Methods of producing dynamic nuclear polarization of liquid 3He by the ‘solid effect’ method have been proposed using ‘enhanced nuclear paramagnetic resonance’ in Van Vleck compounds with singlet ground states. A number of interesting alternatives are discussed: the use of antiferromagnetic resonance, and the application of acoustic magnetic resonance, since acoustic waves can be propagated into the liquid helium itself. Acoustic resonance can be induced using either electronic magnetic resonance or enhanced nuclear resonance.
In each case an external magnetic field is needed, together with radiation at a frequency equal to the sum or difference of an electronic or enhanced nuclear frequency and the nuclear resonance frequency of 3He. The liquid is polarized in opposite directions according to whether the sum or difference of the two frequencies is used.