Fission Fragment Angular Distributions and Cross Sections for Deuteron-Induced Fission

Abstract

Fission-fragment angular distributions and cross sections have been measured with gold surface barrier semiconductor detectors for fission induced in a group of elements from Tl to Pu by deuterons of 7- to 21- MeV energy. Apart from a pronounced dip in the radium region, the differential fission fragment cross-section ratios $\frac{W(174\ifmmode^\circ\else\textdegree\fi{})}{W(90\ifmmode^\circ\else\textdegree\fi{})}$ at 21 MeV show an increasing trend with decreasing $\frac{{Z}^{2}}{A}$, the values ranging from 1.20 for ${\mathrm{Pu}}^{239}$ to 1.58 for natural Tl. Values of ${{K}_{0}}^{2}$ are calculated from the experimentally determined anisotropies for targets in which single-chance fission prevails, i.e., ${\mathrm{Bi}}^{209}$ and nuclides in the plutonium region. When values of ${{K}_{0}}^{2}$ for plutonium are compared with those from available neutron fission data, there is indicated an anomalous suppression of ${{K}_{0}}^{2}$ in the low-energy region below the values extrapolated from the higher energy data by plausible temperature dependences on excitation energy. Saddle-shape calculations and semiquantitative arguments which are reviewed lend added credence to the view that the saddle shape is independent of the mode of formation. The experimentally observed change in anisotropy with the fissionability parameter $x$ is correlated with the effective moments of inertia at the saddle configuration (vis-\`a-vis scission point), indicating that thermodynamic equilibrium is first established near the saddle point in fission. Fission cross sections measured for ${\mathrm{Th}}^{232}$, ${\mathrm{U}}^{233}$, and ${\mathrm{U}}^{238}$ are supplemented by previously determined spallation cross sections to obtain total deuteron reaction cross sections. At 21 MeV the deuteron reaction cross section of heavy elements is found to be 1800 mb. The experimental reaction cross sections are compared with theoretical values calculated with a volume-absorption optical model.