Precision Determination of the Slow Neutron Absorption Cross Section ofB10

The total neutron cross section of ${\mathrm{B}}^{10}$ has been measured at neutron energies between 0.00291 ev and 0.1 ev with the Columbia University crystal spectrometer. The ${\mathrm{B}}^{10}$ absorption cross section was obtained by subtracting the relatively small, known, scattering cross section from the measured total cross section. Two samples enriched to 92.84±0.06 atom% and 99.88±0.01 atom% ${\mathrm{B}}^{10}$ were used for these measurements. The ${\mathrm{B}}^{10}$ absorption cross section obtained with the 92.84% ${\mathrm{B}}^{10}$ sample followed the relation $E^{\frac{1}{2}}{\sigma }_a=612.3\mathrm{}\pm 2.4$ barns ${\mathrm{ev}}^{\mathrm{½}}$ and yielded a value of 3849±15 barns at 0.0253 ev. For the 99.88% ${\mathrm{B}}^{10}$ sample, the ${\mathrm{B}}^{10}$ absorption cross section obeyed the relation $E^{\frac{1}{2}}{\sigma }_a=608.9\mathrm{}\pm 2.3$ barns ${\mathrm{ev}}^{\mathrm{½}}$ and gave a value of 3828±15 barns at 0.0253 ev. A sample of Argonne-Brookhaven standard normal boron, 19.8±0.1% ${\mathrm{B}}^{10}$, was measured simultaneously. The resulting absorption cross section for normal boron was ${\sigma }_a\left(0.0253\mathrm{} \mathrm{ev}\right)=764\mathrm{}\pm 3$ barns and obeyed the relation $E^{\frac{1}{2}}{\sigma }_a=121.5\mathrm{}\pm 0.5$ barns ${\mathrm{ev}}^{\mathrm{½}}$. The average of the results for the two enriched samples yields ${\sigma }_a\left({\mathrm{B}}^{10}\right)=3838\mathrm{}\pm 11$ barns at 0.0253 ev and $E^{\frac{1}{2}}{\sigma }_a\left({\mathrm{B}}^{10}\right)=610.5\mathrm{}\pm 1.7$ barns ${\mathrm{ev}}^{\mathrm{½}}$. The ratio of the above values of the absorption cross sections for ${\mathrm{B}}^{10}$ and normal boron yields a value of 19.9±0.1% ${\mathrm{B}}^{10}$ for the normal sample which is in good agreement with mass spectrometer determinations.