- 15 - Before turning to the biolcgical data it is appropriate to discuss first the radiation field around a particle of Pu02 and thereby dezine the rundamental cuestions that need to be answered by the collateral data from radiobiological studies. The unigue form of tissue irradiation dispiayed by insoluble particles of Pu-239 occurs because, when Pu-239 decays, it emits an alpha particle with an energy of 5.1 MeV. This particle has a range some 40-45 u (0.004 cm) (produces biological damage) in human tissue. of only In other words, a Pu-239 particle in tissue will only irradiate a volume of tissue enclosed in a sphere of 45 u radius. As one moves in-_ ward from the surface of this sphere, the radiation intensity increases geometrically. About half of the alpha particle energy is dissipated at 20 u is 1/8 the total volume). (that is, with a volume that This means that the average dose delivered in the first 20 u is 8 times that delivered in the retaining 20 u. The first column of Table III describes the radiation field around such a particle in soft tissue; e.g., the skin. Since the lung is a Spongy tissue with a large air volume, the range of alpha particles is longer in the lung and consequently the mass of irradiated tissue-is larger. Professor Donald Geesaman made a detailed analysis of plutonium ~ . at Lortnos ata ey weg tote a . ‘la, “ er me vt ‘ . _ , orp te “3 . Eens ° mee. matt, rae ne a . * wtih nhs hag hae Faye se tae ans “oe ’ Ae Ph TY wa. yep tee “ . fe. ¥ oN ere f cote ets cat a gg age ” a 2 ete x nh THe, a ™ “ne woes :, ti gt sie FR : ml AEG fe ye ta the Bcc arf a? she SE Pee? het a Bi. te ea Se tee oy