Fission of Weakly Prolate ¹¹⁹Sn and Weakly Oblate ²⁰⁹Bi Nuclei Induced by 500 and 672 MeV Negative Pions

Fission cross-sections of ¹¹⁹Sn and ²⁰⁹Bi induced by negative pions of two energies 500 and 672 MeV were measured using a CR−39 nuclear track detector. Target-detector stacks were exposed to pion beams at the Brookhaven National Laboratory (USA). Measurement results are compared with the corresponding calculations using the computer code CEM95. Agreement between measurements and calculations is fairly good for the ²⁰⁹Bi target nuclei whereas it is poor for ¹¹⁹Sn at both investigated energies of 500 and 672 MeV. Fission cross−section results of ¹¹⁹Sn and ²⁰⁹Bi are explained using the equilibrium properties of these nuclides including nuclear electric quadrupole moments which determine the shapes of nuclei. A logarithmic dependence of fission cross−section on Z²/A is observed for the above−mentioned reactions and a critical limit of Z²/A is identified with the value of 30 which divides the curve of σ_f versus Z²/A into two regimes, one with weak dependence and the other with strong dependence.