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The international measurement system for radionuclide metrology: A strategy for the future
Applied Radiation and Isotopes 154 (2019) 108838
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
The international measurement system for radionuclide metrology: A strategy for the future
T
Lisa Karama,∗, Steven Judgeb, Wynand Louwc a
Physical Measurement Laboratory, National Institute of Standards and Technology (NIST), USA Ionizing Radiation Department, Bureau International des Poids et Mesures (BIPM), France c Research and International Liaison Department, National Metrology Institute of South Africa (NMISA), South Africa b
HIGHLIGHTS
CCRI is expanding the usefulness of the CIPM MRA the international measurement system in radionuclide metrology. • The CCRI has revised its Strategy to better reflect impacts to its members and stakeholders. • The CCRI Strategy addresses improvements in the state-of-the-art in radionuclide metrology. • The • The CCRI is optimizing CMC claims in radionuclide metrology. ARTICLE INFO
ABSTRACT
Keywords: Calibration and measurement capabilities CCRI Consultative committee KCDB Radioactivity SIR
The Consultative Committee for Ionizing Radiation (CCRI) has coordinated the development of a long-term strategy to support the international measurement system for radionuclide metrology. Supported by the Bureau International des Poids et Mesures (BIPM) and its associated institutional members, the CCRI strategy captures how an NMI/DI can demonstrate capability in radionuclide metrology with the support that is available through the CCRI and the BIPM. The CCRI strategy, specifically the aspects with direct impact on radionuclide metrology, is described. Various projects to ensure that the international measurement system for radioactivity can be sustained in the short and long term will also be outlined.
1. Introduction Users of well-characterized radioactive sources must be able to determine the activity content of materials at an accuracy “fit for purpose” to assure that radionuclides can be used to benefit humankind while ensuring that risks are well-controlled and mitigated, as well as to meet regulatory requirements when imposed. To meet the needs of a diverse cadre of users, from the nuclear pharmacy to environmental monitoring laboratories, the international measurement system (designated “IMS” throughout) for radionuclide metrology, deriving from the decisions of the Comité international des poids et mesures (CIPM, 2018A) and of the Conférence générale des poids et mesures (CGPM, 2019) and based on primary standards developed and realized at National Metrology (or Designated) Institutes (NMIs/DIs), has been well-established. Enabled and supported by the Consultative Committee for Ionizing Radiation (CCRI) (CCRI, 2018), the recognition of the equivalence among these
∗
primary standards world-wide is underpinned by measurement comparison exercises. Particularly for radionuclide metrology, these comparisons, such as those on-going with the International Système de Reference (SIR) (Ratel, 2007; SIR, 2014), continue to provide a solid foundation for a rigorous metrological infrastructure that support the Calibration and Measurement Capabilities (CMCs) that NMIs/DIs use to provide services to their many customers. Maintaining the IMS in radionuclide metrology, however, is becoming increasingly difficult. Challenges in shipping radioactive sources for comparison exercises, for example, can limit the number and breadth of participating laboratories, whereas the greatest rigor can be achieved best by a wide range of participants. Regulatory restrictions on the use of sealed sources, particularly on those that have been held for many years and, therefore, represent a long-standing measurement history, have increased, with the realization of stability issues and increased concerns over security. Finally, demands for new measurements
Corresponding author. E-mail address: [email protected] (L. Karam).
https://doi.org/10.1016/j.apradiso.2019.108838 Received 27 March 2019; Received in revised form 8 July 2019; Accepted 1 August 2019 Available online 05 August 2019 0969-8043/ Published by Elsevier Ltd.
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2. The CCRI and its strategy The focus of the CCRI is to maximize the impact of the work of its participating members on society, including assuring that science develops to meet future needs for the wide-array of users of radionuclide metrology. As an organization of NMIs and DIs with common goals, CCRI's vision is “a world in which ionizing radiation can be used for the benefit of humankind and the environment, confident that the associated risks are constrained by accurate, scientifically-rigorous, measurement.” Its mission, to “discuss, foster, enable and coordinate the development, comparison and promulgation of national measurement standards for ionizing radiation,” is intended to enable all users of ionizing radiation (including radioactivity) to make measurements adequate for their specific needs. The BIPM Ionizing Radiation Department supports the CCRI in its mission and provides a variety of centralized services including providing and coordinating comparisons, organizing knowledge transfer and capacity building activities (such as training courses), coordinating research projects of international interest, and representing the international metrology community. The CCRI, its members, and the BIPM are customer-focused (for the BIPM, the NMIs/ DIs are their primary customers) so that the metrology community can assure that its activities best represent stakeholder needs. The CCRI Strategic Plan (“Strategy”) for 2018–2028 is generally structured to first present the challenges facing ionizing radiation metrology well into the 21st century, then to describe approaches to address these challenges and summarizes the work at the BIPM to facilitate the realization of those approaches (Consultative Committee for Ionizing Radiation (CCRI) ∙ CCRI Strategy Document 2018–2028, CCRI, 2019). Although the Strategy supports all branches of ionizing radiation metrology, radionuclide metrology is a key component that underlies not only radioactivity but also some efforts in dosimetry and in neutron metrology. In fact, of the six main conclusions of the Strategy's review of scientific, economic and social challenges, radionuclide metrology is reflected in five:
Fig. 1. The basic structure of the CCRI, showing its working groups (including Sections I, II and III) and their functions.
and standards as radionuclides find new applications challenge the fiscal and administrative realities of the 21st century. Meeting at least every two years, the CCRI (including its three Sections: x/gamma rays and charged particles, Measurement of radionuclides, and Neutron measurements; Fig. 1), like the other consultative committees of the CIPM, focusses on three objectives (described in CIPM, 2018B):
• progress the state-of-the-art by providing a global forum for NMIs to exchange information about the state-of-the art and best practices, • define new possibilities for metrology to have impact on global •
measurement challenges by facilitavting dialogue between the NMIs and new and established stakeholders, and demonstrate and improve the global comparability of measurements. Particularly by working with the Regional Metrology Organizations (RMOs, 2019) in the context of the CIPM MRA (CIPM MRA, 2019) to plan, execute and monitor Key Comparisons (KCs), and to support the process of CMC review.
