Speaker
Description
Simulations of explosive nucleosynthesis in novae predict the production of the radioisotope $^{22}$Na. Its half-life of 2.6 yr makes it a very interesting astronomical observable by allowing space and time correlations with the astrophysical object. This radioisotope should bring constraints on nova models. It may also help to explain abnormal $^{22}$Ne abundance observed in presolar grains and in cosmic rays. Its gamma-ray line at 1.275 MeV has not been observed yet by the gamma-ray space observatories. Accurate yields of $^{22}$Na are required. At peak nova temperatures, the main destruction reaction $^{22}$Na(p, $\gamma$)$^{23}$Mg has been found dominated by a resonance at E$_{\text{R}}$=0.204 MeV corresponding to the Ex=7.785 MeV excited state in $^{23}$Mg. However, the measured strengths of this resonance disagree by more than a factor 3, see Ref. [1, 2].
An experiment was performed at GANIL facility to measure both the lifetime and the proton branching ratio of the key state at Ex=7.785 MeV. The principle of the experiment is based on the one used in [3]. With a beam energy of 4.6 MeV/u, the reaction $^3$He($^{24}$Mg, $\alpha$)$^{23}$Mg$^*$ populated the state of interest. This reaction was measured with particle detectors (spectrometer VAMOS++, silicon detector SPIDER) and gamma tracking spectrometer AGATA. The expected time resolution with AGATA high space and energy resolutions is 1 fs. Several Doppler based methods were used to analyse the lineshape of $\gamma$-ray peaks.
Our new results will be presented. Doppler shifted $\gamma$-ray spectra from $^{23}$Mg states were improved by imposing coincidences with the excitation energies reconstructed with VAMOS. This ensured to suppress the feeding from higher states. Lifetimes in $^{23}$Mg were measured with a new approach. Proton emitted from unbound states in $^{23}$Mg were also identified. With an higher precision on the measured lifetime and proton branching ratio of the key state, a new value of the resonance strength $\omega\gamma$ was obtained, it is below the sensitivity limit of the direct measurement experiments. The $^{22}$Na(p, $\gamma$)$^{23}$Mg thermonuclear rate has been so reevaluated with the statistical Monte Carlo approach. The amount of $^{22}$Na ejected during novae will be discussed as a tool for better understanding the underlying novae properties. The detectability limit of $^{22}$Na from novae and the observation frequency of such events will also be discussed with respect to the next generation of gamma-ray space telescopes.
References
[1] A.L. Sallaska et al., Phys. Rev. L 105, 152501 (2010).
[2] F. Stegmuller et al., Nuc. Phys. A 601, 168-180 (1996).
[3] O.S. Kirsebom et al., Phys. Rev. C 93, 1025802 (2016).
| Length of presentation requested | Oral presentation: 8 min + 2 min questions (Poster-type talk) |
|---|---|
| Please select between one and three keywords related to your abstract | Nuclear physics - experimental |
| 2nd keyword (optional) | Stellar explosions and mergers - theory |
