The neutron-proton scattering cross section at "zero energy" (${\sigma }_0$), 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 ${\mathrm{H}}_2$O, ${\mathrm{C}}_6$${\mathrm{H}}_6$, C${\mathrm{H}}_3$OH, and ${\mathrm{C}}_7$${\mathrm{H}}_8$. 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 ${\sigma }_0$ 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 ${\sigma }_0=20.491\mathrm{}\pm 0.014$ 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 ${\sigma }_0$ implies changes in the $np$ effective-range parameters towards higher values, by about twice the present standard errors in $a_s$, $a_t$, and $r_{0t}$ and 1 standard error in $r_{0s}$.

NUCLEAR REACTIONS ${}_{}{}^1{}_{}{}^{}\mathrm{H}(n, n)$, $E=132\mathrm{}$ eV; measured ${\sigma }_T$. Cobalt-resonance neutron beam, targets ${\mathrm{H}}_2$O, ${\mathrm{C}}_6$${\mathrm{H}}_6$, C${\mathrm{H}}_3$OH, ${\mathrm{C}}_7$${\mathrm{H}}_8$.

• Received 9 October 1974

DOI:https://doi.org/10.1103/PhysRevC.11.103