial Quality, Inc.

Principles of Neutron Radiography

Neutron radiography is similar to conventional radiography in that both techniques employ radiation beam intensity

modulation by an object to image macroscopic object details. X-rays or -rays are replaced by neutrons as the

penetrating radiation in a through-transmission inspection. The absorption characteristics of matter for x-rays and

neutrons differ drastically; the two techniques in general serve to complement one another.

Neutrons are subatomic particles that are characterized by relatively large mass and a neutral electric charge. The

attenuation of neutrons differs from the attenuation of x-rays in that the processes of attenuation are nuclear rather than

ones that depend on interaction with the electron shells surrounding the nucleus.

Neutrons are produced by nuclear reactors, accelerators, and certain radioactive isotopes, all of which emit neutrons of

relatively high energy (fast neutrons). Because most neutron radiography is performed with neutrons of lower energy

(thermal neutrons), the sources are usually surrounded by a moderator, which is a material that reduces the kinetic energy

of the neutrons.

Neutron Versus Conventional Radiography. Neutron radiography is not accomplished by direct imaging on film,

because neutrons do not expose x-ray emulsions efficiently. In one form of neutron radiography, the beam of neutrons

impinges on a conversion screen or detector made of a material such as dysprosium or indium, which absorbs the

neutrons and becomes radioactive, decaying with a short half-life. In this method, the conversion screen alone is exposed

in the neutron beam, then immediately placed in contact with film to expose it by autoradiography. In another common

form of imaging, a conversion screen that immediately emits secondary radiation is used with film directly in the neutron

beam.

Neutron radiography differs from conventional radiography in that the attenuation of neutrons as they pass through the

testpiece is more related to the specific isotope present than to density or atomic number. X-rays are attenuated more by

elements of high atomic number than by elements of low atomic number, and this effect varies relatively smoothly with

atomic number. Thus, x-rays are generally attenuated more by materials of high density than by materials of low density.

For thermal neutrons, attenuation generally tends to decrease with increasing atomic number, although the trend is not a

smooth relationship. In addition, certain light elements (hydrogen, lithium, and boron), certain medium-to-heavy elements

(especially cadmium, samarium, europium, gadolinium, and dysprosium), and certain specific isotopes have an

exceptionally high capability of attenuating thermal neutrons (Fig. 1). This means that neutron radiography can detect

these highly attenuating elements or isotopes when they are present in a structure of lower attenuation.

Fig. 1 Mass attenuation coefficients for the elements as a function of atomic number for thermal (4.0 × 10

-21

or 0.025 eV) neutrons and x-rays (energy

125 kV). The mass attenuation coefficient is the ratio of the linear

attenuation coefficient, , to the density, , of the absorbing material. Source: Ref 6

Thermal (slow) neutrons permit the radiographic visualization of low atomic number elements even when they are present

in assemblies with high atomic number elements such as iron, lead, or uranium. Although the presence of the heavy

metals would make detection of the light elements virtually impossible with x-rays, the attenuation characteristics of the

elements for slow neutrons are different, which makes detection of light elements feasible. Practical applications of

neutron radiography include the inspection of metal-jacketed explosives, rubber O-ring assemblies, investment cast

turbine blades to detect residual ceramic core, and the detection of corrosion in metallic assemblies.

Using neutrons, it is possible to detect radiographically certain isotopes--for example, certain isotopes of hydrogen,

cadmium, or uranium. Some neutron image detection methods are insensitive to background -rays or x-rays and can be

used to inspect radioactive materials such as reactor fuel elements. In the nuclear field, these capabilities have been used

to image highly radioactive materials and to show radiographic differences between different isotopes in reactor fuel and

control materials. The characteristics of neutron radiography complement those of conventional radiography; one

radiation provides a capability lacking or difficult for the other.

Reference cited in this section

L.E. Bryant and P. McIntire, Ed., Radiography and Radiation Testing, in Nondestructive Testing Handbook,

Vol 3, American Society for Nondestructive Testing, 1985

Neutron Radiography

Harold Berger, Industrial Quality, Inc.

