First spin-parity constraint of the 306 keV resonance in ${}^{35}\mathrm{Cl}$ for nova nucleosynthesis

Of particular interest in astrophysics is the ${}^{34}\mathrm{S}(p,\gamma ){}^{35}\mathrm{Cl}$ reaction, which serves as a stepping stone in thermonuclear runaway reaction chains during a nova explosion. Though the isotopes involved are all stable, the reaction rate of this significant step is not well known, due to a lack of experimental spectroscopic information on states within the Gamow window above the proton separation threshold of ${}^{35}\mathrm{Cl}$. Measurements of level spins and parities provide input for the calculation of resonance strengths, which ultimately determine the astrophysical reaction rate of the ${}^{34}\mathrm{S}(p,\gamma ){}^{35}\mathrm{Cl}$ proton capture reaction. By performing the ${}^{37}\mathrm{Cl}(p,t){}^{35}\mathrm{Cl}$ reaction in normal kinematics at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory, we have conducted a study of the region of astrophysical interest in ${}^{35}\mathrm{Cl}$, and have made the first-ever constraint on the spin and parity assignment for a level at $6677\pm 15$ keV $(E_r=306$ keV), inside the Gamow window for novae.

Figure 1

Experimental setup for the $\left(p,t\right)$ experiment, not to scale. Not shown is the large aluminum plate upstream of the detectors, which was roughly centered in the target chamber.

Figure 2

Triton energy spectrum (for ${\theta }_{\mathrm{lab}}\sim {22}^{\circ }$ in one SIDAR telescope) for both a natural chlorine target (black dashed) and enriched chlorine $\left({}^{37}\mathrm{Cl}\right)$ target (blue solid), showing the locations of select ${}^{33,35}\mathrm{Cl}$ levels, the proton separation energy in ${}^{35}\mathrm{Cl}$ (green dashed line), and the Gamow window for ${}^{34}\mathrm{S}(p,\gamma ){}^{35}\mathrm{Cl}$ in novae (brown hatching). The relative normalization of the spectra is arbitrary and was chosen to enable qualitative comparisons. (a) Full triton energy spectrum; (b) same spectra zoomed into the area around the Gamow window for ${}^{34}\mathrm{S}(p,\gamma ){}^{35}\mathrm{Cl}$ in novae. The peak inside the Gamow window has an energy of $E_x=6677\pm 15$ keV.

Figure 3

Extracted excitation energy for the peak within the Gamow window as a function of detector strip (larger strip numbers correspond to larger laboratory angles). The uncertainties shown for the data are the experimental resolution. The blue band represents the standard deviation of the data.

Figure 4

Angular distribution (black circles) and DWBA calculation (green dash) for the known ${\frac{3}{2}}^{+}{}^{35}\mathrm{Cl}$ ground state. A previous study of this level with a 40 MeV proton beam [14] assigned an $L$-transfer value of zero, consistent with the current results. The digitized Vignon data [14] are shown (blue triangles) for comparison; the divergence at small angles is likely due to a small $L=2$ admixture which is more evident at the lower beam energy.

Figure 5

Angular distribution (black circles) and DWBA calculations $(L=0$, green dashed; $L=1$, blue dotted; $L=2$, red solid; $L=3$, purple dot-dashed) for the peak falling inside the Gamow window, at $6677\pm 15$ keV. The $y$-axis scale is the same as in Fig. 4. The $L=2$ curve has the lowest ${\chi }^2$ value.