Direct determination of short-lived radon daughter products on air filters by liquid scintillation counting using a delayed-coincidence technique

The radioactivity on air-filters is measured by internal scintillation counting following immersion of the filter in a liquid scintillation solution. The resulting transparency of the filter ensures uniform counting efficiency of the nuclides, irrespective of their depth of penetration into the filter medium. The high counting efficiency for both α and β activity which is achieved makes it possible to measure the 1.6 × 10^-4 sec ²¹⁴Bi-²¹⁴Po pair by delayed cocoincidence counting of their scintillation pulses; the relative amounts of ²¹⁴Pb and ²¹⁴Bi are then computed from the decay curve of this activity. Optimum counting conditions have been determined; the sensitivity obtained enables the detection of a 6% variation in the ratio of airborne ²¹⁴Pb and ²¹⁴Bi concentrations at ambient concentration levels of 0.1 dpm ²¹⁴Bi/1 air.