- The continuing need to maintain a robust ionizing radiation measurement system, given the impact on many people's lives - The need for new primary standards for emerging cancer treatments, in both external-beam and internal-source therapies (in addition to the quantification of images) - The need to develop alternatives to the sealed radioactive sources on which many aspects of the measurement infrastructure depends - The need for new comparators for long-lived radionuclides, due to the expansion in nuclear decommissioning and increased concern over NORM - The difficulties of continuing to organize large-scale comparisons due to the complex regulatory framework
To address these objectives, and to facilitate the smooth operation of international ionizing radiation metrology, the CCRI [and its Section II on Measurement of radionuclides, CCRI(II)] is revising its strategic plan from the action-focused version of 2013 (Carneiro et al., 2013) to one more directed toward impacts to NMIs/DIs and to their stakeholders. For the community that makes up the CCRI(II), including the many members and leaders who are also delegate to the International Committee for Radionuclide Metrology (ICRM), developing and following a strategic plan meets several critical goals. Firstly, and most immediately of use to the members, the plan overviews the efforts taking place among the members in the international radionuclide milieu including (but not limited to) the impact and needs for primary standardizations in radioactivity. The strategic plan also provides a means to leverage and support the activities at the BIPM that support the needs of the NMIs/DIs. Finally, and perhaps most critically, the availability of a strategic plan for CCRI provides transparency to the Consultative Committees of other fields of metrology, demonstrating the CCRI's role as a full participant in the CIPM Mutual Recognition Arrangement (MRA).
Nuclear medicine, nuclear decommissioning, nuclear forensics, and naturally-occurring radioactive materials (NORM) are all considered as key sectors informing the basis of CCRI's Strategy, and are described as the main drivers and activities in radionuclide metrology (Fig. 2) to meet the challenges of assuring accurate measurement in the most efficient manner possible. Beyond the aspects specific to radioactivity measurements, several items more commonly considered under the auspices of dosimetry or neutron measurements also rely on sound radionuclide metrology. Source-based brachytherapy (the Strategy describes expanding use of 60Co and 192Ir), for example, although leveraging the gamma rays for therapeutic effect, depend on knowledge of the radioactive content for manufacture, shipping, prescription control, and regulatory compliance. Radionuclide metrology also underpins the
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Fig. 2. Summary of the main activities in radionuclide metrology to fulfill the CCRI strategy.
realization of the primary quantity (fluence) in neutron metrology; the emission from a radionuclide neutron source can be determined by measuring the activity of 56Mn induced in a bath of aqueous manganese sulfate solution (Roberts et al., 2011). Also, in the absence of a reactor facility, many customers rely on calibration back to a neutron-emitting source (such as 252Cf) to assure detector performance.
the POLATOM (Poland), the LNHB (France), the PTB (Germany) and the NPL (UK), the BIPM has decided to implement a variant of the TDCR method; scoping studies have shown that the method can produce consistent results. The aim is to start initial comparison exercises in 2020, and further work is planned to improve the design of the instrument and the techniques to establish a new, long term, comparison service. Such use of secondments at the BIPM supports another of the key thrust areas of the CCRI strategy: to improve stakeholder involvement as a foundation for the development of an inclusive international measurement system for ionizing radiation. In this way, not only can the work at the BIPM be advanced for the benefit of all members (such as in expanding the availability of a centralized reference system for ongoing comparisons) but also provides participating laboratories an opportunity to expand their influence on the future evolutions of the IMS. The great success of the travelling SIR instrument (SIRTI) (SIRTI, 2019; Michotte et al., 2013) for the measurement of very short-lived radionuclides (van Rooy et al., 2020; Galea et al., 2020), beginning with the relatively “long-lived” (t1/2 ~6 h) 99mTc to now being used for the t1/ 11 C, demonstrates the strong influence among the members of 2–20 min the entire radionuclide metrology community within the CCRI in working together to maintain and improve the IMS. The work program at the BIPM, too, relies on the concerted efforts among the members of CCRI working in the context of a strategic approach to leverage capabilities among NMIs/DIs and even across the aisle to other CCs. For example, a key risk to the IMS for radionuclide metrology is the increased regulatory pressures against long-term storage of sealed yet potentially “insecure” radioactive sources such as the 226 Ra sources that have served as the reference sources against which
3. The CCRI strategy and the international measurement system The Strategy delineates the aims of the CCRI in key areas such as progressing the state of the art in the field, improving global acceptance of measurements, building capabilities at smaller NMIs/DIs or those just beginning their efforts in radionuclide metrology, and optimizing the development and publication of CMCs to offer the widest support possible to services provided by metrology laboratories. For example, in the area of enabling the progress of metrology, the CCRI(II) has been working to develop and implement an approach to measuring alphaparticle and beta-particle emitting radionuclides as is currently done for gamma-ray emitting sources within the Système International de Référence (SIR). Building on the work of the Extended SIR Working Group of CCRI(II) (ESWG(II); ESWG, 2019), extensive efforts evaluating the potential of using cross-efficiency liquid scintillator counting (LSC) methods (first proposed at a meeting of the Extended-SIR Working Group in June 2009) or a triple-to-double coincidence ratio (TDCR) counter with Compton spectrometer (Cassette and Do, 2008) have been considered. While recent publications (Cassette and Tartès, 2014) have strongly suggested the disadvantages of the cross-efficiency method, the two approaches have been further evaluated at the BIPM with secondments from the NIST (US) and the NIM (China). In consultation with
