Strengthening Activity Measurement Quality in Radiation Protection from Metrological Science to Reliable End-user Application

Transcription

1 Strengthening Activity Measurement Quality in Radiation Protection from Metrological Science to Reliable End-user Application Franz Josef Maringer a,b,c*, Robert Brettner-Messler a, Michael Kreuziger a a BEV Federal Office of Metrology and Surveying, Arltgasse 35, 1160 Vienna, Austria. b University of Technology Vienna, Atominstitut, Stadionallee 2, 1020 Vienna, Austria c University of Natural Ressources and Applied Life Science Vienna, LLC Labor Arsenal, Faradayg. 3, Arsenal 214, 1030 Vienna, Austria Abstract. In many fields of radiation protection - e.g. internal dosimetry, nuclear medicine, radioecology, NORM - accurate and reliable activity measurements are of fundamental significance on meaningful evaluation of radiation exposure caused by radioactivity. Although the physical quantity activity and its SI unit Becquerel are very easy to define theoretically, the real practical activity measurement of radionuclides in different media is always a highly sophisticated and challenging task. Effective ways to ensure traceability of unit Bq for all radionuclides of interest from national metrological standards to qualified end-user measurements are calibration and/or verification of instruments and methods. The surveillance of competence and quality of activity measurements is additionally ensured by intercomparison exercises on national, regional or international levels. In this paper, recent improvements, developments and practical implementation on the quantification of activity of radionuclides in the field of radiation protection from fundamental metrology to robust end-user applications in Austria are covered. The Austrian experience in verification and calibration of activity measurement instruments is as much addressed as the conclusions on intercomparison exercises on gamma-ray spectrometry and radon measurement methods. Different evaluation methods are shown with regard to the statistical evaluation basis. Reasonable and practically applicable statistical modelling and parameterization based on the results and experience is proposed. Statistically based criteria for the evaluation of conformity of activity measurement instruments and methods with technical and legal requirements are presented. Eventually cooperation within the European and international networks in radionuclide metrology, EURAMET and ICRM respectively, is discussed. KEYWORDS: activity measurement, radionuclide metrology, traceability, quality assurance, internal exposure, intercomparison exercises 1. INTRODUCTION In the last 60 years, the use of radiation sources and radionuclides in nuclear applications, science, medicine and industry have become more and more widespread. It was obvious that a globally harmonised system in radiation protection and radiation dosimetry is required to assure quality and accuracy in exchange of technologies and products. Accurate measurements of ionizing radiation are required in a wide range of industrial and medical applications where they are critical relating to human health and safety. In the field of dosimetry and activity measurements, radiology and nuclear medicine are perhaps the most stringent techniques in its accuracy requirements. Generally this means the uncertainty of radiation measurements in medical applications should not exceed a few percent. The most important measure to ensure the accuracy, quality and reliability of radiation protection measurements is the international networking and traceability of laboratories from top-level metrological institutes to end-user. The international coordinating metrological institute is the International Bureau of Weights and Mesures BIPM. This office acts as the operational unit of the International Metre Convention. Because not all countries are members of the Metre Convention, the International Atomic Energy Agency IAEA metrologically supports countries which are not members of the Metre Convention. * Presenting author, franz-josef.maringer@bev.gv.at 1

