122
LOCKHART, PATTERSON, AND SAUNDERS
100
|
\
|
FILTER 1 ({PC 1478)
—— FILTER 4 (TYPE 6)
RELATIVE RETENTION, %
_
|
\
0
0.00
|
0.25
.
™ een.
L
I
0.50
0.75
N]
RL
1.00
PARTICLE DIAMETER,
Fig. 2—Effect of size on the distribution of particles among filters in
a four-filter pack (air velocity, 145 cm/sec).
active component of the atmospheric dust.’ For these determinations
30-min collections were made, and the filters were counted immediately and simultaneously for 45 min on intercalibrated beta-counting
units. Corrections were made where needed for the small quantities of
longer lived activities that were also collected.
Collections for Radiochemical Analyses
Since the start of 1963, radiochemical analyses have been performed on large collections of long-lived components of the airborne
radioactivity through use of a three-filter pack employing 13- by 17-in.
pieces of the filter media previously described. Sampling was done
over a one~ to three-week period at the rate of about 152 cu ft/min
(4.34 x 104 m® per week) at an average face velocity of 120 linear feet
per minute (61 cm/sec). Little change in flow with time occurred over
collection periods as long as three weeks. The calculated distribution
of particles of various sizes among the three filters is shown in Fig. 3.
Generally, greater penetration of the larger particles occurs in this
pack than in that used for gross beta measurements because of the