0021620 wae If some knowledge of G,(t) is available M(t) can be evaluated from specific activity data. If the average ue of the specific activity of the particles at a given location in the fallout area is /Dp where ap is the activity and My is the mass of single particles, then : M(t) = 1 G~ (t) (a /m ) PP 02) 4 The value of a_/m_ will be sensitive to changes in the radioactive content of the partici8s and to any variation in the radioactive content per particle with particle size. And since the size of the fallout particles changes with downwind distance from ground zero, any variation of the radioactive content: of the particles with size will be reflected in a variation of M¢(t) with downwind distance from ground zero. such variations occur, MQ(t) becomes a point function. When To illustrate how M.(t) could be a point function, consider partic- les that arrive at a distance, x, from the point of detonation and that have fallen from a height, h, directly above the detoration. Let Vy, be the average velocity of the wind that transported the particles the distance, x. Two cases may be considered: for the first case, it will be assumed that the average radioactive concentration varies with the surface area of the particle (i.e. is proportional to the square of the particle diameter, a); for the secomi case, it will be assumed that the concentration is proportional to the volume (or mass) of the particle. For the first case, the average specific activity is e/a, = k/a (13) in which @ is the average diameter of the particle group and k, is a cons- tant. For the larger particles, the falling velocity is approximately proportional to the particle diameter so that the distance at which the particles of diameter d are deposited is x 2 v,b/(k,8) in which ko is a constant. for these particles poE/NU (1s) Combination of Eqs. 12, 13, and 14 gives, 9 1G