-~ % 15 - ‘ Before turning to the biological data it is appropriate to discuss first the radiation field around a particle of Pu02 and thereby define the fundamental questions that need to be answered by the collateral data from radiobiological studies. The unique form of tissue irradiation displayed 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) of only in human tissue. In other words, a Pu-239 particle in tissue will only irradiate a volume of tissue enclosed in a sphere of 45 u radius. ward crom the surface of this sphere, increases geometrically. the radiation intensity About half of the alpha particle energy is dissipated at 20 u ‘is 1/8 the total volume). As one moves in- (that is, with a volume that This means that the average dose @elivered in the-first 20 uis 8 times that delivered in the remaining 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 plutoniun