Neutron cross sections for ${}^3\mathrm{He}$ at epithermal energies

High accuracy, absolute measurements of the neutron total cross section for ${}^3\mathrm{He}$ are reported for incident neutron energies $0.1–400\text{eV}$. The measurements were performed at the LANSCE short-pulse neutron spallation source. Using the previously determined cross section for neutron elastic scattering, $3.367\pm 0.019\mathrm{b}$, we extract a new value for the energy dependence of the ${}^3\mathrm{He}(n,p){}^3\mathrm{He}$ reaction cross section, ${\sigma }_{np}=(849.77\pm 0.14\pm 1.02)E^{-1∕2}-(1.253\pm 0.00{\pm }_{-0.049}^{+0.008})\mathrm{b}$, where the neutron energy is expressed in eV. The first uncertainty is statistical, the second systematic.

Figure 1

Schematic layout of the total cross section experiment (not to scale). Approximate distances (in meters) from the neutron spallation target to the most important elements are indicated at the top.

Figure 2

A typical time-of-flight spectra corresponding to target number 4. The upper spectra correspond to the empty cylinder, the lower to the ${}^3\mathrm{He}$-filled cylinder. The large absorption peaks visible in both spectra result from foils of Co, In, Ta, and Mn which were inserted just upstream of the two target cylinders and used to determine the detector background.

Figure 3

Measured neutron total cross section data (energy averaged) plotted as ${\sigma }_{\text{tot}}\sqrt{E}$ vs neutron energy: target 1 $(+)$, target 2 $(\times )$, target 3 $(*)$, target 4 $(◻)$. The error bars represent statistical uncertainties only. The upper curve is a least-squares fit to ${\sigma }_{\text{tot}}$ using Eq. (11). The lower curve is the ${}^3\mathrm{He}(n,p){}^3\mathrm{H}$ reaction cross section extracted from our data [Eq. (12)]. Also shown are fringe points from the ${\sigma }_{\text{tot}}$ measurement of Ref. 6 (solid squares), and ${\sigma }_{np}$ data from Ref. 8 (circles). The downward curve of ${\sigma }_{np}$ is due to a negative contribution to the reaction cross section [Eq. (7)]; a strict $1∕v$ dependence would be seen as a flat line in this graph. The upward curve of ${\sigma }_{\text{tot}}$ reveals the presence of the scattering cross section.