Astrophysically important ${}^{26}\mathrm{Si}$ states studied with the ${}^{28}\mathrm{Si}$($p,t$)${}^{26}\mathrm{Si}$ reaction. II. Spin of the 5.914-MeV ${}^{26}\mathrm{Si}$ level and galactic ${}^{26}\mathrm{Al}$ production

The ${}^{28}\mathrm{Si}$($p,t$)${}^{26}\mathrm{Si}$ reaction has been studied to resolve a controversy surrounding the properties of the ${}^{26}\mathrm{Si}$ level at 5.914 MeV and its contribution to the ${}^{25}\mathrm{Al}$($p,\gamma$)${}^{26}\mathrm{Si}$ reaction rate in novae, which affects interpretations of galactic ${}^{26}\mathrm{Al}$ observations. Recent studies have come to contradictory conclusions regarding the spin of this level ($0^{+}$ or $3^{+}$), with a $3^{+}$ assignment implying a large contribution by this level to the ${}^{25}\mathrm{Al}$($p,\gamma$)${}^{26}\mathrm{Si}$ reaction rate. We have extended our previous study [Bardayan et al., Phys. Rev. C 65, 032801(R) (2002)] to smaller angles and find the angular distribution of tritons populating the 5.914-MeV level in the ${}^{28}\mathrm{Si}$($p,t$)${}^{26}\mathrm{Si}$ reaction to be consistent with either a $2^{+}$ or $3^{+}$ assignment. We have calculated reaction rates under these assumptions and used them in a nova nucleosynthesis model to examine the effects of the remaining uncertainties in the ${}^{25}\mathrm{Al}$($p,\gamma$)${}^{26}\mathrm{Si}$ rate on ${}^{26}\mathrm{Al}$ production in novae.

Figure 1

A typical particle-identification plot used to distinguish tritons from other reaction products.

Figure 2

The triton energy spectrum from the ${}^{28}\mathrm{Si}$($p,t$)${}^{26}\mathrm{Si}$ reaction observed at ${17}^{°}$. Peaks are labeled with excitation energies from Refs. [11, 14, 23]. The bottom plot shows the deuteron energy spectrum observed at the same angle and demonstrates that our conclusions concerning the 5914-keV level are not influenced by any possible deuteron contamination of our spectra.

Figure 3

The excitation energy extracted for the 5914-keV level is plotted vs. angle for the data obtained in this work and Ref. [14]. Lines have been drawn at the average energy from this work and the energies reported in Ref. [11].

Figure 4

The angular distribution for the 5914-keV level is plotted. Open circles are from this work while filled circles are from Ref. [14]. The dotted and solid curves are DWBA calculations for the population of $0^{+}$ and $2^{+}$ levels, respectively. The dashed line is a FRESCO calculation of the expected angular distribution for populating a $3^{+}$ through a multistep process. The dot-dashed line shows the multistep calculation for populating a $0^{+}$ after reducing the normalization by a factor of ten to directly compare with the data.

Figure 5

The top figure shows the contributions of resonances and direct capture to the ${}^{25}\mathrm{Al}$($p,\gamma$)${}^{26}\mathrm{Si}$ reaction rate. The bottom figure shows the sum of these contributions along with an estimated uncertainty.