The activities induced in nitrogen, phosphorus, copper, and bromine by ($n, 2n$) reactions were determined both by registration of $\beta$ particles and by registration of coincidences between annihilation quanta. The agreement between the results obtained was quite good for phosphorus and copper, somewhat poorer for bromine. For nitrogen it was impossible to obtain accurate results from registration of $\beta$ particles and no comparison could be made. Values for the cross sections of the ($n, 2n$) reactions mentioned and for a number of other reactions are presented. The agreement with earlier measurements is in most cases satisfactory. One exception is the reaction ${}_{}{}^{81}{}_{}{}^{}\mathrm{Br}(n, \alpha ){}_{}{}^{78}{}_{}{}^{}\mathrm{As}$.

• Received 12 October 1964

DOI:https://doi.org/10.1103/PhysRev.137.B878