Neutron-Helium Interaction. I. Scattering of Polarized Neutrons at 1.01 and 2.44 MeV

Precision angular distributions of the asymmetry in the scattering of 1.015- and 2.44-MeV polarized neutrons from helium were determined. The reactions ${\mathrm{Li}}^7(p, n){\mathrm{Be}}^7$ and ${\mathrm{C}}^{12}(d, n){\mathrm{N}}^{13}$ served as neutron sources, and target thicknesses of 46 and 175 keV, respectively, were used. Analyzing angles ranged from 30° to 140°, in 10° increments. A spin-precession solenoid was employed and the helium was contained in a high-pressure scintillation cell. It is demonstrated that asymmetry data alone are sufficient to accurately determine the scattering phase shifts at 2.44 MeV. Here the phase-shift values were found to be 142.1°, 21.2°, and 121.6° for the ${\delta }_0$, ${{\delta }_1}^1$, and ${{\delta }_1}^3$ phase shifts, respectively. $d$ waves were not found to be necessary. Assuming hard-sphere scattering, the $s$-wave phase shift gives a radius of 2.42 F, which is consistent with the values recently obtained by others from both $n$-He and $p$-He scattering data. Since at 1.015 MeV, equally good fits to the present data could be obtained with vaules of ${\delta }_0$ differing by as much as 15°, the hard-sphere formula was used in conjunction with the scattering radius determined at 2.44 MeV to fix the $s$-wave phase shift at 155.5°. The fit to the data then required that ${{\delta }_1}^1=4.4\mathrm{}°$ and ${{\delta }_1}^3=61.2\mathrm{}°$. The presently determined $s$-wave phase shifts are larger than the commonly accepted ones due to Dodder, Gammel, and Seagrave (DGS). At both energies the ${{\delta }_1}^3$ was in excellent agreement with DGS, although ${{\delta }_1}^1$ is about 1.6° below DGS at 1.015 MeV, and 2.8° above at 2.44 MeV.