  The $^3$He(n,p) process is excellent for neutron detection between thermal
and $\sim$4\,MeV because of the high cross section and near-complete energy
transfer from the neutron to the proton. Traditional gaseous $^3$He detectors
using this process typically have high levels of radiogenic backgrounds so that
they cannot measure the small neutron fluxes present in underground labs for
dark matter experiments. I propose a cryogenic liquid $^3$He detector that can
be designed with tiny radiogenic backgrounds and efficiently measure neutron
fluxes in low-flux environments.