Neutron Sources

The excellent discrimination capabilities of neutrons generally refer to neutrons of low energy, that is, thermal neutrons.

The characteristics of neutron radiography corresponding to various ranges of neutron energy are summarized in Table 1.

Although any of these energy ranges can be used for radiography, this article emphasizes the thermal-neutron range,

which is the most widely used for inspection.

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

Type of

neutrons

Energy range, J

(eV)

Characteristics

Cold <1.6 × 10

-21

(<0.01)

High-absorption cross sections decrease transparency of most materials, but also increase

efficiency of detection. An advantage is reduced scatter at energies below the Bragg cutoff,

where neutrons can no longer undergo Bragg reflection.

Thermal 1.6 × 10

-21

to 8.0

× 10

-20

(0.01 to

0.5)

Good discrimination between materials, and ready availability of sources

Epithermal 8.0 × 10

-20

to 1.6

× 10

-15

(0.5 to

)

Excellent discrimination for particular materials by working at energy of resonance. Greater

transmission and less scatter in samples containing materials such as hydrogen and enriched

reactor fuels

Fast 1.6 × 10

-15

to 3.2

× 10

-12

(10

2.0 × 10

)

Good point sources are available. At low-energy end of spectrum, fast-neutron radiography

may be able to perform many inspections done with thermal neutrons, but with a panoramic

technique. Good penetration capability because of low-absorption cross sections in all materials.

Poor material discrimination

In thermal-neutron radiography, an object (testpiece) is placed in a thermal-neutron beam in front of an image detector.

The neutron beam may be obtained from a nuclear reactor, a radioactive source, or an accelerator. Several characteristics

of these sources are summarized in Table 2. For thermal-neutron radiography, fast neutrons emitted by these sources must

first be moderated and then collimated (Fig. 2). The radiographic intensities listed in Table 2 typically do not exceed 10

-5

times the total fast-neutron yield of the source. Part of this loss is incurred in moderating the neutrons, and the remainder

in bringing a collimated beam out of a large-volume moderator.

Table 2 Properties and characteristics of thermal-neutron sources

Type of source Typical

radiographic

intensity,

n/cm

·s

Spatial

resolution

Exposure

time

Characteristics

Radioisotope 10

to 10

Poor to medium Long

Stable operation, low-to-medium investment cost,

possibly movable

Accelerator 10

to 10

Medium Average

On-off operation, medium cost, possibly movable

Subcritical

assembly

to 10

Good Average

Stable operation, medium-to-high investment cost,

movement difficult

Nuclear reactor 10

to 10

Excellent Short Medium-to-high investment cost, movement difficult

Fig. 2

Thermalization and collimation of beam in neutron radiography. Neutron collimators can be of the

parallel-

wall (a) or divergent (b) type. The transformation of fast neutrons to slow neutrons is achieved by

moderator materials such as paraffin,

water, graphite, heavy water, or beryllium. Boron is a typically used

neutron-absorbing layer. The L/D ratio, where L

is the total length from the inlet aperture to the detector

(conversion screen) and D is the effective dimension of the inlet of the coll

imator, is a significant geometric

factor that determines the angular divergence of the beam and the neutron intensity at the inspection plane.

Collimation is necessary for thermal-neutron radiography because there are no useful point sources of low-energy

neutrons. Good collimation in thermal-neutron radiography is comparable to small focal-spot size in conventional

radiography; the images of thick objects will be sharper with good collimation. On the other hand, it should be noted that

available neutron intensity decreases with increasing collimation.

Nuclear Reactors. Many types of reactors have been used for thermal-neutron radiography. The high neutron flux

generally available provides high-quality radiographs and short exposure times. Although truck-mounted reactors are

technically feasible, a reactor normally must be considered a fixed-site installation, and testpieces must be taken to the

reactor for inspection. Investment costs are generally high, but small medium-cost reactors can provide good results.

