Nuclear magnetic relaxation of /sup 3/He gas. II. /sup 3/He-/sup 4/He mixtures
Longitudinal relaxation times T/sub 1/ have been measured in /sup 3/He-/sup 4/He gas mixtures, using pulsed NMR, in the temperature range 0.6-15 K. Helium-3 number densities of the order of 10/sup 24/ atoms m/sup /minus/3/ were used. Relaxation takes place on or near the walls of the Pyrex sample cells and measurements of T/sub 1/ give information about the surface phases. A cryogenic precoating of solid molecular hydrogen was used to reduce the helium-substrate binding energy from approx. 100 K on Pyrex to approx. 13 K for /sup 3/He and 15 K for /sup 4/He. The T/sub 1/ data at high temperatures were similar to those observed previously in the pure /sup 3/He-H/sub 2/ system. The presence of /sup 4/He generally caused T/sub 1/ to rise on cooling below 2 K due to the preferential adsorption of /sup 4/He over /sup 3/He at the surface. However, /sup 3/He atoms that go into quasiparticle states in the superfluid helium film can be an extra source of relaxation. In uncleaned cells, relaxation probably takes place in quasiparticle states at the free surface of the superfluid film, which are bound with an energy of 5.1 /plus minus/ 0.3 K. Baking the Pyrex cells under vacuum and rf discharge cleaning the walls before sealing in the sample gas were found to increase the bulk gas T/sub 1/ by two or three orders of magnitude. In a cleaned, sealed cell of T/sub 1/ of approx. 8 h was measured at 7.7 MHz and 0.8 K. In this case relaxation is probably occurring two or three helium layers away from the helium-hydrogen interface. It may be possible to observe a predicted minimum in the intrinsic dipolar T/sub 1/ of the bulk gas by using a /sup 4/He wall coating to suppress wall relaxation effects (which usually dominate the nuclear relaxation of the bulk gas).
- Research Organization:
- Univ. of Sussex, Brighton (England)
- OSTI ID:
- 6061048
- Journal Information:
- J. Low Temp. Phys.; (United States), Vol. 72:1-2
- Country of Publication:
- United States
- Language:
- English
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640450* - Fluid Physics- Superfluidity