Discovery of ${}^{34g,m}\mathrm{Cl}{(p,\gamma )}^{35}\mathrm{Ar}$ resonances activated at classical nova temperatures

Background: The thermonuclear ${}^{34g,m}\mathrm{Cl}{(p,\gamma )}^{35}\mathrm{Ar}$ reaction rates are unknown due to a lack of experimental nuclear physics data. Uncertainties in these rates translate to uncertainties in ${}^{34}\mathrm{S}$ production in models of classical novae on oxygen-neon white dwarfs. ${}^{34}\mathrm{S}$ abundances have the potential to aid in the classification of presolar grains.

Purpose: Determine resonance energies for the ${}^{34g,m}\mathrm{Cl}{(p,\gamma )}^{35}\mathrm{Ar}$ reactions within the region of astrophysical interest for classical novae to a precision of a few keV as an essential first step toward constraining their thermonuclear reaction rates.

Method: ${}^{35}\mathrm{Ar}$ excited states were populated by the ${}^{36}\mathrm{Ar}{(d,t)}^{35}\mathrm{Ar}$ reaction at $E\left(d\right)=22\mathrm{MeV}$ and reaction products were momentum analyzed by a high resolution quadrupole-dipole-dipole-dipole (Q3D) magnetic spectrograph.

Results: Seventeen new ${}^{35}\mathrm{Ar}$ levels have been detected at a statistically significant level in the region $E_x\approx 5.9–6.7\mathrm{MeV}$ $(E_r<800\mathrm{keV})$ and their excitation energies have been determined to typical uncertainties of 3 keV. The uncertainties for five previously known levels have also been reduced substantially. The measured level density was compared to those calculated using the WBMB Hamiltonian within the $sd\text{−}pf$ model space.

Conclusions: Most of the resonances in the region of astrophysical interest have likely been discovered and their energies have been determined, but the resonance strengths are still unknown, and experimentally constraining the ${}^{34g,m}\mathrm{Cl}{(p,\gamma )}^{35}\mathrm{Ar}$ reaction rates will require further experiments.

Figure 1

Triton position spectra with $E_{\mathrm{beam}}=22$ MeV for the ${}^{36}\mathrm{Ar}{(d,t)}^{35}\mathrm{Ar}$ reaction (black) superimposed on normalized ${}^{13}\mathrm{C}{(d,t)}^{12}\mathrm{C}$ background spectra (gray) for spectrograph angles: (a) ${\theta }_{\mathrm{lab}}={15}^{\circ }$, (b) ${\theta }_{\mathrm{lab}}={20}^{\circ }$, (c) ${\theta }_{\mathrm{lab}}={25}^{\circ }$, and (d) ${\theta }_{\mathrm{lab}}={54}^{\circ }$. Peaks are labeled with $E_x$ in keV.

Figure 2

Background subtracted triton position spectrum with different binning than Fig. 1 for the ${}^{36}\mathrm{Ar}{(d,t)}^{35}\mathrm{Ar}$ reaction with $E_{\mathrm{beam}}=22$ MeV and ${\theta }_{\mathrm{lab}}={54}^{\circ }$ shown in black crosses. (a) shows the middle of the focal plane, and (b) shows the left edge of the focal plane. The solid blue line shows the overall best fit and the constituent exponentially modified Gaussian peaks on top of the flat residual background are shown by the dotted red lines. Peaks are labeled with excitation energy in keV.

Figure 3

Experimental level densities of ${}^{35}\mathrm{Ar}$ and the mirror nucleus ${}^{35}\mathrm{Cl}$ and calculated shell model level densities. Experimental level densities for ${}^{35}\mathrm{Ar}$ with $E_x<5.9$ keV and ${}^{35}\mathrm{Cl}$ are taken from [3].