2 Consultative Committee for Standards of Ionizing Radiations CCRI at the International Bureau of Weights and Measures BIPM The task of the International Bureau of Weights and Measures is to ensure world-wide uniformity of measurements and their traceability to the International System of Units (SI). In 1960, the 11 th General Conference of the Meter Convention (CGPM) decided to establish the Ionizing Radiation Section at the Bureau International des Poids et Mesures, BIPM. The main activities of the BIPM in the field of ionizing radiation are to maintain the international reference standards for dosimetry and activity measurements in the frame of the Mutual Recognition Arrangement CIPM MRA signed in 1999 [ 1 ]. At a meeting held in Paris, the directors of the national metrology institutes of thirty-eight Member States of the BIPM and representatives of two international organizations signed the international arrangement for national measurement standards and for calibration and measurement certificates issued by national metrology institutes. The Mutual Recognition Arrangement provides the technical basis for agreements negotiated for international trade, commerce and regulatory affairs like radiation protection. The standards and protocols used at the BIPM key comparisons and their development and improvement is a major part of the international metrological research and development programme. The ionising radiation section of the BIPM also undertakes calibrations for national laboratories, and participates in international comparisons. International Reference System SIR for gamma emitting radionuclides at BIPM The international reference system [ 2 ], designed and established in 1976, is based on comparison measurements of radionuclide solutions (flame-sealed glass ampoules) including a long-term stable well type ionisation chamber at BIPM. Since its installation over 60 radionuclides have been measured giving a total of more than 650 independent results. The results of the measurements are used to build the efficiency curve of the ionization chamber as a function of gamma-ray energy. The SIR is acting as the international top level metrological comparison system which guarantees the traceability and mutual recognition of national radionuclide activity measurement capabilities. IAEA/WHO Secondary Standards Dosimetry Laboratories SSDL network The IAEA/WHO Network of Secondary Standards Dosimetry Laboratories SSDL includes currently 80 laboratories in 67 Member States, of which over half are developing countries [ 3 ]. The SSDL network is supported by 15 affiliated members - Primary Standard Dosimetry Laboratories, PSDLs the BEV is one of them - and 5 collaborating organizations; these SSDL partners supply scientific and technical support to the Network. The SSDL project has the responsibility to verify that the services provided by the national laboratories follow internationally accepted metrological standards, including the traceability for radiation protection instruments. IAEA's support is accomplished first with the transmission of calibration factors for ionization chambers from the BIPM or PSDLs through the IAEA's Dosimetry Laboratory. For more than 15 years a postal TLD programme has monitored the performance of the SSDLs in the therapy dose range and more recently a comparison has been initiated using ionization chambers. One of the principal goals of the SSDL network is to guarantee that the dose delivered to patients undergoing radiotherapy treatment in the Member States is kept within internationally accepted uncertainty of prescribed levels. During the last years the trend towards the implementation of quality assurance QA according to ISO/IEC international standard in radiotherapy has been based on the criticality of biological response to radiation dose, as the probabilities of tumour control and normal tissue complication are closely related to a correct patient dosimetry [ 3 ]. European Association of National Metrological Institutes EURAMET The European Association of National Metrological Institutes EURAMET is acting as the European regional metrological confederation. Founded in 2007 as successor organisation of EUROMET, it is coordinating the metrological activities of the European national metrology institutes (NMI's) of the 2

3 EU including the European Commission, EFTA and EU Accession States. Other European states may apply for membership, based on certain published criteria. The organisation currently has 37 participating members [ 4 ]. The objective of EURAMET is to promote the coordination of metrological activities and services with the purpose of achieving higher efficiency. EURAMET is working in Technical Committees; on of them is the TC Ionising Radiation. This TC is divided the Sub-Fields photon dosimetry, radioactivity, and neutron measurements. The basic activities are jointly done by co-operative scientific and technical projects. 2. THE AUSTRIAN METROLOGICAL SERVICE IN IONISING RADIATION AT BEV In Austria population of about 8.35 mio. People, member state of the European Union the metrological objectives of the BEV in ionising radiation is based on the Austrian Metrology Act [ 5 ]: Maintenance of the national standards for dosimetry in radiation protection, diagnostic radiology and radiation therapy Verification and calibration of dosimeters used in radiation protection, diagnostic radiology and radiation therapy Metrological examination of personal dosimeters used in radiation protection The BEV dosimetry laboratory [ 6 ] was jointly installed together with the Austrian Research Centers Seibersdorf ARC and has been in operation since To fulfil the international requirements for mutual recognition the BEV dosimetry laboratory participates regularly in international comparisons organized by BIPM, EURAMET and IAEA. The dosimetry branch of the BEV is member of the Comité consultatif des rayonnements ionisants, CCRI. This committee advises the Comité international des poids et measures, CIPM, in Sèvres near Paris in metrological affairs, in the planning and running of international comparison exercises and scientific topics in dosimetry of ionising radiation. Figure 1. Absorbed dose measurements of high energy photons - national primary standard graphite-calorimeter at the LINAC of the hospital Wiener Neustadt, Austria 3

