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 ${\mathrm{He}}_2$ 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 ${}^4\mathrm{He}$.

Figure 1

Diffusion constant $D$ versus temperature $T$ for ${\mathrm{He}}_2$ molecules in superfluid helium, assuming ${\mathrm{He}}^3$ concentrations found in commercially available liquid helium ($X={10}^{-7}$) and purified liquid helium ($X={10}^{-11}$). Also shown are the individual roton and phonon contributions.

Figure 2

Energy levels of the triplet ${\mathrm{He}}_2$ molecule below 21 eV. Also shown is a cycling transition described in the text.