Mass number and excitation energy dependence of the Θeff/Θrig parameter of the spin cut-off factor in the formation of an isomeric pair

The ratio of the effective moment of inertia to the rigid body moment of inertia (Θeff/Θrig) parameter of the spin distribution of the level density was determined for 61 nuclei covering the mass range of 44 to 200. The experimental data for 25 isomeric pairs were compared with the model calculations carried out with the TALYS code. The reduced χ2 values were calculated to describe the deviation of the experimental data from the model calculation as a function of the η=Θeff/Θrig parameter. A term ηd was introduced by calculating the Θeff/Θrig value from the low energy discrete levels of the nucleus. The η/ηd values seem to be independent of the mass number and the η values independent of the excitation energy. The mass number and (N-Z) dependences of the η values were studied. The η values for the odd mass nuclei show a characteristic exponential decrease as a function of the mass number or (N-Z). The η values for the even type of nuclei show more complicated behavior. The odd-even effect of Θeff/Θrig may change the calculated activation cross section significantly.