CALIBRATION OF INSTRUMENTS MEASURING RADON OVER A LARGE ACTIVITY RANGE. N. Michielsen, V. Voisin

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1 Radon in the Living Environment, 011 CALIBRATION OF INSTRUMENTS MEASURING RADON OVER A LARGE ACTIVITY RANGE N. Michielsen, V. Voisin Institut de Protection et de Sûreté Nucléaire, Département de Prévention et d Etude des Accidents, Service d Etudes et de Recherches en Aérocontamination et en Confinement, CEA/Saclay, Bât.389, Gif-sur-Yvette Cédex, France A newly built radon calibration test bench called BACCARA is presented. A method of calibration using a secondary 222 Rn standard is described and examples of calibration are given. With this method, no reference instrument is needed and one obtains a more accurate determination of the 222 Rn volume activity. Key words : calibration test bench, radon, radon standard INTRODUCTION Effective dose from natural sources is mainly delivered by radon and its short-lived progeny (UNSCEAR 1993). With the increased public awareness of the potential risks of indoor radon the need for quality assurance on radon measurements has grown. This has led to the development of radon test chambers for calibration of those instruments. In this report a newly built radon calibration test bench called BACCARA, which is located in the Nuclear Protection and Safety Institute (IPSN) at Saclay, France, is described and examples of calibration are given. DESCRIPTION OF BACCARA Radon activities in BACCARA can be varied, from 40 Bq/m 3 up to 40 kbq/m 3. This large activity range ( 3 orders of magnitude) allows us to calibrate instruments measuring radon in the environment as well as those measuring radon in soil. Fig. 1 shows a picture of a part of the installation. The BACCARA test chamber is made of a 1 m 3 stainless steel cylinder. The concentration of radon, the pressure, the temperature, the humidity and the flow rate are continuously monitored when needed. BACCARA is equipped with stable radon-222 dry sources allowing radon volume activities in the test chamber to be kept constant over the entire indicated activity range. Different types of calibrations are performed with BACCARA. A constant activity concentration can be produced, by a flow through system using a radium source, during a definite period of time and instruments under test are calibrated using a reference instrument. Another method consists of filling the BACCARA chamber once with a known quantity of 222 Rn gas and subsequently calibrating the instruments at decreasing activities caused by radioactive decay. In this paper we will focus on this last method. 93

2 011 Radon in the Living Environment, CALIBRATION USING BACCARA AND A SECONDARY 222 RN STANDARD Radon metrology is usually related to radium-226 standards. But the rate of production of radon by such sources is difficult to ascertain with accuracy and, moreover, it may not be very stable. For this reason, the laboratory responsible for radioactive standard at the AEC (Atomic Energy Commission) has developed a 222 Rn standard. The principle of this primary 222 Rn standard is based on counting, in a defined solid angle, the alpha particles from 222 Rn condensed on a cold point. After absolute measurement this radon is transfer to a glass cell by revaporization and recondensation into this glass cell and provides the secondary 222 Rn gas standard (Picolo 1996). Instruments to be calibrated are placed inside BACCARA and the radon standard is connected to it (Figure 2 ). To increase the transfer of radon from the secondary standard into the volume of BACCARA, a small pump is used in a closed loop for the circulation of air. Moreover, to insure homogeneous volume activity of radon in BACCARA, a fan runs continuously, inside the cylinder, during the experiment. Extensive effort has been made to insure airtightness of BACCARA. This was verified by maintaining the cylinder in overpressure during 3 days, no decrease of pressure was observed. Having a precisely known radon activity makes it possible to perform calibration of radon measuring instruments in the radon chamber without the need of a reference instrument. For a given activity A 0 of the standard transferred in the volume V, the 222 Rn volume activity at any time t is given by : A At () = 0 Ln( 2) t exp( ) T V With T the half life of 222 Rn ( T = ± days ) (1) When an integrated measurement is done, it is not the volume activity of radon that is measured but the integrated activity concentration for an exposure time t exp. This value is calculated as follow : Ln( 2) texp (2) 1 exp( ) A I = 0 T A V Ln( 2) T Relative combined standard uncertainties related to formula (1) and (2), calculated following the Guide to the Expression of Uncertainty in Measurement, are given by : Three sets of calibrations are presented now. 2 2 uc( A( t)) uc( IA) u ( A0 ) u ( V) = = + (3) 2 2 At () I A V A 0 94

