Generation of He 2∗ Clusters via Neutron- He Absorption Reaction Toward Visualization of Full Velocity Field in Quantum Turbulence

For flow visualization study of quantum turbulence in superfluid He, He2∗ excimers are unique tracers which follow only normal-fluid component flow above 1 K. To generate detectable small He2∗ clouds (clusters) required for full-space velocity field measurements, we have adopted a new method based on neutron absorption reaction of He impurities in He and conducted proof-of-principle experiments. Generation of the He2∗ excimers was detected by laser-induced fluorescence using photomultiplier tubes. The fluorescence was observed to increase proportionally to the neutron flux, suggesting that a sufficient amount of He2∗ excimers were generated by neutrons. We also estimated the number of He2∗ excimers possibly generated by γ-rays and found that the relevant contribution was less than 40%. Thus, the majority of the He2∗ excimers was confirmed to be generated via n-He absorption reactions.