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

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Figure 7.3. Four applications of gamma ray attenuation. (a) The

monitoring of slurry in a pipeline. (b) Investigation of scale

deposits in pipelines. (c) Monitoring of density of grout during the

construction of, for example, offshore platforms (ICI Synetix

Tracerco, product information). (d) The

in situ measurement of the density of mineral slurries (Cutmore

et al., 1993).

the density of concrete grout used in the construction of piles supporting

offshore oil platforms is within speci®cation. Grout is pumped under pressure

into the pile and continuously ®lls the space between the radioactive source

and the detector. The density of the grout is directly related to the response of

the detector which is monitored by personnel on the surface. Such a system is

shown schematically in Figure 7.3(c).

The second application is known as the Gammascan

Flooded Member

Detector System. The principle is simple and robust. Flooding of a sub-sea

member of a platform with water will reduce the transmission of the gamma

beam between the source and the detector. The system is normally mounted

on the arms of an ROV and is capable of rapidly monitoring both vertical

and horizontal members.

Other applications of gamma ray transmission are discussed in the

following paragraphs.

Mineral processing

Density gauges are widely used in the minerals processing industry. The real

time analyses of the levels of valuable minerals in process streams generally

employ radioisotope X ray ¯uorescence or g ray preferential absorption

techniques. As the responses of these detectors depend not only on the

composition of the mineral component, but also on the bulk density of the

aqueous slurry, nucleonic density gauges are an essential element of radio-

isotope on-stream analysis systems. The gauge illustrated here (Figure 7.3(d))

is designed for immersion in the mineral slurry.

Coastal engineering

Nucleonic gauges have been adapted for the measurement of the levels of

sediment in rivers and estuaries. Quantitative information on the mobilisation

of sand and sediment under various conditions is important in the investiga-

tion phase of many coastal engineering projects. Either absorption or back-

scatter gauges may be used for this purpose. They may be linked with other

gauges measuring, for instance, depth, salinity and temperature. This infor-

mation, together with position ®xing data from satellite navigation systems,

is integrated into computerised monitoring systems. Further comments are

made in Section 9.3.5.

Radiography

Gamma radiography has long been one of the most important industrial

applications of radioisotopes. The technology evolved out of the widespread

use of X rays in medical imaging and has been adapted to monitoring the

Industrial applications of radioisotopes and radiation192

internal structure of manufactured components and to checking the integrity

of welds.

The scienti®c principles of g radiography are illustrated in Figure 7.4.

Photographic X ray ®lm housed in a suitable cassette holder serves to detect

the transmitted radiation from the radioisotope source. If an inclusion in the

sample under investigation has a higher electron density than the surrounding

material, fewer g rays are transmitted and a shadow image is produced on the

®lm.

A wide range of radiography sources is commercially available. A selection

is listed in Table 7.4. Factors determining the choice of the source are similar

to those for density gauges as discussed above. Particular attention must be

paid to the gamma ray energy. If the g ray transmission is too high, the image

contrast is inadequate. If the transmission is too low, there could be problems

due to a low detector countrate. About 50% transmission is close to

optimum. In practice,

Co,

137

Cs or

192

Ir sources are commonly used.

Computerised tomography (CT)

Computerised tomography (CT) is a sophisticated extension of radiography

in which the detailed internal structure of a component can be obtained in

two or three dimensions by an analysis of the attenuation of the X or g ray

beam in a large number of projections. Cormack and Houns®eld indepen-

dently demonstrated the technique during the early 1960s (Bull, 1981) and

were jointly awarded the Nobel Prize for Medicine and Physiology in 1979.

However, it was not until the coming of suf®ciently powerful computers

during the early 1980s that tomography could be used routinely, ®rst in

medical imaging (Romans, 1995) and later in industry (Section 3.7.2).

When designing a CT instrument, a number of parameters need to be

optimised. They include the energy of the incident beam, the data acquisition

time and the resolution of the image. The criterion for energy selection is

identical to that for density gauges. Data acquisition times should generally

be as short as practicable to permit maximum utilisation of expensive

equipment. Image resolution should be high, but there are intrinsic limits

depending on the wavelengths (energies) of the radiation. Comparison of

computed tomography for medical diagnosis and industrial inspection is

discussed by Martz et al. (1990).

