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

In stars, the fusion of Ne-22 and He-4 may produce either Mg-25, with the emission of a neutron, or Mg-26 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 Ne-22(alpha, gamma)Mg-26 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 Mg-26). The data reported here have been obtained using high intensity He-4(+) beam from the INFN LUNA 400 kV underground accelerator, a windowless, recirculating, 99.9% isotopically enriched Ne-22 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 63 days of no-beam data and 7 days of He-4(+) beam on an inert argon target. The new high-sensitivity setup allowed to determine the first direct upper limit of 4.0 x 10(-11) eV (at 90% confidence level) for the resonance strength. Finally, the sensitivity of this setup paves the way to study further Ne-22(alpha, gamma)Mg-26 resonances at higher energy.