Observation of Helium Excimer Clusters Generated via Neutron-3He Absorption Reaction
Department of Physics, Nagoya University Taku Matsushita
1. Introduction
Turbulence is one of subjects which is scientifically and industrially important but whose detailremains unsolved by its complexity. Therefore, quantum turbulence, where vortices are quantized,
has been expected to be a key to solve the physics of turbulence, in addition to scientific interests on
the nature specific of quantized vortices. To study quantum turbulence existing in superfluid 4He,
recently flow visualization using triplet metastable 4He molecules (excimers) He2* as a tracer wasemerged to be a new method which provides us information on normal-component flow individually[Marakov et al., Phys. Rev. B (2015)]. However, complete observation of the velocity fields inturbulent superfluid 4He requires localized small clouds of He2* excimers, clusters, of which
generation method has not been established so far. Thus, to realize such excimer clusters, we
proposed the use of the neutron absorption reaction 3He(n,p)3H of 3He impurities contained in liquid
4He,where 1H and 3H resulted by the reaction have sufficient energy to excite 4He atoms forexcimer generation. And, we started the proof-of-principle experiments using the neutron beam of
J-PARC/MLF.
As the first step, in previous experiment (Project No. 2016B0111), we found that the application ofneutron beam increases the signal suggesting He2* excimer generation, but the reproducibility andstability were not sufficient for detailed analyses. Therefore, we improved the detection system asshown in the next section. Thus, this experiment was conducted to obtain the proof of excimer
generation via neutron-3He absorption reaction by quantitative analyses of improved experimental
data.

2. Experiment
The sample liquid helium was stored in a stainless Dewar with four quartz optical windows at thebottom part. By depressurizing of it, the temperature of liquid helium was kept at 1.8-1.9 K below
the superfluid transition temperature in order to avoid disturbance by vapor bubbles, and a neutron
beam with a cross section 20  20 mm2 was applied into liquid 4He thorough a pair of the optical
windows.

Commercially-available liquid4He

usually includes 0.3 ppm of 3He as isotopicimpurities. By absorption of a neutron, the 3He atom splits into 1H and 3H with an energy of 764
keV, which is expected to ionize 4He atoms along their track, and generate a cluster consisting oftypically 104 He2* excimers. For detection of these excimers, 905 nm laser pulses with crosssection 8-20 mm2 were applied in the center of the neutron beam in parallel, to excite 640 nm
fluorescence of the excimers. And then, the fluorescence was observed by photomultiplier tubes(PMT) using the bandpass filter selectively, through two optical windows placed in the direction
perpendicular to the neutron and laser beams.
To create the 905 nm laser pulses for fluorescence excitation in our system, 532 nm Nd:YAG laser
pulses are converted via a resonator using a Ti-sapphire crystal. Before this experiment, weimproved this converter for much more stable and larger photon density in pulses than the previous
setup. In addition, for repumping cw laser to help excimers to return the ground state, 1064 nm
1was added to the previous 1085 nm, to cover a wider wavelength range. Faster data acquisition
system for PMT was also prepared to take a statistically meaningful amount of data.
3. Results
As a result, the improved excimer detection system showed a very good performance with highstability and reproducibility during whole the experiment. The excitation laser intensity waslaser sheet, which enables us to visualize thevelocity field in superfluid 4He in future.
The

figureshowsatypicalHe2*

excimer

fluorescence signal observed by PMT, where 2000
events triggered by the 905nm excitation laser pulse
were averaged. Since smaller amount of photons
were always observed accompanying the excitation
pulse even when neutrons were absent, those were

PMT fluorescence signal (arb)confirmed to be sufficient to make an appropriate
 exp(t/)2

 = 45.0 nsec

10-1000subtracted as a background. In the figure, spike

100
200time (nsec)

300

400

signals of each photons were averaged, so that this envelope indicates the probability of photon
emission from the excited state of He2* excimers. The time constant,  in the figure, was obtained
to be 40-46 nsec for various experimental conditions, which corresponds to the lifetime of theexcited state. The value agrees with 48  2 nsec appearing in a literature [Rellergert et al., Phys.
Rev. Lett. (2008)], which shows that this signal is fluorescence of He2* eximers.
When the neutron flux was changed by the selection of upstream collimators and shutters on thebeamline, the intensity of fluorescence was observed to be proportional to the neutron flux, whichclearly indicates that these He2* excimers were generated by neutrons. On the other hand, toconfirm that these He2* excimers are generated via 3He(n,p)3H reaction, contribution of  rayaccompanying the neutron beam must be excluded, because they also generate excimers via theCompton effect on all 4He atoms. By control experiments using the B4C blocker on the beamline,
these  ray contributions were estimated to be 40% at most.

Thus, the main contributor togenerate He2* excimers were confirmed to be the 3He(n,p)3H reaction.
From the obtained PMT signal intensity, the number of He2* excimer clusters in the steady state
are estimated to be at least about 170 clusters/cc for the neutron flux of 3.7  105 n/sec/cm2, which
are considered to be sufficient for tracers to visualize the flow field in superfluid 4He.

4. Conclusion
We have confirmed that He2* excimer clusters are generated via the neutron absorption reactionof 3He impurities existing in liquid 4He, and that they can be detected by the laser-induced
fluorescence. The laser system to excite fluorescence of excimers was also confirmed to havesufficient power towards visualization of complete flow field in turbulent superfluid 4He in future.
Thus, for a next step, we will advance preparation to capture the real image of a He2* excimer
cluster generated by the 3He(n,p)3H reaction, using a video camera with high sensitivity.
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