Another technique that we are developing is based on tracking small He2* clusters created in He II via neutron-3He absorption reactions. A compelling reason for developing this technique is that it will produce full-space velocity-field information that can hardly be obtained using the tracer-line tracking method. Knowing the full-space velocity field is essential in visualizing eddy structures and large-scale flow patterns that in some cases can control the flow properties . In the past, many efforts were devoted in developing PTV methods for mapping the normal-fluid velocity field by tracking seeded micron-sized tracers [1-4]. However, these methods all have inherent issues since the tracer motion can be affected by both the normal fluid and the vortices. Our He2* cluster tracking technique will provide an ultimate solution to the long-lasting quest for unambiguous normal-fluid velocity field measurement.
3He atoms that exist in He II as impurity particles can absorb neutrons (see the reaction schematic in Fig. 1 (a)) . The absorption cross-section σ depends on the energy of the incident neutrons and can be quite large for thermal neutrons that have spins aligned with the 3He atoms (i.e. σ ~ 10-20 cm2) . This reaction produces two charged particles, a proton (1H) and a tritium (3H), that move back to back with a total kinetic energy of 764 keV . The two particles have sufficient energies to ionize and excite ground state 4He atoms along their tracks in He II , which leads to the generation of He2* molecules. The total length of the two tracks is about 80 μm, evaluated based on the known energy deposition rate of 1H and 3H in helium . We also estimate that more than 104 molecules are produced, considering the fraction of the kinetic energy that goes to the generation of He2* molecules . Therefore, every n-3He absorption event in He II creates a cluster of about 104 He2* molecules. Imaging these clusters should be feasible since we have already demonstrated high-quality fluorescence imaging of tiny clouds of He2* molecules in He II (see Fig. 1 (b)) . Due to the small molecular diffusion (~ 10 μm during typical drift time), every cluster can be treated as a single tracer, and a spatial resolution of a few tens of microns is achievable. Tracking these clusters will allow us to map out the full velocity field.
In order to study the generation of He2* clusters in He II as well as to characterize their size and intensity distributions, we have established a collaboration with a group of researchers at Nagoya University (i.e. Prof. Shimizu, Prof. Tsuji, Prof. Wada, et al.) to conduct experiments using the J-PARC neutron facility in Japan . Our plan is to pass a thermal neutron beam with a suitable flux (i.e. ~104/cm2 per pulse) through a He II filled cell (see Fig. 2). By adjusting the concentration of 3He impurity, we will be able to tune the number density of the resulted He2* clusters. For instance, even with a 3He concentration as low as 50 ppm (part per million), about 102 clusters will be produced per 1 cm3 volume of He II, sufficient for velocity-field mapping purpose. This work will also lead to a sensitive mechanism for neutron detection.
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