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 Hg (116 transitions in all). The spectra are interpreted in terms of levels in the compound nucleus 200 Hg. An analysis of the strengths of 88 El transitions indicates a Porter-Thomas distribution, the best fit being achieved-with a X 2 function with 0.96_o.i7 +0-24 degrees of freedom. The mean reduced partial radiation width for transitions to states in 200 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 W. A comparison between resonance capture in 198 Hg and 182 W, where no pairing energy gap exists, shows that the mean partial radiation width is a factor 4 larger for 198 Hg than for the highly distorted 182 W.

Estimation of number of degrees of freedom of nuclear reaction widths distributions

“…An important inequality which imposes a definite lower limit on the variance of an estimate (see for example Kendall Rae et al (1967) have proposed using the method of minimum chi-squared fitting to obtain an estimate of v. This method consists of constructing a histogram of the 17 samples of the experimental quantities xi taking the intervals to be some fraction of X and then comparing this Can. J. Phys.…”

“…The development of the Ge(Li) gamma-ray spectrometer has permitted the acquisition of data of much improved quality which-in turn has led to estimated values of v approaching unity (Rae et al 1967). Recent work by the present authors (Lycklama and Kennett 1967) on the nature of reduced radiation widths for the 31P(n, y)32P reaction revealed that rather large biases in the estimation of v arose for finite sample size.…”

“…Recent work by the present authors (Lycklama and Kennett 1967) on the nature of reduced radiation widths for the 31P(n, y)32P reaction revealed that rather large biases in the estimation of v arose for finite sample size. Fortunately, Rae et al (1967) had a sample totaling 88 widths and hence little bias was expected: however, the dependence of bias upon sample size and, of more importance, the mathematical technique which should be used to obtain an estimate of v, are questions which, because of the nonlinear nature of the problem, have remained unanswered.…”

“…Rae et al (1967) used the L7] oz = _ -I1 GT method of minimization of variance in order to determine an estimate of v for his study of which means that the sample variance, radiation widths. 1 The present study was undertaken in order to [8] SZ = determine the most efficient method for estima-11 -l i = 1 tion quantity v and to investigate the is an unbiased estimate of the true From properties of the standard techniques.…”

Gamma Rays from theHg199(n, γ)Hg200Reaction an

“…Additional data have been obtained through resonance-capture experiments. [26][27][28][29][30] As a result of these measurements, many levels with excitation energies up to 1500 keV have been well established and their spins and parities determined. Above 1570 keV numerous states have been found and spins and parities of several of these levels have been determined through very recent polarization correlation 31 and coincidence experiments.…”