Generation of ${}^4$He${}_2^{\ast }$ Clusters via Neutron-${}^3$He Absorption Reaction Toward Visualization of Full Velocity Field in Quantum Turbulence

For flow visualization study of quantum turbulence in superfluid ${}^4$He, He${}_2^{\ast }$ excimers are unique tracers which follow only normal-fluid component flow above 1 K. To generate detectable small He${}_2^{\ast }$ 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${}_2^{\ast }$ 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${}_2^{\ast }$ excimers were generated by neutrons. We also estimated the number of He${}_2^{\ast }$ excimers possibly generated by $\gamma$-rays and found that the relevant contribution was less than 40%. Thus, the majority of the He${}_2^{\ast }$ excimers was confirmed to be generated via n-${}^3$He absorption reactions.