Academic and Research Staff B. Velocity Distribution of Atoms Evaporating from Superfluid Helium

We have been using molecular-beam techniques to study superfluid liquid helium and other systems at low temperature. The following helium beam experiments are in progress. 1. Measurement of the cross section of 4helium atoms for collision with both 4helium and 3helium at low temperatures (D. E. Oates). 2. Measurement of the velocity distribution of atoms evaporating from helium II at low temperatures (J. W. McWane and R. F. Tinker). 3. Studies of evaporated atoms resulting from heat pulses introduced in the liquid (R. F. Tinker). 4. Studies of diffusion of 3helium in 4helium by observation of evaporating atoms (G. A. Herzlinger). 5. Investigation of motion of ions in helium at low current densities (W. B. Davis, with Dr. C. E. Chase of the Francis Bitter National Magnet Laboratory). 6. Studies of evaporation from superfluid helium films (J. R. Clow). We are continuing and accelerating our work in various kinds of microscopy which includes the computer design of improved electron microscope lenses (D. S. Ofsevit) and their experimental realization (E. H. Jacobsen). We are also developing a scanning neutral molecule "microscope," using water molecules with which we plan to study various biological systems (J. C. Weaver). A. IONS AT LOW DENSITIES IN He II Many experiments on properties of ions in liquid He II have been carried out. In every case, however, the ions were detected with the use of standard electrometer techniques requiring currents in range 10 1 2-10-14 A. Qualitative calculations suggest that space-charge effects may be important in some cases, especially at the lowest temperatures. Application of electron multiplier techniques makes it possible to detect single ions, thereby increasing the sensitivity by a factor greater than 10 4. In order to accomplish this, the ions must be extracted from the liquid and accelerated to a high velocity in a near vacuum. An apparatus is being constructed to test this procedure. Once its feasibility has been demonstrated, a variety of experiments can be carried out; in particular, we hope to study the "neutral excitations" recently reported by Surko and Reif.1