1. M.E.Rose, Phys. Rev. 75 (1949) 213
2. D.G.Haase et al., Nucl. Instr. Meth. A243 (1986) 305
3. W.Heeringa, Proc. 4th intern. works. on Pol.Target Mater. and Techn., Ed. W.Meyer, Bad Honnef (W.-Germany 1984), 129, and Cryogenics 25 (1985) 369
4. M.W.Konijnenberg et al., (ECN, Petten) private communication
5. L.Passell and R.I.Schermer, Phys. Rev. 150 (1966) 146