1. Group-NMT-15 (Z.A.A., D.M.W.), Group C-ADI (D.A.C.), Group ISR-1 (R.C.W.), Los Alamos National Laboratory, Los Alamos, New Mexico 87545; CEA Saclay, DEN/DPC/SCP/LRSI, 91191 Gif sur Yvette Cedex, France (B.S.); and Laboratoire d'Astrophysique, Observatoire Midi-Pyrénées, 14 avenue Edouard Belin, 31400 Toulouse, France (S.M.)