When costs are compared on the basis of available neutron flux (typically, 10

n/cm

· s flux is often available at

collimator entrance, and 10

to 10

n/cm

· s flux is available at the film plane), reactor sources can be less costly than

other sources.

Accelerators. The accelerators most often used for thermal-neutron radiography are:

• The low-voltage type employing the reaction + +

, a (d,T) generator, where n, d, and

T represent the neutron, deuteron (the nucleus of a deuterium atom, D or

, that consists of one

neutron and one proton), and tritium ( ), respectively

• High-energy x-ray machines, in which (x,n) reactions are used, where x represents x-ray radiation

• Van de Graaff accelerators

• More recently, high-energy linear accelerators and cyclotrons to generate neutrons by charged-

particle

reactions on beryllium or lithium targets

Low-Voltage Accelerators. A (d,T) generator provides fast-neutron yields in the range of 10

to 10

n/s. Target

lives in sealed neutron tubes are reasonable (100 to 1000 h, depending on yield), and the sealed-tube system presents a

source similar to that of certain types of x-ray machines.

High-Energy X-Ray Machines. An (x,n) neutron source is a high-energy x-ray source such as a linear accelerator that

can be converted for the production of neutrons by adding a suitable secondary target--for example, beryllium. X-rays

having energies above an energy threshold level cause the secondary target to emit neutrons; in beryllium, the threshold

x-ray energy for neutron production is 2.67 × 10

-13

J (1.66 MeV). Useful neutron radiography has been performed with an

8.8 × 10

-13

J (5.5 MeV) linear accelerator having an x-ray output of 0.17 C/kg · min (650 R/min) at 1 m (3 ft). Changeover

time from neutron emission to x-ray emission for this source was only 1 h. Beam intensities for neutron radiography with

this source were about 5 × 10

n/cm

· s. with reasonable beam collimation.

Van de Graaff Accelerators. Much higher beam intensities have been obtained by the acceleration of deuterons onto

a beryllium target in a 3.2 × 10

-13

J (2.0 MeV) Van de Graaff generator. An intensity of 1.2 × 10

n/cm

· s was achieved

(with medium collimation), and it is estimated that an acceleration voltage of 4.8 × 10

-13

J (3.0 MeV) would improve

beam intensity by a factor of approximately six.

The principle of the Van de Graaff machine is illustrated in Fig. 3. A rotating belt transports the charge from a supply to a

high-voltage terminal. An ion source within the terminal is fed deuterium gas from a reservoir frequently located within

the terminal. A radio-frequency system ionizes the gas, and positive ions are extracted into the accelerator tube. The

terminal voltage of about 3 MV is distributed by a resistor chain over about 80 gaps forming the accelerator tube, all of

which is enclosed in a pressure vessel filled with insulating gas (N

and CO

at 2.0 MPa, or 290 psi).

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

The particle beam is extracted along flight tubes. In a typical neutron reaction, the beam bombards a water-cooled

beryllium target in the center of the water moderator tank, which also serves as a partial shield. The higher-energy

accelerators indicated above can provide neutron yields of 10

n/s and moderated, well-collimated beam intensities of the

order of 10

n/cm

· s.

A few 4.8 × 10

-13

J (3.0 MeV) Van de Graaff generators have recently been placed in service for thermal-neutron

radiography. In one such Van de Graaff system designed for neutron radiography, deuterons (4.8 × 10

-13

J, or 3 MeV; 280

A) are accelerated onto a disk-shaped, water-cooled beryllium metal target. Neutrons in the range of 3.2 to 9.6 × 10

-13

(2 to 6 MeV) are emitted preferentially in the forward direction and are moderated in water. The 4 (solid angle) yield of

5 × 10

n/s produces a peak thermal neutron flux of 2 × 10

n/cm

· s. At a collimator ratio of 36:1, the typical exposure

time for high-quality film (3 × 10

n/cm

) is about 2 h.