4 Furthermore the BEV dosimetry laboratory irradiates reference thermoluminescence dosimeters for the postal dose comparison measurement program of the IAEA within the scope of the Secondary Standard Dosimetry Laboratory SSDL network. The irradiation facilities of the BEV dosimetry laboratory are: Three X-ray equipments with adjustable tube voltage from 5 kv to 320 kv Co-60 teletherapy unit with adjustable beam geometry and irradiation time for therapy dosimetry Panoramic irradiation facility containing four Cs-137 sources with pneumatic transport system and radiation time control for radiation protection dosimetry Reference beam facility containing three Cs-137 and three Co-60 sources with conical ring collimator, pneumatic shutter and irradiation time control for radiation protection dosimetry The realisation of the dosimetric quantities / units is done by: Graphite-cavity ionisation chambers for absolute realisation of the units of air kerma and operational dose equivalent quantities of gamma radiation of Cs-137 and Co-60 Free air-parallel plate ionisation chambers for absolute realisation of the units of kerma and derived dose equivalent quantities of X-rays with energies from 5 kv to 320 kv Graphite Calorimeter for absolute realization of absorbed dose in water, which is derived from absorbed dose to graphite by means of ICRU conversion factors (Figure 1) Secondary standard transfer ionisation chambers with volumes from 0,03 cm³ to cm³ for realisation of the dose rate ranging from natural environmental levels up to dose rates used in radiation therapy In addition, there are high-quality digital current and charge measurement systems in operation. The ionisation current is in the range from 0,1 pa to 100 na. The measurement facilities are in part either self constructed or developed in co-operation with foreign national metrology institutes. The objectives in the radioactivity branch [ 6 ] of the ionising radiation section of the BEV are: Verification of activity meters (in terms of MBq) and contamination monitors (in terms of Bq/cm²) applied for diagnostic, therapy and radiation protection Metrological examination and calibration of gamma-ray spectrometry facilities applied in nuclear medicine, nuclear industry, environmental monitoring Verification and calibration of radon monitors applied in the determination of acitivity concentration of radon-222 in air The radioactivity facilities of the BEV located at the Arsenal, Vienna, are jointly used since 1991 together with ARC Seibersdorf research and the BOKU University of Natural Resources and Applied Life Science Vienna. The radioactivity laboratory consists of well shielded low-level measuring facilities, a radon chamber, and a laboratory for handling medium level of activity. The verification of the activity measurement devices and hence the traceability of the unit Becquerel is provided at BEV by: Three well type ionisation chambers with current/charge measuring devices traceable calibrated by NPL for more than 50 radionuclides for aqueous solutions (Figure 2). Five stationary and two portable high-purity germanium detectors (planar and coaxial) with analogue and digital signal processing and spectrometric deconvolution and evaluation software Three radon monitors traceable to the PTB, Braunschweig, primary radon-222 standard with emanation and calibration chamber (Fig. 2) Radioactive sources of different radionuclides point and volume sources with certified activities for calibration, verification and quality assurance as well as area sources with defined activity and emission rate. 4

5 Figure 2. The Austrian national standard for activity at the BEV radioactivity laboratory, Arsenal, Vienna 3. QUALITY ASSURANCE IN RADIATION PROTECTION MEASUREMENTS IN AUSTRIA To ensure the quality and reliability of activity measurements in radiation protection in Austria, BEV accomplish legal verification and surveillance as well as calibration of activity measurement instruments based on international, European and national standards. For dosemeters and activity measurement instruments verification intervals of two years are generally legally obligatory. In the case of radon measurement methods and gamma spectrometry systems, BEV does these periodically technical quality check-ups by comparison exercises. The recent laboratory comparison in Austria for gamma spectrometry was carried out in 2007 with 12 participants. In this exercise, aqueous solutions of 57 Co, 60 Co and 137 Cs were standardises at the national activity standard of BEV. These activity values act as reference values. Considering the statistical concept of [ 7 ], the given activity concentrations of the several participant laboratories were evaluated on the basis of two acceptance criteria: (1) Measurement value a m (Bq/kg) interval criteria: a 10% + u ) < a < a + (10% + u CRM ( CRM m CRM CRM (2) Measurement uncertainty criteria: U m with: m = am acrm m... difference between measurement and reference value a m... measuring value a CRM... reference value ) 5

