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 and 9.0×{}10^-3 neutrons/electron. Extrapolation to infinite target thickness gives a value of 9.5×{}10^-3 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%.Calibration of the neutron-detecting equipment was made with a RaBe source and checked by measuring the yields, due to electro- and photodisintegration of the deuteron, from a heavy-water target. In addition, yields from thin targets of Be and Cu were observed as a function of electron energy. The data for Be yield a value of (0.018±{}0.003) Mev-barn for the (γ, n) cross section integrated to 17 Mev. The data for Cu were analyzed and combined with other measurements to give an approximate cross section for the Cu(γ, pn) reaction.