Measurement of the Spin Dependence of the ${\mathrm{He}}^3(n, p)\mathrm{T}$ Reaction and of the Nuclear Susceptibility of Adsorbed ${\mathrm{He}}^3$

The spin dependence of the ${\mathrm{He}}^3(n, p)\mathrm{T}$ reaction was investigated by studying the transmission of polarized thermal neutrons through a polarized ${\mathrm{He}}^3$ target which consisted of ${\mathrm{He}}^3$ adsorbed on zeolite. It was found that the interaction cross section was essentially all associated with an $I-\frac{1}{2}=0\mathrm{}$ channel, i.e., $\frac{{\sigma }_{I-\frac{1}{2}}}{{\sigma }_0}=1.010\mathrm{}\pm 0.032$. Once the spin dependence of the cross section was established, polarized neutrons provided a convenient method of studying the nuclear spin susceptibility of ${\mathrm{He}}^3$. Susceptibility measurements were made on our adsorbed-phase target over the temperature range 0.06-1.0°K. It was found that the nuclear susceptibility began to fall below Curie's law at about 0.2°K, indicating the onset of some type of antiparallel ordering in the nuclear spin system.