Elastic Scattering of Fast Neutrons by Tritium andHe3

Differential cross sections have been obtained for the elastic scattering of neutrons by tritium and by ${\mathrm{He}}^3$ at $E_n=1.0, 2.0, 3.5, \mathrm{and} 6.0$ Mev over the angular range 27° to 161° in the c.m. system. The Los Alamos large Van de Graaff accelerator and pulsed-beam time-of-flight facility were employed, and the scattering samples were contained in small thin-walled stainless steel spheres. One-third mole of ${\mathrm{He}}^3$ was contained at 5000 psi and one-half mole of tritium was prepared in the form of Ca${\mathrm{T}}_2$. Absolute cross sections were determined from a comparison with the scattering from a thin shell of C${\mathrm{H}}_2$ at each energy, together with calibration of the relative sensitivity of the detector as a function of energy by the known forward yield of the $\mathrm{T}(p, n){\mathrm{He}}^3$ reaction. The angular distributions for $n$-T scattering are in excellent agreement with the calculations of Bransden, Robertson, and Swan based on a Serber exchange force. The $n$-${\mathrm{He}}^3$ measurements favor qualitatively the Serber rather than the symmetrical exchange force used in the calculations, but the agreement is poorer than that obtained for $n$-T scattering. The polarization of elastically scattered 1-Mev neutrons was found to be less than 5% for both samples, unlike the strong polarization observed in $n$-${\mathrm{He}}^4$ scattering.