GB2056476A - Scintillation detector - Google Patents

Abstract

The surface (3) of a scintillation detector (1) facing a radiation source (2) is concave in its central region (6) to increase multiple reflections in that region and is radiussed in its edge regions (4) to reduce multiple reflections in those regions, so that light yield at all locations in the scintillation detector (1) are equal to improve the resolution of the scintillation detector (1). The scintillation detector (1) is optically coupled to a photoelectrlc transducer (9) by means of a light conducting rod (8) and contained within a tubular aluminium housing (10) having a radiation permeable window (11). Adhesive (13) secures the housing (10) to the light conducting rod (8) and the space (12) between the rod (8) and the housing (10) is filled with MgO. <IMAGE>

Description

SPECIFICATION A scintillation detector The present invention relates to a scintillation detector. Scintillators are used for ascertaining the energetic information of ionised radiation, in which, during energy absorption of individual radiation quanta or charged particles, light flashes or luminescence phenomena are triggered. The intensity of the light produced thereby is proportional to the radiation component in the volume of the scintillator. The light quantity produced with each event is usually amplified in a photomultiplier and analysed according to pulse height in a measuring value processing system connected in series.

The absorption of ionised radiations may occur in any volume element of the scintillator. The light available for the measuring value processing system per energy absorption result is composed of a proportion of primary light irradiated directly by the given volume element to the photomultiplier and a proportion of secondary light resulting from the sum of all the reflections on the internal surface of the scintillator.

The number of multiple reflections varies in dependence upon the position of the volume element in which the energy absorption event occurs within the scintillator volume. Since each reflection is identical with an energy loss, it is necessary to proceed from the fact that, for the absorption of the same energy at various locations in the scintillator volume at the input of the photomultiplier, light pulses of varying intensity occur.

The immediate result of these actions is a reduction of the attainable energy resolution of the radiation to be measured. This undesirable effect increases with diminishing scintillator volume, because the spacings between the defining surfaces of the scintillator causing the reflections are rapidly reduced.

The object of the invention is to develop a scintillation detector the energy resolution of which is substantially independent of the volume of the scintillator and particularly in scintillators having a volume of the order of magnitude less than a cubic centimetre enabling a considerable improvement of the energy resolution.

According to the present invention there is provided a scintillation detector having a photoelectric transducer to which there is optically coupled a rotationally symmetric disc-shaped scintillator, having a volume of less than one cubic centimetre, via a flat light emission surface facing the photoelectric transducer, whereby the scintillator has a circular wall section, the generating line of which has a continuous arcuate transition to the generating surface section of the scintillator remote from the photoelectric transducer, and the surface of the scintillator, remote from the flat light emission surface, is cupshaped and in its central region has a convex shape facing the flat light emission surface, defocusing secondary light and increasing the number of multiple reflections, and said continuously arcuate transition has a concave camber facing the flat light emission surface focusing secondary light and reducing the number of multiple reflections.

The advantages obtained with the proposed scintillation detector consist particularly in that by simple means a substantial improvement of the energy resolution is attained and that the possibility is provided of also using small scintillation detectors for the pulse level analysis and hence to open up new applications for their use.

An embodiment of a scintillation detector is shown in the accompanying drawing and is described in detail hereinafter.

The scintillator 1 consists of a sodium iodide crystal activated with Thallium, which is formed as a circular disc and having a diameter 2r = 3 mm and a thickness d = 1 mm. The surface of the scintillator 1 facing the radiation source 2, and which is the radiation incident surface 3, is radiused in its edge region, i.e. the region of extreme light losses by multiple reflections, in such a manner that the generation curve of the rotationally symmetrical scintillator 1 has a continuous arcuate transition 4 from the axially parallel wall 5 to the defining line of the surface 3 of the scintillator 1 facing the radiation source 2.

The central region 6 of the radiation incident surface 3, thus the region of extremely high light yield is concave. These two measures, radiusing the edge region of the radiation incident surface 3 to reduce the number of mutiple reflections and concave formation of the central region 6 of the radiation incident surface 3 increasing the number of multiple reflections, thus complement each other in such a manner that the light yield at all positions in the scintillator volume if substantially equal.

A photomultiplier 9 is connected to the light emission surface 7 of the circular surface of the scintillator 1 remote from the radiation source 2 via a light conductor 8 comprised of a glass rod of 3 mm diameter.

A cap 10 of aluminium encloses the scintillator 1 and the end of the light conductor 8 is connected to the scintillator 1 by means of a transiucent adhesive. The cap 10 at its flat end face facing the radiation source 2 is adapted as a radiation-permeable radiation window 11, the wall thickness of which is 50 microns. The cavity between the cap 10, with the radiation window 11, and the scintillator 1 with the end of the light conductor 8, is completely filled with a metal oxide powder 12 such as MgO.

The cap 10 is secured at its open end to the light conductor 8 by an adhesive 1 3. The light conductor 8 is enclosed by a light-tight aluminium tube 14, and at its end the internal diameter is enlarged and fitted on the tapered external diameter of the cap 10. The low energetic Gamma or X-rays 1 5 emanating from the radiation source 2 penetrate the radiation window 11 and the magnesium oxide powder 12 and is absorbed in the scintillator 1 comprised of NaJ(TL) crystal. The energy conversion associated therewith, in the locations 16, 17, 18, shown by way of example, each represents a light flash, the light of which is transmitted partly directly and partly via n reflections to the inside surface of the scintillator 1 as light pulses 19 via the light conductor 8 to the photomultiplier 9.

Claims (3)

1. A scintillation detector having a photoelectric transducer to which there is optically coupled a rotationally symmetric disc-shaped scintillator, having a volume of less than one cubic centimetre, via a flat light emission surface facing the photoelectric transducer, whereby the scintillator has a circular wall section, the generating line of which has a continuous arcuate transition to the generating surface section of the scintillator remote from the photoelectric transducer, and the surface of the scintillator, remote from the flat light emission surface, is cup-shaped and in its central region has a convex shape facing the flat light emission surface, defocusing secondary light and increasing the number of multiple reflections, and said continuously arcuate transition has a concave camber facing the flat light emission surface focusing secondary light and reducing the number of multiple reflections.

2. A scintillation detector according to claim 1, in which the scintillator is connected at its flat light emission surface facing the photoelectric transducer to a light conductor by means of a light-permeable adhesive, the scintillator and the light conductor have the same diameter, a cap of aluminium encloses the scintillator and the end of the light conductor connected to the scintillator, the cap at its flat end face facing the radiation source is formed as a radiation permeable window, the space between the cap and the scintillator with the end of the light conductor is filled completely with a metal oxide powder, the cap at its open end is connected to the light conductor by an adhesive, the light conductor is enciosed, impermeable to light, by an aluminium tube, and the tube enclosing the light conductor is fitted onto the end of the cap.

3. A scintillation detector substantially as herein described with reference to and as illustrated in the accompanying drawings.