Double-filter monitor for simultaneous measurement of Radon and its progeny

For simultaneous measurement of radon and its progeny, double filter monitor based on Direct progeny sensors (DRPS) was developed. DRPS is an absorber mounted LR115 detector which measures the progeny concentration. Double filter consisted of two filters, inlet and exit filter, which were utilised in the monitor for progeny atom deposition such that the ambient progeny atoms get deposited on the inlet filter and the progeny formed due to decay of radon sampled inside the delay volume get deposited on the exit filter. DRPS facing the inlet filter would detect the alpha particles emitted during decay of progeny on inlet filter and that will correspond to ambient progeny concentration. Similarly DRPS placed facing the exit filter will detect the alpha particles emitted during the decay of progeny on exit filter, which is related to ambient radon concentration. Thus the system has two filters and two progeny sensors. The loss of progeny atoms formed during the decay of ingressed radon inside the delay chamber may be lost due to plate-out on the walls of the delay chamber. The plate-out loss is characterised by diffusion parameter. Theoretical calculations of transit time of progeny atoms in the delay volume at different flow rates were also used to study the effect of volume of the chamber at different flow rates. The system was calibrated in known activities of radon and its progeny and the calibration factors were measured. Additional advantage of the system is that in-situ equilibrium factor can be measured.