Towards a direct measurement of the E _cm = 65 keV resonance strength in ¹⁷O(p, γ)¹⁸F at LUNA

Abstract

The 17O(p, γ)18F reaction plays a crucial role in several stellar scenarios where the hydrogen burning phases takes place. In particular, in the temperature energy range of interest for AGB nucleosynthesis (20 MK< T <80 MK) the main contribution to the astrophysical reaction rate comes from the elusive 65 keV resonance. Indeed, this resonance strength is at the moment determined only through indirect measurements, with a reported value of ωγ = (1.6 ± 0.3) × 10−11 eV. With typical experimental quantities for beam current, isotopic enrichment and detection efficiency, this strength yields an expected count rate of less than one count per Coulomb, making the direct measurement of this resonance extremely challenging. The Laboratory for Underground Nuclear Astrophysics (LUNA) 400kV accelerator installed in Laboratori Nazionali del Gran Sasso (Italy) provides a unique possibility to directly measure this low resonance thanks to the reduction of cosmic ray background by six orders of magnitude with respect surface laboratories and thanks to an intense, narrow proton beam. To improve the experimental sensitivity, the environmental background was further reduced designing a lead and borated (5%) polyethylene shielding and the absorption of γ − rays emitted by the reaction was minimised by the installation of target chamber and holder made of aluminum. With about 400 Coulomb accumulated on Ta2O5 targets, with nominal 17O enrichment of 90%, the LUNA collaboration has performed the first direct measurement of the 65 keV resonance strength.