Search for 22Na in novae supported by a novel method for measuring femtosecond nuclear lifetimes

Abstract

Classical novae are thermonuclear explosions in stellar binary systems, and important sources of ²⁶Al and ²²Na. While γ rays from the decay of the former radioisotope have been observed throughout the Galaxy, ²²Na remains untraceable. Its half-life (2.6 yr) would allow the observation of its 1.275 MeV γ-ray line from a cosmic source. However, the prediction of such an observation requires good knowledge of its nucleosynthesis. The ²²Na(p, γ)²³Mg reaction remains the only source of large uncertainty about the amount of ²²Na ejected. Its rate is dominated by a single resonance on the short-lived state at 7785.0(7) keV in ²³Mg. Here, we propose a combined analysis of particle-particle correlations and velocity-difference profiles to measure femtosecond nuclear lifetimes. The application of this method to the study of the ²³Mg states, places strong limits on the amount of ²²Na produced in novae and constrains its detectability with future space-borne observatories.

Fig. 1. Identification of γ-ray transitions in ²³Mg.

a The energy of the measured γ rays is plotted as a function of the angle between the γ ray and the ²³Mg emitter. This matrix is conditioned with the detection of an α particle at 5.2 < E_x < 5.4 MeV in the VAMOS++ magnetic spectrometer. The γ-ray transition ($E_{\gamma ,0}=4840.0_{-0.4}^{+0.2}$ keV) from the E_x = 5292.0(6) keV excited state in ²³Mg is clearly observed. Its energy is Doppler shifted. The background observed here is mostly due to random coincidences between γ rays from the Compton background and α particles produced in fusion-evaporation reactions between the beam and ¹²C and ¹⁶O impurities deposited on the target. b Picture of the $\mathtt{\text{AGATA}}$ γ-ray spectrometer used to detect the γ rays emitted during the reaction.

Fig. 2. Angle-integrated velocity-difference profiles.

a The ²³Mg velocity at the time of reaction (β_reac) is shown against the velocity at the time of the γ-ray emission (β_ems), for three excited states. The line corresponds to the prompt γ-ray emission when β_ems = β_reac. The points observed on the left of the line (β_ems < β_reac) correspond to delayed γ-ray emissions. b The corresponding angle-integrated velocity-difference profiles for the three states compared with simulations (continuous lines). It shows unambiguously that the key state (in red) has a lifetime 4 < τ < 40 fs. The red-shaded area corresponds to the simulations with lifetimes within 1σ uncertainty. The horizontal error bars correspond to the width of the bins, which are larger than the real experimental uncertainty, and the vertical error bars to the statistical uncertainty.

Fig. 3. Prediction of the 1.275 MeV γ-ray flux emitted from a nova.

The ²²Na γ-ray emission flux is shown as a function of the white dwarf initial luminosity and the mass-accretion rate. This is calculated from the ²²Na mass-averaged abundance within the ejected shells. Computations were done for a 1.2 M_⊙ ONe white dwarf located 1 kpc from the Earth, using the $\mathtt{\text{MESA}}$ code^–.