Observation of3He crystal nucleation from the superfluid phases

A CCD camera operating at T = 65 K was mounted in the vacuum space of our nuclear demagnetization cryostat. This has allowed us to make observations of³He crystals at temperatures below the superfluid phase transitions, in contrast to direct optical observations, which have so far been limited to T 20 mK.1 The good thermal equilibrium provided by the superfluid allows us to nucleate single crystals of³He in the region of the cell visible to the optical system. This occurs either spontaneously (due either to gravitational pressure gradients or local surface defects) or as a result of a small applied heat pulse.