The yields at several energies for the reactions ${\mathrm{C}}^{12}(\gamma , 2n){\mathrm{C}}^{10}$, ${\mathrm{O}}^{16}(\gamma , 2n){\mathrm{O}}^{14}$, ${\mathrm{F}}^{19}(\gamma , 2n){\mathrm{F}}^{17}$, ${\mathrm{Na}}^{23}(\gamma , 2n){\mathrm{Na}}^{21}$, ${\mathrm{P}}^{31}(\gamma , 2p){\mathrm{Al}}^{29}$, and ${\mathrm{P}}^{31}(\gamma , 2pn){\mathrm{Al}}^{28}$ were measured using the x-ray beam from the University of Illinois 300-Mev betatron. It was found that the ratio of integrated cross sections of the ($\gamma , 2n$) to ($\gamma , n$) reactions in ${\mathrm{F}}^{19}$ and in ${\mathrm{Na}}^{23}$ is of the order of 0.1 and approximately 1-2 orders of magnitude smaller for ${\mathrm{C}}^{12}$ and ${\mathrm{O}}^{16}$. The small ($\gamma , 2n$) yields for ${\mathrm{C}}^{12}$ and ${\mathrm{O}}^{16}$ are consistent with statistical competition between emitted particles if the gamma-ray absorption decreases rapidly with energy above the giant resonance.

• Received 4 May 1959

DOI:https://doi.org/10.1103/PhysRev.116.173