Radioactive Decay and Associated Electrical Changes in Fast-Neutron-Irradiated CdS

Abstract

Changes in resistivity (or conductivity) with time after fast-neutron irradiation have been correlated with the measured decay of radioactivity from CdS. After accounting for effects associated with thermal annealing, the results have shown that induced conductivity results from the $\beta$ decays of ${}_{}{}^{115}{}_{}{}^{}\mathrm{Cd}$ (2.3-day half-life) and ${}_{}{}^{32}{}_{}{}^{}\mathrm{P}$ (14.3-day half-life). Ionization resulting from the absorption of $\beta$ radiation produces the induced conductivity. Absorption of $\gamma$ radiation from the ${}_{}{}^{115}{}_{}{}^{}\mathrm{Cd}$ decay was insignificant. The electron free lifetime ${\tau }_n$ was determined from the induced conductivity, the mobility, the $\beta$- decay rate, the radiation-ionization energy, and the average energy absorbed per $\beta$ particle. Results from 14-MeV neutron irradiations (this study) and reactor fast-neutron irradiations (previous study) have shown that ${\tau }_n$ is dependent on the distance of the electron quasi-Fermi level $E_{\mathrm{Fn}}$, below the edge of the conduction band. When $E_{\mathrm{Fn}}\gtrsim 0.37$ eV ($\rho \gtrsim 2\times {10}^4$ Ω cm), ${\tau }_n$ is of the order of tens of msec. However, for $E_{\mathrm{Fn}}\lesssim 0.37$ eV, ${\tau }_n$ is larger by a factor of ∼${10}^3$. The limiting position of $E_{\mathrm{Fn}}$ in fast-neutron-irradiated CdS is $\approx 0.37$ eV, and depends on the induced defects as well as ionization. The large increase in ${\tau }_n$ with decreasing $E_{\mathrm{Fn}}$ probably arises because the process determining the occupancy of sensitizing centers changes from thermal-equilibrium kinetics to recombination kinetics. These centers are probably associated with fast-neutron-induced defects (or defect clusters). These results, in conjunction with a study of the changes in resistivity with fluence and the characteristics of thermal annealing, have shown that 14-MeV neutrons and reactor fast neutrons produce the same kinds of defects in CdS. Some unique features of these experiments were that resistivities were obtained in the range ${10}^1$-${10}^6$ Ω cm by radiation doping, and that ${\tau }_n$ was determined by self-ionization resulting from the $\beta$ decay of radioisotopes. Also discussed are resistivity inhomogeneities in CdS, and $\gamma$-induced conductivity and photoconductivity experiments in fast-neutron-irradiated CdS.