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are compared sources submitted to the SIR. These radium sources, in use since the SIR was first established in 1976, have supported an unbroken chain of measurements in the on-going ionization chamber comparisons for gamma-ray emitting sources even as the SIR itself has been modernized (Michotte, 2012). However, with the recently enhanced controls on such sources, alternatives for maintaining the linearity of the SIR must be developed. One possibility, the use of 166mHo to replace 226Ra (Jerome et al. presented at ICRM2019) to provide the necessary quality control for the SIR, could be a resolution for the immediate difficulty presented by the loss of being able to maintain the current 226Ra sources for the SIR. For a more long-term solution, however, the BIPM and other members of the CCRI(II) as well as members of the Consultative Committee on Electrical Measurements (CCEM) have been working toward the development of new technological approaches for assuring ionization chamber linearity through improvements in current measurements (Fitzgerald et al. submitted). This effort has implications not only for ionizing radiation measurements but also in the field of electrical measurements. To facilitate these developments, the CCRI(II) and the CCEM have proposed a joint working group between the CCs, perhaps the first time two different CCs have come together to resolve a common problem. The aim is to upgrade the electrical current measurement systems for ionization chambers used in radionuclide metrology so as to reduce the dependence on artifact-based standards and to provide stable traceability to the SI into the next generation.
context of the CIPM MRA on the world-wide stage of metrology. These CMCs are indexed through a classification scheme established in the earliest years of the CIPM MRA so as to make searching for specific capabilities at individual laboratories possible. These claims are vetted by the international metrology community before publications to confirm their validity as supported by results from comparisons, publications in the scientific literature, and other available documentation. In order to properly leverage the use of radioactive sources in international comparisons, as much to reduce the need to ship possibly limited material as to reduce the workload required to support an NMI's/DI's CMC claims, the CCRI has described various strategies intended to optimize the impact of radionuclide measurement comparisons. In addition to the MMM, which allows for a single measurement comparison to support CMCs for several radionuclides using the same primary method, the CCRI has been working to describe its approach to recognizing the broad support primary measurements provide to CMCs covering derived quantities. The CCRI (through the CCRI RMO WG on CMCs) has proposed an interpretation of the definition of a CMC to clearly facilitate the linkage of derived quantities to the primary measurement CMC, considering the clarification that more than one instrument type/method can be listed in a single CMC (explanatory footnotes on the use of the CIPM MRA logo, CIPM MRA, 2018). The CIPM MRA document 5 (CIPM MRA, 2017) defines a CMC as a capability available to customers under normal conditions “as published in the BIPM key comparison database” (Appendix C of the KCDB). Therefore, the CCRI recognizes the importance for NMIs/DIs to be able to publish CMCs in Appendix C for any properly-validated and supported metrological service they make available to customers. However, this does not prevent an NMI/DI from providing services that are not published in Appendix C. In order to be considered in the context of the CIPM MRA, though, such a service must be shown (and clearly documented for any customer or other interested party) to derive from (be traceable to) a CMC published in Appendix C. This traceability could be described in an NMI's/DI's quality management system documentation and be made available to customers through publicly-available literature, catalogs of services or other mechanisms. Using this interpretation, NMIs and DIs are free to publish as few or as many CMCs to best serve their customer base while assuring that all capabilities are metrologically sound through traceability to primary measurements. Toward global comparability, a major focus of the CCRI Strategy, this interpretation of CMCs provides wide-support for services while maintaining the opportunity for laboratories to document derived quantities. In essence, a single international measurement comparison would be able to support a suite of a laboratory's services and, thereby, allowing “the light to shine” far indeed. In order to streamline publication of capabilities so as to optimize the use of Appendix C, the CCRI has also been working to reduce the number of permutations (e.g., activity per unit mass, per unit area, and per unit volume all collapsed into a single quantity, “activity”) of measurement quantities as published in the KCDB. However, the classification of services for ionizing radiation CMCs as published in 2002 have been used to publish over 2700 CMCs in radioactivity (KCDB, 2019) and any change in the classification system would necessarily require a retroactive link so that newly published CMCs could be considered compatible with existing ones. To that end, the CCRI RMO WG on CMCs has proposed a revision of the current services in radioactivity to one in which, while the numbering scheme is unchanged, many quantities (such as the split of activity among per unit “mass,” “area,” and “volume” in addition to “activity”) are dropped or consolidated (such as emission rate or efficiency) (Tables 1 and 2).
