1. See, e.g., D.Feldmann, R.P.Hunt: Z. Instr.72, 259 (1964)
2. M.Fuhrmans, C.Heiden: Cryogenics16, 451 (1976)
3. H.Rogalla: Diploma thesis, University of Münster (1972)
4. Springer Tracts in Modern Physics;K. Stierstadt,1966
5. Springer Tracts in Modern Physics;H. Ullmaier,1975