4He Impurity Effect on the Positive Ion Mobility in Liquid 3He

The behaviour of 4 He impurities has been investigated through positive ion mobility measurements in liquid 3 He at milliKelvin temperatures. In spite of the enormous surface area of the cell, the influence of 4 He lasted for hundreds of hours at temperatures of ~20 mK. However below 10 mK, the 4 He impurities were quickly frozen out on the cell walls and we were free from the impurity problem. Nevertheless the multiple ion signal occasionally appeared even at a few mK, although it disappeared for a number of ionization pulses.