Abstract
Measurements of sulphur isotopes in presolar grains can help to identify the astrophysical sites in which these grains were formed. A more precise thermonuclear rate of the 33S(p,γ)34Cl reaction is required, however, to assess the diagnostic ability of sulphur isotopic ratios. We have studied the 33S(3He,d)34Cl proton-transfer reaction at 25 MeV using a high-resolution quadrupole–dipole–dipole–dipole magnetic spectrograph. Deuteron spectra were measured at ten scattering angles between 10° and 55°. Twenty-four levels in 34Cl over Ex=4.6–5.9 MeV were observed, including three levels for the first time. Proton spectroscopic factors were extracted for the first time for levels above the 33S + p threshold, spanning the energy range required for calculations of the thermonuclear 33S(p,γ)34Cl rate in classical nova explosions. We have determined a new 33S(p,γ)34Cl rate using a Monte Carlo method and have performed new hydrodynamic nova simulations to determine the impact on nova nucleosynthesis of remaining nuclear physics uncertainties in the reaction rate. We find that these uncertainties lead to a factor of ≤5 variation in the 33S(p,γ)34Cl rate over typical nova peak temperatures, and variation in the ejected nova yields of SCa isotopes by ≤20%. In particular, the predicted 32S/33S ratio is 110–130 for the nova model considered, compared to 110–440 with previous rate uncertainties. As recent type II supernova models predict ratios of 130–200, the 32S/33S ratio may be used to distinguish between grains of nova and supernova origin.
Highlights
Introduction for levels withEx(34Cl) > 5.43 MeV, with the reaction crosssection limiting the lowest energies at which measurementsClassical nova explosions occur through thermonuclear igni- have been performed
Excitation energies for 34Cl levels observed in the present work are in excellent agreement with values previously reported in the literature
The observation of new levels in this energy range is not unexpected given that levels from neither the 33S(3He,d) nor the 33S(p, γ) reaction have been previously reported for Ex > 4.63 MeV [30] and Ex < 5.43 MeV [9, 22, 23], respectively
Summary
The focal plane was initially calibrated at each measured angle using well-resolved, known states in 25Al populated through the (3He,d) reaction on the 24Mg target. These low-energy states (1.6 < Ex(25Al) < 3.1 MeV) are known to a precision of better than 1 keV [34]. Different sets of optical model parameters, scaled to the present target mass and beam energy, were adopted in the calculations to assess additional associated systematic uncertainties in the spectroscopic factors (see Section 4.2 as well). Values of the extracted spectroscopic factors were found to be relatively insensitive to these different sets of optical model parameters, with variations of only ≈ 5 − 10% depending on the state
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