Lowenthal G., Airey P. Practical Applications of Radioactivity and Nuclear Radiations

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7.4 Scienti®c and industrial applications of neutrons

7.4.1 Comments on work with neutrons and neutron doses

Neutrons are uncharged sub-atomic particles with a mass similar to that of

protons (Table 1.2). They are of industrial and scienti®c importance because

their applications contribute knowledge of the structure and composition of

materials not readily available from other techniques (L'Annunziata, 1998,

Ch. 1).

Properties of neutrons and of selected neutron sources were brie¯y

described in Sections 1.3.6 and 5.4.4. Applications employing neutrons will

be introduced with a few comments on estimating dose rates to experimenters

working with neutrons. For full information see, for example, Cember (1996,

Chs. 5 and 6).

Table 2.1(a) lists non-dimensional radiation weighting factors w

(see

Table 2.2 for de®nitions), required to calculate equivalent doses due to

nuclear radiations absorbed by operators (Eq. (2.4a)). Table 2.1(a) shows

that weighting factors for neutrons, (w

)

, are functions of the neutron

energy. They reach a maximum of 20 for neutron energies between 0.1 and 2

MeV, when they are equal to w

for a particles. However, they decrease to 5

at higher energies and also for thermal neutrons. Nevertheless, the fact that

)

is at least ®ve times larger than (w

)

, the weighting factor for g rays,

re¯ects the potentially strong interaction of neutrons with tissue material.

Protection against high-¯ux neutrons in research reactors is provided by

thick walls of metal and concrete (Section 1.4.1). Portable sources of the type

described in Section 1.3.6 (Figure 1.6(a)) are designed to ensure that doses to

the operators are well within the international guidelines and are as low as

reasonably achievable (Section 2.7.2 in this book and Martin and Harbison,

1996, Section 8.5).

7.4.2 Industrial applications of neutron sources

Neutron sources

There are three sources of neutrons which are of importance for industrial

applications: nuclear reactors, neutron generators and portable isotopic

sources. The neutron ¯ux densities are in the ranges 10

to 10

n/(cm

6s)

for reactors, 10

to 5610

n/(cm

6s) for accelerators and 10

to 10

n/s for

portable neutron sources.

The energies of moderated neutrons from reactors are predominantly in

7.4 Applications of neutrons 211

the thermal and epithermal range, while those from the other sources are in

the megaelectronvolt range. Neutrons may be classi®ed according to their

energy. A scheme based on the response of commonly used detectors is

included in Section 5.4.4. For a different classi®cation of neutron energies

see, e.g. L'Annunziata, (1998, Ch. 1). A list of applications of neutrons in

science and industry is given in Table 7.6. The three sources mentioned at the

start of this section are now discussed in turn.

(1) Research reactors: Reactors produce an intense ¯ux of neutrons which are

employed principally for:

. the production of radioisotopes for medicine, industry, agriculture and

scienti®c research;

. the neutron activation analysis of materials recovered during geological

research, minerals exploration and environmental monitoring;

. the neutron transmutation doping of silicon for the semiconductor industry;

. the structural determination of materials using neutron diffraction;

. neutron radiography.

(2) High -voltage neutron generators: Deuterium±tritium sealed tube generators

produce pulses of 14 MeV neutrons and are widely used in sophisticated

borehole logging applications. Ionised deuterium gas molecules are accelerated

to 160 kV to bombard a mixed deuterium/tritium (say 560 GBq or 15 Ci) target

and generate neutrons via the

H(d,n)

He reaction. A typical down hole

accelerator produces a ¯ux of about 3610

n/(cm

s) with a thermal/fast ¯ux

ratio of about 1% and a pulse length of, say, 16 ms.

(3) Portable neutron sources: The portable sources in most common use employ

neutrons that are emitted: (a) when a pa rticles interact with the atoms of light

elements (commonly alphas from americium-241 or plutonium-239 interacting

with beryllium atoms and (b) following the spontaneous ®ssion of californium-

252. A third class of source exploits the interaction of gamma rays from

antimony-124 with either heavy water (D

O) or beryllium. The reactions are

listed in Table 7.7. The reader is referred to IAEA (1993) for further details.

