Neutron Yields from Targets Bombarded by Electrons

Abstract

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\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ and 9.0\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ neutrons/electron. Extrapolation to infinite target thickness gives a value of 9.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ 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 \ifmmode\pm\else\textpm\fi{}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\ifmmode\pm\else\textpm\fi{}0.003) Mev-barn for the ($\ensuremath{\gamma}, n$) 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 $\mathrm{Cu}(\ensuremath{\gamma}, pn)$ reaction.