It is important in most radiation experiments to have both a quantitative measure of the amount of radiation involved, and a qualitative description of the radiation which uniquely defines its significant properties. The required quantitative measure is the absorbed dose, which is the amount of energy absorbed per unit mass of the irradiated material. This can be expressed in rads (1 rad = 100 ergs/gm.). The most pertinent qualitative description is that of the spatial distribution of the collisions by which the electrons (or other ionizing and exciting particles) which are responsible for the actual energy dissipation lose their energy. This concept has been termed “linear energy transfer” (LET) and is defined in terms of the energy lost per unit path length by the particle (1, p. 6). The LET is usually written as a function, L(T), of the kinetic energy, T, of the particle and is conveniently expressed in units of kev/µ where µ is 1 micron of path length. The significant information concerning the distri...