Measurement of the neutron-proton total cross section at 132 eV

The neutron-proton scattering cross section at "zero energy" (σ₀), which is an important experimental parameter in the analysis of low energy $np$ data, has been accurately redetermined by transmission measurements at 132 eV neutron energy on H₂O, C₆{H}₆$, C${H}₃$OH, and ${C}₇H₈. We obtain the total cross sections per molecule (132 eV) 44.731 ±{} 0.027, 151.40 ±{} 0.10, 90.37 ±{} 0.08, and 197.20 ±{} 0.34 b, respectively. To evaluate σ₀ we subtract the well-known cross sections of carbon and oxygen, and correct for the small contributions (0.1%) of capture, molecular binding, effective range, and deuterium. Combining all known errors from the experimental and evaluation procedure, we find σ₀=20.491±0.014 b. This is 0.27%, or 2.4 standard errors, above the best previous measurement by Houk, and {~}0.6% above the mean of former data. Our result of σ₀ implies changes in the $np$ effective-range parameters towards higher values, by about twice the present standard errors in aₛ, aₜ, and r₀ₜ and 1 standard error in r₀ₛ.NUCLEAR REACTIONS ¹H(n, n), $E=132$ eV; measured σT. Cobalt-resonance neutron beam, targets H₂O, C₆{H}₆$, C${H}₃$OH, ${C}₇H₈.