The energies of the emitted neutrons are mostly in the range 2 to 10 MeV

(Figure 1.6(b)). The Am/Be, Pu/Be and

252

Cf sources are quasi-point sources,

are compact and transportable and have been incorporated into a range of

nucleonic instruments. These include:

. industrial backscatter gauges for the measurement of liquid surfaces and interfaces

in tanks and reaction vessels;

. soil moisture meters for use in agriculture, hydrology and civil engineering;

. borehole logging tools.

Industrial applications are classi®ed according to the property of the neutrons

Industrial applications of radioisotopes and radiation212

Table 7.6.

Applications of neutrons to science and industry.

Property of the Application

Example

Reference

Radiation

Backscatter Measurements of differences

in the moderating power

(effectively the hydrogen

content) of materials

(a) Monitoring liquid levels in tanks

(b) Measuring moisture in soils, geological strata and bulk

materials. There are speci®c applications to agriculture,

civil engineering and borehole logging (see below)

Section 7.4.2

Neutron±neutron borehole

logging

(a) Porosity measurements

(b) Monitoring the freshwater/saline contact zone in

coastal aquifers and the water/oil interface zone in oil wells

Diffraction Materials structure

Neutron diffraction, being particularly sensitive to H and

the light elements, complements X ray techniques

Section 7.4.3

Neutron

activation±

delayed

Neutron activation analysis;

Production of radioisotopes

Multi element assay;

Radioisotopes used in nuclear medicine, industry,

agriculture

Section 7.4.4

Section 1.4.4

Neutron

activation±

prompt gamma

Borehole logging

Obtain data on the elemental composition of the

surrounding strata, and hence information on the lithology

and groundwater salinity

Section 7.4.2

On-line analysis

Monitor the level of sulphur and the elemental composition

of ash in coal on conveyor belts

Sections 7.2.1

and 7.4.4

Note: The propert ies of neutrons are discussed in Sections 1.3.6 and 5.4.4.

Table 7.7.

Radioisotope neutron sources (Shani, 1990).

Neutron source Neutron yield Type of source

Half life

Reaction

(n/s/Ci

)

Pu±Be

1.7610

a/n

24,360 y

Be + a?

C + n + 5.71 MeV

Am-Be

2.2610

433 y

124

Sb±Be±D

g/n

60.2 d

Be + g ?

2a +n71.67 MeV

H+g ?

H+n7

2.23 MeV

252

4.4

610

Spontaneous

2.65 y

252

Cf (SF)

? 2f + 3.8n + 200 MeV

®ssion (SF)

effective

(Section 5.4.4)

One curie (Ci) is 37

610

Bq i.e. 37 000 MBq.

that is exploited, namely scattering, activation or diffraction. A few com-

ments on radiation protection were included in Section 7.4.1.

Neutron moderation

Neutrons from portable sources are generated at megaelectronvolt energies.

On interacting with surrounding matter they dissipate their kinetic energy,

principally by elastic and inelastic scattering processes. Ultimately they

equilibrate thermally with their surroundings with energies of about 0.025 eV

near room temperature. When con®gured as a backscatter gauge, the

neutrons are detected by a calibrated proportional counter placed adjacent to

the neutron source (Section 5.4.4). The intensity of the neutrons back-

scattered into the proportional counter is determined principally by the

hydrogen concentration in the material of interest, though there are some

complicating factors yet to be mentioned. The process by which kinetic

energy is dissipated is known as neutron moderation.

Elastic scatter conserves the initial kinetic energy of the neutrons, but

redistributes it as in billiard ball collisions. The energy transfer from the

neutrons is greatest for collisions with hydrogen atoms, since protons and

neutrons are of equal mass. The probability for energy transfers decreases

with increasing mass of the target nuclei. In water, fast neutrons are

thermalised in a distance of about 10 cm, predominantly by elastic scatter

with hydrogen atoms. The energy of the recoiling protons is dissipated by

ionisation and excitation of the atoms and molecules of the surrounding

material. Having reached thermal energies, the neutrons are captured, as a

rule by hydrogen atoms to form deuterium with the emission of 2.223 MeV g

rays. This is an example of a neutron capture followed promptly by gamma

ray emission.

