The lifetime of the 7.786 MeV state in 23Mg as a probe for classical novae models

We propose to measure the lifetime of the astrophysically important 7.786 MeV level in 23 Mg. The radiative width of this state is dominating the cross–section of the 22Na(p,γ)23Mg reaction, considered as the main destruction process for the 22 Na produced in novae explosions. The aim of this experiment is to resolve the disagreement between the two existing direct determinations of the rate of this reaction, crucial for the test of novae models.

The study of lifetimes of the order of ≈fs will be possible by analysing the Doppler–Shift–Attenuated (DSA) lineshape of the gamma rays. The position resolution of AGATA will be exploited. The state of interest will be populated via the 24Mg+3He→23Mg+α reaction at 4 MeV/u of beam energy. The reaction channel and excitation energy of interest will be tagged by detecting the α particles in the VAMOS++ spectrometer, which will be oriented at 0° with respect to the beam axis, and in an annular silicon dE–E detector (SPIDER).

The same experiment will provide an evaluation of the branching ratio for proton decay of the considered state in 23 Mg. The lifetime and proton branching ratio obtained from the same experiment will provide a precise determination of the rate of the 22 Na(p,γ)23 Mg reaction, and, hence, add a fundamental brick in the understanding of novae modeling.