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

Simulations of explosive nucleosynthesis in novae predict the production of Na-22, 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 Ne-20/Ne-22 ratio in presolar grains, a possible tool to identify nova grains, also depends on Na-22 produced. Uncertainties on its yield in classical novae currently originate from the rate of the Na-22(p,gamma)Mg-23 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 Mg-23. 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 Mg-23 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, vertical bar M(M1)vertical bar less than or similar to 0.5 mu(N), and proton spectroscopic factors, (CSp)-S-2<10(-2), seem to be beyond the accuracy of the shell model. With the reevaluated Na-22(p,gamma)Mg-23 rate, the Na-22 detectability limit and its observation frequency from novae are found promising for the future space telescopes.