Differential angular distribution of prompt gamma rays from spontaneous fission ofCf252

The differential angular distribution of prompt $\gamma$ rays from spontaneous fission of ${}_{}{}^{252}{}_{}{}^{}\mathrm{Cf}$ has been measured. The source was on a thick backing, and the measurements have been performed in forward and backward geometries. An NaI(Tl) crystal was used as a $\gamma$-ray detector, and the prompt neutrons from fission were rejected by time of flight. The total number of $\gamma$ rays emitted within 12 ns after fission with energies greater than 0.114 MeV is 9.7 ± 0.4 per fission, and the total $\gamma$-ray energy released is (7.0±0.3) MeV per fission. Results from earlier experiments that more $\gamma$ rays are emitted from the light than from the heavy fragment group, are substantiated. The anisotropy $A=\frac{I\left(0\mathrm{}°\right)}{I\left(90\mathrm{}°\right)}-1\mathrm{}$ is small and even negative at low energies, reaches a maximum of about 25% at energies of about 0.50-0.65 MeV, and gets gradually smaller at higher energies. No significant difference in the anisotropy as measured with a nonmagnetic (Pt) and a magnetic (Ni) backing has been found. With the assumption that the angular momentum is aligned in a plane perpendicular to the direction of fission, the results can be consistently described within the statistical model in terms of pure dipole and quadrupole radiation with allowance for stretched $E2\mathrm{}$ cascades from even-even fragments. It is concluded that the root mean square value of the primary angular momentum of the fragments is $J_{\mathrm{rms}}=(6.5\mathrm{}\pm 1.0)\hslash$, the average angular momentum is decreasing $1.0\hslash$ per $\gamma$ ray emitted, and the value of the spin cutoff parameter during $\gamma$-ray deexcitation of the fragments is $\sigma ={2.4}_{-0.5\mathrm{}}^{+0.8\mathrm{}}$. The dipole and the quadrupole components are about equally strong at high $\gamma$-ray energies, the dipole component predominates at low energies, and the quadrupole component at intermediate energies. Statistical dipole and quadrupole transitions (stretched for the last ones) account for 38% and 50% of the $\gamma$ rays, respectively, and stretched $E2\mathrm{}$ transitions in cascades from even-even fragments account for the remaining 12% of the $\gamma$ rays. The quadrupole radiation can be assumed to be $E2\mathrm{}$, and the dipole radiation $M1\mathrm{}$ at low energies and $E1\mathrm{}$ at high energies. While the components of the stretched $E2\mathrm{}$ cascades with energies smaller than 0.6 MeV can be accounted for by the transitions in the ground-state rotational bands of even-even fragments, it is indicated that a large part of the components in the energy range 0.6-0.9 MeV can be associated with vibrational transitions in even-even fragments.