Nuclear magnetic relaxation in gaseous helium
Longitudinal relaxation times T1 have been measured in 'He gas,
using pulsed NMR, for number densities between 3x 1023 and 6x 102' spins/m'
and temperatures between 0.6 and 15K. Relaxation takes place on or near
the walls of the pyrex sample cells and measurements of Tl give information
about the surface phases.
A low temperature amplifier containing GaAs FET devices was
developed to improve the spectrometer sensitivity. An amplifier noise
temperature of 0.9 ± 0.5K was obtained at 1.16 MHz and an NMR signal was
observed at 4.2K with the sensitivity being mainly limited by Johnson
noise in the receiver coil.
Baking the pyrex cells under vacuum and discharge cleaning the
walls before sealing in the sample gas were found to increase the bulk
gas Tl's by two or three orders of magnitude. A cryogenic wall coating
of solid molecular hydrogen was found to delay the formation of a 'He
monolayer on cooling and T, measurements were consistent with a binding
energy of %, 13K for a 3He atom to a hydrogen surface. Once a 3He monolayer
has formed the dipolar interaction between adsorbed spins is
thought to be the dominant source of relaxation in the sealed cells.
The presence of "He generally causes T, to rise on cooling below
2K due to preferential adsorption of "He at the surface. However, 'He
atoms which dissolve in quasiparticle states in the superfluid helium
film can be an extra source of relaxation. In the dirty cells relaxation
probably takes place in quasiparticle states at the free surface of the
saturated helium film, which are bound with an energy of 5.1 t 0.3K. In
a cleaned, sealed cell a T, of ti 8 hours was measured at 7.7 MHz and
0.8K. In this case relaxation is probably occurring 2 or 3 helium
layers away from the helium-hydrogen interface