Study of the ${}^{26}{\mathrm{Al}}^m(d,p){}^{27}\mathrm{Al}$ Reaction and the Influence of the ${}^{26}\mathrm{Al}$ $0^{+}$ Isomer on the Destruction of ${}^{26}\mathrm{Al}$ in the Galaxy

The existence of ${}^{26}\mathrm{Al}$ ($t_{1/2}=7.17\times {10}^5\mathrm{yr}$) in the interstellar medium provides a direct confirmation of ongoing nucleosynthesis in the Galaxy. The presence of a low-lying $0^{+}$ isomer (${{}^{26}\mathrm{Al}}^m$), however, severely complicates the astrophysical calculations. We present for the first time a study of the ${{}^{26}\mathrm{Al}}^m(d,p){}^{27}\mathrm{Al}$ reaction using an isomeric ${}^{26}\mathrm{Al}$ beam. The selectivity of this reaction allowed the study of $\ell =0$ transfers to $T=1/2$, and $T=3/2$ states in ${}^{27}\mathrm{Al}$. Mirror symmetry arguments were then used to constrain the ${{}^{26}\mathrm{Al}}^m(p,\gamma ){}^{27}\mathrm{Si}$ reaction rate and provide an experimentally determined upper limit of the rate for the destruction of isomeric ${}^{26}\mathrm{Al}$ via radiative proton capture reactions, which is expected to dominate the destruction path of ${{}^{26}\mathrm{Al}}^m$ in asymptotic giant branch stars, classical novae, and core collapse supernovae.

Figure 1

(a) Spectrum of the ${}^{26}\mathrm{Al}$ beam measured at 0° in a silicon detector. The main contaminant is a ${11}^{+}$ charge state of the ${}^{26}\mathrm{Mg}$ production beam. (b) Coincidence spectrum measured with the NaI detectors. The inset shows the half-life of the beam determined by the 511 keV coincidences confirming the presence of the isomer.

Figure 2

(a) Apparent excitation energy spectrum of ${}^{27}\mathrm{Al}$ obtained from the ${}^{26}\mathrm{Al}(d,p)$ reaction at ${\theta }_{\text{c.m.}}\sim 6°–12°$. A smooth carbon background has been subtracted. The ${}^{27}\mathrm{Al}$ excitation energy was calculated using the $Q$ value for the ground state. Therefore, states populated by the isomer component of the beam appear shifted down in energy. (b) Data from the ${{}^{26}\mathrm{Al}}^g(d,p)$ reaction in a similar angular range [23] folded with a 120 keV Gaussian, normalized to the state at 3.004 MeV are shown for comparison. (c) Apparent excitation energy spectrum of ${}^{27}\mathrm{Al}$ from the ${{}^{26}\mathrm{Al}}^m(d,p)$ reaction. The spectrum was obtained by subtracting contributions from the ${{}^{26}\mathrm{Al}}^g$ beam measured in Ref. [23].

Figure 3

(a) Angular distribution and DWBA and ADWA fits for the $9/2^{+}$ state in ${}^{27}\mathrm{Al}$ at $E_{\mathrm{ex}}=3.004\mathrm{MeV}$. The data agree with the $\ell =0$ transfer from the $5^{+}$ ground state component observed in Ref. [24]. A spectroscopic factor, $C^{2}S=0.49$ [24] was used to obtain the absolute beam normalization of the cross section. Angular distributions and DWBA calculations for the states in ${}^{27}\mathrm{Al}$ at (b) $E_{\mathrm{ex}}=0.84$, (c) $E_{\mathrm{ex}}=6.8$, and (d) $E_{\mathrm{ex}}=10.2\mathrm{MeV}$. These three states are strongly populated by $\ell =0$ neutron transfers on the isomeric component of the ${}^{26}\mathrm{Al}$ beam.

Figure 4

(a) Upper limits for the rate of the ${{}^{26}\mathrm{Al}}^m(p,\gamma ){}^{27}\mathrm{Si}$ reaction in stellar environments as a function of the temperature. The isomer ($m$) rate (this work) is shown compared to the ground state ($g$) and the recommended NACRE rate [29, 30]. The ($g$) rate was calculated with parameters from Refs. [20, 23, 24]. (b) Ratios between the experimental isomeric ($m$, this Letter) and ground state ($g$, Refs. [20, 23, 24]) rates (solid line) and the isomeric and ground state rates recommended by the NACRE-STARLIB calculations (dashed line) [29, 30, 52].