Mass-yield distribution of fission products from photo-fission of natPb induced by 2.5 GeV bremsstrahlung

The mass-yield distribution of fission products in the 2.5GeV bremsstrahlung-induced fission of natPb has been measured by using a recoil catcher and an off-line γ -ray spectrometric technique in the 2.5GeV electron linac at the Pohang Accelerator Laboratory. The determination of mass-yield distribution involves measurement of cumulative yields of 27 fission products and independent yields of 15 fission products in the photo-fission of natPb. It was found that the mass-yield distribution of fission products in natPb is symmetric with an average mass of 94±0.5 and a full-width at half-maximum of 55±2.0 mass units. The present data along with the literature data in the 50-85MeV bremsstrahlung-induced fission of natPb and 209Bi as well as the data at 2.5GeV for 209Bi show that the average mass of the mass-yield distributions in the bremsstrahlung-induced fission of natPb and 209Bi decreases from 102.63±0.51 at 50-85MeV to 94.5±0.5 at 2.5GeV. The full-width at half-maximum of the mass-yield distribution increases from about 21.5±1.44 mass units at 50-85MeV to 53±2 mass units at 2.5GeV. These observations in the 50MeV to 2.5GeV bremsstrahlung-induced fission of natPb and 209Bi have been explained from the point of view of increase in multi-chance fission probability with increasing excitation energy. The role of the fissility parameter (ZF2/AF) on the full-width at half-maximum is also discussed. (orig.)