Isospin Mixing Reveals ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ Resonance Influencing Nova Nucleosynthesis

The thermonuclear ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ reaction rate is critical for modeling the final elemental and isotopic abundances of ONe nova nucleosynthesis, which affect the calibration of proposed nova thermometers and the identification of presolar nova grains, respectively. Unfortunately, the rate of this reaction is essentially unconstrained experimentally, because the strengths of key ${}^{31}\mathrm{S}$ proton capture resonance states are not known, largely due to uncertainties in their spins and parities. Using the $\beta$ decay of ${}^{31}\mathrm{Cl}$, we have observed the $\beta$-delayed $\gamma$ decay of a ${}^{31}\mathrm{S}$ state at $E_x=6390.2(7)\mathrm{keV}$, with a ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ resonance energy of $E_r=259.3(8)\mathrm{keV}$, in the middle of the ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ Gamow window for peak nova temperatures. This state exhibits isospin mixing with the nearby isobaric analog state at $E_x=6279.0(6)\mathrm{keV}$, giving it an unambiguous spin and parity of $3/2^{+}$ and making it an important $l=0$ resonance for proton capture on ${}^{30}\mathrm{P}$.

Figure 1

Selected portions of the $\beta$-coincident $\gamma$-ray spectrum [black (blue online) line] showing transitions from the 6279- and 6390-keV ${}^{31}\mathrm{S}$ states to the ground state and first two excited states ($J^{\pi }=1/2^{+},3/2^{+},5/2^{+}$, respectively). The bottom two panels also show $\beta \text{−}\gamma \text{−}\gamma$ spectra [gray (green online) line] with additional coincidence conditions on the 1248- and 2234-keV $\gamma$ rays, respectively. Other photopeaks observed from the $\beta$ decay of ${}^{31}\mathrm{Cl}$ are marked with black circles. Double escape peaks are marked with double asterisks.

Figure 2

A simplified ${}^{31}\mathrm{Cl}$ decay scheme focusing on the ${}^{31}\mathrm{S}$ levels at 6279 (IAS) and 6390 keV. The gray (blue online) vertical arrows indicate previously unobserved transitions. Energies and intensities for these transitions are listed in Table 1.

Figure 3

Ratios of the ${}^{30}\mathrm{P}(p,\gamma ){}^{31}\mathrm{S}$ thermonuclear reaction rates calculated for both the new $3/2^{+}$ state at 6390 keV [solid (blue online) line] and the 6280-keV IAS [dashed (green online) line] to the overall Hauser-Feshbach rate [8].