Previous publications have revealed that poled ferroelectrics can be used as detectors of radiation; however, the source of the charge release was speculative. A theoretical derivation using the theory of pyroelectricity is verified by the previously published data and by an especially designed experiment whereby the graph of the normalized charge release as a function of temperature is compared with normalized pyroelectric coefficent data. The experiment verifies quite conclusively that the constant of proportionality K in the equation, i/A = K (dγ/dt), is equal to p/C, where p is the pyroelectric coefficient and C is the specific heat. The appropriate value for K for ceramic lead zirconate titanate, Pb(Zr0.65Ti0.35)O3 + 1 wt% Nb2O5 (65:35PZT), is determined to be 0.6 picocoulombs per square centimeter per rad (PZT).