4. Realizing the international measurement system Measurement comparisons have always served as a strong basis for demonstrating the capability of a metrology lab; so much so that successful participation is considered as crucial in supporting CMCs in the frame of the CIPM MRA as described in the CIPM MRA document 5 (CIPM MRA, 2016). Valid activity comparison results, published in the KCDB may be used to support CMC claims for the same radionuclide, or a corresponding radionuclide as indicated in the Measurement Methods Matrix (MMM) (Karam, 2007; Karam and Ratel, 2016), which is available for preparers and reviewers of CMCs. In March 2019, there were 114 key (including for 70 different radionuclides by the SIR, some with results from before 1980) and 26 supplementary comparisons in radioactivity considered “current” (either published or in process). Although the original CIPM MRA proposal was to have results no older than 10 years in the KCDB, the long-term stability of radionuclide activity measurements has been well demonstrated at the NMIs and at the BIPM. Consequently, during the March 2018 meeting of the CCRI RMO Working Group on CMCs (CCRI RMO WG on CMCs), it was decided to extend the validity of comparisons of radionuclide measurements (including those of the SIR and SIRTI) to 15 years indefinitely. Considering the importance of measurement comparisons to maintain and advance the international measurement system for radionuclide metrology, procedures on how to conduct comparisons are available (CCRI(II), 2003); NMIs/DIs are strongly encouraged to take advantage of previously published comparison documentation to propose and pilot new comparisons to support the wide array of activities needing sound radionuclide metrology. An NMI/DI intending to participate in the CIPM MRA will demonstrate its role in the international measurement system through scientific collaboration and publication, participation in measurement comparisons, and declaration of its capabilities. CMCs as published in Appendix C of the Key Comparison Database (KCDB, 2002) are the primary mechanism by which NMIs/DIs declare their capabilities in the
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Table 1 Classification of services in radionuclide metrology since 2002 (CCRI, 2002). Items planned for deletion are highlighted in grey. Branch
Quantity
Medium
Source
Radionuclide
2 Radioactivity
1 Activity 2 Activity per unit mass 3 Activity per unit area 4 Activity per unit volume 5 Surface emission rate 6 Surface emission rate per unit area 7 Emission rate per unit solid angle 8 Emission rate 9 Efficiency of γ-ray spectrometers (versus energy) 10 Efficiency of ionization chambers 11 Efficiency of contamination monitors
1 Other 2 Gas 3 Liquid 4 Solid 5 Aerosol 6 Reference material: other 7 Reference material: foods 8 Reference material: water 9 Reference material: biological materials 10 Reference material: soils/sediments 11 Reference material: flora 12 Reference material: building materials
1 Single-radionuclide source
Xx-00
2 Multi-radionuclide source 3 Kx-rays
Table 2 Classification of services in radionuclide metrology adopted by the CCRI Section II (June 2019). Added items are indicated in red. Branch
Quantity
Medium
Source
Radionuclide
2 Radioactivity
1 Activity
1 2 3 4 5 6
1 Single-radionuclide source
Xx-00
5 Surface emission rate 8 Emission rate
12 Efficiency
Other Gas Liquid Solid Aerosol Reference material: other
3 Kx-rays
8 Reference material: water 10 11 12 13
Reference Reference Reference Reference
material: material: material: material:
The “International Rules for completing the CMC Tables for Ionizing Radiation” (CCRI, 2010), periodically revised since 2000, are also being updated to reflect the revision to the classification system. Of particular note is the expansion of the description for the validation support (i.e., column “P: Evidence supporting this measurement/calibration service”) included for each CMC and which forms the basis for the metrological review of the claim. Within the revised rules, the critical role that the MMM plays to optimize the comparison support (CGPM, 2018) for a CMC is elucidated: Radioactivity: The “Measurement Methods Matrix”, based on primary measurement methods, should be used to optimize comparison support for CMCs.
soils/sediments flora building materials fauna
The reduction of the service categories to be used and the revisions of the rules for completing CMCs are intended to expand the usefulness of the schemes inherent in the CIPM MRA so that the Arrangement can be used to enable the NMIs/DIs to best serve the needs of the users of metrology. Whether commercial customers or other institutes, stakeholders in nuclear medicine, nuclear forensics, nuclear decommissioning, and the general public can be assured of the most effective distribution of the international measurement system in radionuclide metrology. 5. Conclusions With more than 150 radionuclides that require accurate measurement for diverse applications in nuclear medicine, the nuclear industry, environmental monitoring for the general public and the defense sector, maintaining and advancing the international measurement system for radionuclide metrology is critical to assure the safe and efficient use of radioactive sources. In order to be able to effectively realize such a system, the CCRI has devised a strategy to optimize the range of capabilities supported by limited comparison exercises to better leverage the use of resources at NMIs/DIs and the BIPM, supported the development of alternative technologies to address increased regulatory pressures on the use and distribution of radioactive sources, and has facilitated the knowledge transfer between the BIPM and the NMIs/DIs through secondments and on-site activities such as the key comparisons with the SIRTI. As can be seen from the references herein, the IMS is also strongly supported by the activities of the ICRM – from new measurement techniques, primary standards and nuclear data, to crucial measurement comparisons, all of which are often presented at ICRM conferences and subsequently published for the use of the entire community. In addition to assuring the continued viability of the IMS
• A comparison result from a radionuclide measured using a specific •
2 Multi-radionuclide source
primary method generally cannot support claims for that radionuclide measured by other primary methods. Claims are allowed for radionuclides in the same column only, and according to degree of difficulty (red allowing claims for red, yellow, green; yellow allowing claims for yellow and green; green allowing claims only for green)
Although heavily used in the community, the MMM is not openly available (the current version can be easily obtained from any member of the CCRI(II) or the chairman of the RMO working group on ionizing radiation/radioactivity in any of the regions). Its controlled access allows for “version” control as new results from measurement comparisons and standardizations at NMIs/DIs, particularly in the light of improved technologies, occasionally necessitate changes in the suggested uncertainties reflected in MMM or even the addition of radionuclides not yet listed. 5
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for established NMIs/DIs, this approach helps smaller NMIs/DIs to improve their own capabilities by focusing their efforts on activities (comparisons, collaborations) with the widest possible impact.
