GB2055198A - Detection of concealed materials - Google Patents

Abstract

A hand portable probe 11 contains a radioactive source capsule 16 of low energy gamma rays which penetrate into an object against which the probe is positioned. Backscattered gamma radiation is detected by an annular scintillation crystal 14 light guide 17 and photomultipler 18 or by an annular array of semiconductor detectors (Figs. 4 & 5) positioned to be in contact with or very close to the surface under examination. Materials of low atomic number exhibit low photoelectric absorption and can be detected even when concealed behind steel. <IMAGE>

Description

SPECIFICATION Detection of concealed materials The invention relates to the inspection and analysis of materials within an object, in particular for the detection of concealed materials.

in the inspection and analysis of materials it is known to use various techniques of irradiation with ionising radiation and detection of the effects of scattering or absorption on the radiation or the radiation products of nuclear reactions between the incident radiation and the material under examination.

For example, there is described in Adventures in Experimental Physics 1 975 in an-article by A. Turkevitch et al an alpha scattering analysis instrument used in the NASA Surveyor missions for the first chemical analysis of the lunar surface.

The present invention is concerned with a device for providing an indication of the nature of materials within an object, and especially of materials concealed behind an opaque surface.

For this a source of penetrating radiation, that is X-rays y rays or neutrons, is proposed, together with a detector which is sensitive to backscattered radiation.

It is desired that the device should be portable and for this a radioisotope radiation source is convenient as it requires no power supply and may be contained in a small, rugged capsule. However, it is clear that for such a device a source of as low intensity as possible should be employed and the intensity must, of course, be within the limits imposed by the Radioactive Substances Act 1 960 and the lonising Radiations (Sealed Sources) Regulations 1969.

These desirable features and requirements and restrictions present several problems. Thus, to penetrate into the object under investigation with the maximum radiation intensity, the source should be as close as possible to the surface of the object.

The radiation detector has to intercept the maximum possible radiation backscattered from within the object and the minimum possible radiation either direct from the source or backscattered from any intervening opaque surface behind which the object may be concealed. Some compromise has to be accepted in meeting these requirements, because the maximum backscattered radiation is encountered close to the source and to the surface of the object under inspection, where direct radiation is also at a maximum. It is an object of the present invention to provide a solution to these problems.

The invention provides a device for indicating the nature of materials within an object, which device comprises a housing having a side wall adapted to be placed against an object to be inspected, a radioisotope source of penetrating radiation mounted within the housing close to but spaced from the said side wall, a scintillation or solid state radiation detector located closer to the said side wall than to the source and a collimator between the source and the detector to shield the detector from direct radiation from the source whilst permitting radiation from the source out of the housing transversely of the said side wall, detection and analysis of materials within the object being effected by measurement of radiation backscattered therefrom and detected inAhe radiation detector.

Preferably the radiation detector surface is as close as practicable to the inside surface of the said side wall.

Preferably a radiation detector array is located adjacent the said side wall in an annular configuration surrounding the radioisotope source.

Preferably the housing is constructed in the form of a hand portable probe.

A specific construction of device embodying the invention will now be described by way of example and with reference to the drawings filed herewith, in which: Figure 1 is a diagrammatic sectional view of part of the device on the line 1-1 of Figure 2; Figure 2 is a diagrammatic side sectional view of part of the device; Figure 3 is a diagrammatic side sectional view of another device; Figures 4 and 5 are respectively a diagrammatic sectional view on the line 4--4 of Figure 5, and a diagrammatic side sectional view of part of another device; Figure 6 is a side sectional view similar to Figure 2 of a development of the device shown in Figure 2; and Figure 7 is a block diagram of an electronic signal processing system to which any of the devices may be coupled.

Referring to Figures 1 and 2, the device comprises a cylindrical end housing 11 with a cylindrical side arm 12 attached thereto. The side arm, only part of which is shown in the drawings, provides a convenient handle for holding the device. Close to a side wall 1 3 of the housing 11 is an annular scintillation crystal 14. Within the central circular aperture of the scintillation crystal 14 is a short cylindrical collimator 1 5 in which, at the end remote from the side wa 13, is mounted a capsule 1 6 containing a radioactive material source which in this example is of y radiation. The capsule 1 6 has a tungsten alloy backing which shields against radiation emanating from the source in the direction away from the side wall 1 3.

