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

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prompt gamma ray neutron activation analysis (Section 7.4.4) with the dual

transmission technology for ash and microwave methods for moisture.

7.2.2 Applications based on gamma ray backscatter

Backscatter gauges

In this section attention will be paid to the utilisation of g ray backscatter. A

number of applications are listed in Table 7.1.

Backscatter gauges are frequently used to monitor the levels of liquids in

tanks when transmission measurements are not practical (Figure 7.8(a)). The

intensity of backscattered radiation registered by the gauge depends primarily

on the bulk density of the material in the tank. It is clearly greater below the

liquid±gaseous interface than above.

The variation of the ef®ciency of single Compton backscatter (Figure

7.8(b)) with the angle of the scattered g ray and with the density of the

material in the tank is shown in Figure 7.8(c). The gauges are designed to

optimise the backscatter angle. The countrate response increases with in-

creasing density of the material and is enhanced by low-energy multiply

scattered radiation.

The correct detection of the boundary between liquids of different densities

depends also on the thickness of the tank wall. The walls attenuate both the

primary beam and the backscattered radiation so reducing the sensitivity of

the measurement. Backscatter measurements using g rays at normally avail-

able energies are limited to vessels with wall thicknesses less than about

30 mm steel equivalent or thinner still when using low g ray energies.

Backscatter techniques employing g rays are not particularly suitable for

locating liquid±liquid interfaces where the bulk densities of the two liquids

are similar. This is different for neutron backscatter gauges (Section 7.4.2)

which respond principally to differences in hydrogen concentrations. They

are usually much more sensitive than g ray detectors in de®ning the interface

between immiscible liquids (Figure 7.8(d)).

Borehole logging using backscattered g rays

A range of nuclear techniques, including g ray backscatter, complement

conventional methods in the comprehensive logging of boreholes (IAEA

1971, 1993). The essentials of a backscatter or g±g gauge, were introduced

above and are illustrated in Figure 7.9(a). As discussed above, the gauge

comprises a source and a detector separated by shielding to absorb directly

transmitted radiation.

7.2 Applications of gamma rays 201

Industrial applications of radioisotopes and radiation202

Figure 7.8: Gamma and neutron backscatter gauges. (a) Application of g and n

backscatter gauges to measurements of liquid levels in tanks. (b) Compton back-

scatter. (c) Variation with angle of the ef®ciency of single Compton backscatter for

241

Am,

137

Cs and

Co gamma rays. (d) Neutron scatter by elastic collisions.

The response of g±g gauges depends primarily on the energy of the incident

radiation. At energies above the range 100 to 300 keV (depending on the

atomic number, Figure 3.7(a)), the Compton mechanism dominates and the

detector response depends on the bulk density of the surrounds. At lower

energies, the photoelectric effect becomes increasingly important and the

more so the higher the atomic number of the surrounding material. Hence,

the intensity of the low-energy component of the backscattered g radiation is

dependent on the effective atomic number Z

eff

, i.e. on the composition of the

surrounds.

In practice, measurements are made of the P

ratio which is given by given

by:

= Intensity of Compton scattered radiation (E

> 300 keV)

Intensity of radiation in the low-energy region of the spectrum.

(7.3)

7.2 Applications of gamma rays 203

Figure 7.9. Borehole logging: (a) g±g and (b) neutron backscatter logging techni-

ques.

The numerator is a measure of the bulk density of the surrounding strata,

independent of the composition, whereas the P

ratio is a measure of Z

eff

(i.e.

the composition), independent of density. Clearly detailed logging of the

geological strata requires a number of the complementary techniques

including neutron scatter which will be introduced in Section 7.4.2.

7.2.3 Applications based on X ray ¯uorescence

Introduction

Introductory information about the nature and properties of ¯uorescent

X rays was offered in Sections 3.9.1 and 3.9.2. X rays emitted from excited

atoms carry away the energy that is released when the atomic electrons move

from outer to inner atomic shells. The emitted energies can be predicted from

the differences in the average binding energies of the electrons in their shells.