As with nucleonic density gauges and radiography, the principles of

tomography are ultimately based on Eq. (3.7). This equation was derived to

calculate the attenuation of g rays in material made up of components

differing in their linear attenuation coef®cients m

and thicknesses Dt

. The

sum

can be expressed as the line integral

m dl. This is known as the

1937.2 Applications of gamma rays

Figure 7.4. The principles of gamma radiography (after Charlton, 1986,

Ch. 13).

Table 7.4.

Radioisotopes used as X or

g ray sources. (Based on Charlton, 1986, Table 14.1. For more details, see

Table 3.2 of this book.)

Isotope

Half life Energy (keV) Applications and comments

2.72 y 5.9 KX ray

Low energy X ray analysis. KX rays usefully excited Al to Cr

238

87.8 y 12±17 LX ray XRF applications. KX

rays excited; Mn to Y

109

463 d 22 KX ray

Detector calibration and XRF. KX rays usefully excited Fe to Mo

210

22.2 y 47 KX ray

Used as an energy standard and for XRF

241

432 y 59.5

g ray

Used for continuous gauging applications e.g. ash in coal measure-

ments (Section 7.2.1);

Used with target elements (e.g. copper or silver) to generate

sources of (Section 7.2.3) pure ¯uorescent X rays

XRF applications. KX rays usefully excited I to Lu

170

127 d 84 KX ray

Low-energy radiography

272 d 122 and 136

g rays Mo

ssbauer spectroscopy

H)/Zirconium 12.3 y 5 to 9

Bremsstrahlung sources. The X rays are generated as a conse-

quence

147

Pm/Aluminium 2.6 y

12 to 45

of the deceleration of the negatrons (

particles) emitted by the

Sr/Aluminium 28 y

60 to 150

source (Section 3.8.1)

Radon integral after the mathematician who, in 1917, laid the foundation for

later work on the mathematical reconstruction of tomographic images.

Tomographic systems have gone through several stages or generations of

development. The principle is here illustrated using a ®rst generation system

shown in Figure 7.5 (Zatz et al., 1981). Repeated measurements of the

attenuation of a highly collimated beam of g or X rays are made as the

source±detector system traverses the sample. Similar data are then obtained at

a number of projections by rotating the sample over a range of at least 1808.

This information is accumulated by the computer which uses it to reconstruct

a two or three dimensional image of the internal features of the sample.

These ®rst generation systems produced excellent images free of artefacts.

However, the systems were slow with data acquisition times sometimes

upwards of several hours. More advanced and faster systems were designed

to reduce imaging time by using an incident fan beam arrangement of X or g

rays and multiple detectors. Current instruments are based on those designed

for hospital use. In a fully developed industrial system, more than a million

measurements over 180

are used to develop a single CT image. The

measurements can be made and the image reconstructed and displayed on a

computer screen in about a minute. CT imaging has been applied to the study

of machine components and to the condition of castings. It has been

demonstrated that the CT-based measurements of the dimensions of complex

castings are as accurate as those obtained by conventional means. However,

for very complex or accurate work, imaging times ranging from a few

minutes to a few hours may still be needed.

The size of the component that can be inspected depends on the energy of

the incident beam. At one extreme, X ray sources from linear accelerators up

to 15 MV in energy are being used commercially for a range of applications,

including the examination of rocket motors.

At the other extreme, high-resolution micro-tomographic systems can

provide information complementary to microscopy. Detailed information

can be obtained on the internal structure of materials (such as wood and

polymers) which would be destroyed during normal sample preparation.

Using a low-energy X ray source, images with resolution of 5 mm have been

obtained using small samples of wood (2 mm across). With this resolution,

the cellular structure of the sample can be readily seen (Wells et al., 1992).

High-speed systems are being developed in the steel industry for the real

time gauging and control of products that are manufactured in a continuous

process. This requires very fast computing capability able to generate output

each 1 to 10 ms. Such systems have been demonstrated in the production of

pipes and a range of structural materials with complex cross sections.

Industrial applications of radioisotopes and radiation196

7.2 Applications of gamma rays 197

Figure 7.5. The scanning, rotation con®guration for the ®rst generation CT

scanner (modi®ed from Zatz, 1981).

