Neutron Resonance Spectroscopy. IV. As and Br

Abstract

The Columbia neutron velocity spectrometer with a 200-m flight path was used for total-cross-section measurements using several transmission sample thicknesses each of natural As(100% ${\mathrm{As}}^{75}$, $I=\frac{3}{2}$) and Br (∼50% each ${\mathrm{Br}}^{79}$ and ${\mathrm{Br}}^{81}$, both $I=\frac{3}{2}$). The values of $g{{\Gamma }_n}^0$ for 136 levels in As (to 10 keV) and 157 levels in Br (to 4 keV) have been determined. ${\Gamma }_{\gamma }$ and $J$ values were also determined for many of the stronger levels. The $l=0\mathrm{}$ strength function $S_0$ was found to be (1.70±0.30)×${10}^{-4\mathrm{}}$ for As and (1.20±0.18)×${10}^{-4\mathrm{}}$ for Br. The $S_0$ values are averaged over both spin states and, in the case of Br, $S_0$ is also averaged over the two isotopes. There were many more levels with very small values of $g{{\Gamma }_n}^0$ for both As and Br than expected from the Porter-Thomas distribution for a single channel and a single population. The experimental value of $〈D〉$ for all of the observed levels is 71 eV for As and 25.5 eV for Br. A best Porter-Thomas fit to the stronger levels alone, gives $〈D〉=87\mathrm{}$ eV for As and 36.5 eV for Br. Assuming an approximate theoretical value of (-0.25) for the correlation coefficient between adjacent level spacings, 8 of the observed very weak As levels and 17 of the Br levels were selected as most likely to be unreliable, or due to $l=1\mathrm{}$. Hence 128 levels for As and 140 levels for Br were used for most of the tests of statistical distribution parameters. This gives $〈D〉=75\mathrm{}$ eV for As and 28.4 eV for Br. A considerable number of the weaker levels for each element are probably $p$-wave levels. Statistical analyses are given of the distributions for $g{{\Gamma }_n}^0$, $D^0$, $D^1$, and various correlation co-efficients involving the $g{{\Gamma }_n}^0$ values and the spacings. $D^0$ is the nearest-neighbor level spacing, and $D^1$ is the next-nearest-neighbor level spacing. Comparisons are made with the predictions of various theories.