An automatic static chamber for continuous 220Rn and 222Rn flux measurements from soil

A closed-circuit analytical system for the (quasi)-continuous measurement of radon fluxes from soil consisting of a static accumulation chamber and two radon detectors operated in series is described. The first detector measures the (Rn-220 + Rn-222) activity, the second one the remaining Rn-222 activity after the Rn-220 atoms (half-life 56 s) have decayed when the air passes a delay volume between the two detectors. A step-motor driven cover closes the chamber at the beginning of a flux measurement interval of e.g. 3 h. In between measurements the chamber remains open to maintain soil and vegetation inside the chamber as closely as possible to the conditions outside. A mathematical description of the temporal evolution of Rn activities in the analytical system is presented from which procedures are derived to calculate Rn fluxes (Bq m(-2) s(-1)) from the measured activities in the two detectors. Examples from field tests illustrate the performance of the experimental set-up and possible complications due to technical and/or environmental difficulties. (C) 2003 Elsevier Ltd. All rights reserved.