The accelerator tank for the 4.8 × 10

-13

J (3 MeV) machine measures 5.2 m (17 ft) in length and 1.5 m (5 ft) in diameter.

The weight is 6100 kg (13,500 lb). The dimensions of the water tank are approximately 1 m (3 ft) on each side. Neutron

beams can be extracted through three horizontal beam collimators. Unlike reactors, subcritical multipliers, or (d, T)

accelerators, the Van de Graaff accelerators utilize no radioactive source material and sometimes require less stringent

license processes.

Other acceleration machines or reactions can be used for thermal-neutron radiography. However, those described above

have been most widely used.

Radioactive Sources. There are many possible radioactive sources. The characteristics of several radioisotopes that

are commonly used are summarized in Table 3.

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

Radioisotope Neutron reaction Half-life

Characteristics

124

Sb-Be

( , n)

60 days

Short half-life, high -ray background, low neutron energy easily thermalized,

low cost, high neutron yield

210

Po-Be

( , n)

138 days

Short half-life, low -ray background, low cost

214

Am-Be

( , n)

458

years

Long half-life, easily shielded -ray background, high cost

241

Am-

242

Cm-

( , n)

163 days

Short half-life, high neutron yield, medium cost

244

Cm-Be

( , n)

18.1

years

Long half-life, low -ray background, potential low cost

252

Cf Spontaneous

fission

2.65

years

Long half-life, small size, low neutron energy easily thermalized, high neutron

yield

Radioisotopes offer the best prospect for a portable neutron-radiographic facility, but it should be recognized that the

thermal-neutron intensity is only about 10

-5

of the total fast-neutron yield from the source. Consequently, neutron

radiography using a radioisotope as a neutron source normally requires long exposure times and fast films. For example, a

typical 3.7 × 10

Bq (10 Ci) source would provide a total fast-neutron yield of the order of 10

n/s. The radiographic

intensity would be about 10

n/cm

· s, and a typical exposure time using a fast film/converter-screen combination would

be about 1 h. Californium-252, usually purchased in the form shown in Fig. 4, has been the most frequently used

radioactive source for neutron radiography.

Fig. 4 Cross section of doubly encapsulated

252

Cf source. Source: Ref 6

Subcritical Assembly. Another type of source that has received some attention is a subcritical assembly. This type of

source is similar to a reactor, except that the neutron flux is less and the design is such that criticality cannot be achieved.

A subcritical assembly offers some of the same neutron multiplication features as a reactor. It is somewhat easier to

operate, and safety precautions are less stringent, because it is not capable of producing a self-sustaining neutron chain

reaction.

Reference cited in this section

L.E. Bryant and P. McIntire, Ed., Radiography and Radiation Testing, in Nondestructive Testing Handbook,

Vol 3, American Society for Nondestructive Testing, 1985

Neutron Radiography

Harold Berger, Industrial Quality, Inc.

Attenuation of Neutron Beams

Unlike electrons and electromagnetic radiation, which interact with orbital electrons surrounding an atomic nucleus,

neutrons interact only with atomic nuclei. Usually, neutrons are deflected by interaction with the nuclei, but occasionally

a neutron is absorbed into a nucleus. When a neutron collides with the nucleus of an atom and is merely deflected, the

neutron imparts some of its kinetic energy to the atom. Both the neutron and the atom move off in different directions

from the original direction of motion of the neutron. This process, known as scattering, reduces the kinetic energy of the

neutron and the probability that the neutron will pass through the object (testpiece) in a direction that will permit it to be

detected by a device placed behind the object.

True absorption of neutrons occurs when they are captured by nuclei. The capture of a neutron transforms the nucleus to

the next-higher isotope of the target nucleus and sometimes produces an unstable nucleus that then undergoes radioactive

decay. The probability that a collision between a neutron and a nucleus will result in capture is known as the capture cross

section and is expressed as an effective area per atom. (The capture cross section is usually measured in barns, 1 barn

equaling 10

-24

or 1.6 × 10

-25

in.

