Direct measurement of resonance strengths in 34S(α ,γ )38Ar at astrophysically relevant energies using the DRAGON recoil separator

Background: Nucleosynthesis of mid-mass elements is thought to occur under hot and explosive astrophysical conditions. Radiative α capture on ³⁴S has been shown to impact nucleosynthesis in several such conditions, including core and shell oxygen burning, explosive oxygen burning, and type Ia supernovae.

Purpose: Broad uncertainties exist in the literature for the strengths of three resonances within the astrophysically relevant energy range (E_CM=1.94 -3.42 MeV at T =2.2 GK ). Further, there are several states in ³⁸Ar within this energy range which have not been previously measured. This work aimed to remeasure the resonance strengths of states for which broad uncertainty existed as well as to measure the resonance strengths and energies of previously unmeasured states.

Methods: Resonance strengths and energies of eight narrow resonances (five of which had not been previously studied) were measured in inverse kinematics with the DRAGON facility at TRIUMF by impinging an isotopically pure beam of ³⁴S ions on a windowless ⁴He gas target. Prompt γ emissions of de-exciting ³⁸Ar recoils were detected in an array of bismuth germanate scintillators in coincidence with recoil nuclei, which were separated from unreacted beam ions by an electromagnetic mass separator and detected by a time-of-flight system and a multianode ionization chamber.

Results: The present measurements agree with previous results. Broad uncertainty in the resonance strength of the E_CM=2709 keV resonance persists. Resonance strengths and energies were determined for five low-energy resonances which had not been studied previously, and their strengths were determined to be significantly weaker than those of previously measured resonances.

Conclusions: The five previously unmeasured resonances were found not to contribute significantly to the total thermonuclear reaction rate. A median total thermonuclear reaction rate calculated using data from the present work along with existing literature values using the STARLIB rate calculator agrees with the NON-SMOKER statistical model calculation as well as the REACLIB and STARLIB library rates at explosive and nonexplosive oxygen-burning temperatures (T =3 -4 GK and T =1.5 -2.7 GK , respectively).