Meaurement of spin vs. TKE of $^{144}$Ba produced in spontaneous fission of $^{252}$Cf

TL;DR

This study measures the average spin of $^{144}$Ba fragments from $^{252}$Cf spontaneous fission as a function of TKE, revealing minimal variation and suggesting complex spin-generation mechanisms beyond statistical excitation.

Contribution

First combined use of a twin Frisch-gridded ionization chamber with Gammasphere to correlate fragment properties with gamma-ray data in fission.

Findings

Average spin of $^{144}$Ba varies by only 0.5 $$ across TKE range

Spin appears nearly independent of TKE, indicating complex spin-generation mechanisms

Method enables detailed correlation between fission fragment properties and gamma-ray feeding patterns.

Abstract

We measure the average spin of $^{144}$Ba, a common fragment produced in $^{252}$Cf(sf), as a function of the total kinetic energy (TKE). We combined for the first time a twin Frisch-gridded ionization chamber with a world-class $\gamma$-ray spectrometer that was designed to measure high-multiplicity $\gamma$-ray events, Gammasphere. The chamber, loaded with a $^{252}$Cf(sf) source, provides a fission trigger, the TKE of the fragments, the approximate fragment masses, and the polar angle of the fission axis. Gammasphere provides the total $\gamma$-ray yield, fragment identification through the tagging of decay $\gamma$ rays, and the feeding of rotational bands in the fragments. We determine the dependence of the average spin of $^{144}$Ba on the fragments' TKE by correlating the fragment properties with the distribution of discrete levels that are fed. We find that the average spin only changes by about $0.5$ $\hbar$ across the TKE range of 158-203 MeV. The virtual independence of the spin on TKE suggests that spin is not solely generated through the statistical excitation of rotational modes, and more complex mechanisms are required.