.) The capture cross section varies with neutron energy, atomic number, and mass

number. For thermal neutrons (energy of about 4.0 × 10

-21

J, or 0.025 eV), the average capture cross section varies

randomly with atomic number, being high for certain elements and relatively low for other elements. The cross section

actually varies by isotope rather than element. However, radiographers usually consider an average cross section for an

element. For intermediate neutrons (energies of 8.0 × 10

-20

to 1.6 × 10

-15

J, or 0.5 eV to 10 keV) and for fast neutrons

(energies exceeding 1.6 × 10

-15

J, or 10 keV), the capture cross section is normally smaller than that for thermal neutrons,

and there is much less variation with atomic number. For fast neutrons, most elements are similarly absorbing, and

scattering is the dominant process of attenuation.

In relation to other types of penetrating radiation, many materials interact less with neutrons. Therefore, neutrons can

sometimes be used to inspect greater thicknesses than can be conveniently inspected with electromagnetic radiation. The

combined effect of scattering and capture can be expressed as a mass-absorption coefficient; this coefficient is used to

determine the exposure factor for the neutron radiography of a given object (testpiece). For a given material, attenuation

varies exponentially with thickness, and the basic law of radiation absorption (discussed in the article "Radiographic

Inspection" in this Volume) applies to neutron attenuation as well as to the attenuation of electromagnetic radiation.

Neutron Radiography

Harold Berger, Industrial Quality, Inc.

Neutron Detection Methods

Detection methods for neutron radiography generally use photographic or x-ray films. In the so-called direct-exposure

method, film is exposed directly to the neutron beam, with a conversion screen or intensifying screen providing the

secondary radiation that actually exposes the film (Fig. 5a). Alternatively, film can be used to record an autoradiographic

image from a radioactive image-carrying screen in a technique called the transfer method (Fig. 5b).

Fig. 5 Schematics of neutron radiography with film using the direct-

exposure method (a) and the transfer

method (b). The cassette is a light-

tight device for holding film or conversion screens and film in close contact

during exposure.

Direct-Exposure Method. Conversion screens of thin gadolinium foil or a scintillator have been most widely used in

the direct-exposure method. When bombarded with a beam of neutrons, some of the gadolinium atoms absorb some of the

neutrons and then promptly emit -rays. The -rays in turn produce internal conversion electrons that actually expose

the film; these are directly related in intensity to the intensity of the neutron beam. Scintillators, on the other hand, are

fluorescent materials often made of zinc sulfide crystals that also contain a specific isotope, such as or . In a

neutron beam, these isotopes react with neutrons as follows:

+ +

The particles emitted as a result of these reactions cause the zinc sulfide to fluoresce, which in turn exposes the film.

Gadolinium oxysulfide, a scintillator originally developed for conventional radiography, is now widely used for neutron

radiography.

Scintillators provide useful images with total exposures as low as 5 × 10

n/cm

. The high speed and favorable relative

neutron/gamma response of scintillators make them attractive for use with nonreactor neutron sources. For high-intensity

sources, gadolinium screens are widely used. Gadolinium screens provide greater uniformity and image sharpness (high-

contrast resolution of 10 m, or 400 in., has been reported), but an exposure about 30 or more times that of a scintillator

is required, even with fast films. Excessive background radiation should be kept to a minimum because it can have a

detrimental effect on image quality.

In the transfer method, a thin sheet of metal called a transfer screen, which is usually made of indium or

dysprosium, is exposed to the neutron beam transmitted through the specimen. Neutron capture by the isotope or

induces radioactivity, indium having a half-life of 54 min and dysprosium a half-life of 2.35 h. The intensity of

radioactive emission from each area of the transfer screen is directly related to the intensity of the portion of the

transmitted neutron beam that induced radioactivity in that area. The radiograph to be interpreted is made by placing the

radioactive transfer screen in contact with a sheet of film. The particle and -ray emissions from the transfer screen

expose the film, with film density in various portions of the developed image being proportionally related to the intensity

of radioactive emission.