Scintillation and charge extraction from the tracks of energetic   electrons in superfluid helium-4
  An energetic electron passing through liquid helium causes ionization along its track. The ionized electrons quickly recombine with the resulting positive ions, which leads to the production of prompt scintillation light. By applying appropriate electric fields, some of the ionized electrons can be separated from their parent ions. The fraction of the ionized electrons extracted in a given applied field depends on the separation distance between the electrons and the ions. We report the determination of the mean electron-ion separation distance for charge pairs produced along the tracks of beta particles in superfluid helium at 1.5 K by studying the quenching of the scintillation light under applied electric fields. Knowledge of this mean separation parameter will aid in the design of particle detectors that use superfluid helium as a target material. 
W. Guo, M. Dufault, S.B. Cahn, J.A. Nikkel, Y. Shin, and D.N. McKinsey
physics.ins-det astro-ph.IM cond-mat.other hep-ex