Direct Measurement of the Astrophysical ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ Reaction and Its Influence on the Production of Nuclides toward the End Point of Nova Nucleosynthesis

We have performed the first direct measurement of the ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ reaction using a beam of radioactive ${}^{38}\mathrm{K}$. A proposed $\ell =0$ resonance in the ${}^{38}\mathrm{K}+p$ system has been identified at 679(2) keV with an associated strength of ${120}_{-30}^{+50}\mathrm{meV}$. Upper limits of 1.16 (3.5) and 8.6 (26) meV at the 68% (95%) confidence level were also established for two further expected $\ell =0$ resonances at 386 and 515 keV, respectively. The present results have reduced uncertainties in the ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ reaction rate at temperatures of 0.4 GK by more than 2 orders of magnitude and indicate that Ar and Ca may be ejected in observable quantities by oxygen-neon novae. However, based on the newly evaluated rate, the ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ path is unlikely to be responsible for the production of Ar and Ca in significantly enhanced quantities relative to solar abundances.

Figure 1

MCP versus separator TOF for each beam energy used in the experiment. Only events with $\gamma$-ray energies $>2.4\mathrm{MeV}$ are included. The black ovals indicate the expected location of recoils for each of the three resonances.

Figure 2

BGO hit pattern observed at a beam energy of 715 A keV. The green dashed line corresponds to the center of the windowless gas target, $E_{\mathrm{c}.\mathrm{m}.}=689\mathrm{keV}$.

Figure 3

Contributions of $\ell =0$ resonances to the astrophysical ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ reaction rate. The contributions of the 386 and 515 keV resonances represent upper limits. Also shown is the theoretical rate of Ref. [7], as well as the previous band of uncertainty (shaded region) associated with the ${}^{38}\mathrm{K}(p,\gamma ){}^{39}\mathrm{Ca}$ reaction.