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The international measurement system for radionuclide metrology: A strategy for the future
T
Lisa Karama,∗, Steven Judgeb, Wynand Louwc a
Physical Measurement Laboratory, National Institute of Standards and Technology (NIST), USA Ionizing Radiation Department, Bureau International des Poids et Mesures (BIPM), France c Research and International Liaison Department, National Metrology Institute of South Africa (NMISA), South Africa b
HIGHLIGHTS
CCRI is expanding the usefulness of the CIPM MRA the international measurement system in radionuclide metrology. • The CCRI has revised its Strategy to better reflect impacts to its members and stakeholders. • The CCRI Strategy addresses improvements in the state-of-the-art in radionuclide metrology. • The • The CCRI is optimizing CMC claims in radionuclide metrology. ARTICLE INFO
ABSTRACT
Keywords: Calibration and measurement capabilities CCRI Consultative committee KCDB Radioactivity SIR
The Consultative Committee for Ionizing Radiation (CCRI) has coordinated the development of a long-term strategy to support the international measurement system for radionuclide metrology. Supported by the Bureau International des Poids et Mesures (BIPM) and its associated institutional members, the CCRI strategy captures how an NMI/DI can demonstrate capability in radionuclide metrology with the support that is available through the CCRI and the BIPM. The CCRI strategy, specifically the aspects with direct impact on radionuclide metrology, is described. Various projects to ensure that the international measurement system for radioactivity can be sustained in the short and long term will also be outlined.
1. Introduction Users of well-characterized radioactive sources must be able to determine the activity content of materials at an accuracy “fit for purpose” to assure that radionuclides can be used to benefit humankind while ensuring that risks are well-controlled and mitigated, as well as to meet regulatory requirements when imposed. To meet the needs of a diverse cadre of users, from the nuclear pharmacy to environmental monitoring laboratories, the international measurement system (designated “IMS” throughout) for radionuclide metrology, deriving from the decisions of the Comité international des poids et mesures (CIPM, 2018A) and of the Conférence générale des poids et mesures (CGPM, 2019) and based on primary standards developed and realized at National Metrology (or Designated) Institutes (NMIs/DIs), has been well-established. Enabled and supported by the Consultative Committee for Ionizing Radiation (CCRI) (CCRI, 2018), the recognition of the equivalence among these
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primary standards world-wide is underpinned by measurement comparison exercises. Particularly for radionuclide metrology, these comparisons, such as those on-going with the International Système de Reference (SIR) (Ratel, 2007; SIR, 2014), continue to provide a solid foundation for a rigorous metrological infrastructure that support the Calibration and Measurement Capabilities (CMCs) that NMIs/DIs use to provide services to their many customers. Maintaining the IMS in radionuclide metrology, however, is becoming increasingly difficult. Challenges in shipping radioactive sources for comparison exercises, for example, can limit the number and breadth of participating laboratories, whereas the greatest rigor can be achieved best by a wide range of participants. Regulatory restrictions on the use of sealed sources, particularly on those that have been held for many years and, therefore, represent a long-standing measurement history, have increased, with the realization of stability issues and increased concerns over security. Finally, demands for new measurements
Corresponding author. E-mail address: [email protected] (L. Karam).
https://doi.org/10.1016/j.apradiso.2019.108838 Received 27 March 2019; Received in revised form 8 July 2019; Accepted 1 August 2019 Available online 05 August 2019 0969-8043/ Published by Elsevier Ltd.
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2. The CCRI and its strategy The focus of the CCRI is to maximize the impact of the work of its participating members on society, including assuring that science develops to meet future needs for the wide-array of users of radionuclide metrology. As an organization of NMIs and DIs with common goals, CCRI's vision is “a world in which ionizing radiation can be used for the benefit of humankind and the environment, confident that the associated risks are constrained by accurate, scientifically-rigorous, measurement.” Its mission, to “discuss, foster, enable and coordinate the development, comparison and promulgation of national measurement standards for ionizing radiation,” is intended to enable all users of ionizing radiation (including radioactivity) to make measurements adequate for their specific needs. The BIPM Ionizing Radiation Department supports the CCRI in its mission and provides a variety of centralized services including providing and coordinating comparisons, organizing knowledge transfer and capacity building activities (such as training courses), coordinating research projects of international interest, and representing the international metrology community. The CCRI, its members, and the BIPM are customer-focused (for the BIPM, the NMIs/ DIs are their primary customers) so that the metrology community can assure that its activities best represent stakeholder needs. The CCRI Strategic Plan (“Strategy”) for 2018–2028 is generally structured to first present the challenges facing ionizing radiation metrology well into the 21st century, then to describe approaches to address these challenges and summarizes the work at the BIPM to facilitate the realization of those approaches (Consultative Committee for Ionizing Radiation (CCRI) ∙ CCRI Strategy Document 2018–2028, CCRI, 2019). Although the Strategy supports all branches of ionizing radiation metrology, radionuclide metrology is a key component that underlies not only radioactivity but also some efforts in dosimetry and in neutron metrology. In fact, of the six main conclusions of the Strategy's review of scientific, economic and social challenges, radionuclide metrology is reflected in five:
Fig. 1. The basic structure of the CCRI, showing its working groups (including Sections I, II and III) and their functions.
and standards as radionuclides find new applications challenge the fiscal and administrative realities of the 21st century. Meeting at least every two years, the CCRI (including its three Sections: x/gamma rays and charged particles, Measurement of radionuclides, and Neutron measurements; Fig. 1), like the other consultative committees of the CIPM, focusses on three objectives (described in CIPM, 2018B):
• progress the state-of-the-art by providing a global forum for NMIs to exchange information about the state-of-the art and best practices, • define new possibilities for metrology to have impact on global •
measurement challenges by facilitavting dialogue between the NMIs and new and established stakeholders, and demonstrate and improve the global comparability of measurements. Particularly by working with the Regional Metrology Organizations (RMOs, 2019) in the context of the CIPM MRA (CIPM MRA, 2019) to plan, execute and monitor Key Comparisons (KCs), and to support the process of CMC review.
