Coincident electrofission of 238U at q

Abstract

We have measured the coincidence cross sections $^{238}\mathrm{U}$(e,e'f) at three values of momentum transfer (q\ensuremath{\approxeq}0.26, 0.40, and 0.55 ${\mathrm{fm}}^{\mathrm{\ensuremath{-}}1}$) over the range from 5 to 23 MeV in excitation energy; these values of q correspond approximately to the first maxima of the electric dipole, quadrupole/monopole, and octupole form factors, respectively. The angular distribution of fission fragments is peaked along the direction of q near fission threshold, indicating the dominance of K=0 states, but becomes isotropic above 10 MeV. We have integrated our cross sections over fission angle and compared them to recent calculations of multipole strength derived in the quasiparticle random phase approximation. Using model-dependent form factors generated from the Tassie model and the quasiparticle random phase approximation, we have extracted strength functions for the lowest electric multipoles. The deduced E1 strength agrees well with photofission results for reasonable choices of the model form factors. The E2/E0 distribution displays resonant structures at 10 and 13 MeV which agree in position with the collective E2 and E0 strength, respectively, predicted in the quasiparticle random phase approximation. If we make such an assignment for the multipolarity of these two structures, the observed E0 strength agrees with the theoretical prediction, whereas the E2 strength is low by a factor of 2--3. Higher order multipoles also contribute substantially to the cross section at our highest value of q.