Mar 1, 2004 - Abstract. Among the general requirements to accredit a radon measuring service, outlined in the international standards as EN ISO/IEC 17025, ...
The technical framework supporting the accreditation action of the ENEA ION-IRP radon service M. Calamosca, S. Penzo ENEA-ION-IRP, Montecuccolino, V. dei Colli 16, 40136 – Bologna (Italy) Abstract. Among the general requirements to accredit a radon measuring service, outlined in the international standards as EN ISO/IEC 17025, the technical requirements play the main role in the quality assurance of a testing service. The reliability of the assessment of the radon air concentration in workplaces and dwelling is determined by many factors the most important being the physical measuring method chosen. Our Service uses the own ION-IRP radon dosimeter, specifically designed and tested to be used as a routine referenced and traceable device to monitor 222Rn. It is composed by a closed with air gap holder and a CR-39 ATD. In this paper the whole spectrum of the dosimeter’s technical specifications which can seriously affect the measurements validity are introduced and discussed. Moreover, because the etched track dosimetry consists of many phases, and all affect the quality of the service, we are going to discuss the problems arising from the detector storage, the shipping to and from the client, the visualization of the track by the chemical etching, and finally the analysis at the optical microscope, including the hardware and software features. A series of standard operative procedures have been edited on the basis of the knowledge of our measuring system, in order to periodically check and maintain the due quality level of the measurement. This documentation will be the basis of the validation scheme we are going to use to get the approval as an accredited laboratory for the measurement of radon air concentration (ARL).
1. Introduction By the issue of the D.Lgs. 241/00 [1] the current national radioprotection legislation extends the control to the exposures due to natural occurring radioactive materials (NORM), with a particular attention to radon exposures in workplaces. In the underground workplaces, the employer shall determine the annual mean 222Rn air concentration before the deadline of the 1st March, 2004: if the concentration exceeds the enforced limit of 500 Bq/m3, a remedial action is needed whether or not an effective dose over 3 mSv could be actually associated to the radon exposure. It is mandatory [1], nevertheless very sensible, that the radon measurement is provided by an accredited radon laboratory using an approved measurement devices. The risk due to exposure to radon is primarily related with the inhalation of its decay products (DP), which are not in equilibrium with their parent. By assuming, according to EU Directives and ICRP-65, the equilibrium factor F = 0.4, the radon risk is conveniently measured by determining only the 222Rn rather than the DP air concentration. To accomplish the 222Rn concentration measurement many methods are available [2], ranging from continuous radon monitor to Alpha Track Detectors (ATD), or from Electrets to Activated Charcoal devices. All the mentioned measurement systems can provide radon measurements with the quality required by a testing service to meet the European Standard. Anyway ATD with a passive diffusive closed holder is the most popular and internationally widespread technique. Table I shows the distribution of the 46 devices used by the 39 laboratories that have participated at the 2002 NRPB Intercomparison of Passive Radon Detectors. Note as the 77% of the laboratories attending the Intercomparison did use ATDs, and among these 74% have used the CR39. Table I. Measurement device protocols distribution at the 2002 NRPB Intercomparison. Measurement Device ATD - CR-39 ATD - Makrofol ATD - LR-115 Electrets Activated Charcoal
N° 28 3 7 3 5
% 60.8 6.5 15.2 6.5 10.8
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Table II. – Results obtained by the different protocols at the 2002 NRPB Intercomparison. Measurement Device ATD - CR-39 ATD - Makrofol ATD - LR-115 Electrets Activated Charcoal
A 50 % 33.3 % 28.6 % 33.3 % 40 %
B 28.6 % 33.3 % 42.9 % 33.3 % 40 %
Rating C 3.6 % 33.3 % 20 %
D 3.6 % 14.3 % -
E 14.6 % 14.3 % 33.3 % -
Table II shows the overall results of the 2002 NRPB Intercomparison; the different measurement techniques are so almost equally distributed in the rating, to infer that the quality of the measurement cannot be elicited from the kind of protocol used rather it has to be attributed to the quality assurance programme (QA) supported by the testing laboratory. The radon testing Laboratory QA must provide for the editing of standard operative procedures (SOP), a system’s documentation of the quality control procedures (QC) so as to comply with the European Standard ISO/IEC 17025/2000. Of particular importance are the technical requirements, which are peculiar to each specific measurement protocol adopted by the Radon testing Service (RS). This introduces the prime technical problems which, on the basis of our experience, affect the quality of the measurement of radon concentration by means of a CR-39 ATD system. 