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Generation of \(^4\)He\(_2^*\) 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\(_2^*\) 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.

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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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Correspondence to T. Matsushita.

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Matsushita, T., Sonnenschein, V., Guo, W. et al. Generation of \(^4\)He\(_2^*\) Clusters via Neutron-\(^3\)He Absorption Reaction Toward Visualization of Full Velocity Field in Quantum Turbulence. J Low Temp Phys 196, 275–282 (2019).

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  • He\(_2^*\) clusters
  • Neutron-\(^3\)He absorption reaction
  • Quantum turbulence