Decay properties of 22Ne + α resonances and their impact on s-process nucleosynthesis

Abstract

The astrophysical s-process is one of the two main processes forming elements heavier than iron. A key outstanding uncertainty surrounding s-process nucleosynthesis is the neutron flux generated by the 22 Ne(α, n) 25 Mg reaction during the He-core and C-shell burning phases of massive stars. This reaction, as well as the competing 22 Ne(α, γ) 26 Mg reaction, is not well constrained in the important temperature regime from ∼0.2-0.4 GK, owing to uncertainties in the nuclear properties of resonances lying within the Gamow window. To address these uncertainties, we have performed a new measurement of the 22 Ne( 6 Li, d) 26 Mg reaction in inverse kinematics, detecting the outgoing deuterons and 25,26 Mg recoils in coincidence. We have established a new n/γ decay branching ratio of 1.14(26) for the key E x = 11.32 MeV resonance in 26 Mg, which results in a new (α, n) strength for this resonance of 42(11) µeV when combined with the well-established (α, γ) strength of this resonance. We have also determined new upper limits on the α partial widths of neutron-unbound resonances at E x = 11. 112, 11.163, 11.169, and 11.171 MeV. Monte-Carlo calculations of the stellar 22 Ne(α, n) 25 Mg and 22 Ne(α, γ) 26 Mg rates, which incorporate these results, indicate that both rates are substantially lower than previously thought in the temperature range from ∼0.2-0.4 GK.

The Study of Key Reactions Shaping the Post-Main Sequence Evolution of Massive Stars in Underground Facilities

“…The new evaluation of the reaction rate, based on spin-parity assignments by Jayatissa et al (2020) combined with data from Ota et al (2020), resulted in lower rates than previous evaluations, especially at low temperatures (see Figure 7). The lower rate is also the result of excluding an excited state at E x 11.112 MeV, corresponding to E α,lab 598 keV, observed by Massimi et al (2017) and not observed in these studies.…”

“…The continued interest in this reaction is demonstrated by two very recent experimental studies by Ota et al (2020) and Jayatissa et al (2020) with α transfer reactions: Ota et al (2020) studied the 22 Ne( 6 Li, d) 26 Mg in inverse kinematics, detecting outgoing deuterons and 25,26 Mg in coincidence. In addition, Jayatissa et al (2020) studied the 22 N( 7 Li, t) 26 Mg reaction.…”

Reevaluation of the Ne22(α,γ)Mg26 and Adsley1, Battino2, Best3 et al. 2021Phys. Rev. C460310View full textAdd to dashboardCite

“…In addition to the Giesen and Talwar 22 Ne( 6 Li,d) 26 Mg data, the more recent 22 Ne( 6 Li,d) 26 Mg results from Texas A&M at 7 MeV/u (corresponding to a 6 Li beam energy of 42 MeV) [16] and the sub-Coulomb barrier transfer results at 1 MeV/u [15] both fail to observe a strongly populated state at around E x = 11.17 MeV. In the case of the sub-Coulomb transfer measurement, the non-observation of the state could be due to the state of interest having a high spin [15].…”

“…The other experimental study used the 22 Ne( 6 Li,d) 26 Mg transfer reaction in inverse kinematics at a beam energy of 7 MeV/u [16]. In this case the deuteron ejectiles were detected in an array of silicon detectors and the 25 Mg and 26 Mg recoils were detected at the focal plane of the MDM magnetic spectrograph.…”

“…The higher-energy measurement from Texas A&M (at 7 MeV/u) is at an energy between the Giesen and Talwar measurements, and no strongly populated state at E x = 11.17 MeV is observed. In this measurement [16], the energy resolution was around 230 keV (FWHM) and it is possible that a weakly populated state in close proximity to the strongly populated state at E x = 11.31 MeV could have been missed. However, the results of Talwar suggest that the E x = 11.17-and 11.31-MeV states should have almost identical population strengths so this explanation for the non-observation of the state may be discounted.…”