Trace Detection of Metastable Helium Molecules in Superfluid Helium by   Laser-Induced Fluorescence
  We describe an approach to detecting ionizing radiation that combines the special properties of superfluid helium with the sensitivity of quantum optics techniques. Ionization in liquid helium results in the copious production of metastable helium molecules, which can be detected by laser-induced fluorescence. Each molecule can be probed many times using a cycling transition, resulting in the detection of individual molecules with high signal to noise. This technique could be used to detect neutrinos, weakly interacting massive particles, and ultracold neutrons, and to image superfluid flow in liquid He-4. 
D. N. McKinsey, W. H. Lippincott, J. Nikkel, and W. Rellergert