  Purpose: To deduce the angular momenta of fission fragments based on the
observed isomeric yield ratios (IYR) in 25-MeV proton-induced fission of 238U
and to compare these using Wilson's model. Method: A surrogate model of GEF has
been developed to generate properties of primary fission fragments. Based on
the excitation energy and angular momentum of fission fragments from GEF, an
energy versus angular momentum matrix is reconstructed using a set of
parameters. With such matrices as input, TALYS is used to calculate the
de-excitation of the fission fragments, from which the IYRs are obtained. By
varying one of the parameters, the root-mean-square angular momentum (Jrms),
which determines the angular momentum distribution of the matrix,
Jrms-dependent IYRs are obtained. Considering all primary fission fragments
contributing to the IYR for a given fission product, the average angular
momentum of those fragments is estimated. Results: Data of 31 IYRs in
proton-induced fission of 238U were analysed. As a result, the average Jrms,
equivalent to average angular momentum Jav, with uncertainties of 24 fission
products, are presented. Considering the neutron emissions of the primary
fission fragments, the Jav as a function of the primary fission fragment is
presented. A mass dependency of Jav is observed in the proton-induced fission
of 238U. Moreover, the Jav for A larger than 131 could be described by the
parameterisation proposed by J. Wilson. In general, higher Jav are observed in
the present work compared to those from Wilson et al. This is likely due to the
higher excitation energy of the fissioning nuclei in this work compared to
Wilson's. Furthermore, systematic measurements of the Jav of fission products
in the symmetric mass region are presented for the first time. A decreasing
trend with mass numbers is observed, which can not be explained by the proposal
in Wilson's paper.
