Symposium on Solid State Conductivity IV. Solid State Radiation Detectors, with Particular Reference to Low Dose rates

The use of single crystals of photoconductor has been investigated at gamma dose rates of ( a ) 1 to 300 r/min for supervoltage beam measurements, ( b ) 10 to 300 r/hr for intracavitary probes, and ( c ) at very low dose rates of the order of 1 mr/hr for radiation protection or for the detection of isotopes in the human body. Results are presented of measurements on CdS and CdSe crystals from various industrial sources. It is possible to obtain crystals which pass 0·1 to 1 μ A per r/min ( 60 Co) with a dark current some 3 orders of magnitude smaller; a few have been tested which give up to 40 μ A per r/min, but the more sensitive crystals are more difficult to obtain. Results of measurements on single crystals have shown that linearity with dose rate, reasonably short time constants, and small dependence on temperature are possible with CdS, but CdSe has not yet been found to be so good in the two last-mentioned properties. Radium dose rates of 1·3 to 2 mr/hr have been detected to an accuracy of about 20% within 5 to 10 min of beginning the exposure. Such dose rates would enable the distribution of a few tens of microcuries of 131 I to be displayed, by an array of single crystals of CdS, and the methods of achieving this are discussed. Present photoconductors show considerable promise for these and other applications, with little improvement in properties but with supplies made more readily available.