Charge Transport in Liquid Helium at Low Temperatures

In an experiment to investigate the possibility of using superfluid helium as a detection medium for low energy solar neutrinos, we have studied the currents produced by a radioactive source in a helium cell having a liquid/vacuum interface at 50 mK. A number of phenomena have been observed that appear not to have been described in the literature. These include the following: 1) The current at very low voltages in a cell having a free surface can be 100 times greater than in a filled cell. This additional current is associated with Penning ionization of metastable triplet dimers in surface states. 2) There is a large amplification of current in modest electric fields with a free surface present in the cell. This is the result of charges accelerated across the vacuum having sufficient energy to produce ionization and additional free charges upon hitting a liquid surface. The amplification becomes sufficiently large that breakdown occurs at potential differences across the vacuum of less than 1000 V. The dependence on 3He concentration of these phenomena has been studied