Helium burning of22Ne

Abstract

The 2ZNe(c~,7)26Mg and 22Ne(cq n)2SMg reactions were investigated for E~(lab) from 0.71 to 2.25 MeV. Neon gas enriched to 99% in 22Ne was recirculated in a differentially pumped gas target system of the extended type. The 7-ray transitions were observed with Ge(Li) detectors and the neutrons with 3He ionization chambers. A previously known resonance at ER(lab)= 2.05 MeV was verified and 15 new resonances were found in the energy range covered, with the lowest at ER(lab)=0.83 MeV. Information on resonance energies, widths, strengths, 7-ray branching ratios, as well as J" assignments, is reported. The energy range investigated corresponds to the important temperature range of T9 from 0.3 to 1.4 (109 K), for which the astrophysical rates were determined for both reactions. The results show that the ratios of the rates for 22Ne(c~,n)2SMg and 22Ne(c~, 7)26Mg are significantly smaller than the previously adopted values, e.g., by at least a factor of 60 near T9=0.65. Thus, the 22Ne(e, n)25Mg reaction will likely play a smaller role as a neutron source for s-process nucleosynthesis, than has frequently been assumed.

Elastic scattering of protons from15N

“…The target chamber was very similar to the one shown in Ref. [9]'s Fig. 2a, except that additional view ports were available at θ lab = 135, 150, and 165 degrees.…”

“…This gas target has been used extensively in the past, see, e.g., Refs. [8,9] and references therein for further details. Nitrogen gas, enriched to 99 percent 15 N and kept at a pressure of ∼0.25 Torr, was used throughout the experiment.…”

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

“…Despite several attempts to measure the reaction cross section [52][53][54][55], these experiments failed to reach the low α energies of astrophysical relevance, mostly because cosmic-ray induced background starts to dominate the experimental signatures below E α = 0.5 MeV. A series of indirect measurements via α-particle transfer reactions have been performed to overcome this limitation [56,57].…”

“…[58], and upper limits are from Ref. [52]. In Table VII Below 0.2 GK the reaction rate is dominated by the natural parity resonances at 19.86 and 72.66 keV.…”

“…Previous evaluations [52] have assumed that all states observed by neutron spectroscopy below the lowest observed resonance at E α = 832 keV can contribute to the reaction rate. This approach overestimates this contribution because in the 22 Ne(α, n) 25 Mg reaction the 26 Mg compound states are formed 26 Mg, obtained from nuclear resonance fluorescence [40] experiments, were used by Longland [59] to improve the calculation of the reaction rate.…”