Consultative Committee on Ionizing Radiation: Impact on Radionuclide Metrology

Abstract

In response to the CIPM MRA, and to improve radioactivity measurements in the face of advancing technologies, the CIPM’s consultative committee on ionizing radiation developed a strategic approach to the realization and validation of measurement traceability for radionuclide metrology. As a consequence, measurement institutions throughout the world have devoted no small effort to establish radionuclide metrology capabilities, supported by active quality management systems and validated through prioritized participation in international comparisons, providing a varied stakeholder community with measurement confidence.

1. Introduction

Since the creation of the Bureau International des Poids et Mesures (BIPM) and the Comité International des Poids et Mesures (CIPM) in 1875 by the Conférence Diplomatique du Mètre (which resulted in the Meter Convention), the performance of metrology has evolved from an activity focused inward (at the single laboratory level) to outward as an interactive activity among laboratories around the world. In 1959, and in the context of the need to compare national radium standards with international standards, the Comité Consultatif pour les Étalons de Mesure des Rayonnements Ionisants (CCEMRI, which would become the Comité Consultatif pour les Rayonnements Ionisants, CCRI, in 1999) met for the first time (CCEMRI, 1960). At this meeting, the importance of interlaboratory cooperation in the difficult task of measurement and standardization of ionizing radiations, including radioactivity, and the role of BIPM in the verification of those measurements and standards, was discussed. The establishment, also during that first meeting, of a permanent subcommittee to concentrate specifically on radioactivity measurements, was within the rules of the CIPM (CIPM, 1952). This permanent subcommittee, eventually to be called the CCRI Section Two or CCRI(II) (CCRI Section II, 2015), was tasked with not only focusing on the emission of various particles and energies from unstable atoms, but also facilitating interactions among the various member institutions representing ionizing radiation measurement laboratories from around the world. The BIPM (tasked, also at that first CCEMRI meeting, with establishing “international standards for measurement of [radio]activity…taking into account the results of national, international, and other laboratories and institutions”) and the Radium Institute were included among those first member institutions, as was the International Commission on Radiation Units and Measurements (ICRU), as major a stakeholder at the time as it is today. The role of the CIPM and the CCRI in the creation of the ionizing radiation section of the BIPM has been described by Terry Quinn, previous Director of the BIPM (Quinn, 2011).

As international metrology became better defined, and as governments began to use the results of metrology to enable trade and commerce, the parent organization for which the CCRI operates, the CIPM, adopted a “Mutual Recognition Arrangement” (CIPM MRA) under which the consultative committees in all fields currently operate (CIPM, 2015). Under the auspices of the CIPM MRA, which was intended to enable the recognition of measurements across borders to facilitate trade, individual laboratories [National Measurement Institutes (NMIs) and Designated Measurement Institutes (DIs) ] began to publish their calibration and measurement capabilities (CMCs) to document their capabilities, including in radionuclide metrology (Karam, 2007). Under this new structure, the preexisting structure of the CCEMRI (becoming the CCRI with the adoption of the CIPM MRA) permitted a logical extension to a strategic model (with the CCRI defining the GOALS of ionizing radiation metrology; the Sections, including CCRI(II), defining the technical deliverables needed to support those goals; and the various working groups and individual laboratories performing the actual tasks to achieve the needed deliverables). Such a structure allows the CCRI to work efficiently while remaining responsive to stakeholder needs in a timely fashion.

With the signing of the CIPM MRA in 1999, the function of the CCRI(II) as facilitator of international radionuclide metrology expanded to serve as the strategic coordinator between the NMIs and the BIPM, with input and insights from wide stakeholder (user) communities (CIPM MRA, 2015a). At the same time, growing interest among existing laboratories (academic and government) to play a greater role in international trade and commerce led them to more widely demonstrate and perform their radionuclide metrology capabilities, leading to their eventual designation by official National Metrology Institutes (many of which do not have ionizing radiation measurement capabilities) to provide the national radioactivity measurements and standards needed to support environmental monitoring, nuclear energy, industrial, and medical applications of radioactivity.

As a consequence of the CIPM MRA, and to improve radioactivity measurements in the face of advancing technologies, the CCRI(II) and its working groups continue in a strategic approach to the realization and validation of measurement traceability for radionuclide metrology. As is described in the short, medium and long-term Action Plans in the CCRI Strategic Plan for the Period 2013-2023, the importance of increased dialogue among all stakeholders (including between NMIs and DIs) as well as increased efficiencies in all aspects of radionuclide metrology (quicker publication of comparisons reports, improvements in radionuclide decay schemes, expansion of capabilities at the BIPM to accommodate a wider range of submitted radioactive sources, harmonization of uncertainties, etc.) are balanced with the need to address immediate applications (such as post-Fukushima monitoring) as well as establishing the infrastructure required to address un-anticipated needs in the user community and arising from improved technologies and knowledge (Table 1) (Carneiro, et al., 2013). The ultimate goal of the CCRI Strategic Plan to enable the CCRI to become the “undisputed hub for ionizing radiation global metrology” is dependent on close collaborations with its institutional stakeholders and in direct dialogue with end-users.

