Direct Radiative Capture of Protons by ${\mathrm{O}}^{16}$ and ${\mathrm{Ne}}^{20}$

A study has been made of the ${\mathrm{O}}^{16}(p, \gamma ){\mathrm{F}}^{17}$ and ${\mathrm{Ne}}^{20}(p, \gamma ){\mathrm{Na}}^{21}$ reactions by counting the positron activities of ${\mathrm{F}}^{17}$ and ${\mathrm{Na}}^{21}$ following proton bombardment of oxygen and neon targets. The ${\mathrm{O}}^{16}(p, \gamma ){\mathrm{F}}^{17}$ cross section was measured at a proton bombarding energy of 616 kev to be 0.29±0.03 microban and the ${\mathrm{Ne}}^{20}(p, \gamma ){\mathrm{Na}}^{21}$ cross section was measured at 1100 kev to be 1.3±0.7 microbarns. These cross sections are consistent with the reaction process in each case being one of direct radiative capture. In addition, the energy dependence of the ${\mathrm{O}}^{16}(p, \gamma ){\mathrm{F}}^{17}$ cross section from 275 kev to 616 kev was also consistent with the direct-capture hypothesis.

Both of these reactions are believed to be important at the thermal energies effective in stars. For such energies the cross-section parameter $S_0$ was estimated to be $S_0=5\mathrm{}\pm 1$ kev-barns for ${\mathrm{O}}^{16}(p, \gamma ){\mathrm{F}}^{17}$, and $S_0\sim 80$ kev-barns for ${\mathrm{Ne}}^{20}(p, \gamma ){\mathrm{Na}}^{21}$.