  A detailed investigation on the relative isotopic distributions has been
carried out for the first time in case of even-even correlated fission
fragments for the $^{235}$U($n_{th}$,$f$) fission reaction. High-statistics
data were obtained in a prompt $\gamma$ ray spectroscopy measurement during the
EXILL campaign at ILL, Grenoble, France. The extensive off-line analysis of the
coincidence data have been carried out using four different coincidence
methods. Combining the results from 2-dimensional $\gamma-\gamma$ and
3-dimensional $\gamma-\gamma-\gamma$ coincidence analysis, a comprehensive
picture of the relative isotopic yield distributions of the even-even
neutron-rich fission fragments has emerged. The experimentally observed results
have been substantiated by the theoretical calculations based on a novel
approach of isospin conservation, and a reasonable agreement has been obtained.
The calculations following the semi-empirical GEF model have also been carried
out. The results from the GEF model calculations are found to be in fair
agreement with the experimental results.
