The authorsw ould like to expresst heir sincereg rati-tude to Dr. T. Ohkawa and the staff of the General Atomic Company for their great encouragementT.h is study was stimulated by the suggestiono f Dr. H. Nakamura of the Japan Atomic Energy ResearchI n-stitute and by his discussions.T he authorsw ould also like to thank Drs. M. Yoshikawa and S. Mori and the staff at the Japan Atomic Energy ResearchIn stitute for their continuing encouragementth roughoutt his work;