They are radioactive isotopes of Xenon, a noble or inert gas present in the atmosphere to a small extent. It is used to measure blood flow of lungs and brain.Theoretically, half-life span (radioactivity) of X-133 is 5 days, and, X-127 is 36 days, but, because it is an inert gas it can not be accumulated, thus its actual half-life span = 0. It is exhaled within few minutes. The mother should wait just for few minutes before resuming BF.