Activation cross section and isomeric cross-section ratio for the (n, 2n) reaction on 191Ir

The Ir-191(n,2n)Ir-190 cross section was measured by means of the activation technique at four neutron energies in the range 10.0-11.3 MeV. The quasimonoenergetic neutron beam was produced via the H-2(d,n)He-3 reaction at the 5.5 MV Tandem Van de Graaff accelerator of NCSR "Demokritos." The cross section for the population of the second high spin (11(-)) isomeric state was measured along with the sum of the reaction cross section populating both the ground (4(-)) and the first isomeric state (1(-)). Statistical model calculations taking into account pre-equilibrium emission were performed and compared with all the available experimental data. The experimentally deduced excitation function for populating both the ground and first isomeric state was well reproduced by the theoretical calculations. The statistical model calculations of populating the second metastable state revealed its strong dependence on the details of the introduced level scheme, as well as on the ratio between the effective moment of inertia and the rigid body moment of inertia of the residual nucleus. The experimental cross-section data corresponding to the second isomeric state and to the isomeric ratio were reproduced in a quite consistent way by the calculations.