First direct limit on the 334 keV resonance strength in 22Ne(α,γ)26Mg reaction

In stars, the fusion of 22Ne and 4He may produce either 25Mg, with the emission of a neutron, or 26Mg and a γ ray. At high temperature, the (α,n) channel dominates, while at low temperature, it is energetically hampered. The rate of its competitor, the 22Ne(α,γ)26Mg reaction, and, hence, the minimum temperature for the (α,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}=334keV (corresponding to E_{x}=10949keV in 26Mg). The data reported here have been obtained using high intensity 4He+ beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched 22Ne gas target, and a 4π bismuth germanate summing γ-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 4He+ beam on an inert argon target. The new high-sensitivity setup allowed to determine the first direct upper limit of 4.0×10−11 eV (at 90% confidence level) for the resonance strength. Finally, the sensitivity of this setup paves the way to study further 22Ne(α,γ)26Mg resonances at higher energy.