6 and: U = 2 u 2 2 u = u m + u CRM u... combined uncertainty of measurement and reference value U... expanded (k = 2) combined uncertainty u m... uncertainty of measurement u CRM... uncertainty of reference value Both criteria must be fulfilled for each radionuclide. The application of these two criteria onto the results of the several laboratories participating the comparison exercise [ 8 ] shows for (a) 57 Co: Criteria (1) achieved by 9 of 12 laboratories; criteria (2) achieved by 5 of 12 laboratories (Figure 3) (b) 60 Co: Criteria (1) achieved by 11 of 12 laboratories; criteria (2) achieved by 11 of 12 laboratories (c) 137 Cs: Criteria (1) achieved by 9 of 12 laboratories; criteria (2) achieved by 5 of 12 laboratories deviation from BEV reference value BEV M L A J K B I H N C O E laboratory code upper limit criteria (1) reference value lower limit criteria (1) Figure 3. Laboratory comparison exercise for gamma spectrometry measurement systems in Austria, Co activity result The result shows the necessity of this exercise. The advantage of this kind of comparison is the highly practical procedure of processing a typical sample. The laboratories with not acceptable results in the comparison exercise have improved their calibrations and laboratory procedures. For radiation protection measurements an annual period for the technical surveillance of gamma spectrometry systems is legally regulated by the Austrian Metrology Act [ 5 ]. Similar comparison exercises are obligatory carried out for internal dosimetry activity measurement methods (e.g. whole body counters) and radon measuring instruments. 6

7 4. CONCLUSIONS: EUROPEAN AND GLOBAL PERSPECTIVES IN RADIATION PROTECTION METROLOGY It is obvious that future progress in radiation protection metrology can only be achieved as well as in established as in newly created co-operative metrological networks: The world-wide exchange of ideas, science, technology, service and products in nuclear and radiation protection applications increasingly demands harmonised metrological standards in dosimetry and radiometry. At regional level: EURAMET, the European association in measurement standards, is addressing this issue by developing a closer collaboration between its national metrological member institutes (NMIs) in all fields of metrology including ionising radiation. This strategy is based on closer collaboration in research as well as increased sharing of major facilities and rationalisation of calibration and measurement services. The aim is a European Research Area in Metrology (MERA). A joint metrology research and development program the European Metrology Research Programme EMRP [ 9 ] - started in The national metrology institutes of Croatia, Czech Republic, Hungary, Slovakia and Austria additionally formed the subregional metrological cooperation DUNAMET [ 10 ] coordinating metrological activities and services complementary and in addition to EURAMET. This general necessities leads to specific objectives in metrology in ionising radiation and radioactivity for radiation protection purpose in the near future: 1.) Establishment of a joint international traceability network for activity measurement instruments in nuclear medicine (similar to the SSDL) 2.) Joint financing and co-operative use of high energy photon and particle generator facilities (e.g. LINACs) for metrological science supporting upcoming therapy applications 3.) Establishment of a joint international traceability network for NORM measurements e.g. radon measurement instruments 4.) Increased technical co-operation and support of developing and emerging countries for improvement of radiation protection instruments metrology 5.) Co-ordination of metrological improvements in radiation protection and co-ordination of technological efforts in the frame of IRPA, IAEA and BIPM / Metre Convention REFERENCES [ 1 ] BIPM-Homepage, (Sept 2008) [ 2 ] Guy Ratel. The Système International de Référence and its application in key comparisons. Metrologia 44 (2007). S7 S16 [ 3 ] IAEA-Homepage, (Sept 2008) [ 4 ] EURAMET-Homepage, (Sept 2008) [ 5 ] Mass- und Eichgesetz MEG (Metrology Act), BGBl. (Federal Law Gazette) I 137/2004. (in German) [ 6 ] BEV-Homepage, section Metrology, (Sept 2008) [ 7 ] T. Linsinger. Application Note 1: Comparison of a measurement result with the certified value. European commission Joint Research Centre, Institute for reference materials and Measurements (IRMM), Geel, July 2005 [ 8 ] Robert Brettner-Messler, Franz Josef Maringer, Peter Michai, Michael Kreuziger. Ergebnis der Vergleichsmessung für Gammaspektrometriemesssysteme BEV-GAMMARING (Result of the gamma spectrometry comparison exercise) Report BEV-PTP, Dec 2007 (in German) [ 9 ] EMRP at the EURAMET-Homepage: [ 10 ] DUNAMET: Metrologia 1999, 36, 485 7