3He-4He liquid mixtures investigated by neutron imaging technique at low temperatures*

Helium is a unique element which exhibits a variety of different phases and unusual behaviors. It can be found in nature in two stable isotopic forms: $^{3}$He and $^{4}$He. One of the most profound quantum mechanical effects, superfluidity, occurs below 2.17 K in liquid helium $^{4}$He and 0.003 K in liquid $^3$He. There are also interesting phenomena occurring in mixtures of the two isotopes. One demonstrative example is the finite solubility of liquid $^{3}$He (a Fermi system) in superfluid $^{4}$He (a Bose system) even at T = 0 K. This is the basic principle in the operation of a $^{3}$He-$^{4}$He dilution refrigerator capable of continuously producing 2 mK. While much has been done in studies of the thermodynamical, quantum properties of liquid helium mixtures, there has not been any attempt to visualize the dynamics of $^{3}$He in liquid $^{4}$He. Presented results of neutron imaging experiments on 0.3 bar liquid $^{3}$He-$^{4}$He mixtures, at 1.5 K have shown a clear diffusion of $^{3}$He driven by the difference in chemical potential. The data were taken for over 12 hours using a high resolution CCD camera.