The thermal neutron absorption cross section of ${\mathrm{A}}^{40}$ was determined by measuring in calibrated proportional counters the amount of 110-min ${\mathrm{A}}^{41}$ produced from samples of argon irradiated in a neutron flux whose magnitude was measured by $\beta -\gamma$ coincidence counting of gold monitors. The result was 0.53±0.03 barn.

${\mathrm{A}}^{42}$ was produced from ${\mathrm{A}}^{40}$ by two successive neutron captures and it was detected through its 12.5-hr ${\mathrm{K}}^{42}$ daughter. A lower limit of 3.5 years was set on its half-life. From this result the thermal neutron absorption cross section of ${\mathrm{A}}^{41}$ is calculated to be greater than 0.06 barn.

It was also shown that the thermal neutron absorption cross section of ${\mathrm{A}}^{38}$ is 0.8±0.2 barn.

• Received 16 May 1952

DOI:https://doi.org/10.1103/PhysRev.87.886