Abstract:
Cubic boron nitride having an impurity profile as set out below has been found to exhibit excellent thermoluminescent properties with a high gamma radiation sensitivity and low light sensitivity: 
     
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     Impurity     Content - less than______________________________________Carbon       1100 ppmOxygen       1500 ppmSilicon       100 ppmPhosphorus    30 ppmTitanium      30 ppmBeryllium      1 ppm______________________________________

Description:
This application is a continuation of application Ser. No. 257,345, filed Oct. 13, 1988 now abandoned. 
    
    
     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 &#34;Electrical Conductivity, Absorption and Luminescence Measurements in Cubic Boron Nitride&#34;, 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 ppmOxygen       1500 ppmSilicon       100 ppmPhosphorus    30 ppmTitanium      30 ppmBeryllium      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        1000Oxygen        less than 1500Silicon       10Phosphorus    10Titanium      0,3Beryllium     less than 0,1______________________________________ 
    
     The CBN may be made by methods known int he 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. 
     
                                           TABLE I__________________________________________________________________________CBN Impurity - ppm   γ    LightItem   Sensitivity    Sensitivity          C   O.sub.2                   Al  Si  P   S  Mg   Ca  Ti   Cr As Fe__________________________________________________________________________A  1328  31     1000              &lt;1500                   20   10 10  &lt;30                                  10000                                        200                                           0.3  3  1  3B  13.5  28     1000               5000                   100 300 30  &lt;30                                  10000                                        200                                           0.3  3  0.3                                                      1C  71    149   30000              15000                   30  1000                           30  &lt;30                                  30000                                       10000                                           30   3  1  3D  48.3  58.6  10000              15000                   30  300 10  &lt;30                                  30000                                        100                                           1    1  1  1E  1040  611    5000               3000                   30  100 10  &lt;30                                  30000                                       3000                                           30   3  1  5F  640   911    3000              15000                   1000                       3000                           100 &lt;30                                  20000                                       1000                                           100  30 1  100__________________________________________________________________________ 
    
     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, phosphorus, titanium and beryllium impurity profile essentially the same as that of the CBN produced.