- The continuing need to maintain a robust ionizing radiation measurement system, given the impact on many people's lives - The need for new primary standards for emerging cancer treatments, in both external-beam and internal-source therapies (in addition to the quantification of images) - The need to develop alternatives to the sealed radioactive sources on which many aspects of the measurement infrastructure depends - The need for new comparators for long-lived radionuclides, due to the expansion in nuclear decommissioning and increased concern over NORM - The difficulties of continuing to organize large-scale comparisons due to the complex regulatory framework
To address these objectives, and to facilitate the smooth operation of international ionizing radiation metrology, the CCRI [and its Section II on Measurement of radionuclides, CCRI(II)] is revising its strategic plan from the action-focused version of 2013 (Carneiro et al., 2013) to one more directed toward impacts to NMIs/DIs and to their stakeholders. For the community that makes up the CCRI(II), including the many members and leaders who are also delegate to the International Committee for Radionuclide Metrology (ICRM), developing and following a strategic plan meets several critical goals. Firstly, and most immediately of use to the members, the plan overviews the efforts taking place among the members in the international radionuclide milieu including (but not limited to) the impact and needs for primary standardizations in radioactivity. The strategic plan also provides a means to leverage and support the activities at the BIPM that support the needs of the NMIs/DIs. Finally, and perhaps most critically, the availability of a strategic plan for CCRI provides transparency to the Consultative Committees of other fields of metrology, demonstrating the CCRI's role as a full participant in the CIPM Mutual Recognition Arrangement (MRA).
Nuclear medicine, nuclear decommissioning, nuclear forensics, and naturally-occurring radioactive materials (NORM) are all considered as key sectors informing the basis of CCRI's Strategy, and are described as the main drivers and activities in radionuclide metrology (Fig. 2) to meet the challenges of assuring accurate measurement in the most efficient manner possible. Beyond the aspects specific to radioactivity measurements, several items more commonly considered under the auspices of dosimetry or neutron measurements also rely on sound radionuclide metrology. Source-based brachytherapy (the Strategy describes expanding use of 60Co and 192Ir), for example, although leveraging the gamma rays for therapeutic effect, depend on knowledge of the radioactive content for manufacture, shipping, prescription control, and regulatory compliance. Radionuclide metrology also underpins the
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Fig. 2. Summary of the main activities in radionuclide metrology to fulfill the CCRI strategy.
realization of the primary quantity (fluence) in neutron metrology; the emission from a radionuclide neutron source can be determined by measuring the activity of 56Mn induced in a bath of aqueous manganese sulfate solution (Roberts et al., 2011). Also, in the absence of a reactor facility, many customers rely on calibration back to a neutron-emitting source (such as 252Cf) to assure detector performance.
the POLATOM (Poland), the LNHB (France), the PTB (Germany) and the NPL (UK), the BIPM has decided to implement a variant of the TDCR method; scoping studies have shown that the method can produce consistent results. The aim is to start initial comparison exercises in 2020, and further work is planned to improve the design of the instrument and the techniques to establish a new, long term, comparison service. Such use of secondments at the BIPM supports another of the key thrust areas of the CCRI strategy: to improve stakeholder involvement as a foundation for the development of an inclusive international measurement system for ionizing radiation. In this way, not only can the work at the BIPM be advanced for the benefit of all members (such as in expanding the availability of a centralized reference system for ongoing comparisons) but also provides participating laboratories an opportunity to expand their influence on the future evolutions of the IMS. The great success of the travelling SIR instrument (SIRTI) (SIRTI, 2019; Michotte et al., 2013) for the measurement of very short-lived radionuclides (van Rooy et al., 2020; Galea et al., 2020), beginning with the relatively “long-lived” (t1/2 ~6 h) 99mTc to now being used for the t1/ 11 C, demonstrates the strong influence among the members of 2–20 min the entire radionuclide metrology community within the CCRI in working together to maintain and improve the IMS. The work program at the BIPM, too, relies on the concerted efforts among the members of CCRI working in the context of a strategic approach to leverage capabilities among NMIs/DIs and even across the aisle to other CCs. For example, a key risk to the IMS for radionuclide metrology is the increased regulatory pressures against long-term storage of sealed yet potentially “insecure” radioactive sources such as the 226 Ra sources that have served as the reference sources against which
3. The CCRI strategy and the international measurement system The Strategy delineates the aims of the CCRI in key areas such as progressing the state of the art in the field, improving global acceptance of measurements, building capabilities at smaller NMIs/DIs or those just beginning their efforts in radionuclide metrology, and optimizing the development and publication of CMCs to offer the widest support possible to services provided by metrology laboratories. For example, in the area of enabling the progress of metrology, the CCRI(II) has been working to develop and implement an approach to measuring alphaparticle and beta-particle emitting radionuclides as is currently done for gamma-ray emitting sources within the Système International de Référence (SIR). Building on the work of the Extended SIR Working Group of CCRI(II) (ESWG(II); ESWG, 2019), extensive efforts evaluating the potential of using cross-efficiency liquid scintillator counting (LSC) methods (first proposed at a meeting of the Extended-SIR Working Group in June 2009) or a triple-to-double coincidence ratio (TDCR) counter with Compton spectrometer (Cassette and Do, 2008) have been considered. While recent publications (Cassette and Tartès, 2014) have strongly suggested the disadvantages of the cross-efficiency method, the two approaches have been further evaluated at the BIPM with secondments from the NIST (US) and the NIM (China). In consultation with
