Gamma-Ray Spectra from Radiative Capture of Thermal and Resonance Neutrons in Mercury and Tungsten

Abstract

A 20-cc lithium-drifted germanium detector has been used in conjunction with the R.P.I. Electron Linac Neutron Velocity Selector to examine the high-energy gamma-ray spectra arising from thermal and resonance neutron capture in mercury, and from resonance capture in tungsten. The effective gamma-ray energy resolution of 12-18 keV allowed 29 transitions to be examined in the energy range 4.67 to 8.02 MeV following capture of thermal neutrons and resonance neutrons of energy 34, 130, and 175 eV in $^{199}\mathrm{Hg}$ (116 transitions in all). The spectra are interpreted in terms of levels in the compound nucleus $^{200}\mathrm{Hg}$. An analysis of the strengths of 88 $E1$ transitions indicates a Porter-Thomas distribution, the best fit being achieved with a ${\ensuremath{\chi}}^{2}$ function with $0.96_{\ensuremath{-}0.17}^{}{}_{}{}^{+0.24}$ degrees of freedom. The mean reduced partial radiation width for transitions to states in $^{200}\mathrm{Hg}$ above the pairing energy gap is shown to be significantly higher than the corresponding mean width for transitions to collective states below the pairing energy. No such effect is obvious in the case of resonance capture in $^{183}\mathrm{W}$. A comparison between resonance capture in $^{198}\mathrm{Hg}$ and $^{182}\mathrm{W}$, where no pairing energy gap exists, shows that the mean partial radiation width is a factor 4 larger for $^{198}\mathrm{Hg}$ than for the highly distorted $^{182}\mathrm{W}$.