Abstract
The NeNa and the MgAl cycles play a fundamental role in the nucleosynthesis of asymptotic giant branch stars undergoing hot bottom burning. The Na23(p,γ)24Mg reaction links these two cycles and a precise determination of its rate is required to correctly estimate the contribution of these stars to the chemical evolution of various isotopes of Na, Mg and Al. At temperatures of 50≲T≲110MK, narrow resonances at Ep=140 and 251keV are the main contributors to the reaction rate, in addition to the direct capture that dominates in the lower part of the temperature range. We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA). We have used two complementary detection approaches: high efficiency with a 4π BGO detector for the 140keV resonance, and high resolution with a HPGe detector for the 251keV resonance. Thanks to the reduced cosmic ray background of LUNA, we were able to determine the resonance strength of the 251keV resonance as ωγ=482(82)μeV and observed new gamma ray transitions for the decay of the corresponding state in Mg24 at Ex=11931keV. With the highly efficient BGO detector, we observed a signal for the 140keV resonance for the first time in a direct measurement, resulting in a strength of ωγ140=1.46−0.53+0.58neV (68% CL). Our measurement reduces the uncertainty of the Na23(p,γ)24Mg reaction rate in the temperature range from 0.05 to 0.1GK to at most −35%+50% at 0.07GK. Accordingly, our results imply a significant reduction of the uncertainties in the nucleosynthesis calculations.
Highlights
We present new measurements of the strengths of these resonances at the Laboratory for Underground Nuclear Astrophysics (LUNA)
Asymptotic Giant Branch (AGB) stars provide a major contribution to the synthesis of the elements in the cosmos and to the chemical evolution of stellar clusters and galaxies
The uncertainties of the 23Na(p, γ )24Mg and the 22Ne(p, γ )23Na reaction rates account for the major nuclear contribution to the uncertainty of predicted 23Na and 24Mg yields from these massive AGB stars [3]
Summary
Asymptotic Giant Branch (AGB) stars provide a major contribution to the synthesis of the elements in the cosmos and to the chemical evolution of stellar clusters and galaxies. The. 23Na(p, γ )24Mg reaction rate in this temperature range is dominated by two resonances at Ep = 140 and 251 keV, where Ep is the proton beam energy in the laboratory system throughout this paper. The upper limit on the strength of the 140 keV resonance [9] does not exclude this resonance from being the dominant contribution to the reaction rate at T ∼ 80 − 90 MK. The uncertainty of the 23Na(p, γ )24Mg reaction rate in the discussed temperature range, associated with the upper limit of the 140 keV resonance strength and the 33% relative uncertainty of the 251 keV resonance strength, provide a strong motivation for an experimental study of these resonances
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