DISTRIBUTION OF AIRBORNE RADIOACTIVITY <a > Sy 121 SSS THT TT A POO te gl FILTER 1 (IPC 1478) FILTER 2 (TYPE 5G) FILTER 3 (TYPE 6) SUPPORTING GRID (STAINLESS-STEEL SCREEN) N10 PRESSURE GAUGE TO VACUUM PUMP Fig. 1—Schematic of a filter holder with a three-filter pack. The initial, and poorest, filter (or, in the case of four-filter packs, the first two filters) was IPC 1478 paper, the next filter was type 5G cellulose—glass fiber paper, and thefinal filter was type 6, a so-called “absolute filter” of cellulose—asbestos paper. The calculated relation of particle size to the distribution of particles among filters in a four- filter pack under standard operating conditions of 280 to 290 linear feet per minute (145 cm/sec) is shown in Fig. 2. This information is based on the measuredfilter retentivity toward monodisperse H,SO, aerosol particles (spherical particles of 1.8 g/cm®* density) over a size range of 0.3 to 1.2 1 in diameter; values below 0.3 1 have been inferred from retentivity measurements of the extremely small natural radioactive aerosols containing RaB + C (*4Pb + *!4Bi) activity. Determinations of the distribution of gross fission-product beta activity among the filters in a pack were made by countingthefilters in sequence on the same counter unit after allowing a minimum of seven days for decay of natural radioactivity (10.6-hr ThB). Sufficiently long counting times were employed to give good statistical accuracy. So that collections of reasonable size could be obtained, the collection periods were generally of two to four days duration. In a few cases collections of RaB + C activity were made to obtain information on the size distribution of this natural radioactivity, which is known to become rapidly attached to the extremely small nonradio-