Imaging Fluorescence of He2* Excimers Created by Neutron Capture in Liquid Helium II

Abstract

We show unequivocal evidence for formation of 𝐻𝑒 2 * excimers in liquid He II created by ionizing radiation produced through neutron capture. Laser beams induced fluorescence of the excimers. The fluorescence was recorded at a rate of 55.6 Hz by a camera. The location of fluorescence was determined with an uncertainty of 5 m. The technique provides an opportunity to record the flow of 𝐻𝑒 2 * excimers in a medium with very small viscosity and enables measurement of turbulence around macroscopic liter size objects or vortex matter in three dimensions.The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.

Intermittency of velocity circulation in quantum turbulence

“…While this technique has been applied in finite temperature superfluid 4 He [51][52][53], the interpretation of PIV measurements in this system remains unclear [54,55]. As an alternative, variants of the particle tracking velocimetry (PTV) technique have been used in most recent studies of 4 He [54][55][56][57][58][59][60][61][62]. To our knowledge, no attempts have been made to compute the velocity circulation in superfluid experiments.…”

Tracking normal fluid flow in He II with unsupervised machine learning

“…The density of excimer clusters can be increased by a factor of 10x by increasing the concentration of 3 He accordingly. The increased density of 3 He would be ~1% of the roton density thus not expected to affect the hydrodynamics of He II at 1.7 K. 1 With more excimer clusters, the spatial resolution will be improved. However, with more excimer clusters to track, the correlation metric may provide ambiguous results (cluster confusion).…”

“…Previously, we demonstrated the ability to produce copious 𝐻𝐻𝐻𝐻 2 * excimers in He II using neutron capture, to excite the excimers with lasers and to observe the resulting fluorescence. 1 The demonstration was performed in a quiescent environment (i.e., one with very little fluid motion) of an enclosed glass bulb. Acquisition of such images taken during flow around objects is expected to provide data to test and validate theoretical 2 and computer models that describe the development, intensity, and internal structure of turbulent flow beyond simple systems (e.g., flow over a smooth flat plate).…”

“…A detailed description of the experimental setup is given in Ref. [1]. Briefly, a cryostat with windows transparent to infrared light contains a glass bulb that itself contains a mixture of 3 He and 4 He in the ratio of 1 to 1818 (pressure at 300 K is 67 kPa).…”

“…The experimental setup for the present experiment differs from Ref. [1] in two ways. First, a 6.4-mm thick plate of boron-nitride (BN) (an effective neutron absorber) was placed on the front surface of the cryostat to block the neutron beam from entering either the top or alternately the bottom half of the glass bulb, thus, forming a neutron shadow (Fig.…”