Figure 3.16(a) shows a simpli®ed diagram of the electronic shells for the

element nickel (Z = 28).

X ray ¯uorescent spectra are characteristic of excited atoms and are

extensively used in analysing the composition of material for a wide range of

elements. Reference will be made in this section to X ray ¯uorescence (XRF)

analysis and to portable XRF gauges. X ray energies are listed in Table 3.2.

A more comprehensive listing has been posted on the Internet (see Chu et al.

(1999) and Table A3.1, Appendix 3).

X ray ¯uorescence analysis

X ray ¯uorescence analysis (XRF) is a technique for the analysis of a wide

range of elements without the requirement for chemical pre-treatment. The

sample is irradiated with an X ray beam of the required energy and the

characteristic ¯uorescent X rays are detected (Figures 7.10(a) and (b)).

An X ray generator with a monochromator is the usual source since

intensities are several orders of magnitude greater than the alternative radio-

isotope sources. Monochromatic X rays yield increased analytical sensitivity

by minimising the degree of overlap of the ¯uorescent X ray energies of

interest with unwanted peaks from scattered X rays. The ef®ciency of

emission of ¯uorescent radiation from an element reaches a maximum when

the excitation energy is close to, but just above the absorption edge of

interest.

There are basically two methods of detection: energy-dispersive X ray

spectroscopy (EDS) using high-resolution semiconductor detectors or wave-

length-dispersive X ray spectroscopy (WDS). Details are beyond the scope of

Industrial applications of radioisotopes and radiation204

Figure 7.10. X ray ¯uorescence analysis. (a) Schematic representation

of XRF analysis system. (b) Features of an XRF spectrum

(after Jaklevic

et al., 1977). (c) Schematic diagram of the head of a portable XRF analyser

showing the location of the source, the

sample, the balanced ®lter and the scintillation crystal. (d) The linear attenuatio

n coef®cients of the balanced cobalt and nickel ®lters

in the vicinity of the copper K

X ray emission. (e) The pass band, i.e. the change in the detector response

when the balanced cobalt

and the nickel ®lters are interchanged.

Nal(Tl)

this text. Readers are referred to Lachance and Chaisse (1995) for a

comprehensive account of XRF.

Portable X ray ¯uorescence gauges

Portable XRF analysers have been developed for ®eld applications by

replacing the X ray generator and monochromator with radioisotope sources

and X ray ®lter systems. The lower intensity is offset by portability. The

principle is here illustrated by the assay of copper. Two factors will be

considered: (a) the choice of an isotopic source to optimise the emission of the

copper KX rays and (b) the use of balanced ®lters to minimise interference by

unwanted peaks.

(1) Excitation energies: The X ray energy emitted by the isotopic source and

intended to trigger the copper KX rays should be greater than, but as close as

practical to 9.0 keV, the energy of the K absorption edge of copper. A number

of radionuclides emitting X rays or g rays at a suitable energy are commercially

available and are listed in Table 7.4. In this case one can use either

238

1/2

= 87 y), emitting X rays in the range 12 to 17 keV or

109

Cd (T

1/2

= 1.2 y)

emitting X rays in the range 22 to 25 keV.

(2) Balanced ®lters: Although the correct choice of the excitation source will

maximise the yield of the K ¯uorescent X rays, interference from other elements

will also be present. Interfering X ray energies are not easily identi®ed because

portable XRF analysers use NaI(Tl) detectors which are of relatively low energy

resolution. They cannot resolve individual KX rays but only show their

average. Also, the entrance window of the detector must be thin enough to

transmit the required X ray energies with suf®cient intensit y.

A portable X ray ¯uorescence (XRF) analyser is shown schematically in

Figure 7.10(c). In this design, pairs of balanced ®lters are used to minimise

the effects of extraneous emissions. When copper is monitored, cobalt and

nickel ®lters are used.