3 Radon in the Living Environment, 011 The first one involved five radon passive detectors. One of them is an electret with a radon diffusion chamber. The third one is a film (LR115) that is directly exposed. The three others are LR115 films placed in a radon diffusion chamber. Those detectors were placed inside BACCARA and the calibration was done using a secondary 222 Rn standard activity, A 0 = 3.90 kbq with a relative standard deviation of 1.5 %. After exposure, the films were sent to the constructor for reading and results of the integrated volume activity with its uncertainty were sent back to us. The second set also involved LR115 film detectors placed in a diffusion chamber. This time, the constructor wanted to determine their calibration factor which is defined as : Where n tr is the traces density. CF = n tr I A Six detectors were exposed inside BACCARA where the integrated 222 Rn activity concentration was I A = 988 ± 27 kbq.h.m -3. The last set concerned an ionisation chamber that continuously measures the 222 Rn volume activity. It was exposed in BACCARA to a 222 Rn volume activity ranged from 8.4 ± 0.3 Bq.m -3 to 3727 ± 135 Bq.m -3. RESULTS AND DISCUSSION The results obtained for the first set of calibration are listed in Table 1. The first column shows the detectors' number. I A is the integrated activity concentration calculated with equation 2 and the third column contains the values given by the constructors. The last column gives the relative differences between the instrument and the calculated value, that is, (I Ainst - I A )/I A. For the three first detectors, this relative difference is not significant. The fifth detector is given a value 11% too low. And the value of the fourth detector is 50 % too low, but for this detector, the result depends strongly on the equilibrium factor because the film is directly exposed, instead of the others that have a diffusion radon chamber. Eventhough we did not determine the equilibrium factor in BACCARA, it is probably very low because the presence of the fan induces particles deposition. One can suppose that the calibration of those type of films needs another type of calibration method including radon decay products monitoring. For the second set of calibration, the mean calibration factor was found to be CF = 1.97 ± 0.10 traces.cm -2 / kbq.h.m -3. Uncertainties on I A and CF are equal to two times the standard deviation. One should remark that the error on the calibration factor is only 5% for k=2. Figure 3 shows the result of the ionisation chamber. 222 Rn volume activity measured by the instrument is plotted against the theoretical value. The results follow the bisecting line except for activities under 40 Bq.m -3. A better view of those differences is given in Fig. 4. The large dispersion for low activities are mainly due to statistical fluctuations. A systematic error of 11 ± 2 Bq.m -3 was found. 95

4 011 Radon in the Living Environment, CONCLUSION BACCARA coupled with a secondary 222 Rn standard of a known activity, makes it possible to calibrate radon detectors without the need of reference instrument. That allows us to obtain a 222 Rn volume activity or an integrated 222 Rn concentration with a standard deviation better than 2 % even for volume activities as low as 10 Bq.m -3. With the present experimental set-up, precise calibration is possible for instruments measuring volume activity of radon, included films detectors coupled with a diffusion chamber that do not depend on the equilibrium factor. Future work will focus on creating radon and radon decay products reference atmospheres with different equilibrium factors in order to calibrate radon detectors sensitive to this parameter and also to test instruments measuring radon decay products. REFERENCES [1] GUIDE TO THE EXPRESSION OF UNCERTAINTY IN MEASUREMENT (1993). ISO, ISBN [2] Picolo J.L. Absolute measurement of radon-222 activity. Nucl. Instr. Meth. 1996; A386: [3] UNSCEAR. Ionisation Radiation Sources and Biological Effects Report to the General Assembly with Annexes (New York : UNSCEAR Publications),

5 Radon in the Living Environment, 011 Table 1: Results of the first set of calibration Instrument Number I A (kbq.h.m -3 ) error (k=2) I Ainst (kbq.h.m -3 ) error (k=) relative difference % error (k=2) 1 électret 490 ± ± 61 no significant 2 Film 490 ± ± 103 no significant 3 Film 490 ± ± 95 no significant 4 Film 490 ± ± ± 5 5 Film 490 ± ± ± 6 97

6 011 Radon in the Living Environment, Figure 1 : BACCARA BACCARA pump ventilator standard cell Instrument in test Figure 2 : Schematic diagram of BACCARA with the secondary 222 Rn standard 98

7 Radon in the Living Environment, Value of the tested instrument (Bq/m3) Figure 3 : Calibration curve 222Rn Volume Activity in BACCARA (Bq/m3) 99

8 011 Radon in the Living Environment, Relative Difference (%) Rn Volume Activity in BACCARA (Bq/m3) Figure 4 : Relative differences between the instrument and the theoretical 222 Rn volume activity 100