First direct limit on the 334 keV resonance strength in the $^{22}$Ne({\alpha},{\gamma})$^{26}$Mg reaction

TL;DR

This study provides the first direct experimental upper limit on the resonance strength of the 334 keV resonance in the $^{22}$Ne($\a,\gamma$)$^{26}$Mg reaction, crucial for understanding stellar nucleosynthesis processes.

Contribution

The paper introduces a high-sensitivity experimental setup that directly measures the resonance strength, improving upon previous indirect methods and enabling future studies of similar nuclear resonances.

Findings

Established an upper limit of 4.0×10^{-11} eV for the resonance strength.

Demonstrated ultra-low background detection with less than 0.5 counts/day.

Enabled future direct measurements of higher-energy resonances.

Abstract

In stars, the fusion of $^{22}$Ne and $^4$He may produce either $^{25}$Mg, with the emission of a neutron, or $^{26}$Mg and a $\gamma$ ray. At high temperature, the ($\alpha,n$) channel dominates, while at low temperature, it is energetically hampered. The rate of its competitor, the $^{22}$Ne($\alpha$,$\gamma$)$^{26}$Mg reaction, and, hence, the minimum temperature for the ($\alpha,n$) dominance, are controlled by many nuclear resonances. The strengths of these resonances have hitherto been studied only indirectly. The present work aims to directly measure the total strength of the resonance at $E$_{r}$\,=\,$334$\,$keV (corresponding to $E$_{x}$\,=\,$10949$\,$keV in $^{26}$Mg). The data reported here have been obtained using high intensity $^4$He$^+$ beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched $^{22}$Ne gas target, and a 4$\pi$ bismuth germanate summing $\gamma$-ray detector. The ultra-low background rate of less than 0.5 counts/day was determined using 67 days of no-beam data and 7 days of $^4$He$^+$ beam on an inert argon target. The new high-sensitivity setup allowed to determine the first direct upper limit of 4.0$\,\times\,$10$^{-11}$ eV (at 90% confidence level) for the resonance strength. Finally, the sensitivity of this setup paves the way to study further $^{22}$Ne($\alpha$,$\gamma$)$^{26}$Mg resonances at higher energy.