The BIPM and the International Reference System (SIR) for activity measurements

Bureau International des Poids et Mesures Decay Data Evaluation Project Workshop, 8th October 2012 Laboratoire National de Métrologie et d’Essais, Paris The BIPM and the International Reference System (SIR) for activity measurements

Bureau International des Poids et Mesures Decay Data Evaluation Project Workshop, 8th October 2012 Laboratoire National de Métrologie et d’Essais, Paris I. Presentation of the BIPM

Some relevant dates for the International System of Units (SI) (1/2)…  20 May 1875 : Establishement of the Metre Convention (Convention du Mètre); in 1899 the 1st CGPM sanctioned the international prototypes for the meter and the kilogram. Prototypes of the kilogram and the metre

Some relevant dates for the International System of Units (SI) (2/2)…  1901 : Giorgi showed that it is possible to combine the mechanical units of this metre–kilogram–second system with the practical electric units to form a single coherent four-dimensional system by adding to the three base units, a fourth base unit of electrical nature, such as the ampere or the ohm, and rewriting the equations occurring in electro-magnetism in the so-called rationalized form.  1946 : the adoption of a four-dimensional system based on the metre, kilogram, second and ampere are endorsed by the CIPM.  1954 : The 10th CGPM approved the introduction of the ampere, the kelvin and the candela as base units.  1960 : The name International System of Units (SI) is given to the system by the 11th CGPM.  1971 : At the 14th CGPM the number of base units is brought to 7 by adding the mole (unit for amount of substance).

Metre Convention  An intergovernmental treaty which, inter alia, established the BIPM;  Affirmed international support for the International System of Units- the SI;  Created the General Conference of Weights and Measures which meets every 4 years and is responsible for all major decisions affecting the SI, the BIPM and links with other intergovernmental bodies etc;  BIPM is on an international territory at Sèvres with 76 staff ;  €12.5 M (2012) budget funded by 56 full Members States and 37 Associates of the General Conference of of Weights and Measures.

BIPM’s Mission THE ROLE OF THE BIPM IS WORLDWIDE UNFORMITY OF MEASUREMENT. It achieves this through providing the necessary scientific and technical basis for such uniformity and by collaborating with other institutions and organisations that have related missions.

BIPM’s political role  Working with Governments, National Metrology Institutes, and the accreditation community so as to maintain confidence in the world measurement system for science and trade;  To act on behalf of NMIs and Member States in dealings withinternational and intergovernmental bodiessuch as the World Trade Organisation, World Meteorological Organisation, World Health Organisation, the International Federation of Clinical Chemistry, International Laboratory Accreditation Co-operation, International Bureau for Legal Metrology etc;  Extend thegeographical coverageof the Metre Convention amongst developing countries.

BIPM’s main technical roles  Maintain thekilogram - needed for the next 5/10 years until replaced, probably by Watt Balances or others means;  DisseminateCo-ordinated Universal Timebased on weighted averages of clocks from many National Metrology Institutes;  Maintain uniqueworld reference facilitiese.g. SIR (ionizing radiation and isotopes), ozone spectrophotometers;  Maintaintravelling standardsto compare fixed national references e.g. Arrays of Josephson Junctions for the volt, Quantum Hall devices for the ohm, graphite calorimeter for dosimetry quantities etc;  Coordinate internationalcomparisonsandnetworkse.g. organic chemistry reference materials for laboratory medicine;  Promote traceable, accurate measurementfor physical, engineering, chemical and medical measurements worldwide.

SCHEME OF EQUIVALENCE OF NATIONAL MEASUREMENT STANDARDS THROUGH KEY COMPARISONS EUROMET key comparisons SIM APMP key comparisons B I P M and Consultative Committee (CC) key comparison other regional key comparisons other regional key comparisons 250+ institutes participate in CC activities More than 810 key comparisons have been identified BIPM NMI participating in BIPM / CC key comparisons NMI participating in BIPM / CC key comparisons and in regional key comparisons NMI participating in regional key comparisons NMI participating in neither BIPM / CC nor regional key comparisons but making bilateral comparisons

CIPM MRA (1/3) In 1999 the CIPM developed an MRA between NMIs to address technical barriers to trade caused by lack of traceability and equivalence. Complying with the MRA means that an NMI calibration certificate is acceptable world-wide with a validated accuracy.

