Generation of He clusters via neutron-He absorption reaction toward visualization of full velocity field in quantum turbulence

Matsushita, Taku*; Sonnenschein, V.*; Guo, W.*; Hayashida, Hirotoshi*; Hiroi, Kosuke  ; Hirota, Katsuya*; Iguchi, Tetsuo*; Ito, Daisuke*; Kitaguchi, Masaaki*; Kiyanagi, Yoshiaki*; Kokuryu, Shoma*; Kubo, Wataru*; Saito, Yasushi*; Shimizu, Hirohiko*; Shinohara, Takenao   ; Suzuki, So*; Tomita, Hideki*; Tsuji, Yoshiyuki*; Wada, Nobuo*

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