The measurement principle is illustrated in Figure 7.10(d) which shows the

attenuation for cobalt and nickel ®lters as a function of energy near the

copper KX ray emission. The attenuation factors have been normalised by

allowing for differences in the thicknesses of the ®lters. The K absorption

edge for cobalt (7.71 keV) is less than, and the K absorption edge for nickel

(8.33 keV) is greater than that of the average energy of the ¯uorescent copper

KX ray (8.05 keV).

The measurement involves recording the difference in countrates when one

®lter is replaced by the other. This de®nes a narrow pass band (7.71 to 8.33

keV) which includes the copper KX ray energy (Figure 7.10(e)). The relative

thicknesses of the two ®lters are adjusted so that the attenuation on either

206 Industrial applications of radioisotopes and radiation

side of the pass band is balanced. By using the balanced ®lters, a good

measurement of the level of copper is obtained in spite of potential inter-

ference from other elements in the sample which would not be resolved by the

sodium iodide detector. A list of balanced ®lters of elements frequently used

for KX ray detection is reported by Charlton (1986, Table 14.2).

Applications to the mineral processing industry

Reference was made above to the role of nucleonic density gauges in the real

time analyses of the levels of valuable minerals in process streams. The

identi®cation of these levels is often made using radioisotope X ray ¯uores-

cence techniques. Since the complex minerals processing stream is the target,

a complex spectrum of ¯uorescent X rays is produced.

In many industrial environments, the challenge is to monitor the desired

economically important elements using ruggedised systems based on sodium

iodide detectors which are of limited resolution. An example of such an

approach is the use of balanced ®lters described above. A range of more

elaborate techniques has been developed for applications in the minerals

industry (see Watt, 1972, 1973).

7.3 Scienti®c and industrial applications of beta particles and electrons

7.3.1 Attenuation of beams of beta particles and electrons

Whereas g rays are uncharged electromagnetic radiations, b radiations are

charged particles interacting with matter primarily through Coulomb interac-

tion with the outer electrons of its atoms (Section 3.3.1). The range, t cm, of a

beam of mono-energetic electrons depends principally on the energy of the

electrons and the density, r g/cm

, of the material in which they move. It was

shown in Section 3.3.5 that the product t6r g/cm

known as surface density,

has the useful property of being only weakly dependent on the Z number of

the absorbing element. With b particles having short ranges in solids (Figure

3.2(b)), their attenuation is used for the measurement of the thickness of ®lms

in the paper, plastics and rubber industries. Comprehensive listings of the

ranges of electrons have been published in regular reports (Berger and

Seltzer, 1964) and on the Internet (Berger, 1999).

Frequently used commercially available b particle sources include

1/2

= 10.73 y, E

(max) 672 keV) and

147

Pm (T

1/2

2.62 y, E

(max) 225 keV)

(Table 8.2). The detectors are saturation ionisation chambers ®lled with

argon at two or more atmospheres pressure to increase their ef®ciency

(Section 6.3.4). Ionisation chambers are used because they can cope with

7.3 Applications of beta particles and electrons 207

high particle ¯uxes without the need for dead time corrections (Section

4.5.2).

Applications in paper manufacture

Nucleonic gauges using such sources are ®tted routinely to new paper

manufacturing plants and are often retro®tted to older units. They may be

used to continuously monitor the thickness of paper at speeds up to 400 m/s.

Optimum ranges depend on the source chosen (0.03 to 1 g/cm

with

Kr, and

0.015 to 0.175 g/cm

with

147

Pm).

On-line processing of the output data from the detectors allows the

collection and display of short-term trends and overall average variations in

the thickness of the paper. The output data may be used to control the speed

and inclination of the rollers to ensure that the paper thickness always

remains within tolerance. Using microwave or infrared techniques, the

moisture level is monitored to about 0.2% and controlled by automatically

varying the amount of evaporative heating. Through the use of these control

systems, the paper is manufactured within tighter tolerances than would

otherwise be possible so gaining a higher quality product and a reduction in

the use of materials and energy.

7.3.2 Industrial applications of beta particle backscatter

Assuming saturation thickness (Section 3.3.3), the ef®ciency of the back-

scatter of b particles increases with the energy of the incident radiation and

with the electron density i.e. the effective Z number of the backscattering

material (Figure 3.5).