Table 1.

Actions Planned in Radionuclide Metrology to Support the Strategy for the CCRI

ID	Action plan for 2013-2015 (short/current term)
a	Harmonize stringency in uncertainties
c	Increase meaningful dialogue between NMIs and DIs
d	Dosimetry for diagnostic imaging - identify metrology needs
f	Evaluation and improvements of the CIPM MRA
g	Stakeholder workshop (both institutional and end-user)
i	New challenges for radiation protection dosimetry (operational quantities)
k	Activity (SIRTI) comparisons
m	Consistent radionuclide decay schemes
n	New needs in public security, health, and industry
p	Extend SIR to pure α and pure β emitters
q	SIRTI for more short-lived radionuclides
t	Standards for contaminated environment or foodstuffs (e.g., post Fukushima)
u	Standards for nuclear forensics
v	Shorten the time to publication of comparisons reports
ID	Action plan for 2016-2019 (medium term)
g	Extension of the SIR to α-emitters
h	Molecular imaging measurement needs
j	Brachytherapy – LDR comparisons
k	Climate change needs for low-level measurements standards and tracers
l	Anthropogenic and natural radionuclides standards for the environment and the industry (NORM, wastes,…)
m	Single atom counting techniques for activity-mass connection
ID	Action plan for 2020-2023 (long term)
a	Standardization methods for new radionuclides
b	Introduction of new biologically related quantities
d	Evaluate non-reactor based methods of radionuclide production

In responding to the CIPM MRA and the resulting strategic planning of CCRI(II), measurement institutions throughout the world have devoted significant effort to the establishment and maintenance of quality systems, organized their priorities in participation in international comparisons, and encouraged dialogue with each other and with users. This has allowed the community to better address the needs of not only international metrology but also those of the stakeholders for whom the metrology forms a solid foundation for their own requirements in meeting regulations and assuring quality to their customers.

2. Expanding the International Radionuclide Metrology Infrastructure: Increased Dialogue and Awareness

Among the four strategic initiatives of the CCRI Strategic Plan, the “focus on stakeholders” is a key driver for many of the activities of the CCRI(II) and its members. Fundamental to such a focus is the establishment of a metrology infrastructure for radioactivity at measurement laboratories. Although radioactivity measurements have been done since the early part of the 20^th century, and the recognition of the importance of its metrology led to creation of the CCEMRI in the mid-20^th century, the regulatory and safety considerations of handling radioactive materials are not always within the capacity of a National Metrology Institute. In fact, the number of countries choosing to designate a laboratory for ionizing radiation measurements is significant (see “CIPM MRA: List of Participants”) (CIPM MRA, 2015b). Although these designated institutions may have been performing radioactivity measurements for decades, the concept of “metrology” – the science of measurement along with all its aspects – may not have been a consideration in the past.

For more than forty years (since 1974), the International Committee on Radionuclide Metrology (ICRM) has provided an opportunity for laboratories “new” to the concept of metrology of radionuclides to interact directly with members of the CCRI(II) in a wider, scientifically-focused forum. Recognizing that a growing number of these laboratories were taking on increasing roles in providing measurements to support users in achieving measurement-quality assurance, the 2011 meeting of the ICRM in Tsukuba, Japan, presented the work of several laboratories new to the ICRM. The National Technical University of Athens, Greece (NTUA), the Center for Physical Sciences and Technology, Lithuania (CPST), the Center for Technology of Radiation Safety and Metrology National Nuclear Energy Agency, Indonesia (PTKMER-BATAN), the Norwegian Radiation Protection Authority (NRPA), and the Turkish Atomic Energy Authority (TAEK-SANAEM) were presented, and the critical activities they provide in radioactivity measurement standards to address the needs of the communities in their respective countries were summarized; both the NRPA and the TAEK-SANEM are the Designated Institutes (DI) for Ionizing Radiation in their respective countries (Karam, et al., 2012). More recently (since 2011), in the Sistema Interamericano de Metrologia (SIM), the Regional Metrology Organization for the Americas, the need for additional national laboratories for the measurement of ionizing radiation has been recognized as key in the establishment of an international measurement system in which all users can have confidence (SIM, 2015). In the last few years, in addition to the reestablishment of radionuclide metrology at the National Research Council (NRC) in Canada, the Saint Kitts and Nevis Bureau of Standards (StKNBS, an NMI) and the Laboratorios Tecnogestión - Ministerio de Industria, Energía y Minería in Uruguay (MIEM-LSMRI, a DI) have recently come on-board to provide national standards in ionizing radiation, while the NMI of Chile, the Instituto Nacional de Normalización, is preparing to designate the Comisión Chilena de Energía Nuclear (CCHEN) for ionizing radiation (including radioactivity) measurements. While none of these American laboratories yet have CMCs (and the NRC is only recently preparing to submit any in radioactivity), they have been establishing their internal Quality Management Systems (QMS) and participating in international exchanges (from participating in the SIM metrology working group meetings to hosting site visits and sending personnel to established laboratories for training).

