The thermal neutron capture cross section of ${\mathrm{C}}^{13}$ has been determined from the amount of ${\mathrm{C}}^{14}$ formed in pile-irradiated samples of graphite, barium carbonate, and carbon dioxide. Because of the impurity reaction, ${\mathrm{N}}^{14}(n, p){\mathrm{C}}^{14}$, of comparatively high cross section, the results were consistent only when samples were used which were enriched in ${\mathrm{C}}^{13}$.

The average isotopic cross section determined for the enriched samples was 1.4±0.2 millibarns. The final value of the cross section in these samples corrected for the estimated residual nitrogen impurity is 0.9±0.2 millibarn.

• Received 22 March 1954

DOI:https://doi.org/10.1103/PhysRev.95.92