Inelastic scatter occurs when the neutrons lose some of their kinetic energy

to form metastable states in nuclei. Normally, the excited nuclei promptly de-

excite to the ground state with the emission of g rays and possibly nucleons.

The element in question can often be identi®ed from the energy of the emitted

g rays. This is the basis of prompt gamma neutron activation analysis

(PGNAA) which will be discussed in more detail in Section 7.4.4. Inelastic

scattering will only occur when the energy of the neutron is above a threshold

value. The values vary from element to element but are generally in the range

0.5 to 7 MeV. The emission of an inelastic scattering gamma ray is not

necessarily a prompt process. An interesting example is the excitation of

rhodium-103 to a metastable state rhodium-103m (T

1/2

56.12 min), which de-

excites with the emission of 40 keV g rays.

The ef®ciency with which fast neutrons dissipate their kinetic energy

2157.4 Applications of neutrons

(elastically and/or inelastically) and slow down is known as the moderating

power of the surrounding medium.

Neutron backscatter gauges

A neutron backscatter gauge comprises a source of fast neutrons (commonly

Am/Be) and a proportional counter using helium-3 or boron tri¯uoride

which responds only to backscattered neutrons down to thermal energies

(Figure 7.8(a), see also Charlton, 1986, p. 278). Thermal and epithermal

neutrons can be detected by helium-3 proportional counters whereas boron

tri¯uoride detectors are sensitive only to thermal neutrons. Both detectors

record an almost zero response to fast neutrons. Hence, as noted earlier,

unshielded proportional counters can be placed adjacent to the fast neutron

source without recording a signi®cant background reading (Section 5.4.4).

Neutron backscatter gauges are routinely used in the oil re®ning and

chemical industries to monitor the levels of liquids in tanks and to detect

the interfaces between different liquids. With hydrogen being a highly

ef®cient neutron moderator, the rate of detection of backscattered, therma-

lised neutrons is used as a measure of the concentration of water or

hydrocarbon in the tank. For the arrangement shown in Figure 7.8(a), it is

possible to monitor not only the level of the surface of the oil, but also its

interface with the denser water. As shown schematically in the ®gure, it

would be more dif®cult to accurately locate such an interface with an

externally located g backscatter gauge which responds principally to

differences in bulk density.

Neutron moisture meters

Soil moisture meters are specialised backscatter gauges. They comprise a

source of fast neutrons and a detector responding only to thermalised

neutrons con®gured into a portable ®eld instrument. The probe is lowered

into the borehole when the countrate can be related to the moderating power

of the soil in the sphere of in¯uence around the probe which is typically of the

order of 0.5 m radius. The countrate is usually normalised to that observed

when the probe is suspended in a water-®lled portable standard drum.

The moderating power of the soil is determined principally by its total

hydrogen content. Both soil moisture and organic hydrogen are present. A

calibration step employing an oven dried sample of soil to monitor the level

of bound hydrogen [H] is necessary before the water content can be deduced

from the neutron meter readings. Additional corrections are needed for the

percentage of thermal neutrons which are absorbed by other components of

the soil (see below), and are therefore not recorded by the detector (Long-

Industrial applications of radioisotopes and radiation216

worth, 1998, Ch. 5). Quantitative estimates are based on the concept of the

macroscopic thermal neutron absorption cross section S

, which is de®ned as

= n

+ n

+  (7.4)

where n

, etc. is the atom fraction of element x in the soil and s

a,x

, etc. is the

corresponding neutron absorption cross section. Values of n

are obtained

from the chemical analysis of the soil and s

a,x

from tabulated values (see e.g.

Figure 1.3) or via the Internet (e.g. http://t2.lanl.gov/data/map.html).

Problems may arise because very minute traces of strong neutron absorbers

can have a signi®cant effect on S

and are not readily assayed with suf®cient

accuracy. One approach is to measure the response of the neutron moisture

meter to soil of known values of the hydrogen concentration [H] and S

in a

standard calibration drum.

