A soft beta-activity was found in old ${\mathrm{P}}^{32}$ samples from the Harwell reactor. It was established that it definitely is due to a P isotope, probably ${\mathrm{P}}^{33}$, thus confirming the findings by Jensen and Sheline. The amount of ${\mathrm{P}}^{33}$ and ${\mathrm{P}}^{32}$ samples from a 14-day irradiation of sulfur has been found to 1.4±0.2 percent. The following data were found: half-life 25±2 days, maximum beta-energy 246±5 kev, number of gamma-rays at most 3.5 percent of the betas, cross section for the presumed ${\mathrm{S}}^{33}\mathrm{}(n,p)\mathrm{}{\mathrm{P}}^{33}$ reaction 0.03 barn. From energy considerations the latter reaction was found to be exothermic, whereas the reaction leading to ${\mathrm{P}}^{32}$ is endothermic, and this was supported by getting a P activity as rich as 30 percent in ${\mathrm{P}}^{33}$ by means of a pure thermal neutron flux. The cross section for the ${\mathrm{S}}^{33}\mathrm{}(n,p)\mathrm{}{\mathrm{P}}^{33}$ reaction for thermal neutrons was found to be 2.3×${10}^{-3\mathrm{}}$ barn.

• Received 23 June 1952

DOI:https://doi.org/10.1103/PhysRev.88.573