Neutron and fragment yields in proton-induced fission of 238U at intermediate energies

Abstract

The primary fission fragment mass and kinetic energy distributions, and neutron multiplicities as function of fragment mass have been measured in the proton-induced fission of 238 U at energies E p ¼ 20, 35, 50 and 60 MeV using time-of-flight technique. Pre-scission and post-scission neutron multiplicities have been extracted from double differential distributions. The fragment mass dependence of the post-scission neutron multiplicities reveals the gross nuclear shell structure effect even at the higher proton energies we measured. The yields of neutron-rich fission products in the fission of 238 U by 25 MeV protons were measured using an ion guide-based isotope separator technique. The results indicate enhancement for superasymmetric mass division at intermediate excitation energy of the fissioning nucleus. The experimental results have been analysed in the framework of a time-dependent statistical model with inclusion of nuclear friction effects in the fission process. #

First isomeric yield ratio measurements by direct ion counting and implications for the angular momentum of the primary fission fragments

“…The shadowed regions depict the experimental results with the respective uncertainties. For all studied nuclides, except for the case of 81 Ge, calculations for one or two neutrons emitted from the primary fragments were performed, as these emissions were considered most probable [76]. For 81 Ge, the emissions of zero and one neutrons were instead investigated.…”

“…Several different de-excitation paths of primary fragments can lead to the production of a specific fission product, depending on the number of emitted neutrons. In the current investigation, for a specific isomeric pair, we take into account the mass dependence of post-scission neutron multiplicity [76], so that only the fragments that are most probable to contribute to this observable are considered.…”

“…The values of the post-scission neutron multiplicities of the fission fragments were taken from Refs. [76] and [82] for nat U and 232 Th, respectively.…”

Promptγ-ray spectroscopy of the neutron-rich124Cd

“…The proton-beam energy of 25 MeV was chosen as a compromise between a high fission cross section and producing a low-energy compound nucleus, thereby minimizing prompt neutron evaporation. The mass distribution of this reaction is symmetric, centered around mass ∼ 120 and an average of 6 neutrons were evaporated [9] per fission. The secondary fission fragments therefore remain neutron rich due to the high N/Z ratio of the compound nucleus.…”

Measurements of isomeric yield ratios of fission products from proton-induced fission onnatU and232Th via direct ion counting

“…The total average number and the TKE of the fissioning system were taken from [14] and [15] respectively. One to three neutrons emitted from each fragment were investigated for each case, whereas the extreme cases of none or all neutrons emitted from the same fragment were not considered due to their low probability.…”