GB2232482A

GB2232482A - Scintillator

Info

Publication number: GB2232482A
Application number: GB8912130A
Authority: GB
Inventors: John Myatt
Original assignee: British Nuclear Fuels PLC; UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority; Sellafield Ltd
Priority date: 1989-05-26
Filing date: 1989-05-26
Publication date: 1990-12-12
Also published as: GB8912130D0

Abstract

A scintillator (10) particularly suitable for detecting alpha emissions within a fluid consists of a large number of thin parallel spaced-apart sheets (16) of a scintillation plastic, coupled optically at one edge to a photomultiplier tube (12). For use in a liquid the sheets (16) may have metallized surfaces to enhance internal reflection of light. Two such scintillators may face each other, with the sheets of scintillation plastic interdigitated. <IMAGE>

Description

Scintillator The invention relates to a scintillator for detecting radiation.

It is well known that certain materials such as crystals of thallium-doped sodium iodide, or of anthracene, emit light when hit hy alpha, beta or gamma radiation.

This light can be sensed by a photomultiplier, and so the radiation monitored.

According to the present invention there is provided a scintillator comprising a photomultiplier tube, and a multiplicity of thin sheets of scintillation material arranged parallel to but spaced apart from each other, one edge of each sheet being optically coupled to the photomultiplier.

The sheets are preferably of a plastics scintillation material, and are desirably between 0.5 and 10 mm thick, preferably between 1 and 2 mm thick. The sheets are preferably spaced apart by gaps wider than the thickness of the sheets, desirably at least 1 mm wider. The plates must be thick enough to be effectively rigid and hence not sag in use, but this will depend on their orientation. Where the sheets will be in a vertical plane coupled to the photomultiplier at their top edges then they may be only 1 mm thick.

The scintillator is particularly suitable for detecting alpha particles emitted by radioactive nuclides in a fluid, as a large surface area of scintillation material is exposed to the fluid, and light due to irradiation by alpha particles at any part of any sheet will be sensed by the photomultiplier. If the fluid is a liquid rather than a gas it is preferable to metallize the surface of the sheet to reduce light loss on reflection from the surfaces.

For distinguishing alpha particles from gamma rays it is desirable to arrange two such scintillators next to each other with the two sets of sheets interdigitated, and with the two photomultipliers connected to an anti-coincidence detector. Alpha particles, being of short range, will affect only one sheet of one set and so will be detected, whereas gamma rays will affect both sets and so will be cancelled out.

The invention will now be further described by way of example only and with reference to the accompanying drawing which shows a longitudinal sectional view through a scintillator 10. The scintillator 10 includes a photomultiplier tube 12, whose window end 13 is circular and of diameter 75 mm. The photomultiplier tube 12 is connected electrically to a power supply and pulse detection unit 8 by a cable 9. A polymethylmethacrylate (perspex) coupling block 14 is glued to the window end 13 of the tube 12; the coupling block 14 is of circular cross-section and diameter 75 mm at the end attached to the photomultiplier tube 13, and is of square cross-section of side 75 mm at the opposite end.Several square sheets 16 of scintillation plastic NE 102 (from NE Technology, Beenham, Reading, RG7 5PR, England) each of side 75 mm and 1 mm thick are glued along one edge to the square end of the coupling block 14, so the sheets 16 extend parallel to each other and are equally spaced apart. The actual number of sheets 16 depends on their thickness and their spacing; there might be nineteen sheets 16 each of width 1 mm with gaps 3 mm between them, the outermost sheets 16 being fixed 1 mm in from the edge of the block 14 (only nine sheets 16 are shown in the drawing).

In operation the scintillator 10 is arranged so that a gas which may contain a small amount of an alpha-emitting nuclide occupies the gaps between the sheets 16. Any alpha particle incident on a sheet 16 will cause the emission of light within the sheet 16 and the light will propagate along within the sheet 16 (being reflected by the surfaces of the sheet 16) and so into the coupling block 14 and hence into the photomultiplier 12. Hence a corresponding electrical pulse will be detected by the unit 8. The concentration of the radioactive nuclide in the gas can therefore be assessed.

Where it is desired to use the scintillator 10 to detect alpha emissions from within a liquid it is preferable to metallize the surfaces of the sheets 16 to maximize internal reflection of light within the sheets 16 when the sheets 16 are immersed in the liquid. The scintillator 10 then operates as described above.

It will be appreciated that the scintillator 10 can be modified in various ways while remaining within the scope of the invention. For example the sheets 16 may be joined together by an end plate (not shown) at the opposite end to the coupling block 14; the coupling block 14, the two outermost sheets 16 and the end plate then define a square duct across which the other sheets 16 extend, and this duct may form part of a pipe through which a fluid is to flow.

For another example, the coupling block 14 might be replaced by a cylindrical coupling block (not shown) of uniform diameter 75 mm, and the sheets 16 be of constant length but of different widths so that in cross-section their outer edges lie on a circle of diameter 75 mm; indeed such a set of plates 16 might be glued directly onto the window end 13 of the photomultiplier tube 12 and the coupling block be dispensed with.

It will also be appreciated that alpha, beta and gamma radiation will all be detected by the scintillator 10. As mentioned earlier, where the scintillator 10 is exposed to a significant background gamma flux, alpha particles can readily be distinguished from gamma rays by arranging two such scintillators 10 facing each other with the two sets of sheets 16 interdigitated, and with the photomultipliers 12 connected to an anti-coincidence circuit. Gamma rays being of long range will pass through sheets 16 of both the scintillators 10, so causing simultaneous pulses from both photomultiplier tubes 12 which are blocked by the anti-coincidence circuit. Alpha particles will be detected by only one scintillator 10.

Claims

1. A scintillator comprising photomultiplier tube, and a multiplicity of thin sheets of scintillation material arranged parallel to but spaced apart from each other, one edge of each sheet being optically coupled to the photomultiplier.

2. A scintillator as claimed in Claim 1 wherein the scintillation material comprises a plastics material.

3. A scintillator as claimed in Claim 1 or Claim 2 wherein the sheets are between 1 and 2 mm thick.

4. A scintillator as claimed in any one of the preceding Claims wherein the surfaces of the sheets are metallized.

5. A scintillator as claimed in any one of the preceding Claims wherein the two outermost sheets define opposed walls of a duct for through flow of a fluid, the other sheets extending across the duct.

6. A scintillator substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.