1. AT. Goshaw et al., Phys. Rev. Lett. 43, (1979) p.1065;
2. P.V. Chliapnikov et al., Phys. Lett. B141, (1981) p.276;
3. V.Hedberg, thesis, Univ. Lund, LUNFD6(NFFL-7037) (1987);
4. T. Akesson et al., AFS, Phys. Lett. 192B (1987) p.463;
5. J J Aubert et al.(EMC), CERN -EP/88 -33, March 1988;