Photofission of ²⁰⁹Bi at intermediate energies

Summary

The reaction yields of 63 radionuclides with the mass number A = 56 − 135 produced in the photofission of ²⁰⁹Bi by bremsstrahlung of end-point energies (E₀) from 450 to 1100 MeV have been measured using a catcher foil technique with the aid of intensive chemical separation. The charge distribution was well reproduced by a Gaussian function with the most probable charge (Z_p) expressed by a linear function of A, i.e., Z_p = RA + S, and with the A-independent full width at half maximum (FWHM_CD). The charge distribution parameters R, S and FWHM_CD were independent of E₀ above 600 MeV, reflecting the resonance nature in photonuclear reactions at intermediate energies. The weighted mean values at E₀ ≥ 600 MeV were R = 0.421 ± 0.001, S = 0.6 ± 0.1 and FWHM_CD = 2.1 ± 0.1 charge unit (c.u.). The numbers of pre- and post-fission neutrons were deduced to be ν_pre = 12 ± 1 and ν_post = 1.4 ± 0.3, respectively, assuming the Unchanged Charge Distribution (UCD). Based on the charge distribution parameters, the symmetric mass yield distributions with the most probable mass A_p of 96 ± 1 mass unit (m.u.) and the width FWHM_MD of 33 ± 1 m.u. were also obtained. The characteristics of the charge and mass yield distributions are discussed by referring to those for ¹⁹⁷Au based on the results of calculations using the Photon-induced Intranuclear Cascade Analysis code combined with the Generalized Evaporation Model (PICA3/GEM).