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xenon-133

n.
  1. A radioisotope of xenon having gamma emissions and a physical half-life of 5.27 days and used in the study of pulmonary function and organ blood flow.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.