Acta Facultatis Ecologiae - Technická univerzita vo Zvolene

142The chamber background measured at the stableflow of the inactive air through the chamber is about0.1 cpm.As the second radon monitor was used Alpha-GUARD monitor (AG) developed by GenitronCompany [7]. In AlphaGUARD cylindrical ionizationchamber in combination with DSP-technologiesis used for radon concentration measurement.The active volume of the ionization chamberis equal to 0.56 liters. The measured gas enters theionization chamber via a large surface glass fiberfilter in a diffusion mode. The sensitivity of the detectoris 0.05 cpm at 1 Bq·m –3 of 222 Rn concentration.The background signal due to an internal detectorcontamination is less than 1 Bq·m –3 . Alpha-GUARD was operated in the flow mode at the flowrate of the air through the monitor of 0.6 l·min –1 .The third radon monitor (LSCH) is built on thebasis of the large scintillation chamber. The chamberhas a cylindrical shape. The inner diameter is120 mm and its length is 400 mm. The volume ofthe chamber is 4.5 liters. To achieve good geometricalconditions for radon detection the inner spaceof the chamber is divided into 9 sectors. The wallsof the sectors are coated with ZnS(Ag). Scintillationsare collected through two glass windowsplaced at the ends of the chamber to two photomultipliertubes. At the flow rate of the air throughthe chamber of 0.5 l·min –1 and with an aerosol filterclosely outside the chamber inlet the sensitivity ofthe scintillation chamber is 0.3 cpm at 1 Bq·m –3 of222Rn concentration. The chamber background measuredat the stable flow of the inactive air throughthe chamber is about 2.4 cpm.RESULTS AND DISCUSSIONFor the comparison of an operation of radonmonitors the radon concentration was measured inthe same unventilated room. During the measurementthe air was being sucked in with the flow rateof 0.6 l·min –1 through one liter scintillation chamberand simultaneously through AlphaGUARD andwith the flow rate of 0.5 l·min –1 through a large scintillationchamber. For the large and one liter scintillationchamber the radon activity concentrationscorresponding to the time interval of 2 hours werecalculated on the basis of every 30 minutes recordsaccording to the method published by Ward andBorak [4]. AlphaGUARD monitor was operatedin flow mode with a 10-minute lasting measuringcycle. On the basis of the recorded data the averageradon activity concentrations corresponding to thetime interval of 2 hours were calculated.In Fig. 1 the air supply line for one liter scintillationchamber and AlphaGUARD is shown andin Fig. 2 for the large scintillation chamber. In theFig. 3 there are shown courses of the radon activityconcentration measured by all three monitors.For an illustration there was chosen such case ofmeasurement where the course of the radon activityconcentration was not regular during the wholemeasured interval. The radon activity concentrationshows noticeable changes. The regular variationswith the low amplitude and low average valuesof the radon activity concentration measuredan 27/3/2007 – 1/4/2007 and 6/4/2007 – 13/4/2007correspond to the unventilated room. In the timeperiod from 1/4/2007 till the 6/4/2007 an additionalradon source to the room was brought in.The regular variations showed larger amplitudesthan in previous days. In the time interval from8:00 a.m. till 10:00 a.m. of April 4 th , the room wasventilated by opening the windows.Further, as can be seen in Fig. 3, the coursesof the radon activity concentration measured bymeans of all monitors are almost identical. Even insome details it is possible to find the same responsesof all the monitors. All three devices recordeda growth during the 1/4/2007 and also an intenseFig. 1 The continuous monitoring system for the 222 Rn measurement in indoor air on the basis of one literscintillation chamber and AlphaGUARD monitor