2. The ENEA ION-IRP Radon Service The ENEA Radioprotection Institute (IRP) has recently implemented its dosimetric services by establishing a new radon air concentration testing laboratory, with the aim to fulfill the oncoming demand of a quality measurement as required by the law. To meet this aim a new integrated radon measurement system has been completely designed, beginning from a new ATD dosimeter. The technique adopted by the IRP RS [3] consists of the use of a PADC (CR-39) detector, the same used by our neutron personal dosimetry Service, to be exposed within a newly designed holder® of the closed with air gap type. The rationale of the research [3, 4] devoted to this task has been throughout justified by the QA of the RS, taking into account how the exactness of the measure can be affected by a poorly designed or not well known holder. The radon measurement by etched track detectors consists of a multi-step procedure, beginning from the location of the holder in the workplace and ending by editing the test result. Every step could affect the quality of the test. According to our measurement system, the following items are deemed particularly critical. • Unbiased sampling of the actual mean 222Rn environment concentration; • homogenous deposition of the 222Rn short lived DP onto the internal holder’s walls, so as to achieve a homogeneous tracks density in the reading area of the detector; • non significant device response (sensitivity) dependence on environmental conditions; • non sensitivity dependence on workplace equilibrium factor; • laboratory background measurements both at the acceptance phase and during the storage; • field background measurements by using transit devices; • repeatability of the etching conditions; • accuracy and precision of the analysis phase performed at the optical microscope (OM); • expert data analysis software. To summarize, the main technical items which can affect the RS quality using ATD are: 1) the holder response, 2) the detector, 3) the etching conditions, 4) the OM analysis and 5) the final data analysis software. 3. Holder Quality Controls A representative radon sampling must provide a good estimate of the 222Rn exposure to workers passing a significant time in the tested workplace. So the location of the device is as important as the
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influence of the dosimeter’s response on environmental parameters. While the location is one of the tasks of the residential service provider, the testing laboratory is quite involved in assuring the independence of the response on the exposure parameters. 3.1. Equilibrium Factor and 220Rn Dependence The main environmental parameter affecting the radon measurement is the varying presence of the unattached and attached 222Rn and 220Rn DP fractions. To avoid an overestimation of radon exposure, the holder design must prevent the inlet of the DPs into the effective volume of the chamber and at the same time slow down the diffusion of the 220Rn, so that it decays before entering into the chamber. This aim is generally obtained by adopting closed or filtered holder. Our holder was designed specifically to meet this aim without using a physically closed diffusive chamber. The actual air gap between the cup and the lateral walls, obtained by opening the device, allows a faster diffusion of the radon into the effective volume than the permeation across the walls. At the same time it acts as a very effective abatement for every sort of aerosol granulometry, belonging both to the thermodynamic range (unattached fraction) and the aerodynamic one (attached fraction).
Internal to external concentration ratio [x100]
Our holder’s equilibrium kinetic between internal and external 222Rn, 220Rn, and their DP concentrations have been theoretically studied (FIG. 1). The equilibrium constants and the asymptotic concentration values are derived by only taking into account the diffusive time and the physical decay behaviour. If the capture of DP laden particles, due to the diffusion, sedimentation and electrostatic attraction processes occurring during the passive movement along the air gap is also considered, the overestimation of the radon by using this device is to be deemed really non significant.
100
Rn-222 Pb-214
10
Po-218 Rn-220 Bi-214
1
0,1
Bi-212 Pb-212
0,01
0,001 0,0001 FIG. 1.
0,001 222
0,01
0,1 1 Time [h]
10
100
1000
Rn, 220Rn and their DPs equilibrium kinetics withins the IRP holder.
The highest internal 220Rn concentration doesn’t reach the 3 % of the external one, so that, also taking into account its usual minor environmental presence, the overestimation is negligible. This check should be executed as an acceptance test and whenever a substantial holder’s modification occurs
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3.2. Permeation Time Dependence The indoor radon concentration could change rapidly during the 24 hours because of the influence of many parameters, including ventilation. A passive radon measuring device underestimates the mean concentration value unless it is characterized by a short permeation time or a long exposure time. For example if the permeation time is greater than 7 h the asymptotic ratio between internal and external concentrations will always be less than 95 %. The physically closed devices, if not carefully designed, could be characterized by high permeation times, with the consequence of decreasing the dosimeter response. Moreover the sensitivity of the dosimeter could depend on the term of the exposure, causing a bias when calibration and field exposures are not similar. This check should be executed as an acceptance test and whenever a substantial holder’s modification occurs.
Radon concentration [Bq\m^3]
250
200
External Radon
Tm=33.6 h
IRP Dosemeter Tm=0.5 h
Tm=8 h
150
100
50
0 0
10
20
30
40
50 60 Time [h]
70
80
90
100
FIG. 2. Theoretical comparison among the responses of IRP holder and of higher permeation time ATDs to an oscillating 222Rn concentration (Average 100 Bq/m3, period 12 h). The air gap provided in our holder determines a very short permeation time, 0.5 h, (FIG. 2), and a condition of almost equilibrium (>99%) inside the chamber. This specification assures that the maximum geometric sensitivity is independent of the term of exposure. In theory very short exposures (
