Neutron Yields from Targets Bombarded by Electrons

The total neutron yields from thick targets bombarded by electrons were measured as a function of electron energy for the range 10 to 36 Mev. Targets ranging in thickness from one to six radiation lengths of C, Al, Cu, Ta, Pb, and U were used. The yields for 1- and 6-radiation-length targets of Pb at 34 Mev are 2.1×${10}^{-3\mathrm{}}$ and 9.0×${10}^{-3\mathrm{}}$ neutrons/electron. Extrapolation to infinite target thickness gives a value of 9.5×${10}^{-3\mathrm{}}$ neutron/electron. The yield, comparing targets of one radiation length, from C is about 10 times smaller and that from U two times greater than the yield from Pb. An explanation of the relative $Z$-dependence of the yield in terms of known photonuclear cross sections is successful to within a factor of 1.5. The absolute accuracy of the results is estimated to be ±15%.