While b particle transmission techniques are extensively used in industry

to measure the thickness of paper and other ®lms, b particle backscatter

methods are well suited to measurements of the thickness of thin coatings

on substrates that are thick enough for saturation backscatter (Section

3.3.3). The radiation source in a backscatter gauge is a pure b emitting

isotope with an energy chosen to suit the application. Examples include

Ni,

147

Pm and

204

Tl (Table 8.2). A practical gauge incorporates a micro-

processor to convert the reading into a meaningful measure of coating

thickness.

Since the ef®ciency of backscatter from a material is proportional to its

electron density, the best de®ned applications are those in which a material of

high atomic number is laid on one of preferably much lower atomic number

or vice versa (Charlton, 1986, Ch. 14). Applications include the monitoring

of the thickness of electroplated gold and the thickness of plastic coatings on

Industrial applications of radioisotopes and radiation208

metals (see Table 7.5); suitable sources for these examples are

204

Tl and

respectively.

7.3.3 Special applications: electron microscopy

Knowledge of the relationships between the properties, structure and com-

patibility of materials under a wide range of conditions underpins many of

the processing, manufacturing and heavy engineering sectors of industry.

Electron microscopy, which involves the application of many of the scienti®c

principles discussed in this book, has been used to advance knowledge in

these areas. In simple terms, an electron microscope comprises a source of

electrons and a series of electro-magnets, which perform functions similar to

that of lenses in optical microscopes.

The fundamental difference between optical and electron microscopes is

the mechanism of formation of the contrasts resulting in the image. In light

optics, images result from differences in absorption of the light illuminating

the object; in electron microscopy they arise from the complex processes of

electron scattering and diffraction.

In both types of instruments, the Rayleigh criterion requires that the limit

of resolution, d, is a function of the incident wavelength given by d =kl/a

where k is a constant, l is the wavelength of the incident radiations and a is

the aperture of the microscope, all in consistent units. For optical micro-

scopes, the resolution lies in the range 0.2 and 0.5 mm (2000 to 5000 A

leading to a maximum magni®cation of around 10006.

The wavelengths of electrons decrease as their energies increase i.e. with

increasing accelerating voltage. Electron wavelengths are very short, only

0.09 A

at 20 keV and 0.025 A

at 200 keV, and so substantially less than the

average interatomic spacings which are about 2 A

. With modern high-

resolution techniques, well maintained and properly aligned transmission

electron microscopes can resolve at the atomic scale. However, due to a

number of instrumental effects, the resolution for scanning electron micro-

scopes is limited to between 50 and 200 A

, which corresponds to magni®ca-

tions of about 100 0006.

The interaction of higher energy electrons (> 5 keV) with the atoms of the

specimen leads to the generation of X rays. These comprise both bremsstrah-

lung (Section 3.8.1) and ¯uorescent X rays (Section 3.9.1). The latter are used

to map the distribution of elements in the sample. In addition, structural

information may be obtained through the analysis of electron diffraction

patterns. Readers seeking further information are referred to works by

Hunter et al. (1993), Williams and Carter (1996) and Watt (1997).

7.3 Applications of beta particles and electrons 209

Table 7.5.

Industrial applications of beta particles and electrons.

Property of the Application

Example

Reference

Radiation

Transmission

Thickness measurements Thickness control in paper manufacturing industry

based on the attenuation of transmitted

Kr b

Section 7.3.1

particles

Backscatter

Thickness measurements

Section 7.3.2

of coatings

Absorption

 Chemical processing; Irradiation by electron beams or the derived

 Sterilisation;

bremsstrahlung is an alternative to gamma ray

 Radiation curing. treatment. Electron beam irradiation

complements Section 7.6

UV as a curing agent in the printing and coating

industries

Special applications Electron microscopy Investigation of

materials characteristics, composition

and structure

Section 7.3.3

Note: The properties of beta particles are discussed in Section 3.3 and the

decay data for sealed beta particle sources are presented in

Table 8.2.