The energies and the absolute intensities of the $\gamma$-rays produced by neutron capture in fluorine, sodium, magnesium, aluminum, and silicon have been measured with the aid of a pair spectrometer. Direct transitions to the ground states of the product nuclei are predominant in the $\gamma$-ray spectra of fluorine and aluminum. The neutron binding energy in ${\mathrm{F}}^{20}$ is 6.63±0.03 Mev and in ${\mathrm{Al}}^{28}$, 7.724±0.010 Mev. The $\gamma$-ray representing the direct transition to the ground state of ${\mathrm{Na}}^{24}$ was not detected. The interpretation of the magnesium spectrum presents some difficulties. A weak $\gamma$-ray with the binding energy of a neutron in ${\mathrm{Mg}}^{25}$ was detected, but none could be found to correspond to the direct transition to the ground state in ${\mathrm{Mg}}^{26}$. In silicon, most captures are due to ${\mathrm{Si}}^{28}$ which produces ${\mathrm{Si}}^{29}$ by the emission of two $\gamma$-rays in cascade. Very weak $\gamma$-rays representing direct transitions to the ground states in ${\mathrm{Si}}^{29}$ and ${\mathrm{Si}}^{30}$ were detected. Their energies are 8.51±0.04 (${\mathrm{Si}}^{29}$) and 10.55±0.05 Mev (${\mathrm{Si}}^{30}$).

• Received 20 April 1951

DOI:https://doi.org/10.1103/PhysRev.83.519