DISTRIBUTION OF AIRBORNE RADIOACTIVITY 125 sults from three-filter packs have been obtained by solution of three simultaneous linear equations of the form “Ay = 0.86 x, + 0.12 x, + 0.00 x; A» = 0.14 x, + 0.80 x, + 0.20 x; A; = 0.00 x, + 0.08 x, + 0.80 x; where A,, Aj, and A; are the measured activity distributions on the three filters; x,, x,, and x; are the relative amounts of activity in three size groupings; and the coefficients of these terms represent the theoretical distribution of particles of these sizes among thefilters. Analyses of data from several collections of different types are shown in Table 1. The large, but not unexpected, differences between the Table 1— APPROXIMATE PARTICLE-SIZE DISTRIBUTIONS DETERMINED FROM THE RELATIVE RADIOACTIVITY OF FILTERS IN A THREE-FILTER SYSTEM Collection data Collection dates Fission products (Beta) Nov. 23 to 26, 1962 (76 hr) Relative Filter Order activity, % IPC 1478 1 72.62 + 0.21* Type 6 3 2.74 + 0.05 Type 5G 2 24.64 + 0.08 ‘Distribution Particle Activity, 1.0 82,2 size, pu 0.3 0.05 % 16,0 1.8 (Total activity, 981.1 counts/min) Jan. 25 to 28, 1963 (75 hr) IPC 1478 Type 5G Type 6 1 2 3 51.82 + 0.17 45.03 + 0.18 3.15 + 9.03 1.0 0.3 0.05 53.7 47.1 —0.77 (Total activity, 1314.8 counts/min) Natural activity (RaB + C) Oct. 19, 1962 (30 min) IPC 1478 Type 5G Type 6 1 2 3 7.0 + 0.4 53.9 + 0.7 39.1 + 0.6 1.0 0.3 0.05 0.25 56.5 43.2 (Total activity, 457.0 counts/min) *Standard deviation, co, based on counting statistics. sizes of the aerosol particles with which fission-product and natural radioactivity (RaB+C) are associated are clearly evident. These analyses also showed that the coefficients that had been derived from filter calibrations with H,SOQ, (or DOP) smokes were not entirely satisfactory, as demonstrated by the negative value obtained in one case, and hinted that greater difficulties would be encountered in the solution of four-filter collections. These suspicions were justified. The Applied Mathematics Staff at the U. S. Naval Research Laboratory (NRL) has devised a scheme which would give better values for