PRODUCTION AND CHARACTERISTICS OF ‘25Xe:

Radionuclides of the noble gases are exten sively used to assess ventilation and blood flow in clinical and investigative studies. Xenon-133 is most commonly used but is not optimal for these in vivo studies. Xenon-125 has better phys ical characteristics and can be produced with a cyclotron by a 1271(p,3n)1t5Xe reaction; this reaction results in a maximum of 25 mCi/gm/ cm2/pA-hr for 31-MeV protons. Under actual pro duction conditions, 1 1 mCi/pA-hr were collected. Xenon-125 decays by electron capture with a 17.2-hr half-life and contributes less radiation per dose of radioactivity than 133Xe. The radia tion dose to the lungs from 133Xe and 155Xe is 5.0 and 1.8 mrads/mCi-min, respectively. The radiation dose per usable photon for 125Xe is only 0.3 of 183Xe. The principal photons of ‘25Xe,188 keV (55%) and 243 keV (29%), are more intense and are in an energy range that is more advantageous for imaging than the 81 keV (35%) of 133Xe. These physical proper ties o