- (5) The particles settle by Stoke's law,.. This assumption is ‘always challensed but no suitable substitute has yet been suggested. [In the range of particle sises of interost it 1s prebably as good s lew as any S&tion of it. (6) The arca at the surface covered typarticles falling from a givon height 4s proporticnal to the square of the time of fall. This assumption exmreeses the fact of divermnce which oy arise from any number of reasons. No mumerical value is: aselimed to the divergence, (7) The net radial distance that a particle travels is proportional to the time of fall, “ With tho abows essumtions one cbtaines the dose index in the follcwing inelegent wy. «$ & given point "P* on the surface (Fig, 1) activo rar- ticles can arrive fran certain discrete altitudes of the cloud determined by the intersection with the hodograrh of the radius vector from ground sero through the point... In the example these are the altitudes 79,000 ft at "2" and 39,000 ft at "B". The hodograph is customarily draw for a fall rete of 5,000 £& per hour so thet particles with that fall rate which start from 39,000. f& at nominally sero time will land at "B* in 7,4 hours, These particles will havo a definite diameter "4". We have said that the activity broughs down by any particle igproportional to its area; 1.0.,. the dose index "0D" contains a factor *a%, . The remainder of the argunent concerns the tims. . Two other aseumytios contain time terms, the natural decay and the area of deposition. ow tis dose rate is expressible in activity per unit area. For examls, cme mega- curie rer square mile is roughly equivalent to 3 r/hr about 3 f above 4 fission fragment deposition. . The dose rate is therefore expressible as rateee A wc It “ t q) and the integratod dose, apart from other factors, is expressible as 2.2 dose«<= ¢ :- (2) We chowe to define the dese index as D2 (3) t Do?