Destruction of the cosmic $\gamma$-ray emitter ${}^{26}\mathrm{Al}$ in massive stars: Study of the key ${}^{26}\mathrm{Al}$($n,p$) reaction

The ${}^{26}\mathrm{Al}(n,p){}^{26}\mathrm{Mg}$ reaction is the key reaction impacting on the abundances of the cosmic $\gamma$-ray emitter ${}^{26}\mathrm{Al}$ produced in massive stars and impacts on the potential pollution of the early solar system with ${}^{26}\mathrm{Al}$ by asymptotic giant branch stars. We performed a measurement of the ${}^{26}\mathrm{Al}(n,p){}^{26}\mathrm{Mg}$ cross section at the high-flux beam line EAR-2 at the n_TOF facility (CERN). We report resonance strengths for eleven resonances, nine being measured for the first time, while there is only one previous measurement for the other two. Our resonance strengths are significantly lower than the only previous values available. Our cross-section data range to 150 keV neutron energy, which is sufficient for a reliable determination of astrophysical reactivities up to 0.5 GK stellar temperature.

Figure 1

(a) Reaction chamber with silicon detectors and Al sample. (b) $\mathrm{\Delta }E\text{−}E$ telescope setup to identify protons emitted in ${}^{26}\mathrm{Al}(n,p)$ reactions.

Figure 2

(a) $\mathrm{\Delta }E$ vs $E$ energy across the entire neutron energy range from thermal (25 meV) to 150 keV. Clear groups of events can be observed for ${}^{26}\mathrm{Al}(n,p_1)$ and ${}^6\mathrm{Li}(n,t)$ reactions. ${}^{26}\mathrm{Al}(n,p_1)$ events are spread over two groups as some protons do not get stopped in the $\mathrm{\Delta }E\text{−}E$ system. (b) Same as (a) but for events corresponding to neutron energies in the resonance region (1–150 keV).

Figure 3

(a) ${}^{26}\mathrm{Al}(n,p_1)$ count spectrum over the whole neutron energy range from thermal ($2.5\times {10}^{-5}$ keV) to 150 keV. (b) Zoom into the resonance region. The positions of present and previously observed resonances are marked by black lines and listed in Table 1.

Figure 4

Stellar reaction rate compared with previous measurements by Trautvetter et al. [11], Koehler et al. [10], and theoretical calculations and evaluations [12, 22, 23].