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

The stellar reaction rates of radiative alpha-capture reactions on heavy isotopes are of crucial importance for the gamma 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 alpha-induced reaction cross section measurements on iridium isotopes carried out at first close to the astrophysically relevant energy region. Thick target yields of Ir-191(alpha,gamma)Au-195, Ir-191(alpha,n)Au-194, Ir-193(alpha,n)Au-196m, Ir-193(alpha,n)Au-196 reactions have been measured with the activation technique between E-alpha = 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 alpha + nucleus optical potential gives a good description of the experimental data.