Neutron Resonance Spectroscopy. IV. As and Br

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% As⁷⁵, I=3/2) and Br ({~}50% each Br⁷⁹ and Br⁸¹, both I=3/2). The values of gΓₙ⁰ for 136 levels in As (to 10 keV) and 157 levels in Br (to 4 keV) have been determined. Γ_γ and J values were also determined for many of the stronger levels. The $l=0$ strength function S₀ was found to be (1.70±{}0.30)×{}10^-4 for As and (1.20±{}0.18)×{}10^-4 for Br. The S₀ values are averaged over both spin states and, in the case of Br, S₀ is also averaged over the two isotopes. There were many more levels with very small values of gΓₙ⁰ 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 $〈D〉=87$ 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 $l=1$. Hence 128 levels for As and 140 levels for Br were used for most of the tests of statistical distribution parameters. This gives $〈D〉=75$ 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 gΓₙ⁰, D⁰, D¹, and various correlation co-efficients involving the gΓₙ⁰ values and the spacings. D⁰ is the nearest-neighbor level spacing, and D¹ is the next-nearest-neighbor level spacing. Comparisons are made with the predictions of various theories.