Moisture gauges are widely used in agricultural research for the monitoring

of soil moisture variations in the root zone. They have also found widespread

use

. in civil engineering to measure the moisture levels in bulk material employed for

the co nstruction of roads and earth-®lled dams;

. in the concrete and glass industries to monitor the moisture levels in sand; and

. in the iron and steel industry to determine the moisture levels in coke and sinter

mixtures.

Borehole logging with neutrons

The scienti®c principles underpinning the neutron moisture meter have been

extended and applied to the logging of oil ®elds and groundwater bores

(Figure 7.9(b)). The purpose of this section is to indicate the level of

sophistication resulting from decades of development, principally in the oil

industry. Major applications include measurements of the

hydrocarbon volumes and viscosity;

porosity and permeability; and

grain size and mineralogy.

Reference will be made to the measurement of hydrocarbon volumes and of

porosity.

(1) Hydrocarbon volumes: Hydrocarbon volumes in oil be aring strata are normally

obtained by conventional borehole logging. However, where there is a need for

fast, accurate measurements of the hydrocarbons prompt g neutron activation

analysis (PGNAA) methods are used (Section 7.4.4). In practice it is the carbon/

oxygen ratio which is monitored. This ratio varies from zero in the absence of

hydrocarbons to a maximum value when the surrounding strata are saturated

7.4 Applications of neutrons 217

with hydrocarbons. Complications arise when the petroleum occurs in carbo-

nate rocks such as calcite [CaCO

] and dolomite [CaMg(CO

)

(2) Porosi ty: Porosity is the fraction of the total volume occupied by pore space and

is an important parameter in the evaluation of the oil bearing potential of a

formation. The porosity can be determined from the average bulk density if the

average matrix density is known. The bulk density can be estimated using g ray

backscatter (Section 7.2.2). The matrix density, which is the physical density of

the material comprising the formation, can be calculated from knowledge of the

mineral abundances.

In practice, there is a very limited range of minerals commonly found in

many oil bearing strata. It is therefore often possible to infer the mineral

composition from multi element analyses. Such analyses can be obtained, for

instance using PGNAA techniques, supplemented by estimates of potassium,

uranium and thorium levels obtained from g ray logging.

Direct estimates of porosity can be obtained from measurements of the

lifetimes of epithermal neutrons (Wilson et al., 1989). A down hole deu-

terium±tritium (D±T) generator is used to produce pulses of 14 MeV neutrons

with a pulse time of about 16 ms and a repetition rate of 5 kHz (i.e. every

200 ms). The response of the detector to epithermal neutrons is recorded at

microsecond intervals. The energies of the neutrons are moderated by the

surrounding strata. The greater the porosity of the strata, the greater the level

of hydrocarbons and hence the greater the moderating power of the

surroundings and the shorter the lifetime of the epithermal neutrons.

To apply this technique it is necessary to monitor the energy loss of

epithermal neutrons free from thermal neutrons. This is achieved by ®tting a

He detector with a gadolinium cover to absorb the thermal component.

Neutron radiography

This technique uses neutrons to obtain information about the internal

structure and distribution of material under investigation. As with g ray

radiography (Section 7.2.1) the neutrons produce an image on a photographic

®lm (via protons scattered by the neutrons out of hydrogen atoms), which

re¯ects the pattern of opaqueness of the object to the radiation. However, the

mechanisms of generating the images are different.

On the one hand, a g radiograph records variations in the linear attenua-

tion of g rays within the object. On the other, the response of the photo-

graphic ®lm to an incident neutron beam re¯ects differences in the ef®ciencies

of scattering and absorption processes. The greater the extent of scattering or

the greater the absorption of neutrons, the less the response on the ®lm.

Materials with a high hydrogen content or with a high neutron absorption

Industrial applications of radioisotopes and radiation218

cross section S

will be seen as regions of high opaqueness on the ®lm. There

are many occasions when gamma and neutron radiography can provide

complementary information.

