Scattering of 64 eV to 3 keV Neutrons from Polyethylene and Graphite and the Coherence Length Problem

We measured the neutron scattering intensity ratios from polyethylene (${\mathrm{CH}}_2$) relative to graphite (C) at several discrete final energies, of narrow widths ($\sim 3\mathrm{eV}$) between 64 eV and 3 keV. The final energies were selected using a ${}^{238}\mathrm{U}$ filter. This experiment was carried out to search for any anomaly in the $n\mathrm{\text{−}}p$ scattered intensities from ${\mathrm{CH}}_2$ caused by the neutron coherence length. The scattered intensity ratios were found to conform to conventional expectations and no anomaly was observed.

Figure 1

Schematic view of the layout of the scattering experiment. The ${}^{238}\mathrm{U}$ filter is 7.6 cm thick. The Ta target to $n$-detector distance is 25.5 m.

Figure 2

Time of flight spectrum of scattered neutrons from a 1.2 mm thick ${\mathrm{CH}}_2$ sample, filtered by a 7.6 cm thick ${}^{238}\mathrm{U}$ plate. A ${}^6\mathrm{Li}$-Glass $n$-detector was used. The length of the flight path is 25.5 m. The peaks are labeled in energy units of eV.

Figure 3

Scattering intensity ratios from ${\mathrm{CH}}_2$ and C. The upper two lines are conventionally calculated ratios, showing the single (dashed line) and total, including MS (solid line). The bottom dotted line is the single scattering result where the $n\mathrm{\text{−}}p$ cross section is reduced by 40%.