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Title: Nuclear magnetic relaxation in gaseous helium
Author: Lusher, C. P.
Awarding Body: University of Sussex
Current Institution: University of Sussex
Date of Award: 1985
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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
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solid-state physics Solid state physics Chemistry, Inorganic