Understanding 22Na cosmic abundance by measuring lifetimes in 23Mg

given by Chloé Fougères

at the Maison d’hôtes at 11h00

Abstract:
Simulations of novae explosive nucleosynthesis predict the production of the radionuclide ²²Na. Its half
life of 2.6 yr makes it a very interesting astronomical observable by allowing space time correlation with
the astrophysical object. Radionuclide ²²Na should provide constraints on nova models. Its synthesis may
also help to explain abnormal ²²Ne abundance observed in presolar grains and in galactic cosmic rays.
Its gamma-ray line at 1.275 MeV has not been observed yet by the astronomical observation campaigns.
Hence accurate yields of ²²Na are required. Within nova temperatures range, the main destruction reaction
²²Na(p,γ)²³Mg is dominated by a resonance at 0.213 MeV leading towards ²³Mg excited state at 7.786 MeV.
However the measured strengths of this resonance are in disagreement.
An experiment was performed at GANIL facility to measure the lifetime of the key state at 7.786 MeV
with an expected resolution of 1 fs. The experiment will be presented. With a beam energy of 4.6 MeV/u,
the reaction ³He(²⁴Mg,α)²³Mg* populated the state of interest. This reaction was tagged thanks to particle
detectors (VAMOS++ spectrometer, silicon detector SPIDER) and gamma tracking spectrometer AGATA.
The key state has two decay channels (gamma deexcitation, proton emission) which must be characterized.
The expected time resolution of 1 fs is made possible with AGATA because of its high space and energy
resolution. Several Doppler based methods will be used for analysing lineshape of measured redshifted
gamma peaks. SPIDER detector will allow to evaluate the branching ratio for proton decay. In fine, a new
value of ²²Na(p,γ)²³Mg resonance strength ωγ is expected with a higher precision.
The analysis procedures and some preliminary results will be presented. Produced light particles are
identified within SPIDER data, as well as Doppler shifted gamma rays from ²³Mg excited states. Gamma
spectra are improved by coincidence constrains on reconstructed α excited energies with VAMOS, ensuring
so to focus on states directly populated. A new method based on Doppler correction leads to estimate the
velocity of the gamma emitting ²³Mg ions. A test case with Monte Carlo simulations of the experiment is
done to measure a known lifetime of ²³Mg at 2.052 MeV.