1. H. Schlichting and D. Menzel, Rev. Sci. Instrum. 64, 2013 (1993).
2. Constant flow cryostat, Air Products, APD-Cryogenics, P.O. Box 2802, Allentown, PA 18105.
3. H. Schlichting (private communication).
4. K. Wandelt, S. Daiser, R. Miranda, and H.-J. Forth, J. Phys. E17, 22 (1984).
5. A. Winkler, M. Hausenblas, M. Leisch, and K. Rendulic, Vacuum 40, 39 (1990).