The Thermal Accommodation of Atomic Hydrogen on Liquid HELIUM-4 and HELIUM-3.

An apparatus is described for the creation of atomic hydrogen in situ in a low-temperature environment provided by e.g. a dilution refrigerator. Measurements of the source performance and estimates of the maximum density obtainable with such a source are presented. A maximum flux of ~8 times 10^ {14} H atoms/s is obtained from a very simple source construction utilizing an 8 T solenoid to extract the atoms from the source region. The thermal accommodation coefficient alpha has been measured for H atoms on liquid ^3{ rm He} and ^4{rm He} surfaces, in order to determine the effectiveness with which H atoms may be cooled to ultra-low temperatures. It is found that the thermal accommodation coefficient decreases approximately as alpha = 0.50 cdot T as the temperature is reduced to zero for both ^3{rm He} and ^4{rm He} surfaces. The presence of small amounts of ^3{rm He} on the surface of ^4{ rm He} was also found to have little effect on the thermalization of the gas. Prospects for cooling H gas to temperatures below 50 mK by heat exchange with the walls of a container appear to be poor.