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1DUKE UNIVERSITY, DEPARTMENT OF PHYSICS, DURHAM, NORTH CAROLINA
†
Present address: Department of Physics, West Virginia University, Morgantown, West Virginia.
*
This research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command and by the Office of Ordinance Research, Department of the Army. Reproduction in whole or in part is permitted for any purpose of the United States Government.