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

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 ¹⁹¹Ir(α,γ)¹⁹⁵Au, ¹⁹¹Ir(α,n)¹⁹⁴Au, ¹⁹³Ir(α,n)^196mAu, ¹⁹³Ir(α,n)¹⁹⁶Au 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.