On the Conductivity Produced in CdS Crystals by Irradiation with Gamma-Rays

By combining the theory of secondary photo-currents developed by Hilsch, Pohl, Schottky, and others, with the model of a crystal phosphor by Johnson and Seitz and by Riehl and Schoen a theory of the photo-conductivity in CdS crystals is given. The theory agrees qualitatively with the results of previous and present experiments. A quantitative energy scheme, which is the basis for this theory, has been determined from measurements of the emission and absorption spectra of these crystals. According to the new determined sign of the thermal e.m.f.: CdS against Cu, the photo-current in CdS is predominantly carried by electrons. The dark current follows the usual law: $log\sigma \sim \frac{1}{T}$ with the constants $\epsilon =0.67\mathrm{}$ ev and ${\sigma }_0=1.7\mathrm{}·{10}^{-3\mathrm{}}$ mho ${\mathrm{cm}}^{-1\mathrm{}}$. In accordance with the theory the final current reached after prolonged irradiation is proportional to the square root of the intensity of irradiation. The final current follows Ohms law. The initial slope of the decay curve is proportional to the intensity of irradiation. Under periodical irradiation with gamma-rays the amplitude of the resulting alternating current slowly increases to a final value. If the electric field is temporarily removed the original conductivity is slowly restored. When a CdS crystal is heated and then cooled to liquid air temperature irradiation produces a small primary current of less than ${10}^{-13\mathrm{}}$ amp. at first. With continuous irradiation and after a slight delay this current suddenly increases until it reaches a constant value of ${10}^{-8\mathrm{}}$ amp.