Slow Neutron Velocity Spectrometer Studies of Cu, Ni, Bi, Fe, Sn, and Calcite

Abstract

The thermal cross section of Cu is well matched by the curve $\sigma =(7.8+0.54\mathrm{}{E}^{-\frac{1}{2}})$. Microcrystalline interference is observed in the low energy region. The cross section decreases slowly from 8 to 7.3 between 2 and 100 ev, then decreases more rapidly until at zero timing it is 5.3. A small dip is observed near 3000 ev. The thermal cross section of Ni is well matched by the curve $\sigma =(17.0+0.77\mathrm{}{E}^{-\frac{1}{2}})$. Microcrystalline interference effects are observed at low energies. The Ni cross section is approximately constant at 17.2 from 3 to 50 ev. A small dip is observed with a minimum near 100 ev. Above 100 ev the cross section drops rapidly to 8 at zero time of flight. Bismuth shows no $\frac{1}{v}$ slope in the thermal region because of microcrystalline interference effects. The free cross section of Bi is (9.0±0.2) and is constant over the energy region investigated. The thermal cross section of Fe is well matched by the curve $\sigma =(11.0+0.35\mathrm{}{E}^{-\frac{1}{2}}$. Microcrystalline interference effects are observed in the low energy region. The cross section of Fe remains approximately constant at 11 from 3 to 300 ev, then decreases to 4.8 at zero time of flight. The cross section of Sn in the thermal region rises because of microcrystalline interference effects which mask any $\frac{1}{v}$ slope. The free cross section is (4.9±0.1). There is a slight dip in the transmission curve of tin around 6 ev which may or may not be real. Other small dips near 50 and 100 ev are probably real. The cross section increases near zero time of flight indicating that there are probably resonances above 1000 ev. The cross section of a large Nicol Prism decreases from the additive CaC${\mathrm{O}}_3$ cross section of 20 at short wave-lengths to about 3.5 at 0.005 ev. The residual cross section cannot be explained by theory.