Fast-Neutron Irradiation Effects in CdS

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Abstract
The effects of fast‐neutron irradiation on the room‐temperature dark resistivity and Hall mobility in CdS have been determined. Single crystals ranging from 0.5–10Ω·cm and from 107–109Ω·cm were irradiated in a reactor at 30°±5°C. For fluences of 1014 neutrons/cm2 and 1017 neutrons/cm2 the Hall mobility decreases by 3%–5% and 20%–30%, respectively. At one day after irradiation to > 1017 neutrons/cm2, the crystals are still n‐type and the resistivity approaches a limiting value of ∼2×104Ω·cm, independent of the initial resistivity. The corresponding limiting electron quasi‐Fermi level is at ∼0.37 eV below the conduction band. Following irradiation, three separate time‐dependent processes occur which cause changes in the resistivity. Two of these are temperature‐independent and follow first‐order kinetics with half‐lives of ∼1 and ∼14 days, respectively. The third process is much slower and is temperature‐dependent. The room‐temperature annealing rate for this process depends on the sample and also on either the electron quasi‐Fermi level or the defect concentration. Initial changes in the postirradiation resistivities are primarily results of the first two processes and are attributed to self‐ionization of the sample by radioactive decay of transmutated atoms produced during the irradiation. The major radioactive decay process (14 day half‐life) results from the beta irradiation from radioactive 32P; the other process is tentatively identified as resulting from the 115Cd beta decay. The temperature‐dependent process is attributed to the annealing of radiation‐induced defects, which are probably more complicated than simple vacancies or interstitials.

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