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

Abstract

In stars, the fusion of ^{22}Ne and ^4He 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_{text {r}} = 334 keV (corresponding to E_{text {x}} = 10949 keV in ^{26}Mg). 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 ^{22}Ne gas target, and a 4pi 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 ^4He^+ 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.