3. Quality Management Systems

The International Organization for Standardization (ISO) was formed in 1947 “to facilitate the international coordination and unification of industrial standards” (ISO, 2015). Since that time and for the foreseeable future, the need to document the performance of radionuclide metrology, especially in its applications for the user community, remains a high priority for measurement laboratories. For example, the National Institute of Standards and Technology (NIST) QMS for ionizing radiation describes the structure of the quality management of, and procedures for, calibration services and reference material production (NIST, 2014). The QMS of the NIST satisfies the requirements for confidence in measurements specified within the CIPM MRA. The NIST QMS incorporates the requirements of ISO/IEC 17025 (General requirements for the competence of testing and calibration laboratories) and ISO Guide 34 (General requirements for the competence of reference material producers). The NIST QMS is scoped to cover the suite of measurement services offered to customers: calibrations and reference materials. The ISO/IEC 17025 is commonly used as a conformity assessment standard by accreditors to address customers’ needs including for meeting regulatory requirements (an important consideration in radioactivity applications).

Despite the usefulness of an active QMS for customers and to document a level of measurement competence, the maintenance of a QMS does require an active commitment to implement and maintain. To demonstrate measurement traceability for example, which is a usual requirement, may necessitate participation in more interlaboratory comparisons than might otherwise be considered (either for practical or administrative reasons). Nevertheless, the efforts invested in establishing and maintaining a QMS not only provide customer confidence as to the system’s vitality (how well it works) and relevance (how well it works for the application), but also provides the laboratory with a mechanism to evaluate and improve methods.

4. International Comparisons: Key and Supplementary

Participation in international comparisons is recognized in the CIPM MRA as an important, though not unique, approach to validating calibration and measurement capability claims (CIPM MRA-D-04, 2013). Successful participation in international comparisons are also important demonstrations of the vitality of an institution’s QMS, and enable customers and other institutions to assess the applicability of a laboratory’s measurement capabilities; such comparisons form the basis for the determination of equivalency in the context of the CIPM MRA (CIPM MRA-D-05, 2014).

The Système International de Référence (SIR) supports a significant, on-going key comparison for radioactivity. It is a simple, highly stable system consisting of two commercial re-entrant ionization chambers filled with nitrogen at pressure (2.0 mPa; Ratel, 2007). In service at the BIPM since 1976, the SIR continues to provide a basis for the international comparison of more than 60 gamma-ray-emitting radionuclides, with plans to extend the scope to beta-- and alpha-particle-emitting radionuclides as indicated, both considered near-term actions (with alpha-particle emitting radionuclides to follow into the medium term) by the strategic plan of the CCRI (SIR, 2014). However, for very short-lived gamma-ray emitting radionuclides [such as for Tc-99m, T_1/2 = 6.0067 h; u = 0.0010 h (all nuclear data are taken from Bé et al., 2004)], submitting samples for measurement in the SIR is prohibited except for laboratories at the closest proximity to the BIPM. To address the need of a wide stakeholder community for measurements of such short-lived radionuclides, the working group established by the CCRI(II) to address this problem developed, at the BIPM, a transportable “transfer instrument” (a well-type NaI(Tl) crystal calibrated against the SIR and for which a ⁹⁴Nb reference source is used to monitor stability; Michotte and Fitzgerald, 2010), which could be located at different laboratories around the world to act as the “on-site” SIR for comparisons of short-lived gamma-emitting radionuclides (BIPM.RI(II)-K4.Tc-99m, 2015). Figure 1 shows the reach of this Tc-99m K4 (indicating the transportable SIR chamber) comparison beyond the geographically limited Tc-99m K1 (indicating the BIPM-located SIR system), and the closeness in degrees of equivalence achieved between the K1 and the K4. The expansion of the scope of this comparison to other short-lived radionuclides such as F-18 (T_1/2 = 1.8288; u = 0.0003 h) and even C-11 (T_1/2 = 20.370 min, u = 0.029 min), currently underway, will enable global international validation of measurements for these medically significant radioisotopes.

Figure 1.

Degrees of equivalence for Tc-99m, and world distribution of measurements. BIPM.RI(II)-K1.Tc-99m is the comparison of the activity of Tc-99m using the SIR located at the BIPM; BIPM.RI(II)-K4.Tc-99m is the comparison of the activity of Tc-99m using the traveling instrument and host laboratory. Refer to text for further detail.