7.4.3 Neutron diffraction

The French physicist Louis de Broglie postulated, back in 1924, that

according to quantum theory, elementary particles can, in appropriate

circumstances, be diffracted like waves. It can be shown that the wavelength l

of a stream of particles, each of mass m and velocity v, is inversely related to

its momentum p (=mv) with the Planck constant h as the constant of

proportionality:

l = h/p = h/mv, (7.5)

all in consistent units. When applied to neutrons, Eq. (7.5) can be written

l = h/(2m

)

1/2

, (7.6)

where E

is the kinetic energy of the neutrons. For thermal neutrons one has

= 1.67610

727

kg, E

= 0.02561.6610

719

J and, also, h = 6.63610

734

Js.

Substituting into Eq. (7.6) one obtains

l = 1.8 A

, (7.7)

which is of the order of interatomic distances in numerous materials at room

temperature (Hey and Walters, 1987, Ch. 3). Neutron wavelengths, within

the range 0.1 to 20610

710

m, are accessible. Longer wavelengths are

produced by cooling the neutrons, e.g. to liquid helium temperatures (& 4K),

so lowering their kinetic energies and increasing l (Eq. (7.6)). Shorter

wavelength neutrons are generated using hot graphite at up to 2400 K with a

corresponding increase in kinetic energy. With these capabilities, it is possible

to employ neutron diffraction for investigations of the structure of solids with

dimensions ranging from atomic spacings to those characteristic of large

polymers or biological molecules.

Neutron diffraction techniques complement conventional diffraction

methods using X rays. X rays of similar wavelengths to neutrons interact

principally with atomic electrons and therefore preferentially with high Z

number atoms. In contrast, neutrons interact preferentially with low Z

number nuclei. Neutron diffraction techniques are more useful than X rays in

locating the position of hydrogen atoms in samples and are widely employed

for the investigation of the structure of biological materials. The technique

has also been used to study the crystal structures of new magnetic and

7.4 Applications of neutrons 219

semiconducting materials which are being developed for advanced commu-

nications.

7.4.4 Neutron activation analysis (NAA)

An overview

Neutron activation analysis techniques are extensively used in geology,

geochemistry and environmental science for multi elemental analysis. NAA

involves: (a) irradiation of the sample in a research reactor (Sections 1.3.6

and 1.4.1), or with a neutron generator or portable neutron source, (b)

detection and measurement of the level of radioactive isotopes resulting

from the neutron activation of the target material (Section 1.4.3); and (c)

calculation of the concentration of each activated component of the

sample.

The energies of the emitted g rays have to be measured using a high-

resolution g ray spectrometer to distinguish between the often large number

of activated nuclides. The sensitivity of the analysis depends on the neutron

absorption cross section of the isotopes of interest, the neutron ¯ux density

and the effect of interference from other elements in the sample. Minimum

detectable levels for some 90 elements when using NAA in optimum condi-

tions are listed by Charlton (1986, Table 15.2).

Prompt neutron activation analysis

Neutron interactions with numerous elements lead to prompt (n,g) reactions

(Section 7.4.2). The analytical technique based on measuring the energies and

intensities of the emitted g rays is known as prompt gamma ray neutron

activation analysis. Measurements must be made during neutron irradiation.

They cannot therefore be undertaken within the core of a nuclear reactor and

are restricted to portable neutron generators or isotopic sources (Table 7.7).

However, they can be applied when the half lives of the daughters are either

too short or too long for delayed or normal NAA.

PGNAA is widely used in borehole logging (Section 7.4.2) where it is

necessary to monitor a wide range of elements including hydrogen, carbon

and oxygen (IAEA, 1993). Quantitative and qualitative aspects of neutron±

gamma borehole logging have been discussed by Charbucinski et al. and by

Mikesell et al. in IAEA (1990b). Some gamma ray energies listed by the latter

group are presented in Table 7.8 for illustration.

PGNAA is also routinely applied to monitoring the elemental composition

of coal and other materials on conveyor belts (James, 1990). As noted in

Industrial applications of radioisotopes and radiation220