Generation of $$^4$$4He$$_2^*$$2∗ Clusters via Neutron-$$^3$$3He Absorption Reaction Toward Visualization of Full Velocity Field in Quantum Turbulence

For flow visualization study of quantum turbulence in superfluid \(^4\)He, He\(_2^*\) excimers are unique tracers which follow only normal-fluid component flow above 1 K. To generate detectable small He\(_2^*\) clouds (clusters) required for full-space velocity field measurements, we have adopted a new method based on neutron absorption reaction of \(^3\)He impurities in \(^4\)He and conducted proof-of-principle experiments. Generation of the He\(_2^*\) 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 He\(_2^*\) excimers were generated by neutrons. We also estimated the number of He\(_2^*\) excimers possibly generated by \(\gamma \)-rays and found that the relevant contribution was less than 40%. Thus, the majority of the He\(_2^*\) excimers was confirmed to be generated via n-\(^3\)He absorption reactions.