Nuclei in the Cosmos (NIC XVII)

Understanding the abundance of 22Na in novae by measuring femtosecond lifetimes in 23Mg with a new method

19 Sept 2023, 16:05

15m

Oral Novae and X-ray bursts Novae and X-ray bursts, Type IA supernova and the p-process

Speakers

Chloé FOUGÈRES (Argonne National Laboratory)Dr François DE OLIVEIRA SANTOS (Grand Accélérateur National d'Ions Lourds (GANIL, CEA/DRF-CNRS/IN2P3)

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. Its $\gamma$-ray line at 1.275 MeV has not been observed yet by the $\gamma$-ray space observatories. 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. Hence 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 0.204 MeV corresponding to the $E_x=$7.785 MeV excited state in ${}^{23}$Mg. However, the different determinations of the strength of this resonance disagree, resulting in uncertainties of one order of magnitude for the expected mass of ${}^{22}$Na ejected in novae [1].

An experiment was performed at GANIL facility to measure both the lifetime and the proton branching ratio of the key state at $E_x=$7.785 MeV. The principle of the experiment is based on the one used in [2]. With a beam energy of 4.6 MeV/u, the reaction ${}^3$He(${}^{24}$Mg,$\alpha$)${}^{23}$Mg${}^{\ast }$ populated the state of interest. This reaction was measured with particle detectors (magnetic spectrometer VAMOS++, silicon detector SPIDER) and $\gamma$-ray tracking spectrometer AGATA. The expected time resolution with AGATA high space and energy resolutions is 1 fs. Particle and $\gamma$-ray emissions were analyzed with a new simulation code EVASIONS to determine the spectroscopic properties of the key state.

Our new results [3] 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, down to the femtosecond, were measured with a new approach based on particle - $\gamma$-ray correlations and velocity-difference profiles. Protons 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 direct measurement experiments. The ${}^{22}$Na($p,\gamma$)${}^{23}$Mg thermonuclear rate has been reevaluated with the statistical Monte Carlo approach. The amount of ${}^{22}$Na ejected during novae has been proven to be a tool for better understanding the underlying novae properties. Thanks to the highly-accurate rate, derived here, robust estimates of the detectability limit of ${}^{22}$Na in novae have been determined with respect to the next generation of $\gamma$-ray space telescopes, and the detection of the ${}^{22}$Na $\gamma$-ray line found promising in the coming decades.

References
[1] C. Fougères et al, EPJ Web Conf 279, 09001 (2023).
[2] O. S. Kirsebom et al, Phys. Rev. C 93, 1025802 (2016).
[3] C. Fougères et al, arXiv:2212.06302, 09001 (2022).

Presentation materials

3. Presentation_NICXVII_CFougeres.pdf