The problem of isotope impurity in liquid 3He

1
The problem of isotope impurity in liquid 3He

• Static impurities. Freezing and evaporation of
  impurity film.
• Dynamic impurity. Motion of ions along
  surface.
• Pure Fermi liquid.
• Multimobility motion of ions in liquid 3He.

2
Behaviour of 4He isotopic impurities in liquid 3He

• The concentration of 3He in 4He is usually 10-5
  and may be cleaned up to 10-11, while
  concentration of 4He in 3He 10-2
• The compressed area around the ions attracts 4He
  atoms and repels 3He atoms
• How do 4He impurities affect the structure of the
  ions and the results of measurements of ion
  mobility in liquid 3He?
• What real concentration of 4He atoms do we have
  to deal with at low temperatures (1-10 mK)?

3
Experimental technique and ions emission.

• Measurements were made in dilution fridge with
  copper nuclear demagnetization stage.
• Thermal contact through a Pt-Ag sintered powder
  heat exchanger (180 m2 surface area)
• Thermometers - Pt NMR and melting curve.
• Ion emission by short ( 20 ms) high voltage
  pulses from W tip.
• The ionization current10-12A, which corresponds
  to 105 ions.
• The measurement in the cell used four operating
  grids.
• The concentration of impurities (0.05 4He) was
  found by mass spectrometer.

Atoms adhere on positive ions near emission tip.
4
Multipeak mobility of positive ions in 3He

• At both studied pressures (3 and 28.5 bar) we
  observed the existence of multipeak behaviour,
  which was explained by differing ion mobilities.
  This indicates different numbers of impurity 4He
  atoms in the ions immediate surroundings.
• It suggests that the signal depends on the number
  of ions with different surrounding structures.
• The compressed area around ion attracts less
  quantum atoms of 4He.

For an impurity concentration of 0.1 the
probability to find a pure complex and a complex
with one impurity atom within N16, 32 or 64
atoms of the ion surrounding is approximately
0.999N N0.001 or 0.984 0.016 (for 16
atoms) 0.968 0.032 (for 32 atoms) and 0.93
0.064 (for 64 atoms). It means that at
equi-probable places the atoms in the snowball,
the amplitude of second peak (one impurity atom)
must be negligibly small. Experimentally it was
observed that the second peak may be higher than
the peak corresponding to pure 3He.
5
Freezing of impurities on a cell wall and their
evaporation.

• We found that the second (and higher order) peaks
  exist only at certain temperatures and prehistory
  conditions.
• The tip and draft area are cleared of the 4He
  impurity atoms in a few hours by cooling the
  liquid to 13 mK or less at pressure of 3 bar, or
  to less than 5 mK at 28.5 bar. The disappearance
  of 4He may be explained as the freezing of the
  impurity atoms into a solid or liquid film on the
  cell surface.
• Experiments in a cell with insufficient surface
  area showed the existence of 4He impurities at
  all temperatures.

• Static impurities

6
Process of 4He impurities freezingon the cell
walls.
The measurements in virgin liquid The 4He atoms
began to freeze on the cell walls, but
concentration of impurities is still very high.

• Change of signal shapes before and after
  impurities freezing.

After cooling to low temperature all the 4He
atoms freeze on to the cell walls. After many
shots measurements are not affected by the
impurities.

• Static impurities

7
Evaporation of freezing 4He impurity atoms.

• The heating of the cell to high temperatures
  evaporated the 4He impurities into the bulk and
  we observed the multi-peak mobility. For example,
  a sample at P3 bar sitting at 0.4 K for an hour
  resulted in renovation of impurities
  concentration.

In the right diagram we reconstructed the
shape of the multipeak signal as sum of
independent batches of ions with different
mobility.

• Static impurities

8
Dynamic impurities.

• Shot evaporation of atoms 4He after freezing of
  impurities.
• The reduce of area of the second peak gives us
  information about the depletion of the 4He.
• S2 0.5exp(-i/48), where i is number of shoots
  .
• The number of evaporated 4He atoms is not less
  then 25 105.
• The upper limit of numbers of atoms on the tip
  surface is ?0.13mm2/Satom.10-6m2/10-19 m2
  1013 atoms.
• The average waiting time between measurements was
  about two hours (i.e. 104 s). The time between
  two sets of sweeps ( 102 s) was not enough to
  renew the tip film.

The exhausting He-4 film from tips area. For
first 32 shoots S1 0.079, Ampldel, S20.050
S1S20.129   For second 32 shoots S1 0.122,
Ampldel, S20.029 S1S20.151   For last
curve S0.137 The common area (number of
charges) S1S2 const
9
Processes of 4He evaporation from tip area, cell
sweeping and film renewal.
The next shots The tip and drift area are swept
of the 4He impurities. The rate of 4He creep is
not enough for the tip film to renew.
After a long waiting time between measurements
4He covers the tip and surrounding wall.
First shots 4He impurities evaporate from hot
area near tip, we observe ions polluted by 4He
impurities.
After a long waiting time a 4He film recovers.

• Dynamic impurities

10
CONCLUSIONS

• In liquid 3He at different pressures (high - 28.6
  and 32.34 bar, and low 3 bar) in the temperature
  region between 30 and 14 mK, we observed the
  existence of multipeak mobility of positive ions.
  This behaviour can be explained as the motion of
  ions with different levels of 4He 'pollution'.
• The quickest batch of ions corresponds to the
  clusters surrounded only by pure 3He.
• The positive ions attract 4He more effectively
  than 3He. Thus ions in liquid 3He serve as a
  detector of isotopic 'pollution'.
• The 4He impurities freeze on the surface of cell
  during cooling. The temperature of effective
  cooling is order of 10 mK. After this we had pure
  3He in temperature range less 60 mK. (The
  concentration of 4He reduced by at least in two
  orders). Warming liquid to a temperature of 0.4 K
  renovates the equilibrium impurity concentration.
  
• We observed the existence of impurity mobility
  for high pressures down to the temperature of the
  SF transition. The frozen 4He atoms evaporated
  from the tip surface during the emission of ions.
  The local concentration quickly reduces with
  sweep repetitions. We have a highly effective
  process moving impurity atoms from the tip to the
  collector by ions. This process supports by high
  mobility of the 4He film along the tip for the
  next sweep.