Alternating current measurements of the impedance changes induced by ultraviolet, infrared, x-ray, and gamma-ray irradiation in (Zn:Cd)S luminescent powders are reported. These results are in agreement with previously reported dc measurements, and indicate that the observed impedance changes are primarily due to the change of the electron density in the conductivity band. Whereas the capacity changes show a monotonic increase, the $Q$ values ($\frac{X}{R}$) show a pronounced minimum with increasing intensity. A model consisting of radiation sensitive and nonsensitive powder portions is proposed, and theoretical calculations based on such a model are shown to be in agreement with most of the experimental results. This ac measurement technique can be used to measure small exciting intensities (down to almost $\frac{1}{1000}$ erg/sec ${\mathrm{cm}}^2$).

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