These expansion experiments involve the superfluidity properties of the helium isotope ⁴He. It is well known that at the temperature of 2.17 K ⁴He undergoes a transition signalled by a specific heat anomaly, whose characteristic shape has led to the name "lambda point", being given to the temperture at which it occurs. This temperature marks the transition between two different forms of liquid ⁴He. Above the lambda point helium behaves like a low-viscosity low-density liquid. Below the transition temperature helium is capable of being viscous and non-viscous at the same time, a contradiction which is the essence of the "two-fluid model" (Tisza, 1938).

According to this model, helium in the superfluid phase behaves as if it were a "mixture" of two liquids, one, the normal fluid, possessing an ordinary viscosity, and the other, the superfluid, being capable of frictionless flow past obstacles and through narrow channels. The validity of the two-fluid model was demonstrated in an experiment by Andronikashvili (1946), in which a pile of equally spaced thin metal discs were suspended by a torsion fibre in order to be able to perform oscillations in liquid helium. These experiments confirmed the prediction that the superfluid fraction would have no effect on the torsion pendulum. Andronikashvili found that helium is almost entirely superfluid below 1 K. London (1938) suggested that the lambda point marks the onset of "Bose-Einstein condensation", a phenomenon in which an ideal gas of particles with a resultant spin of zero (bosons) at low temperatures tends to occupy the lowest single-particle energy level of the system. The condensate is associated with the superfluid fraction of helium below the lambda point, while the normal component corresponds to the elementary (thermal) excitations of the whole system.

In the present experiments the idea is to produce a superfluid beam by expanding helium, cooled to temperatures in the range 1.3 - 2.2 K, through a nozzle (aperture diameter of about 1-5 micron) into a vacuum. A bath cryostat is needed for reaching and maintaining a nozzle temperature in the above range, the upper limit being determined by the normal boiling point point of ⁴He. To decrease the temperature one simply pumps the vapour above the liquid ⁴He bath away. An electric valve above this pump, operated according to the readings of a pressure gauge, allows to maintain a fairly constant nozzle temperature below 4.2 K.

Extensive measurements of the angular and velocity distributions led to the identification of two modes of operation. At temperatures above about 2.0 K and source pressures below about 10 bar the liquid jet appears to break up shortly after leaving the nozzle into a beam of droplets with a broad angular and velocity distributions. At lower temperatures and higher pressures the distributions become very sharp. Optical examination reveals a pencil-like beam, which is composed of uniformly sized micron-diameter droplets consisting of more than 10⁹ atoms.

Some remarkable changes in the beam properties are seen at the superfluid transition temperature (≈ 2.2 K). These new operating conditions open up the possibility to embed very large molecules or even nano-crystals inside these huge droplets.

Robert Grisenti, who carried out these experiments as a postdoc in Göttingen, is now constructing a helium liquid jet at the Physics Department at the University of Frankfurt to be used as a high density α - particle target to be inserted into the heavy ion accelerator at the GSI in Darmstadt as part of the Panda project. The Panda project will study antiproton - nucleon and antiproton - nuclei reactions with high precision and high luminosity at the future international accelerator FAIR in Darmstadt.