Yields of correlated fragment pairs in spontaneous fission of ${}^{252}$Cf

Independent yields of 139 individual secondary (appearing after neutron evaporation) fragment pairs of five different charge splits ${(Z}_L{/Z}_H=$46/52, 44/54, 42/56, 40/58, and 38/60) have been experimentally measured by detecting the coincidences between prompt $\gamma$ rays emitted in the spontaneous fission of ${}^{252}\mathrm{Cf}.\mathrm{}$ Nuclear charge and mass distributions of fission fragments that follow from the measured yields of individual fragment pairs are consistent with similar previously known more integral data. Another type of data extracted from the yields of fragment pairs is the multiplicity distributions of prompt neutrons emitted in the five above charge divisions of ${}^{252}$Cf. For the measured charge splits, about 70% of the fission events where $>~7$ neutrons are evaporated from the fission fragments occur for the Mo-Ba split of ${}^{252}$Cf. Mass and excitation energy distributions of primary Ru-Xe, Mo-Ba, and Zr-Ce fragments were deduced from a least squares fit to the yield pattern of secondary fragment pairs. For the Ru-Xe and Zr-Ce splits, the experimental data are well fitted by assuming one fission mode with average total kinetic energy $〈\mathrm{TKE}〉$ values close to the value of $〈\mathrm{TKE}〉$ known for the ${}^{252}\mathrm{Cf}$ spontaneous fission. For the Mo-Ba split, a successful fit could be obtained only with the assumption that, in addition to this “normal” fission mode, a second mode with a remarkably lower value of $〈\mathrm{TKE}〉$ of 153 MeV contributes to this charge split. This is 36 MeV lower than for the normal mode. These data indicate that in mode two the barium nuclei are hyperdeformed (3:1 axis ratio) at scission. Mean angular momentum values of Mo-Ba fission fragments observed in pairs together with various partners have been deduced from the measured populations of different spin levels of the fragment nuclei. These angular momentum values are discussed in terms of their dependence on the primary fragment excitation energy and presence of two fission modes.