CIPM MRA (2/3)  Signed October 1999, by 38 NMIs and two international organisations; to date by 89 Institutes and three international organisations (IAEA, IRMM and WMO)  Covers ‘Metre Convention’ countries;  A new category of Associates to the General Conference on Weights and Measures (now 37) was created to involve other countries that were not (yet) full members;  Provides a data base covering the world’s NMIs and “designated institutes” that shows National capabilities and differences between National Standards – more than 25,000 CMC entries already;  www.bipm.org

CIPM MRA (3/3) TO MEET THE REQUIREMENTS OF THE MRA, AN NMI WILL NEED: To have itscalibration and measurement capabilities(CMCs)validated by others;  To take part inkey comparisonsthat give confidence in day-to day measurements at the NMIs;  To implement and allow others to understand itsquality/managementsystems.

BIPM current activities (1/3)

BIPM current activities (2/3) •  MASS: maintain the kilogram; provide copies for Member States; agree better ways of measuring air density corrections and surface contamination; •  TIME: TAI/UTC; two way time transfer and algorithms; collaborate with NMIs, GPS, Glonass and Galileo systems to harmonise world time. Issues of how to compare new cold atom and ion clocks- current time scales and satellite transfer methods are not good enough for air navigation requirements; •  ELECTRICITY: meet NMI needs for travelling and reference standards in voltage, resistance and capacitance.

BIPM current activities (3/3)  IONIZING RADIATION: x and γ reference chambers.WHO/IAEA dosimetry network; world reference system for radionuclides;  CHEMISTRY: reference network for atmospheric ozone monitors; new work in bio-analysis and organic chemistry with NMI “virtual” networks. MoU with WHO, WMO to set priorities;  KEY COMPARISON DATABASE: approved capabilities of the world’s NMIs;  METROLOGIA: editorial control of contracted out publication;  JOINT COMMITTEES: ISO; ILAC; developing country work; laboratory medicine; Regional Metrology Organisations;  KNOWLEDGE TRANSFER: workshops; unique calibrations/characterisations that do not duplicate NMI services; secondments/exchanges with NMIs; provide unique international experience for guest workers from NMIs;  NEW PROJECTS: Calculable Capacitor; Watt Balance.

Bureau International des Poids et Mesures Decay Data Evaluation Project Workshop, 8th October 2012 Laboratoire National de Métrologie et d’Essais, Paris II. The International Reference System (SIR) for activity measurements

1. Recall of the Rôle of the BIPM : to establish and warrant the uniformity of the measurements carried out in NMIs, to contribute to the improvement of the precision of measurements every time it is feasible and required by the applications, to develop and use new measurement techniques, maintain the perennial characteristics of measurements, initiate the dissemination of units. These objectives should be met by all BIPM Departments and in particular by the Ionizing Radiation Department. The International Reference System (SIR) for activity measurements

Measurements of Radionuclides : In the field of the measurements of radionuclides this is achieved by a regular organization of international comparisons of activity concentration measurements : since its establishment at the BIPM in 1961 the ionizing radiation department has organized 41 such comparisons, which represents an average of one comparison every 15 months. In the most recent comparisons the following radionuclides have been measured :54Mn, 65Zn, 85Kr, 89Sr, 109Cd, 125I (2x), 133Ba,137Cs,152Eu,192Ir, 204Tl. The International Reference System (SIR) for activity measurements 18

2. Measurements of Radionuclides : Most significant drawbacks : international comparisons of activity measurements are : very demanding to implement, time consuming, and requiring important financial and personal ressources. Consequently, international comparisons are: not very frequent, not accessible to all laboratories, only a few radionuclides can be measured. The International Reference System (SIR) for activity measurements 19

International Reference System : Presentation For all these reasons the International Reference System was established in 1976. Advantages : simple to use (measurement in ionization chambers), quick measurements, stability [long–lived reference sources (Ra) are used], precision of the results (about 0.1 %), robustness of the results. The International Reference System (SIR) for activity measurements 20

