Study of Positive Helium Ions in Superfluid Helium-4
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Study of Positive Helium Ions in Superfluid Helium-4 S. Sirisky1 · Y. Xing1
· G. M. Seidel1 · H. J. Maris1
Received: 11 July 2019 / Accepted: 12 November 2019 © Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract We report on studies of positive ions in superfluid helium-4. In addition to the positive ion that has been previously studied, we have been able to detect two more positive ions with slightly different mobility, and the results show some evidence for a third additional ion. Keywords Liquid helium · Positive ion · Mobility
1 Introduction The properties of ions in liquid helium-4 have been of interest for many years. The negative ion is believed to be an electron trapped in a bubble formed in the liquid. This general type of structure was first proposed by Ferrell [1] to explain the anomalously long lifetime of positronium in liquid helium and was later suggested by Careri et al. [2,3] as the structure for an electron in the liquid. To explain the structure of the positive ion, Atkins [4] proposed what has become known as the “snowball” model. He pointed out that the electric field around a positive ion polarizes the neighboring helium atoms and results in a pressure large enough to form a solid region close to the ion. When the liquid is in the normal fluid phase, the mobility of the ions is limited by the viscous drag exerted on the ion. In the superfluid phase, the mobility is determined by the rate at which the moving ion loses momentum as a result of collisions with the phonon and roton thermal excitations. Above a critical velocity, ions can lose momentum as a result of the nucleation of quantized vortices. In this paper, we report on a further investigation of positive ions in superfluid helium-4. We have detected three positive helium ions with slightly different mobility, and possibly a fourth. As far as we are aware, there is only previous experiment in which more than one positive ion has been detected in liquid helium-4 [5]. A very small signal from an ion with a mobility 8 to 10% higher than the normal positive
B 1
Y. Xing [email protected] Department of Physics, Brown University, Providence, RI 02912, USA
123
Journal of Low Temperature Physics
(a)
(b)
Fig. 1 Cross-sectional view of the experimental cell. The cell is filled with liquid helium up to a level between the grids G0 and G1. A radioactive source S emits alpha particles into the helium vapor above the liquid. A voltage applied to grid G0 maintains a plasma in the region between G0 and the source S. The plasma is laterally confined by the nylon structure that is shown in part (b). Positive ions entering the liquid from the plasma are able to pass into the drift region below G2 when a positive voltage pulse is applied to grid G1. After moving down the cell, the ions pass through the Frisch grid F and result in a pulse of current arriving at the collector C. The function of the different components in the cell and the measurement of the mobility of the ions are described in more detail in the text
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