Understanding the cosmic abundance of 22Na: Lifetime measurements in 23Mg

Simulations of explosive nucleosynthesis in novae predict the production of ²²Na, a key astronomical observable to constrain nova models. Its gamma-ray line at 1.275 MeV has not yet been observed by the gamma-ray space telescopes. The ²⁰Ne/²²Ne ratio in presolar grains, a possible tool to identify nova grains, also depends on ²²Na produced. Uncertainties on its yield in classical novae currently originate from the rate of the ²²Na(p, γ)²³Mg reaction. At peak novae temperatures, this reaction is dominated by a resonance at E_R=0.204 MeV, corresponding to the E_x=7.785 MeV excited state in ²³Mg. The resonance strengths measured so far disagree by one order of magnitude. An experiment has been performed at GANIL to measure the lifetime and the proton branching ratio of this key state, with a femtosecond resolution for the former. The reactions populating states in ²³Mg have been studied with a high resolution detection set-up, i.e. the particle VAMOS, SPIDER and gamma tracking AGATA spectrometers, allowing the measurements of lifetimes and proton branchings. We present here a comparison between experimental results and shell-model calculations, that allowed us to assign the spin and parity of the key state. Rather small values obtained for reduced M1 matrix elements, M(M1) ≲ 0.5 µ_N, and proton spectroscopic factors, C²S_p<10⁻², seem to be beyond the accuracy of the shell model. With the reevaluated ²²Na(p, γ)²³Mg rate, the ²²Na detectability limit and its observation frequency from novae are found promising for the future space telescopes.