DISTRIBUTION OF AIRBORNE RADIOACTIVITY ne INFERRED FROM BEHAVIOR OF | ———~7 | i © RELATIVE RETENTION, % | FILTER 1 (IPC 1478) — —- — FILTER 2 (TYPE 5G) FILTER 3(TYPE 6) NATURAL RADIOACTIVE AEROSOLS ~ | 123 — 25 0 0.25 0.50 0.75 1.00 PARTICLE DIAMETER, p Fig. 3—Effect of size on the distribution of particles among filters in a three-filter pack (air velocity, 61 cm/sec). lower air velocity with consequent lessening of the effects of particle capture through impaction on the filter fibers. Radiochemical analyses were performed by previously described techniques? for sr, “sr—™y, *y, cs, “4Cce— “py, "pm, and *!°Ph; radioactivity determinations were made by beta counting on equipment that had been standardized against similar mounts containing known quantities of standardized radionuclides. Routinely, corresponding nuclides from each of the filters in a pack were counted sequentially on the same unit to minimize uncertainties in counter performance or calibration. RESULTS Measurements of Gross Fission-product Beta Activity A rough idea of the changes occurring in the contribution of the larger particles to the total fission-product radioactivity in the air at ground level since termination of atmospheric nuclear testing is shown