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

Abstract

The mass-yield distribution of fission products in the 2.5 GeV bremsstrahlung-induced fission of Pb-nat has been measured by using a recoil catcher and an off-line gamma-ray spectrometric technique in the 2.5 GeV 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 (na)tPb. 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 Bi-209 as well as the data at 2.5 GeV for Bi-209 show that the average mass of the mass-yield distributions in the bremsstrahlung-induced fission of natPb and Bi-209 decreases from 102.63 +/- 0.51 at 50-85MeV to 94.5 +/- 0.5 at 2.5 GeV. 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 2.5 GeV. These observations in the 50MeV to 2.5 GeV bremsstrahlung-induced fission of Pb-nat and Bi-209 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 (Z(F)(2)/A(F))on the full-width at half-maximum is also discussed.