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are compared sources submitted to the SIR. These radium sources, in use since the SIR was first established in 1976, have supported an unbroken chain of measurements in the on-going ionization chamber comparisons for gamma-ray emitting sources even as the SIR itself has been modernized (Michotte, 2012). However, with the recently enhanced controls on such sources, alternatives for maintaining the linearity of the SIR must be developed. One possibility, the use of 166mHo to replace 226Ra (Jerome et al. presented at ICRM2019) to provide the necessary quality control for the SIR, could be a resolution for the immediate difficulty presented by the loss of being able to maintain the current 226Ra sources for the SIR. For a more long-term solution, however, the BIPM and other members of the CCRI(II) as well as members of the Consultative Committee on Electrical Measurements (CCEM) have been working toward the development of new technological approaches for assuring ionization chamber linearity through improvements in current measurements (Fitzgerald et al. submitted). This effort has implications not only for ionizing radiation measurements but also in the field of electrical measurements. To facilitate these developments, the CCRI(II) and the CCEM have proposed a joint working group between the CCs, perhaps the first time two different CCs have come together to resolve a common problem. The aim is to upgrade the electrical current measurement systems for ionization chambers used in radionuclide metrology so as to reduce the dependence on artifact-based standards and to provide stable traceability to the SI into the next generation.
context of the CIPM MRA on the world-wide stage of metrology. These CMCs are indexed through a classification scheme established in the earliest years of the CIPM MRA so as to make searching for specific capabilities at individual laboratories possible. These claims are vetted by the international metrology community before publications to confirm their validity as supported by results from comparisons, publications in the scientific literature, and other available documentation. In order to properly leverage the use of radioactive sources in international comparisons, as much to reduce the need to ship possibly limited material as to reduce the workload required to support an NMI's/DI's CMC claims, the CCRI has described various strategies intended to optimize the impact of radionuclide measurement comparisons. In addition to the MMM, which allows for a single measurement comparison to support CMCs for several radionuclides using the same primary method, the CCRI has been working to describe its approach to recognizing the broad support primary measurements provide to CMCs covering derived quantities. The CCRI (through the CCRI RMO WG on CMCs) has proposed an interpretation of the definition of a CMC to clearly facilitate the linkage of derived quantities to the primary measurement CMC, considering the clarification that more than one instrument type/method can be listed in a single CMC (explanatory footnotes on the use of the CIPM MRA logo, CIPM MRA, 2018). The CIPM MRA document 5 (CIPM MRA, 2017) defines a CMC as a capability available to customers under normal conditions “as published in the BIPM key comparison database” (Appendix C of the KCDB). Therefore, the CCRI recognizes the importance for NMIs/DIs to be able to publish CMCs in Appendix C for any properly-validated and supported metrological service they make available to customers. However, this does not prevent an NMI/DI from providing services that are not published in Appendix C. In order to be considered in the context of the CIPM MRA, though, such a service must be shown (and clearly documented for any customer or other interested party) to derive from (be traceable to) a CMC published in Appendix C. This traceability could be described in an NMI's/DI's quality management system documentation and be made available to customers through publicly-available literature, catalogs of services or other mechanisms. Using this interpretation, NMIs and DIs are free to publish as few or as many CMCs to best serve their customer base while assuring that all capabilities are metrologically sound through traceability to primary measurements. Toward global comparability, a major focus of the CCRI Strategy, this interpretation of CMCs provides wide-support for services while maintaining the opportunity for laboratories to document derived quantities. In essence, a single international measurement comparison would be able to support a suite of a laboratory's services and, thereby, allowing “the light to shine” far indeed. In order to streamline publication of capabilities so as to optimize the use of Appendix C, the CCRI has also been working to reduce the number of permutations (e.g., activity per unit mass, per unit area, and per unit volume all collapsed into a single quantity, “activity”) of measurement quantities as published in the KCDB. However, the classification of services for ionizing radiation CMCs as published in 2002 have been used to publish over 2700 CMCs in radioactivity (KCDB, 2019) and any change in the classification system would necessarily require a retroactive link so that newly published CMCs could be considered compatible with existing ones. To that end, the CCRI RMO WG on CMCs has proposed a revision of the current services in radioactivity to one in which, while the numbering scheme is unchanged, many quantities (such as the split of activity among per unit “mass,” “area,” and “volume” in addition to “activity”) are dropped or consolidated (such as emission rate or efficiency) (Tables 1 and 2).