A light guide 1 7 provides an optical path for directing the light from scintillations in the crystal 1 4 onto a photomultiplier indicated diagrammatically at 18.

For detection and indication of backscattered radiation, any suitable conventional equipment may be used and is conveniently mounted in the side arm 1 2. However, the device of this example is particularly intended as a search device for detecting low atomic number materials, especially organic materials, concealed behind surfaces such as the metal skin of motor vehicles. For this, indication of a step change in backscattered intensity is particularly suitable and this is achieved by providing two rate meters to both of which is fed the backscatter count. The ratemeters have different rates of response to change in count rate. Under steady count rate conditions, their outputs will settle into balance and a detector subtracting one output from the other will give a zero indication.If the backscatter count rate changes, a difference between the ratemeter outputs appears, owing to their different rates of response to the change, this difference dying away with time. Differences thus detected are used, in this example, to change the pitch of an audible indicator tone.

The radioactive source and detector system are chosen according to the particular nature of investigation to be carried out by the device.

Thus, for search for low atomic number materials low energy y radiation is chosen.

Although the scattering cross-section in this case is substantially independent of atomic number, the photelectric absorption rises steeply with increasing atomic number. The net effect is therefore that more y-radiation is backscattered from low atomic number material than from high atomic number material.

The energy of the y-radiation is chosen to provide penetration of the thickness of the concealing surface expected to be met in the search. Thus Co57 provides y rays of energy 120 KeV which is adequate to penetrate steel up to -sith inch (3 mm) thick. Cs137 provides y rays of energy 662 KeV which is adequate to penetrate steel up to - inch (6 mm) thick. Co60 provides y rays at energies of 1.17 MeV and 1.33 MeV which can penetrate steel up to 2 inch (12 mm) thick. It will be appreciated that the higher the y radiation energy the greater will have to be the weight of collimator.

At low energies y radiation is backscattered with very nearly the same energy as the incident radiation. It is therefore important that the detector is screened from receiving radiation direct from- the source and that a minimum of radiation backscattered from the surface against which the device is placed should reach the detector.

The configuration of the device of this example meets these requirements particularly well. The arrangement enables the detector surface to be positioned as close as possible to actual contact with the surface under examination. This, together with the collimator configuration and recessed source, is effective to minimise incidence on the detector of direct radiation from the source and radiation scattered from the surface. An important characteristic in this respect is that, whilst both source and detector are ideally positioned as close as possible to the surface under examination, for practical reasons the detector has to be closer to the surface than the source.The annular configuration of the detector, with centrally located source, provides efficient interception of backscattered radiation such that, in the example using Co57 source, a 3 millicurie source provides adequate intensity for a typical backscatter count rate of 10,000 counts per second. In this example a 1% change in count rate is the limit of sensitivity, this corresponding, by way of example, to the change in count rate produced by a package of about 25 grams of Heroin concealed behind -sith inch (3 mm) thick steel.

Figure 3 shows a modification of the device in which the source capsule 1 6a and collimator 1 spa are embedded in a cylindrical block of plastic or crystal scintillator 1 4a. This arrangement avoids the need for a separate light guide. The photomultiplier 18a mounted in a side arm may be positioned in any convenient orientation with respect to the scintillator 1 4a. It will be noted that the collimator 1 5a extends across the back of the space for the source capsule 1 6a so astro provide a firm base support for the capsule 1 6a.

Figures 4 and 5 illustrate a further modified device, in which source capsule 1 6b and collimator 1 sub are surrounded by an annular array of circular semi-conductor detectors 21.

Figure 6 shows a development of the device of Figure 2. Similar components are referenced with the same reference numerals distinguished by suffix "c". The light guide 1 7c is of perspex and couples to a window 31 of a photomultiplier (not shown in Figure 6). The space between aluminium housing 1 C and the combined light guide 1 7c and annular Na I (T1) scintillation crystal 1 4c is filled with magnesium oxide powder 32 to reflect light from the scintillation crystal 1 4c back into the light guide 1 7c.A thin layer of magnesium oxide is included between the scintillation crystal 1 4c and the side wall 1 3c, the consequent disadvantage of a small displacement away from optimum closeness of the crystal 1 4c to the side wall 1 3c being well worthwhile when weighed against the benefit of reflection back into the light guide 1 7c of light emitted from the crystal 1 4c towards the side wall 1 3c.

