Design of a continuous digital-output environmental radon monitor

A new field instrument for the continuous measurement of radon concentrations has been developed to investigate the magnitude and variability of environmental levels of 222Rn. Passive diffusion of radon, but not its daughters, occurs through an open pore polyurethane foam into a sensitive volume where a static electric field directs the positively ionized radon daughter products to a central collecting electrode. Pulses in a ZnS(Ag) scintillator, resulting from the alpha emission of 218Po and 214Po, are observed with a photomultiplier tube and counted using standard NIM electronics. The detector unit has been fabricated into a small, convenient package for indoor air sampling without the use of air movers or pumps. The unit is unobtrusive and acceptable into a daily routine without disrupting normal family or business activities. The monitor can detect as little as 0.5 pCi/l for a 40-min. count (α = .05). The equilibrium detection efficiency of the instrument is 0.7 cpm/pCi/l. (U.S.)