Low-lying resonances in ${}^{26}\mathrm{Si}$ relevant for the determination of the astrophysical ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction rate

The ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction plays a key role in accurately modeling and understanding the nucleosynthesis of the long-lived radioisotope ${}^{26}\mathrm{Al}$ observed throughout the galaxy by $\gamma$-ray telescopes via the detection of its 1.809 MeV $\gamma$-ray line. The ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction is responsible for redirecting the flux of nuclear material away from the ground state of the long-lived radioisotope ${}^{26}\mathrm{Al}$ (${}^{26}\mathrm{Al}^g$) in favor of its short-lived isomer (${}^{26}\mathrm{Al}^m$) which bypasses the emission of the 1.809 MeV $\gamma$ ray, but is observed in, for example, an excess of the isotopic abundance of ${}^{26}\mathrm{Mg}$ in meteorites. Uncertainties in the ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction rate are dominated by the nuclear properties of low-lying proton-unbound states in ${}^{26}\mathrm{Si}$. A high-sensitivity spectroscopic study of ${}^{26}\mathrm{Si}$ was performed at the John D. Fox Accelerator Laboratory at Florida State University, using a neutron/$\gamma$-ray coincidence measurement with the ${}^{24}\mathrm{Mg}({}^3\mathrm{He},n\gamma ){}^{26}\mathrm{Si}$ reaction. The present measurement solves previous discrepancies in the existence and location of the relevant resonances in ${}^{26}\mathrm{Si}$. Furthermore, the high sensitivity of the study allowed for a direct estimate of the 3${}_3^{+}\gamma$-partial width. The present experimental information combined with previous works provide an updated rate of the ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction at nova temperatures.

Figure 1

Neutron-gated ToF spectrum obtained in the present experiment with the CATRiNA detector placed at ${40}^{\circ }$. Neutron groups corresponding to states in ${}^{26}\mathrm{Si}$ as well as groups from ${}^{16}\mathrm{O}$ present in the target are labeled (states in ${}^{26}\mathrm{Si}$ are given in units of MeV). The dashed vertical line indicates the proton separation energy in ${}^{26}\mathrm{Si}$.

Figure 2

Reconstructed $Q$-value spectrum of the ${}^{24}\mathrm{Mg}({}^3\mathrm{He},n\gamma ){}^{26}\mathrm{Si}$ reaction for all CATRiNA detectors. The dashed vertical line indicates the proton separation energy in ${}^{26}\mathrm{Si}$.

Figure 3

The neutron-$\gamma$ matrix built using data from the CATRiNA detectors (neutron $Q$ value in the $y$ axis) and the FSU Clovers ($\gamma$-ray energy in the $x$ axis). Several well-known transitions in ${}^{26}\mathrm{Si}$ are identified. The horizontal red dashed line indicates the proton separation energy ($S_p$) in ${}^{26}\mathrm{Si}$.

Figure 4

Neutrons in coincidence with $\gamma$ rays at $E_{\gamma }=4092$ keV showing a state at $Q=-5.83$ (15) MeV, which corresponds to $E_{ex}=5.90$ MeV in ${}^{26}\mathrm{Si}$.

Figure 5

$\gamma$ rays in coincidence with neutrons from resonance states in ${}^{26}\mathrm{S}$ (states below red line in Fig. 3) with energies below 2 MeV (top) and above 2 MeV (bottom).

Figure 6

Portion of the $\gamma$-ray spectrum around $E_{\gamma }=1741$ keV corresponding to the $3_3^{+}$ resonance state in ${}^{26}\mathrm{Si}$. A neutron gate with neutrons that populated states near $E_x=5.9$ MeV, $Q$ value $=-5.85\pm 0.15$ MeV is applied (gray). An additional gate on the PH values centered on $\mathrm{PH}=800\pm 250$ keVee, corresponding to ${\mathrm{E}}_n=3.4$–3.6 MeV, was applied (blue). A linear background fit to the blue data is shown by the solid red line.

Figure 7

ToF vs PH correlation matrix obtained in the measurement of the ${}^{24}\mathrm{Mg}({}^3\mathrm{He},n\gamma ){}^{26}\mathrm{Si}$ reaction for the CATRiNA detector placed at ${40}^{\circ }$. An additional gate on the PH values centered on $\mathrm{PH}=800\pm 250$ keVee (shown in black), was applied to all the neutron detectors. This gate corresponds to neutron energies of $E_n=3.4$–3.6 MeV, that populate states around $E_{ex}\approx 5.9$ MeV in ${}^{26}\mathrm{Si}$.

Figure 8

Contribution of the $0_4^{+}$ states to the total ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ resonant reaction rate. The relocation of the $0_4^{+}$ state, from $E_{ex}=5.946$ MeV (in black, Ref. [26]) to $E_{ex}=5.889$ MeV (in blue, this work), raises the contribution of the $0_4^{+}$ resonance up to $6\pm 2\%$.

Figure 9

Rate of the ${}^{25}\mathrm{Al}(p,\gamma ){}^{26}\mathrm{Si}$ reaction calculated in this work. The total and individual contributions of the three resonances within the Gamow window for nova temperatures are shown along with the direct-capture contribution obtained from Ref. [28].

Figure 10

Comparison of the present calculated reaction rate (numerator) to those reported by Wrede [17], Liang et al. [19], Iliadis et al. [29], and Matic et al. [34] (blue, red, green, and cyan, respectively) in the JINA REACLIB database [43] (denominator).