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arXiv:nucl-ex/0503006 · Physical Review Letters

Trace Detection of Metastable Helium Molecules in Superfluid Helium by Laser-Induced Fluorescence

Daniel N. McKinsey, W. H. Lippincott, James A. Nikkel, Wade G. Rellergert

Published Sep 8, 2005• 30 citations
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Abstract

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 He2 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 4He.

DOI
10.1103/physrevlett.95.111101

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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 He2 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 4He.
We present data that show a cycling transition can be used to detect and image metastable He2 triplet molecules in superfluid helium. We demonstrate that limitations on the cycling efficiency due to the vibrational structure of the molecule can be mitigated by the use of repumping lasers. Images of the molecules obtained using the method are also shown. This technique gives rise to a new kind of ionizing radiation detector. The use of He2 triplet molecules as tracer particles in the superfluid promises to be a powerful tool for visualization of both quantum and classical turbulence in liquid helium.
Metastable helium molecules generated in a discharge near a sharp tungsten tip immersed in superfluid 4He are imaged using a laser-induced-fluorescence technique. By pulsing the tip, a small cloud of He(2*) molecules is produced. We can determine the normal-fluid velocity in a heat-induced counterflow by tracing the position of a single molecule cloud. As we run the tip in continuous field-emission mode, a normal-fluid jet from the tip is generated and molecules are entrained in the jet. A focused 910 nm pump laser pulse is used to drive a small group of molecules to the first excited vibrational level of the triplet ground state. Subsequent imaging of the tagged molecules with an expanded 925 nm probe laser pulse allows us to measure the flow velocity of the jet. The techniques we developed provide new tools in quantitatively studying the normal fluid flow in superfluid helium.
Long-lived phosphorescence has been observed emanating from superfluid helium following excitation by ionizing radiation. This extreme ultraviolet light is predominantly from the radiative decay of metastable ${\mathrm{He}}_{2}(a{}^{3}{\ensuremath{\Sigma}}_{u}^{+})$ molecules and is detected through frequency down-conversion and photon counting techniques. This allows measurement of the phosphorescence lifetime in a low-molecular-density regime that is not dominated by two-body reactions. A phosphorescence lifetime of $13\ifmmode\pm\else\textpm\fi{}2 \mathrm{s}$ is observed.
We study the time evolution of the cloud of Dy atoms dispersed in superfluid helium and investigate the possibility of using these atoms as tracers for the imaging of superfluid-helium flows. The atoms are produced by pulsed laser ablation and are visualized via their laser-induced fluorescence. It turns out that the observation time is limited by the optical pumping of the atomic population into a nonabsorbing metastable state which is also responsible for the saturation of the fluorescence yield observed earlier.
We show in two demonstration experiments that metastable helium molecules can be visualized and used as tracers to quantitatively study the ∞ow of the normal component in super∞uid liquid 4 He using a laser-induced-∞uorescence technique. The methods we developed are useful in studying the role of the normal ∞uid component in quantum turbulence. A proposed experiment on imaging the normal-∞uid velocity pro- flle during the transition of quantum turbulence in a counter∞ow channel is discussed.