Measurement of Gamma Ray Induced Secondary Electron Current from Various Elements

Knowledge of the angular distribution and absolute efficiency for gamma ray induced secondary electron currents is necessary in the development of fast radiation detectors, in dosimetry and in transient radiation effects studies. The angular distribution of the emergent forward electron current and the absolute quantum efficiency for 1.25 MeV gamma ray induced secondary electron current has been measured for elements having atomic number 6, 13, 29, 48, and 82. To ensure electron equilibrium, targets of thickness about equal to the extrapolated range of a 1.25-MeV photoelectron were used. The non-equilibrium thickness of one-tenth the extrapolated range was also used. A significant difference between the total target current and the total forward current was observed. This difference increased with atomic number and target thickness, as would be expected from results of electron backscattering experiments. Forward currents, calculated as a function of atomic number and target thickness, using Mar´ s universal electron transmission formula and taking into account both Compton and photoelectric effects, agree well with experimental results.