124 Pages
English

Results of the 1997 European Commission intercomparison of passive radon detectors

-

Gain access to the library to view online
Learn more

Description

Nuclear energy and safety
Environment policy and protection of the environment

Subjects

Informations

Published by
Reads 27
Language English
Document size 5 MB

ISSN 1018-5593
EUROPEAN
COMMISSION
Results of the 1997
European Commission
intercomparison
of passive radon
detectors
EUR 18035 EN EUROPEAN COMMISSION
Edith CRESSON, Member of the Commission
responsible for research, innovation, education, training and youth
DG XII/F.6 — RTD: Energy, radiation protection
Contact: Mrs Anna Karaoglou
Address: European Commission, Rue de la Loi 200, B-1049 Brussels
Tel. (32-2) 29-65415; fax (32-2) 29-66256 European Commission
Results of the 1997 European Commission
intercomparison of passive
radon detectors
S. P. Naismith, C. B. Howarth and J. C. H. Miles
National Radiological Protection Board
Chilton, Didcot, Oxon, 0X11 ORQ
United Kingdom
Directorate-General
Science, Research and Development
1998 EUR 18035 EN LEGAL NOTICE
Neither the European Commission nor any person acting on behalf of the Commission
is responsible for the use which might be made of the following information.
A great deal of additional information on the European Union is available on the Internet.
It can be accessed through the Europa server (http://europa.eu.int).
Cataloguing data can be found at the end of this publication.
Luxembourg: Office for Official Publications of the European Communities, 1998
ISBN 92-828-3467-0
© European Communities, 1998
Reproduction is authorised provided the source is acknowledged.
Printed in Belgium CONTENTS
Page
1 INTRODUCTION 1
2 MEETING TO DISCUSS INTERCOMPARISON 2
3 LABORATORY EXPOSURE AND MEASUREMENT FACILITIES 3
4 PASSIVE RADON DETECTORS 4
5 LOGISTICAL ARRANGEMENTS 5
6 EXPOSURES 6
7 RESULTS AND DISCUSSION 7
8 ACKNOWLEDGEMENTS 10
9 REFERENCES
TABLES
1 Parameters measured and controlled in the NRPB radon chamber 11
2 Laboratories participating in the intercomparison2
3 Air treatment during laboratory exposures3
4 Exposure durations and magnitudes4
5 Radon results from passive detectors5
6 EER results from passives6
7 Radon results ranked by category 17
8 Minimum standard deviations acheived by different standard detector types 18
FIGURES
1 The NRPB radon exposure chamber9
2 Detectors during ane 20
3 Radon and EER concentrations: exposure 11
4n and EER:e 22
5 Radon and EER: exposure 33
6 Aerosol size distributions during exposures 2 and 34
7 Temperature: exposure 15
8 Humidity: exposure 16
9 Passive detector results with standard deviations: radon, transit detectors 27
10er results with standard errors: radon, exposure 18
11e detectors withd:,e 2 29
12 Passiver results with standard errors: radon, exposure 3 30
13e detectors withd deviations: EER, transit detectors1
14 Passiver results with standard errors: EER, exposure 12
15e detectors withd: EER,e3
16 Passiver results with standard errors: EER, exposure4
APPENDICES
A Characteristics of passive radon detectors submitted for the intercomparison 35
Β Standard designs of detector 111 1 Introduction
Radon is the largest and most variable contributor to the radiation dose to the public. Surveys of
radon levels in homes and other buildings have been carried out throughout Europe to determine the
magnitude of average exposures and to identify situations where excessive exposures occur. Almost
all of these surveys have been carried out using passive etched-track detectors exposed over long
periods to take account of the short-term variations in radon levels.
In order to ensure the quality of these measurements, it is important to compare different detectors
exposed side by side. Although the etched track technique is a very simple one in principle, it has
been found that it is difficult in practice to maintain good quality control. The Radiation Protection
Research Programme of the European Commission provided funds for intercomparisons of active
and passive radon and radon decay product measurement techniques in 1982, 1984 and 1987 (Miles
and Sinnaeve 1988). More limited exercises were held in 1989, 1991 and 1995 for passive radon
detectors only (Miles and Olast, 1990, Whysall et al, 1996, Mües et al, 1996, Wellman et al, 1998).
Each intercomparison has been more popular than the one before, with 48 laboratories submitting 57
sets of detectors to the last intercomparison. Many laboratories regard such an exercise as an
important check on the international comparability of radon measurement results and on their quality
control procedures.
In 1996 the European Commission agreed to sponsor a series of three annual intercomparisons to be
held at NRPB. The contract also provided for a steering meeting to be held before the first
intercomparison and a final meeting open to all participants to be held after the last of the three. The
steering meeting took place in early 1997, followed by the first intercomparison, which is reported
here. Sixty two laboratories submitted 74 sets of 40 passive detectors. After exposure to radon and
its decay products, the detectors were returned to their originating laboratories for assessment.
Participants reported the estimated exposure for each detector before they were notified of the
exposures given to the detectors. The intercomparison included three laboratory exposures at
different equilibrium factors. 2 Meeting to discuss intercomparison
A meeting was held in January 1997 to discuss the series of three intercomparisons and recommend
how they should be conducted. Those present were Hans Vanmarcke (SCK/CEN, Belgium), Anders
Damkjaer (Risoe National Laboratory, Denmark), Jack Madden (RPH, Ireland), Jon Miles, Jayesh
