Cross section of α-induced reactions on iridium isotopes obtained from thick target yield measurement for the astrophysical γ process

Abstract The stellar reaction rates of radiative α -capture reactions on heavy isotopes are of crucial importance for the γ process network calculations. These rates are usually derived from statistical model calculations, which need to be validated, but the experimental database is very scarce. This paper presents the results of α -induced reaction cross section measurements on iridium isotopes carried out at first close to the astrophysically relevant energy region. Thick target yields of 191 Ir( α , γ ) 195 Au, 191 Ir( α ,n) 194 Au, 193 Ir( α ,n) 196 m Au, 193 Ir( α ,n) 196 Au reactions have been measured with the activation technique between E α = 13.4  MeV and 17 MeV. For the first time the thick target yield was determined with X-ray counting. This led to a previously unprecedented sensitivity. From the measured thick target yields, reaction cross sections are derived and compared with statistical model calculations. The recently suggested energy-dependent modification of the α  + nucleus optical potential gives a good description of the experimental data.