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