Bhakta, Stuart Naismith and Chris Howarth (NRPB, UK).
The meeting felt that the general format of laboratory exposures as used in previous
intercomparisons was appropriate and should be retained. It was decided that 10 transit controls
should be used instead of the 5 used in previous exercises. Tests of saturation and ageing might be
included in later intercomparisons. If finance allows, these tests should be in addition to the three
standard exposures.
Radon concentrations in the laboratory and thoron decay product concentrations in the exposure
chamber should be monitored and reported in addition to the parameters previously monitored. At
the end of each exposure, detectors should be left exposed in the laboratory for 3 days instead of 1
day before being sealed in radon-proof bags. Detectors of the Karlsruhe KfK A design should not be
dismantled for shipping as had been done in the previous intercomparison. Participants should be
encouraged to e-mail results or send them on disk. A template for results should be e-mailed to them.
Late reported results should be shown in a separate table and marked on the graphs.
It was decided that sets of detectors should be ranked in an additional table. The mean standard
deviation and the mean percentage difference between the reported result and the reference value
should be calculated separately for each set. Sets which record <10% for both SD and difference will
be ranked as category A, sets <15% for both as category B, sets <20% as category C and others as
category D. The table should not identify sets, but list characteristics such as open or closed,
detector material, and common detector designs.
It was agreed that the final meeting open to all participants should be held over an afternoon of one
day and the morning of the following day to allow participants to attend with only one overnight stay. Speakers should be invited to give short presentations on different topics. The topics suggested
were:
Intercomparison operation and results
Detector design
Etching
Quality control
Automatic counting
How I improved my results
International outlook for radon
3 Laboratory exposure and measurement facilities
NRPB maintains a 43 m3 walk-in radon chamber (Miles and Strong, 1989), shown in Figure 1. This
facility is the European regional reference laboratory for radon measurements under an
intercalibration and intercomparison scheme organised by IAEA. The chamber is of the static type:
radon is continuously released inside the chamber by radon sources, so there is no need to ventilate
the chamber. All of the exposures were carried out in this chamber.
The chamber contains a radon atmosphere which can be varied (and held stable) from around
200 Bq m"3 to 8000 Bq m"3, depending on the use of various dry and liquid radium-226 sources. A
radon concentration of about 3000 Bq m"3 is normally maintained in the chamber, and the
concentrations of radon and its decay products are continuously monitored. The aerosol conditions
and the equilibrium factor in the chamber are altered as required for different studies and calibrations.
Table 1 shows the parameters measured and controlled in the chamber.
Three different values of the equilibrium factor (F) between radon and its decay products were
obtained for the three laboratory exposures in the intercomparison. To obtain a high value of F in the
chamber, the aerosol generator was used to maintain a high aerosol concentration. This reduces the
plate-out of radon decay products onto room surfaces. A low equilibrium factor was obtained by
running an electrostatic precipitator to remove aerosols and decay products. By running the precipitator fan at low speed an intermediate equilibrium factor was obtained.
The radon concentration in the chamber was continuously monitored using an ATMOS 12 ionisation
chamber. This instrument is calibrated every 6 months using a radon gas source obtained from the
UK National Physical Laboratory. One such calibration was carried out immediately after the
intercomparison, and showed that there had been no significant change in the sensitivity of the
instrument. Radon decay products were sampled on a Millipore AA filter and their concentrations
determined using a spectrometry system (Cliff, 1990). All monitored data was automatically
transferred to a database. Radon and radon decay product exposures were calculated later.
The total ambient aerosol concentration and size distribution within the radon chamber were
monitored using a TSI 3025 condensation nucleus counter connected to a TSI 3040 serial diffusion
battery via a TSI 3042 switching valve. The equipment was mounted outside the chamber and the
aerosol was sampled inside the chamber at a distance of 30 cm from the wall in order to minimise
any plate-out effects. Measurements were carried out every two hours during each day of the
exposures, each measurement consisting of four samples, the results being averaged. These average
values were then used to calculate the aerosol size distribution using a program developed at NRPB
based on the Twomey (1975) method.
4 Passive radon detectors
Two types of passive detector are commonly used, one consisting of a track detector within a closed
container, which allows radon-222 to diffuse into it (closed) and the other which consists of naked
track-detecting material exposed to the ambient atmosphere (open). The closed detector excludes
radon decay products which are present in the ambient atmosphere, and records only those alpha
particles generated by the radon entering the container and the decay products formed from it. This
form of detector therefore provides a result which is related to the true average radon gas
concentration during the time of exposure.