I. Pepe et al., POSITRON-ANNIHILATION IN LIQUID-HELIUM AND LIQUID ARGON UNDER AN ELECTRIC-FIELD, Journal of physics. B, Atomic molecular and optical physics, 28(16), 1995, pp. 3643-3659
We have made new measurements of the lifetimes at 4.29 K and 2.92 K in
He I, and at 2.14 K and 1.67 K in He II, without and with an electric
field. At 4.19 K we have a new value of the lifetime, 2.01 +/- 0.002
ns, leading to a Z(eff) of 3.53 at zero field. In contrast to earlier
reports, Z(eff) decreases very slightly as the temperature is lowered
and the density increases. The orthopositronium lifetime at 4.19 K is
98.1 +/- 0.2 ns, at zero field, independent of field up to 18 kV cm(-1
). At 4.19 K the free positron decay rate falls exponentially with inc
reasing field to a value 5% below its zero-field value, the drop-off r
ate being 0.36 +/- 0.04 cm amagat V-1. From 28 through 50 V cm(-1) ama
gat(-1) the decay rate of the free component rises slowly as expected
because of the increasing positronium fraction over the same range of
electric field. In liquid argon there is positronium enhancement begin
ning at or below 3 V cm(-1) amagat(-1), which contrasts with 160 V cm(
-1) amagat(-1) for argon gas. At least two lifetimes make up the long
triplet component, suggesting different degrees of thermalization, or
different states of positronium within the liquid. The effect of the e
lectric field is to enhance the longer-lived, or more thermal componen
t, in strong contrast with the findings of Charlton et al for the gas.
The argon results suggest positron behaviour similar to that of elect
rons in the liquid phase, namely, such particles move in a conduction
band subject to small effective scattering and diffusion cross section
s. An analytic model in which the scattering length is an adjustable p
arameter yields the onset of positronium enhancement just below 3 V cm
(-1) amagat(-1) when a(s) = -0.043 a(0), corresponding to an elastic s
cattering and diffusion cross section of 0.034 pi a(0)(2). Consequence
s for positron thermalization are discussed. A model of positronium lo
sing energy slowly in the liquid without bubble formation is suggested
.