Automatic alpha/beta analyser for air filter samples -absolute determination of radon progeny by pseudo-coincidence techniques

The alpha/beta analyser uses five gas flow proportional counters: one alpha, one beta and one background detector above the sample, and one beta and one background detector below the sample. The counting efficiencies for 210Bi (beta) and 210Po (alpha) are 75% and 42% respectively. The sample changer unit can handle 25 large-area (up to 240 mm diameter) air filters. All combinations of coincidences and pseudo-coincidences between the five detectors can be pre-selected for analysis, thus permitting identification of 222Rn or 220Rn progeny. In addition to total alpha and beta measurements for monitoring purposes, non-destructive assessment of 212Pb and 210Pb have proved valuable in prolonged studies. Beta/alpha pseudo-coincidence techniques provide a procedure to achieve absolute counting efficiencies for short-lived 222Rn progeny, a procedure which also serves as a permanent check on instrument stability. (Author)