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.