4. Realizing the international measurement system Measurement comparisons have always served as a strong basis for demonstrating the capability of a metrology lab; so much so that successful participation is considered as crucial in supporting CMCs in the frame of the CIPM MRA as described in the CIPM MRA document 5 (CIPM MRA, 2016). Valid activity comparison results, published in the KCDB may be used to support CMC claims for the same radionuclide, or a corresponding radionuclide as indicated in the Measurement Methods Matrix (MMM) (Karam, 2007; Karam and Ratel, 2016), which is available for preparers and reviewers of CMCs. In March 2019, there were 114 key (including for 70 different radionuclides by the SIR, some with results from before 1980) and 26 supplementary comparisons in radioactivity considered “current” (either published or in process). Although the original CIPM MRA proposal was to have results no older than 10 years in the KCDB, the long-term stability of radionuclide activity measurements has been well demonstrated at the NMIs and at the BIPM. Consequently, during the March 2018 meeting of the CCRI RMO Working Group on CMCs (CCRI RMO WG on CMCs), it was decided to extend the validity of comparisons of radionuclide measurements (including those of the SIR and SIRTI) to 15 years indefinitely. Considering the importance of measurement comparisons to maintain and advance the international measurement system for radionuclide metrology, procedures on how to conduct comparisons are available (CCRI(II), 2003); NMIs/DIs are strongly encouraged to take advantage of previously published comparison documentation to propose and pilot new comparisons to support the wide array of activities needing sound radionuclide metrology. An NMI/DI intending to participate in the CIPM MRA will demonstrate its role in the international measurement system through scientific collaboration and publication, participation in measurement comparisons, and declaration of its capabilities. CMCs as published in Appendix C of the Key Comparison Database (KCDB, 2002) are the primary mechanism by which NMIs/DIs declare their capabilities in the
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Table 1 Classification of services in radionuclide metrology since 2002 (CCRI, 2002). Items planned for deletion are highlighted in grey. Branch
Quantity
Medium
Source
Radionuclide
2 Radioactivity
1 Activity 2 Activity per unit mass 3 Activity per unit area 4 Activity per unit volume 5 Surface emission rate 6 Surface emission rate per unit area 7 Emission rate per unit solid angle 8 Emission rate 9 Efficiency of γ-ray spectrometers (versus energy) 10 Efficiency of ionization chambers 11 Efficiency of contamination monitors
1 Other 2 Gas 3 Liquid 4 Solid 5 Aerosol 6 Reference material: other 7 Reference material: foods 8 Reference material: water 9 Reference material: biological materials 10 Reference material: soils/sediments 11 Reference material: flora 12 Reference material: building materials
1 Single-radionuclide source
Xx-00
2 Multi-radionuclide source 3 Kx-rays
Table 2 Classification of services in radionuclide metrology adopted by the CCRI Section II (June 2019). Added items are indicated in red. Branch
Quantity
Medium
Source
Radionuclide
2 Radioactivity
1 Activity
1 2 3 4 5 6
1 Single-radionuclide source
Xx-00
5 Surface emission rate 8 Emission rate
12 Efficiency
Other Gas Liquid Solid Aerosol Reference material: other
3 Kx-rays
8 Reference material: water 10 11 12 13
Reference Reference Reference Reference
material: material: material: material:
The “International Rules for completing the CMC Tables for Ionizing Radiation” (CCRI, 2010), periodically revised since 2000, are also being updated to reflect the revision to the classification system. Of particular note is the expansion of the description for the validation support (i.e., column “P: Evidence supporting this measurement/calibration service”) included for each CMC and which forms the basis for the metrological review of the claim. Within the revised rules, the critical role that the MMM plays to optimize the comparison support (CGPM, 2018) for a CMC is elucidated: Radioactivity: The “Measurement Methods Matrix”, based on primary measurement methods, should be used to optimize comparison support for CMCs.
soils/sediments flora building materials fauna
The reduction of the service categories to be used and the revisions of the rules for completing CMCs are intended to expand the usefulness of the schemes inherent in the CIPM MRA so that the Arrangement can be used to enable the NMIs/DIs to best serve the needs of the users of metrology. Whether commercial customers or other institutes, stakeholders in nuclear medicine, nuclear forensics, nuclear decommissioning, and the general public can be assured of the most effective distribution of the international measurement system in radionuclide metrology. 5. Conclusions With more than 150 radionuclides that require accurate measurement for diverse applications in nuclear medicine, the nuclear industry, environmental monitoring for the general public and the defense sector, maintaining and advancing the international measurement system for radionuclide metrology is critical to assure the safe and efficient use of radioactive sources. In order to be able to effectively realize such a system, the CCRI has devised a strategy to optimize the range of capabilities supported by limited comparison exercises to better leverage the use of resources at NMIs/DIs and the BIPM, supported the development of alternative technologies to address increased regulatory pressures on the use and distribution of radioactive sources, and has facilitated the knowledge transfer between the BIPM and the NMIs/DIs through secondments and on-site activities such as the key comparisons with the SIRTI. As can be seen from the references herein, the IMS is also strongly supported by the activities of the ICRM – from new measurement techniques, primary standards and nuclear data, to crucial measurement comparisons, all of which are often presented at ICRM conferences and subsequently published for the use of the entire community. In addition to assuring the continued viability of the IMS
• A comparison result from a radionuclide measured using a specific •
2 Multi-radionuclide source
primary method generally cannot support claims for that radionuclide measured by other primary methods. Claims are allowed for radionuclides in the same column only, and according to degree of difficulty (red allowing claims for red, yellow, green; yellow allowing claims for yellow and green; green allowing claims only for green)
Although heavily used in the community, the MMM is not openly available (the current version can be easily obtained from any member of the CCRI(II) or the chairman of the RMO working group on ionizing radiation/radioactivity in any of the regions). Its controlled access allows for “version” control as new results from measurement comparisons and standardizations at NMIs/DIs, particularly in the light of improved technologies, occasionally necessitate changes in the suggested uncertainties reflected in MMM or even the addition of radionuclides not yet listed. 5
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for established NMIs/DIs, this approach helps smaller NMIs/DIs to improve their own capabilities by focusing their efforts on activities (comparisons, collaborations) with the widest possible impact.
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