Ionization Measurement ofB10(n,α)Li7

A thin layer of boron on the wall of a cylindrical ionization chamber exposed to slow neutrons has been used to study the ionization produced by the recoiling particles of the transmutation ${\mathrm{B}}^{10}(n,\alpha ){\mathrm{Li}}^7$. Polonium alpha-particles were used for calibration, and the ionization electron pulses were amplified and photographed on an oscilloscope screen. Calculated peak shapes taking into account wall effect and layer thickness were fitted to the experimental number pulse-height curves, and from this information the ionization values were deduced. The ratio of ionization produced by the ${\mathrm{He}}^4$ and ${\mathrm{Li}}^{7*}$ recoils was observed to be 1.878±0.014 as compared to the energy ratio 1.7529 determined from conservation of momentum. This difference can be accounted for with the assumption that the average energy per ion pair varies with velocity. The implications of this assumption are discussed.