Nuclear Pairing Energy of Transition NucleusPu240

Abstract

Fission-fragment angular distributions are measured for the fission of ${\mathrm{Pu}}^{239}$ induced by monoenergetic neutrons of energies 150, 400, 475, 550, 600, 700, 800, 900, 1000, 1100, 1200, 1350, and 1500 keV (energy resolution, ±25 keV). Theoretical calculations are utilized to determine the parameter ${K_0}^2$ at each excitation energy from the observed angular distributions. The values of ${K_0}^2$ rise sharply from a value of 5-6, for neutron energies less than or equal to 600 keV, to an approximately constant value of 13, for neutron energies in the range 1.3-2.0 MeV. The break in the ${K_0}^2$ curve occurs at about 2.2 MeV above the fission threshold, and is interpreted as the beginning of two-quasiparticle excitations of the highly deformed transition nucleus ${\mathrm{Pu}}^{240}$. This result suggests that the pairing energy gap is increased for large nuclear deformations.