Precision Measurement of theBe9(γ, n)Cross Section

Measurements have been made of the photoneutron cross section for ${\mathrm{Be}}^9$ using $\gamma$ rays from radioactive sources of ${\mathrm{Y}}^{88}$ (1.85 Mev) and ${\mathrm{Sb}}^{124}$ (1.69 Mev). The values obtained at these two energies are (6.54±0.31)×${10}^{-28\mathrm{}}$ ${\mathrm{cm}}^2$ and (12.62±0.69)×${10}^{-28\mathrm{}}$ ${\mathrm{cm}}^2$, respectively. The limiting factors in the accuracy of these measurements are the uncertainties in the $\gamma$-ray source strength calibration and in the NBS No. 2 Ra-$\gamma$-Be neutron source standard, and the lack of precise knowledge concerning the ${\mathrm{Sb}}^{124}$ decay scheme. A new measurement of the $\gamma$-ray branching ratios in the ${\mathrm{Sb}}^{124}$ decay has also been made.