Yield ofXe133in the Thermal Neutron Fission ofU235

The yield of 5.27-day ${\mathrm{Xe}}^{133}$ in the thermal neutron fission of ${\mathrm{U}}^{235}$ was determined accurately by a method in which the fission rate was measured with a double chamber fission counter, and in which the disintegration rate of the chemically separated Xe was measured by gas phase proportional counting. Five separate determinations gave a mean value of (6.62±0.15) percent, where the indicated error is the 99: 1 probable error. A considerable number of possible sources of error was investigated in detail and taken into account when making the measurements and calculations.