Parametric resonances associated with zero-field level crossings of the $2^3{S}_1$ level of ${\mathrm{He}}^4$ have been investigated using transverse optical pumping. A steady-state technique based on the $n=0\mathrm{}$, $p=0\mathrm{}$ resonance (Hanle effect) is used to measure spin-state lifetimes and determine the dependence of the thermal relaxation time on the state of the active helium discharge. The $n=0\mathrm{}$, $p=1\mathrm{}$ resonance is quite strong and suitable for measuring weak magnetic fields. A sensitivity of 1.0 × ${10}^{-8\mathrm{}}$ G is demonstrated for this measurement technique.

• Received 24 October 1972

DOI:https://doi.org/10.1103/PhysRevLett.29.1642