Neutron scattering from the free surface of liquid He has been used to investigate the distribution of 3He atoms in extremely dilute 3He-4He solutions at temperatures down to 0.08 K In particular evidence has been sought and found for the existence of Andreev trapping states for 3He just below the surface. The experiments were conducted in ISIS at the Rutherford Appleton Laboratory using the CRISP instrument (a general purpose reflectometer). The neutron scattering from the surface of 4 He (industrial and high purity) also was investigated and compared with dilute 3He-4He solutions. The 3He surface layer was observed for the first time using a small-angle neutron scattering (SANS) technique. The thickness of the layer is strongly dependent on temperature and 3He concentration. Even in the case of industrial 4He and even at 2.3 K (higher than superfiuid transition) a substantial amount of 3He atoms was found close to the surface. In the case of extremely pure 4He the 3He atoms on the surface were not observed. At low temperature (0.4 K) a very thin layer of 3He atoms was observed and the calculated thickness (based on our experimental results) is approximately ~10 A of pure 3He on the top of the mixture. It is followed by a diffusive interface area (also observed experimentally) of 3He-4He mixture of thick layer approximately ~200 A, just below that almost pure 3He thin layer on the top. With increase of the concentration the thickness of the top layer increases until it reaches a limit above which the 3He starts dissolving in the bulk superfiuid liquid 4He. Neutron critical opalescence from 3He gas/vapour was observed for the first time and the information for 3He density was obtained in a broad range of temperatures 1.4 - 50 K. The data was compared with the results gathered by using different techniques. Far from the critical temperature all methods yield similar results. However, near the critical temperature of Tc = 3.32 K, the densities obtained from neutron transmission results are lower than other published results.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:664265 |
| Date | January 2012 |
| Creators | Vasilev, Nikolay |
| Publisher | Lancaster University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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