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.

As by-products of this work, it was determined that: (1) the independent yield of ${\mathrm{Xe}}^{133}$ in ${\mathrm{U}}^{235}$ fission is not greater than 0.1 percent of the chain yield; (2) the independent yield of 9.15-hr ${\mathrm{Xe}}^{135}$ is about 3.5 percent of the total yield of the mass-135 chain; and (3) the half-life of ${\mathrm{I}}^{133}$, as measured by a procedure which circumvented most of the interference from other nuclides, is 20.8±0.2 hours.

• Received 9 June 1953

DOI:https://doi.org/10.1103/PhysRev.91.1458