In the example of Figure 6 collimation at 1 4c is provided by tungsten alloy material.

Figure 7 shows the electronic signal processing arrantKement, which is a modification of that described ab < , for providing an audible and/or visual indication when the device detects the presence of low atomic number material concealed behind a thin layer of a relatively high atomic number material such as steel.

In Figure 7 an indication of the electronic signal output waveform is shown adjacent each component.

The signal from the detector is amplified by amplifier 41 and fed to pulse shaper 42 and ratemeter 43 in a manner conventional in the art of processing electrical signals indicative of nuclear events. The output of ratemeter 43 is a signal level indicative of a rate of receipt of pulses from the pulse shaper 42. The signal level output from the ratemeter 43 only changes when the pulse rate changes.

Such changes in pulse rate are indicated by differentiator 44 which provides a negative-going spike, such as 47, whenever the ratemeter level increases.

A fall in ratemeter level will, of course, produce a positive going spike from the differentiator 44, but this is not used in the detection circuit and is cut-off as shown at 48 to reduce as far as practicable the recovery time of the differentiator 44.

Negative going spikes, such as 47, are tested by adjustable discriminator 45 against a trip level 49 and indicator 46 is triggered whenever the 'change irr the pulse rate exceeds a predetermined amount. This predetermined amount is adjustable by changing the setting of the trip level 49 of the adjustable discriminator 45. It is particularly convenient for the iridicator 46 to be an audible alarm, but visual indication as an alternative or in addition can readily be provided.

It will be appreciated that, in the arrangement illustrated in Figure 7, the differentiator 44 performs the same function as the twin ratemeter arrangement described above.

The invention is not restricted to the details of the foregoing examples. For instance, instead of y radiation X-rays or neutrons may be employed with appropriate adaptation of the source and detector system. The choice of radiation is, of course, influenced by the nature of the analysis to be carried out, the radiation giving the most appropriate back scattering effect for the material to be examined being selected from the extensive published data on nuclear interactions and their cross sections.

Claims (10)

1. A device for indicating the nature of materials within an object, which device comprises a housing having a side wall adapted to be placed against an object to be inspected, a radioisotope source of penetrating radiation mounted within the housing close to but spaced from the said side wall, a scintillation or solid state radiation detector located closer to the said side wall than to the source and a collimator between the source and the detector to shield the detector from direct radiation from the source whilst permitting radiation from the source out of the housing transversely of the said side wall, detection and analysis of materials within the object being effected by measurement of radiation backscattered therefrom and detected in the radiation detector.

2. A device as claimed in Claim 1, wherein the radiation detector surface is as close as practicable to the inside surface of the said side wall.

3. A device as claimed in Claim 1 or Claim 2, wherein a radiation detector array is located adjacent the said side wall in an annular configuration surroundirtg the radioisotope source.

4. A device as claimed in any one of the preceding claims, wherein the housing is constructed in the form of a hand portable probe.

5. A device as claimed in any one of the preceding claims, wherein the radiation detector comprises an annular scintillation crystal coupled via a light guide to a photoelectric detector.

6. A device as claimed in Claim 5, wherein the light guide and scintillation crystal are surrounded by magnesium oxide powder.

7. A device as claimed in Claim 6, wherein a thin layer of the magnesium oxide powder extends between the scintillation crystal and the said side wall of the housing.

8. A device substantially as hereinbefore described with reference to, and illustrated in, Figures 1 and 2, or Figure 3, or Figures 4 and 5, of the drawings filed herewith.

9. A device substantially as hereinbefore described with reference to, and illustrated in, Figure 6 of the drawings filed herewith.

10. A device substantially as hereinbefore described with reference to, and illustrated in, any of Figures 1 to 6 together with Figure 7 of the drawings filed herewith.