With more than 150 different radionuclides of interest to various customers either for principal use (such as P-32 in biochemical research, Co-60 for measurement assurance for industrial processing, Y-90 for health applications, or Cs-137 used to track environmental contaminations) or as potential contaminants in radioactive samples (particularly important in medicine, where potential contaminants may lead to medical consequences), participation in interlaboratory comparisons beyond the SIR remains a key path to validating measurement capabilities and establishing degrees of equivalence. As part of its role in facilitating the validation of measurement traceability for radionuclide metrology and in defining the technical deliverables needed to achieve the goals of international radionuclide metrology, the CCRI(II) has provided guidance and has actively led international comparisons following a strategic approach to choose key comparisons more efficiently while allowing laboratories to optimize the number of comparisons which are needed to support their own and their customers’ needs. In addition to the development and use of the Measurement Methods Matrix (MMM; previously referred to as the “Generic Groupings Table,” as in Karam, 2007) to categorize and group radionuclides by radiation type and primary measurement method (along with the relative difficulty of measurement and expected range of uncertainty), the Key Comparison Working Group of CCRI(II) [KCWG(II)] first proposed a 10-year comparison plan for maximizing the coverage of radionuclides and primary measurement methods on the MMM through international comparisons to reflect compliance with the stipulations of the CIPM MRA (Table 2) (CCRI, 2007). Although the plan covers 10 years (from the date of the latest CCRI(II) meeting), years 3 through 10 are considered relatively flexible and, as circumstances (e.g., unavailability of material, unexpected stakeholder needs, etc.) determine, changes are made in a timely manner. For example, despite the original plan to run a key comparison for Cs-137 in 2015 (piloted by the NIST), availability (gratis) of source material for Ra-223 presented an unexpected opportunity to perform a key comparison for this radionuclide instead, meeting an immediate and recognized need of key stakeholders in the healthcare sector.

Table 2.

Status of 10-year Plan for International Comparisons in Radioactivity After the 2015 Meeting of CCRI(II)

Nuclide	Suitable for SIR	Suitable for ESIR	Year	Pilot Lab
68Ge/68Ga	YES		2014	NIST
222Rn	YES		2015	LNE-LNHB
223Ra	YES		2015/16	NPL
35S		YES	2016	PTB
109Cd	YES		2017	BIPM
229Th	YES		2018	To be determined
123mTe	YES		2019	To be determined
133Xe	YES		2020	LNE-LNHB
152Eu	YES		2021	To be determined
90Y		YES	2022	To be determined
241Am	YES	YES	2023	To be determined

While comparisons of the activity of non-complex samples (such as, for example, a single radionuclide in a solution of defined volume and well-known physical and chemical characteristics) may depend (mostly) on the physics of measurement (and the decay characteristics of the radionuclide being measured), a comparison of activity in a complex matrix (soils, biological tissues, etc.) will depend on not only the physics and decay characteristics, but also on the additional attributes of the matrix material itself (and its potential interactions with the measurand). This aspect is particularly an issue for secondary measurement techniques, such as gamma-ray spectrometry of whole samples, which are important for a variety of applications (and many stakeholders). Although the CCRI(II) has already accepted supplementary comparisons of these types of measurements (Outola, et al., 2008; Shakhashiro and Sansone, 2009), which have often been run for the development of a reference material and not particularly to demonstrate measurement capability, the issue of “coverage” (in other words, extent of applicability) of participation in such a supplementary comparison to measurements in other types of samples remains. Regional representatives who are members of the Regional Metrology Organization Working Group on Calibration and Measurement Capabilities of the CCRI (CCRI RMO WG CMCs), which covers topics not only affecting Section II (radionuclide metrology) but also Sections I (x and gamma rays, charged particles) and III (neutron measurements), have recently begun to consider a potential approach to optimize supplementary comparisons, patterned after the concept of the MMM, so that a single, appropriately organized comparison may serve to support claims for a collection (as opposed to a single) of sample types.

5. Final Thoughts

Since the signing of the CIPM MRA, the influence of the CCRI(II) on the activities of member institutions has become more significant. In addition to the growing membership (due to designations of ionizing radiation measurement laboratories by NMIs and DIs as national regulations begin to require metrological traceability and validated measurement capabilities for commerce and trade) in the radionuclide metrology community, the responsibilities of these measurement laboratories to maintain quality management systems and to participate activity in international comparisons have also expanded. Through strategic planning, and facilitated by expert advice (such as the recently published special issue of the journal Metrologia, which describes the evaluation of uncertainties in radionuclide metrology), the technical expertise of the CCRI(II) is enabling NMIs and DIs to meet the metrological needs for a wide variety of applications, supporting quality assurance and regulatory compliance around the world.