(γ, 2n) Reactions in Light Elements

The yields at several energies for the reactions ${\mathrm{C}}^{12}(\ensuremath{\gamma}, 2n){\mathrm{C}}^{10}$, ${\mathrm{O}}^{16}(\ensuremath{\gamma}, 2n){\mathrm{O}}^{14}$, ${\mathrm{F}}^{19}(\ensuremath{\gamma}, 2n){\mathrm{F}}^{17}$, ${\mathrm{Na}}^{23}(\ensuremath{\gamma}, 2n){\mathrm{Na}}^{21}$, ${\mathrm{P}}^{31}(\ensuremath{\gamma}, 2p){\mathrm{Al}}^{29}$, and ${\mathrm{P}}^{31}(\ensuremath{\gamma}, 2pn){\mathrm{Al}}^{28}$ 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 ($\ensuremath{\gamma}, 2n$) to ($\ensuremath{\gamma}, n$) 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 ($\ensuremath{\gamma}, 2n$) 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.