Angular Momentum of Primary Products Formed in the Spontaneous Fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$

The measured intensities of intraband cascading transitions in the ground-state bands of 21 high-yield even-even fission products have been analyzed by two methods to determine the magnitude of the intrinsic angular momentum of the primary products formed in the spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$. The first method was to quantitatively compare the intensities of the intraband transitions observed in fission with those reported in the literature for in-beam (particle, $\mathrm{xn}$) reactions for which the primary angular-momentum distribution was determined by optical-model calculations. The second method was based on a simple statistical-model analysis of the angular momentum distribution throughout the neutron evaporation and the pre-ground-state-band $\gamma$-ray transition phases of the deexcitation process. The two methods gave reasonably similar results, with the former method yielding a somewhat larger primary angular momentum for the fragments. The general conclusions from the statistical-model analysis are that: (1) The average angular momentum of the products is $\overline{l}\approx (7\mathrm{}\pm 2)\hslash$; (2) the heavy fission products have $\approx 20\%$ greater angular momentum than the light products; (3) the more symmetric the mass division the lower the initial angular momentum; and (4) there are only small changes in angular momentum [$\sim \mathrm{}(1-2)\mathrm{}\hslash$] with changes in fragment kinetic energy. An important feature of these results is that the fragment angular momentum does not correlate with the number of neutrons evaporated by the fragment. Additional measurements have been made to study the angular distribution of individual prompt $\gamma$ rays. In all observed cases the $2^{+}\to 0^{+}$ ground-state transitions were forward-peaked with respect to the fission axis, and this is consistent with the assumption that the angular momentum is aligned in a plane perpendicular to the direction of fission. The results are discussed in terms of a quasistatistical model in which the neck width at scission is approximately constant.