Collision Phenomena in Helium and Atomic Hydrogen: Quantum Gases at Low Temperature.

We have developed a low temperature atomic helium beam apparatus, which is cooled inside of a dilution refrigerator. A low energy beam is produced by heat pulse evaporating atoms from a sub-kelvin helium film coating a resistor. The beam is detected with a sensitive resistive bolometer 8.3cm away. Using this system, we have measured integral cross sections for the elastic scattering of ^4 He from ^4He and ^3He from ^3 He at sub-kelvin collision energies. Very large integral cross sections are observed for ^4He- ^4He scattering between 0.5K and 1.25K, which are more than an order of magnitude larger than a simple classical prediction, but in agreement with the quantum theory using the most recent analytical form for the He-He interatomic potential. We have performed a careful analysis of our measurement technique and have reached conclusions about the accuracy and feasibility of current and future experiments involving ^3 He beams. We have also conducted a theoretical and experimental study of the development of the helium beam via the heat pulse technique, and we have found that under certain conditions dramatic cooling in the beam is achieved. We have demonstrated beam cooling to below 700muK and have shown that cooling below 100muK is feasible with a properly designed source. Some of the potential applications and prospects for these beams in future research are presented as well. Finally, we used the sensitive bolometer developed for the beam experiments to study two-body surface recombination in spin-polarized atomic hydrogen (Hdownarrow ) from the standpoint of heat release in this highly exothermic reaction. We discovered that less than 4% of the total heat released is deposited locally on the surface at the point of recombination. This result, that only a tiny fraction of heat is deposited locally and the rest distributed throughout the enclosing cell, will have important consequences for the design and interpretation of future Hdownarrow experiments.