Deducing angular momenta in nuclear fission of 238U

• Background: Angular momenta of fission fragments considerably higher than that of the fissioning nucleus have been observed several times, raising the question of how these high angular momenta are generated. J.Wilson et al. have proposed that the angular momentum is generated after scission, from the collective motion of nucleons in the ruptured neck of the fissioning system. This conclusion is based on observed average angular momenta deduced from γ-ray spectroscopy, which seems to suggest that the angular momentum of a fission fragment is independent from that of the complementary fragment. Willson et al. also propose a model for the angular momentum as a function of mass for fission products in the low and high mass peak. However, there is no systematic study, and hence no model, for the angular momenta of fission fragments in the symmetric mass region.Purpose: To deduce the angular momenta of fission fragments based on the observed isomeric yield ratios in25-MeV proton-induced fission of 238UMethod: A surrogate model of the fission code GEF has been developed to generate properties of primary fission fragments. Based on the excitation energy (Ex) and angular momentum (J) of fission fragments from GEF, an energy versus angular momentum (Ex − J) matrix is reconstructed using a set of parameters. With such matrices as input, the reaction code TALYS is used to calculate the de-excitation of the fission fragments, including the population of the isomers, from which the isomeric yield ratios are obtained. By varying one of the parameters, the root-mean-square angular momentum (Jrms) determining the angular momentum distribution of the matrix, Jrms-dependent isomeric yield ratios are obtained. By matching the calculated ratios with the experimental value, the most likely Jrms for that fragment is obtained. Repeating this procedure for all fragments contributing to anisomeric yield ratio for a given fission product, makes it possible to deduce the average angular momentum of the contributing fragments.Results: Data of 31 isomeric yield ratios were analysed in 25-MeV proton-induced fission of 238U. In seven cases, no conclusive result for the angular momentum could be obtained. However, the average Jrms with uncertainties are presented for 24 different fission products. For the purpose of comparing the result with those of Willson et al., also the average of the average angular momentum (Jav ) with uncertainties is presented.Conclusion: A mass dependency of the average angular momentum is observed in the proton-induced fission of 238U. Moreover, the average angular momenta for mass numbers A≥ 132 could be described by the parameterisation proposed by J. Wilson. However, a significant discrepancy between the Jav of 130Sn and 131Te between the data and the model suggests a different lower limit for the validation of the parameterisation compared to that suggested by Willson et al. In general, higher Jav for A≥ 132 is observed in the present work compared to those from Willson et al.. This is likely due to the higher excitation energy of the fissioning nuclei in this work compared to Wilssons’. Furthermore, systematic measurements of the average angular momenta of fission products in the symmetric mass region are presented for the first time. In this region, a decreasing trend with mass number is observed.

Subject headings

NATURVETENSKAP  -- Fysik -- Subatomär fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Subatomic Physics (hsv//eng)

Keyword

Angular momentum

fission

model

Publication and Content Type

vet (subject category)

ovr (subject category)