Abstract
We have investigated the rate
v
at which negative ions nucleate charged vortex rings in a series of extremely dilute superfluid
3
He/
4
He solutions. Measurements of
v
were made at a pressure P = 23 bar (23 x 105 Pa) for temperatures,
T
, electric fields,
E
, and
3
He/
4
He isotopic ratios,
x
3
, within the ranges: 0.33 <
T
<0.61 K, 1.0 x 10
4
<
E
< 1.5 x 10
6
V m
-1
, 2.1
x
10
-8
<
x
3
< 1.7 x 10
-7
. A few data were also recorded at other pressures within the range 19 <
P
< 25 bar. For each concentration, and also for nominally pure
4
He (
x
3
= 1.9 x 10
-10
),
v
was measured for the same set of
E
and
T
. For all the chosen values of
x
3
and
P
, the form of
v
(
E, T
) was qualitatively much the same, and considerably more complicated than for pure
4
He. It was found that
v
became equal to the nucleation rate in pure
4
He for large , but that for smaller values of
E
at low
T
. The
3
He-influenced contribution to the overall nucleation rate, A
v
=
v
—
v
0
, passed through a pronounced maximum at a value of
E
that increased with increasing
T
; but the magnitude of A
v
itself decreased rapidly with increasing
T
. Plots of
v
against
x
3
for fixed
P, E
and
T
show a marked upward curvature for the lower values of
E
and
T
, but become linear within experimental error above
ca
. 0.5 K. A model is proposed (in two variants) in which the complicated behaviour of
v
(
E, T
) is accounted for in terms of changes in the average occupancy by
3
He atoms of trapping states on the surface of the ion, it being proposed that the nucleation rate
v
1
, due to ions each having one trapped
3
He atom, is very much greater than
v
0
for bare ions. The nonlinearities in
v
(
x
3
) are interpreted in terms of the simultaneous trapping of two (or more)
3
He atoms on a significant fraction of the ions. It is shown that the model can be fitted closely to the experimental data, thereby yielding numerical values of
v
1
)of the
3
He binding energy on the ion, and of a number of other relevant quantities. From the form of
v
1
(
E
), it is deduced that the addition of a
3
He atom to a bare ion affects its propensity to create vortex rings in two ways: the critical velocity for the process is reduced by
ca
. 4 m s
-1
, and the rate constant is increased by a factor of
ca
. 10
3
. The implications of these results for microscopic theories of the vortex nucleation mechanism are discussed.
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