Experimental study of Ar38+α reaction cross sections relevant to the Ca41 abundance in the solar system

In massive stars, the $^{41}\mathrm{Ca}(n,\ensuremath{\alpha})^{38}\mathrm{Ar}$ and $^{41}\mathrm{K}(p,\ensuremath{\alpha})^{38}\mathrm{Ar}$ reactions have been identified as the key reactions governing the abundance of $^{41}\mathrm{Ca}$, which is considered as a potential chronometer for solar system formation. So far, due to experimental limitations, the $^{41}\mathrm{Ca}(n,\ensuremath{\alpha})^{38}\mathrm{Ar}$ reaction rate is solely based on statistical model calculations. In the present study, we have measured the time-inverse $^{38}\mathrm{Ar}(\ensuremath{\alpha},n)^{41}\mathrm{Ca}$ and $^{38}\mathrm{Ar}(\ensuremath{\alpha},p)^{41}\mathrm{K}$ reactions using an active target detector. The reactions were studied in inverse kinematics using a 133-MeV $^{38}\mathrm{Ar}$ beam and $^{4}\mathrm{He}$ as the active-gas target. Both excitation functions were measured simultaneously in the energy range of $6.8\ensuremath{\le}{E}_{\mathrm{c}.\mathrm{m}.}\ensuremath{\le}9.3$ MeV. Using detailed balance the $^{41}\mathrm{Ca}(n,\ensuremath{\alpha})^{38}\mathrm{Ar}$ and $^{41}\mathrm{K}(p,\ensuremath{\alpha})^{38}\mathrm{Ar}$ reaction rates were determined, which suggested a 20% increase in the $^{41}\mathrm{Ca}$ yield from massive stars.