Keywords:superfluid, fluorescence, laser excitation
Metastable He2* molecules (excimers) are a newly developed tracer for visualization of the superfluid 4He velocity distribution used in investigations for the field of quantum turbulence. Due to their relatively long excited state lifetime of 13s they can be repeatedly interrogated by interacting with a laser for fluorescence imaging. For full 3D flow-field mapping a distributed cloud of localized clusters of the excimer are required, which may be generated via the 3He(n,p)3T neutron absorption reaction. The resulting recoils of the reaction create a short track of ionized and excited Helium atoms, which form a cluster of approximately 104 excimer molecules via recombination. Electrons ejected by Compton-scattering of gamma-rays may create excimers as well, though with lower density. At the neutron beamline (BL22) at J-PARC MLF cold neutrons are produced via proton impact on a target, which results in a neutron beam accompanied by intense gamma radiation. In previous experiments fluorescence emitted by the excimers could clearly be detected via use of photomultiplier tubes (PMT). From these results it was however not clear whether the dominant excimer generation process was neutron absorption or Compton scattering. In a recent experiment at the beginning of this year we obtained new results using an image intensifier enhanced CCD camera. While individual He2* clusters could not be seen directly, a clear intensity distribution related to the exciting laser’s beam profile was observed during irradiation. To distinguish between the effects of gamma and neutron irradiation, different types and thicknesses of absorbers (Pb, Bi) were placed into the upstream neutron beam. Their respective neutron absorption and gamma ray scattering cross sections were then used to estimate the ratio between the two He2* generation processes.