An experimental setup for creating and imaging 4He2 * excimer cluster tracers in superfluid helium-4 via neutron-3He absorption reaction

V Sonnenschein; Y Tsuji; S Kokuryu; W Kubo; S Suzuki; H Tomita; Y Kiyanagi; T Iguchi; T Matsushita; N Wada; M Kitaguchi; H M Shimizu; K Hirota; T Shinohara; K Hiroi; H Hayashida; W Guo; D Ito; Y Saito

doi:10.1063/1.5130919

An experimental setup for creating and imaging ⁴He₂ ^* excimer cluster tracers in superfluid helium-4 via neutron-³He absorption reaction

Rev Sci Instrum. 2020 Mar 1;91(3):033318. doi: 10.1063/1.5130919.

Authors

V Sonnenschein¹, Y Tsuji¹, S Kokuryu¹, W Kubo¹, S Suzuki¹, H Tomita¹, Y Kiyanagi¹, T Iguchi¹, T Matsushita², N Wada², M Kitaguchi², H M Shimizu², K Hirota², T Shinohara², K Hiroi³, H Hayashida⁴, W Guo⁵, D Ito⁶, Y Saito⁶

Affiliations

¹ Department of Energy Engineering, Nagoya University, Nagoya, Aichi 464-8603, Japan.
² Department of Physics, Nagoya University, Nagoya, Aichi 464-8602, Japan.
³ J-PARC Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan.
⁴ Comprehensive Research Organization for Science and Society, Tokai, Ibaraki 319-1106, Japan.
⁵ National High Magnetic Field Laboratory, Tallahassee, Florida 32310, USA.
⁶ Institute for Integrated Radiation and Nuclear Science, Kyoto University, Kumatori, Osaka 590-0494, Japan.

PMID: 32259963
DOI: 10.1063/1.5130919

Abstract

For the purpose of future visualization of the flow field in superfluid helium-4, clusters of the triplet state excimer ⁴He₂ ^* are generated along the micro-scale recoil tracks of the neutron-absorption reaction n + ³He → ³T + p. This reaction is induced by neutron irradiation of the ³He fraction contained in natural isotopic abundance liquid helium with neutron beams either from the Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility (JPARC)/Materials and Life Science Experimental Facility or from the Kyoto University Institute for Integrated Radiation and Nuclear Science. These ⁴He₂ ^* clusters are expected to be ideal tracers of the normal-fluid component in superfluid helium with several advantageous properties. Evidence of the excimer generation is inferred by detection of laser induced fluorescence emitted from the ⁴He₂ ^* clusters excited by a purpose-built short pulse gain-switched titanium:sapphire (Ti:sa) laser operating at a wavelength of 905 nm. The setup and performance characteristics of the laser system including the Ti:sa and two continuous wave re-pumping lasers are described. Detection at the fluorescence wavelength of 640 nm is performed by using optical bandpass filtered photomultiplier tubes (PMT). Electrical noise in the PMT acquisition traces could successfully be suppressed by post-processing with a simple algorithm. Despite other laser-related backgrounds, the excimer was clearly identified by its fluorescence decay characteristics. Production of the excimer was found to be proportional to the neutron flux, adjusted via insertion of different collimators into the neutron beam. These observations suggest that the apparatus we constructed does function in the expected manner and, therefore, has the potential for groundbreaking turbulence research with superfluid helium.