Finally, on-line applications of tomography to the milling of timber are

also being developed. It is now possible to tomograph an uncut log and

mathematically reconstruct the grain patterns on veneers sawn at any angle.

When fully developed, it will be possible to ensure that each log is milled in a

way that optimises the use of the valuable resource.

Column scanning

The scanning of large distillation columns in the oil re®ning industry is a long

established example of the application of g ray density pro®ling. The columns

are typically tens of metres high and two to three metres in diameter and are

designed to separate the lighter petroleum fractions from the crude feed

stock. They comprise an interconnected series of trays spaced at intervals of

about one metre. Columns operate continuously and should require a

minimum of maintenance.

Failure to meet the design speci®cation of a column arises from a number

of causes, notably the collapse of individual trays, the development of

blockages leading to the ¯ooding of trays and the formation of foams in the

vapour phases. A number of these features are illustrated schematically in

Figure 7.6. Many of these problems can be diagnosed using g ray transmis-

sion techniques with no interference to plant operation.

The radioactive source and the detector are lowered in parallel on opposite

sides of the column to develop a scan such as illustrated in the ®gure.

However, a full understanding of the results of such scans in terms of column

malfunction requires extensive experience. Many thousands of scans are

performed annually, the vast majority by international service companies.

Other applications of radiotracers in the oil re®ning industry will be discussed

in Section 8.4.2.

On-line measurement of ash in coal

(1) Dual isotopes applications: The applications described in the previous

sections involved monitoring the density and the distribution of material

between the radiation source and the detector. However, there is often a need

for additional information. For instance, for the optimum performance of

coal ®red boilers in the electricity generating industry, it is desirable that the

feedstock be blended to a constant calori®c value. Although the total ash and

moisture contents must be known, the detailed elemental composition of the

ash may be less important.

A widely used gauge employs two g ray emitting radionuclides,

137

Cs (662

keV) and

241

Am (59.5 keV) as shown schematically in Figure 7.7. Attenuation

of the 662 keV g rays from

137

Cs is due mainly to Compton scatter. As shown

Industrial applications of radioisotopes and radiation198

earlier (Section 3.4.6), Compton attenuation at that energy is almost indepen-

dent of the composition of the material, so that the 662 keV g rays can be

used to monitor the mass of coal on the conveyor belt as it passes through the

radiation beam.

By contrast, the attenuation of the 59.5 keV g rays from

241

Am is due far

more to photoelectric than to Compton interactions (Figure 3.7(a)) and

hence increases rapidly with increasing concentrations of elements of higher

atomic number such as iron and silicon (Eq. (3.3)). Coal heaped loosely on to

conveyor belts is of fairly low density, permitting the transmission of some of

the 59.5 keV g rays through relatively thick layers. However, it may require

241

Am activities of order 10 to 20 GBq to ensure that the detected signal is

strong enough for suf®ciently precise measurements.

The percentage of ash in coal is calculated from a computer analysis of the

transmission data of both the

137

Cs and the

241

Am g rays. The overall

uncertainty in the determination is normally in the range of 0.7 to 1.5 weight

per cent (Cutmore et al., 1993). Because the information is obtained in real

7.2 Applications of gamma rays 199

Figure 7.6. Gamma transmission scanning of a distillation column illustrating

identi®able features (after Charlton, 1986, Ch. 13).

time, the outputs from a series of gauges installed on different conveyor belts

can be used to optimise the blend of coal fed into the power station furnaces.

(2) Moisture in coke and coal, hybrid technology: The on-line monitoring of

the calori®c value of coke and coal requires knowledge of the moisture

content. Gamma transmission or backscatter gauges cannot be used to

distinguish between the hydrogen in moisture and that associated with the

organic component of the coal. One approach to overcome this dif®culty has

been to develop a gauge based on a combination of microwave phase shift

and g ray transmission techniques (Cutmore et al., 1991).

(3) Multi element analysis of coal: A still higher level of sophistication

makes use of an elemental on-line analysis of the coal. Gauges have been

developed commercially and are available from nuclear equipment manufac-

turers (Section 4.3.3) which can determine ash, moisture, density as well as

the levels of key elements such as sulphur. The operating principles combine

Industrial applications of radioisotopes and radiation200

Figure 7.7. The `on-belt' analysis of ash in coal using the dual-energy gamma

transmission technique (after Cutmore et al., 1993).