3. International Reference System : Presentation Drawbacks : all g emitters are not measurable, e.g. when the radioactive half-life is too short (less than a few days) or when the photons energy is too low, the measurements are relative, not absolute. Since the CIPM Mutual Recognition Arrangement (CIPM MRA) has been signed the SIR has become the cornerstone of the process enabling the evaluation of the degrees of equivalence for all laboratories participating in comparisons of activity measurements in view of establishing their capabilities in calibrations and measurements (CMCs). The International Reference System (SIR) for activity measurements 21

International Reference System : Operating procedure The International System of Reference is based on two commercial re-entrant well-type ionization chambers filled with 2 MPa nitrogen. The two chambers are operated with a negative polarization voltage of - 500 V. The measurements rely on the principle of the Townsend balance with stepwise compensation. They consist of measuring precisely the time necessary to charge a capacitor of well-known capacitance to a precisely defined constant voltage. The International Reference System (SIR) for activity measurements 22

International Reference System : Operating procedure The collected current for the radionuclide iIi – If = CnU / tiis compared to the current obtained for a Ra source (T1/2 = 1600(7) a) IRa – If = CnU / tRa. The leakage current amounts typically to about 100 fA. The International Reference System (SIR) for activity measurements 23

International Reference System : Operating procedure Five radium sources with increasing activity, in a geometrical progression of ratio 3 are available, with a total activity of 16.4 MBq. The radium source for the measurement is chosen so that titRa. This set of radium sources enables to cover an activity range for the measured radionuclides from a few hundred kBq to a few hundred MBq. The International Reference System (SIR) for activity measurements 24

International Reference System : Operating procedure Introducing the concept of equivalent activity, Ae, which is the radionuclide activity necessary to produce in the chamber a current equal to that produced by the radium source of the highest activity the ratio between the current produced by the radium source and that produced by the radionuclide can be written: The International Reference System (SIR) for activity measurements 25

International Reference System : Operating procedure with t0, reference time for the radium sources: 01-01-1976,tr, reference time for the sample from the laboratory,tm, measurement time at the BIPM, Fj,a constant, evaluated every year, to transform the measured current, obtained with the source of radium used to match the current of the sample, into the current which would have been produced by the highest–activity radium source. The International Reference System (SIR) for activity measurements 26

International Reference System : Corrections for Impurities For every measured radionuclide, i, which emits g’s detected in the chamber, the collected current has the expressionor, in the presence of several impurities The International Reference System (SIR) 27

International Reference System : Corrections for Impurities If the contribution to the current due to the main radionuclide, rad, is separated from those due to the impurities, i, the expression becomes or The International Reference System (SIR) for activity measurements 28

International Reference System : Corrections for Impurities The equivalent activity of the main radionuclide can then be written As a consequence the correction due to the impurities can only be taken into account if the equivalent activities for the impurities are known. The International Reference System (SIR) 29

International Reference System : SIR Efficiency Curve The detection efficiency of the chamber for a given radionuclide can be written in function of the emission probabilities and of the energies of the radiations detected as Under the hypothesis that the detection efficiency varies monotonously with the energy as The International Reference System (SIR) for activity measurements 30

International Reference System : SIR Efficiency Curve The detection efficiency becomesor with The International Reference System (SIR) for activity measurements 31

International Reference System : SIR Efficiency Curve The current produced in the ionization chamber by the sample has the expression As this current is proportional to the sample activity, the detection efficiency of the chamber is simply equal to The International Reference System (SIR) for activity measurements 32

International Reference System : SIR Efficiency Curve For convenience the constants K and K’ are so chosen that the energies are expressed in MeV and the activities in kBq; then the efficiency becomes Which leads, for the factor f, to the expression The International Reference System (SIR) for activity measurements 33

International Reference System : SIR Efficiency Curve Pure g emitters are relatively seldom. Generally the samples are decaying at the same time by b emission and the bremsstrahlung should also be taken into account. Therefore the general expression for the f factor becomes The International Reference System (SIR) for activity measurements 34

The International Reference System (SIR) for activity measurements 35

The International Reference System (SIR) for activity measurements Dimensions of the radium sources : L = 12 mm Ø = 1,5 mm 40

Practical operation of the SIR

The BIPM and the International Reference System (SIR) for activity measurements