Generation of \(^4\)He\(_2^*\) Clusters via Neutron-\(^3\)He Absorption Reaction Toward Visualization of Full Velocity Field in Quantum Turbulence
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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.
KeywordsHe\(_2^*\) clusters Neutron-\(^3\)He absorption reaction Quantum turbulence
We thank S. Murakawa, R. Nomura and Y. Okuda for permitting the use of their dewar for this project. Neutron experiments have been performed at the beamline B-4 in KUR at the Institute for Integrated Radiation and Nuclear Science, Kyoto University (Project No. 29P7-13), and at the beamline BL22 in MLF of J-PARC Center as a non-proprietary use experiment (Project No. 2017B0220). We acknowledge the Cryogenics Section in J-PARC Center for supplies of liquid He and N\(_2\).
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