Effects of Gamma Radiation on Germanium

High-purity $n$- and $p$-type samples of Ge have been exposed to $\gamma$ rays from a 900-curie ${\mathrm{Co}}^{60}$ source. The extrinsic electron concentration of $n$-type material decreases at a rate only ∼${10}^{-3\mathrm{}}$ of that for fast-neutron irradiation. Two electron trapping levels which are apparently identical to those produced by fast neutrons have been observed. Extended exposure converts $n$-type material to $p$-type, indicating the presence of an acceptor level at ∼0.26 ev above the valence band. This energy is somewhat larger than that found in material irradiated by fast neutrons. Interstitial clustering and nonuniform defect distribution are believed to lower the energy levels produced by fast neutrons; hence energetic photons apparently produce randomly distributed defects throughout the volume of the specimen. Exposure of $p$-type material causes a decrease in extrinsic hole concentration with a removal rate much less than that for $n$-type Ge. The cross section for atomic displacements obtained from these results is ∼1.5×${10}^{-26\mathrm{}}$ ${\mathrm{cm}}^2$. This value is in reasonable agreement with the calculated cross section expected for the Compton electrons and photoelectrons produced by ${\mathrm{Co}}^{60}$ $\gamma$-ray absorption in Ge.