THERMOLUMINESCENT MATERIAL

Abstract: A material capable of thermoluminescence comprising cubic boron nitride which, if it contains one or more of the impurities set out below, contains said impurities in an amount not exceeding that specified in the content column: Inpurity Content (ppm) Carbon 1100 Oxygen 1500 Silicon 100 Phosphorus 30 Titanium 30 Beryllium 1

Description: BACKGROUND OF THE INVENTION This invention relates to a thermoluminescent material useful in the detection, counting or otherwise measuring of nuclear radiation. Nuclear radiation is radiation that has its genesis within the nucleus of the atom. Examples of such radiation are radiation by X-rays, alpha particles, neutrons, protons, electrons and gamma rays. It has been proposed to use diamond of low nitrogen content as a thermoluminescent material in the detection of nuclear radiation - European Patent Publication No 0195678. Such diamond particles are available from natural sources or may be produced synthetically. Whilst diamond has been shown to be an excellent human tissue equivalent detector of ionising radiation cubic boron nitride with an effective atomic no of 6.8 is an even closer tissue equivalent material. Cubic boron nitride (CBN) has also been described in the literature as having thermoluminescent properties. Reference in this regard may be had of the paper by A. Halperin and A. Katzir entitled "Electrical Conductivity, Absorption and Luminescence Measurements in Cubic Boron Nitride", Phys. Chem. Solids, 1975, Vol. 36, pp. 89-93. SUMMARY OF THE INVENTION It has now been found that CBN having an impurity profile as specified below exhibits thermoluminescent properties with high gamma radiation sensitivity and low light sensitivity. The CBN has an impurity profile as set out below: Impurity Content - less than Carbon 1100 ppm Oxygen 1500 ppm Silicon 100 ppm Phosphorus 30 ppm Titanium 30 ppm Beryllium 1 ppm The CBN may contain other impurities, but these six impurities must not be present in any greater amount than that specified. The CBN will typically be made from hexagonal boron nitride (HBN) which has a similar impurity profile. The CBN has utility in the detection, counting or other measurement of nuclear radiation, particularly gamma radiation. The invention provides the CBN for such use. According to another aspect of the present invention, there is provided a body for use in detecting, counting or otherwise measuring nuclear radiation comprising a mass of the CBN bonded into a coherent form or held against movement in a suitable container. According to yet another aspect of the invention, there is provided a method of making a bonded mass of the CBN including the steps of placing a mass of the CBN in contact with a mass of a second phase, exposing the CBN and second phase to conditions of elevated temperature and pressure suitable for compact manufacture thereby producing a compact comprising the CBN bonded into a coherent form and having the second phase uniformly dispersed therethrough and optionally removing substantially all the second phase from the coherent form. DETAILED DESCRIPTION OF THE INVENTION An example of a particularly suitable CBN has an impurity profile as set out below: Impurity Content (ppm) Carbon 1000 Oxygen less than 1500 Silicon 10 Phosphorus 10 Titanium 0,3 Beryllium less than 0,1 The CBN may be made by methods known in the art using an HBN as starting material. This HBN will typically have an impurity profile similar to that of the CBN. The CBN may be made using an alkaline earth metal nitride such as a mixture of calcium nitride and magnesium nitride, as catalyst. Particles produced using this catalyst are generally white. Other catalysts such as a mixture of lithium nitride and calcium nitride (producing orange particles) or a mixture of lithium nitride and magnesium diboride (producing black particles) may also be used. The CBN body when in bonded, coherent form may have a size which renders it easily mountable in a suitable detection device or apparatus. Exposing the CBN to nuclear radiation causes electrons or holes to be trapped at lattice imperfections within the CBN crystal structure. Heating the CBN then causes at least some of these electrons or holes to be released from their traps, return to stable energy states and emit light, i.e. to luminesce. This property may be utilised in the detection and/or measurement of the nuclear radiation, particularly gamma radiation. The temperature to which the CBN should be subjected to cause it to luminesce will be above ambient and below 500°C. The preferred temperature is 200 to 500°C, more preferably 250 to 400°C. The CBN body when in bonded, coherent form may be produced using known and standard CBN compact manufacture conditions. Essential to this method is that the second phase, in producing the compact, does not become significantly radioactive when subjected to nuclear radiation. Examples of a suitable second phase are silicon and glass. The CBN body when in bonded, coherent form will generally be produced in disc form and may be used in this form. Alternatively, the disc may be cut by known techniques into a number of other bodies having a variety of shapes. When the CBN body comprises a mass of CBN particles held against movement in a suitable container, the CBN particles will generally be in discrete, unbonded form. An example of a suitable container consists of two co-operating parts which may be clipped or otherwise secured together to define a cavity which receives the CBN particles. The cavity should be filled to an extent that little or no movement of the particles is possible. One possible use for the container thus filled is as a detector in radiotherapy. The container must, of course, be made of a material which is transparent to the radiation and does not luminesce when heated after being subjected to the radiation. The container may also be a vial. CBN particles were produced using a variety of sources of HBN. The CBN particles thus produced were subjected to gamma radiation and thereafter heated and the thermoluminescent response determined. In particular, the gamma radiation sensitivity of each CBN material as well as the light sensitivity of each CBN material was determined. The results are set out in Table I. It will be noted that the CBN having the impurity profile of the invention (Item A) had the desirable combination of high gamma sensitivity and low light sensitivity when compared with all the other CBN particles. In each example above, there was no detectable, i.e. less than 0,1 ppm, beryllium in the CBN. Further in each example above, the HBN source material had a carbon, oxygen, silicon, phosphorous, titanium and beryllium impurity profile essentially the same as that of the CBN produced.

Claims: A material capable of thermoluminescence comprising cubic boron nitride which, if it contains one or more of the impurities set out below, contains said impurities in an amount not exceeding that specified in the content column: Impurity Content (ppm) Carbon 1100 Oxygen 1500 Silicon 100 Phosphorus 30 Titanium 30 Beryllium 1 A material according to claim 1 wherein the cubic boron nitride which has an impurity profile set out below: Impurity Content (ppm) Carbon about 1000 Oxygen less than 1500 Silicon about 10 Phosphorus about 10 Titanium about 0.3 Beryllium less than 0.1 A mass of material as claimed in claim 1 or 2 bonded into a coherent form. A method of detecting or measuring nuclear radiation including the steps of subjecting material according to any one of claims 1 to 3 to nuclear radiation, heating the irradiated material to a temperature above ambient and below 500 o C to cause the material to luminesce and detecting or measuring the thermoluminescent response. A method according to claim 4 wherein the material is heated to a temperature in the range 200 o C to 500 o C. A method according to claim 5 wherein the material is heated to a temperature in the range 250 o C to 400 o C. A method according to any one of claims 4 to 6 wherein the radiation detected or measured is gamma radiation.

Inventors: DAVIES, GEOFFREY JOHN, KEDDY, REX JAMES, NAM, TOM LEONG
Applicants: DE BEERS INDUSTRIAL DIAMOND DIVISION (PROPRIETARY) LIMITED
Application year: 1988
Main classification: C09K11
IPC codes: C01B21, C09K11, G01T1