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Bureau International des Poids et Mesures The BIPM and the International Reference System (SIR) for activity measurements Bureau International des Poids et Mesures Decay Data Evaluation Project Workshop, 8 th October 2012 Laboratoire National de Mtrologie et dEssais, Paris 1 Slide 2 Bureau International des Poids et Mesures I. Presentation of the BIPM Bureau International des Poids et Mesures Decay Data Evaluation Project Workshop, 8 th October 2012 Laboratoire National de Mtrologie et dEssais, Paris 2 Slide 3 Bureau International des Poids et Mesures Some relevant dates for the International System of Units (SI) (1/2) 20 May 1875 : Establishement of the Metre Convention (Convention du Mtre); in 1899 the 1st CGPM sanctioned the international prototypes for the meter and the kilogram. Prototypes of the kilogram and the metre 3 Slide 4 Bureau International des Poids et Mesures 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 metrekilogramsecond 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). 4 Slide 5 Bureau International des Poids et Mesures 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 Svres 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. 5 Slide 6 Bureau International des Poids et Mesures BIPMs 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. 6 Slide 7 Bureau International des Poids et Mesures BIPMs 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 with international and intergovernmental bodies such 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 the geographical coverage of the Metre Convention amongst developing countries. 7 Slide 8 Bureau International des Poids et Mesures BIPMs main technical roles Maintain the kilogram - needed for the next 5/10 years until replaced, probably by Watt Balances or others means; Disseminate Co-ordinated Universal Time based on weighted averages of clocks from many National Metrology Institutes; Maintain unique world reference facilities e.g. SIR (ionizing radiation and isotopes), ozone spectrophotometers; Maintain travelling standards to 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 international comparisons and networks e.g. organic chemistry reference materials for laboratory medicine; Promote traceable, accurate measurement for physical, engineering, chemical and medical measurements worldwide. 8 Slide 9 Bureau International des Poids et Mesures SCHEME OF EQUIVALENCE OF NATIONAL MEASUREMENT STANDARDS THROUGH KEY COMPARISONS B I P M and Consultative Committee (CC) key comparison other regional key comparisons APMP key comparisons SIM EUROMET 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 9 Slide 10 Bureau International des Poids et Mesures 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. 10 Slide 11 Bureau International des Poids et Mesures 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 worlds NMIs and designated institutes that shows National capabilities and differences between National Standards more than 25,000 CMC entries already; www.bipm.org 11 Slide 12 Bureau International des Poids et Mesures CIPM MRA (3/3) To have its calibration and measurement capabilities (CMCs) validated by others; To take part in key comparisons that give confidence in day-to day measurements at the NMIs; To implement and allow others to understand its quality/management systems. TO MEET THE REQUIREMENTS OF THE MRA, AN NMI WILL NEED: 12 Slide 13 Bureau International des Poids et Mesures BIPM current activities (1/3) 13 Slide 14 Bureau International des Poids et Mesures 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. 14 Slide 15 Bureau International des Poids et Mesures BIPM current activities (3/3) IONIZING RADIATION: x and 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 worlds 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. 15 Slide 16 Bureau International des Poids et Mesures II. The International Reference System (SIR) for activity measurements Bureau International des Poids et Mesures Decay Data Evaluation Project Workshop, 8 th October 2012 Laboratoire National de Mtrologie et dEssais, Paris 16 Slide 17 Bureau International des Poids et Mesures 1.Recall of the Rle 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 17 Slide 18 Bureau International des Poids et Mesures 2.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 : 54 Mn, 65 Zn, 85 Kr, 89 Sr, 109 Cd, 125 I (2x), 133 Ba, 137 Cs, 152 Eu, 192 Ir, 204 Tl. The International Reference System (SIR) for activity measurements 18 Slide 19 Bureau International des Poids et Mesures 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 Slide 20 Bureau International des Poids et Mesures 3.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 [longlived 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 Slide 21 Bureau International des Poids et Mesures 3. International Reference System : Presentation Drawbacks : all 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 Slide 22 Bureau International des Poids et Mesures 3.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 Slide 23 Bureau International des Poids et Mesures 3.International Reference System : Operating procedure The collected current for the radionuclide i I i I f = CnU / t i is compared to the current obtained for a Ra source (T 1/2 = 1600(7) a) I Ra I f = CnU / t Ra. The leakage current amounts typically to about 100 fA. The International Reference System (SIR) for activity measurements 23 Slide 24 Bureau International des Poids et Mesures 3.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 t i t Ra. 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 Slide 25 Bureau International des Poids et Mesures 3.International Reference System : Operating procedure Introducing the concept of equivalent activity, A e, 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 Slide 26 Bureau International des Poids et Mesures 3.International Reference System : Operating procedure with t 0, reference time for the radium sources: 01-01-1976, t r, reference time for the sample from the laboratory, t m, 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 highestactivity radium source. The International Reference System (SIR) for activity measurements 26 Slide 27 Bureau International des Poids et Mesures 3.International Reference System : Corrections for Impurities For every measured radionuclide, i, which emits s detected in the chamber, the collected current has the expression or, in the presence of several impurities The International Reference System (SIR) 27 Slide 28 Bureau International des Poids et Mesures 3.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 Slide 29 Bureau International des Poids et Mesures 3.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 Slide 30 Bureau International des Poids et Mesures 3.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 Slide 31 Bureau International des Poids et Mesures 3.International Reference System : SIR Efficiency Curve The detection efficiency becomes or with The International Reference System (SIR) for activity measurements 31 Slide 32 Bureau International des Poids et Mesures 3.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 Slide 33 Bureau International des Poids et Mesures 3.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 Slide 34 Bureau International des Poids et Mesures 3.International Reference System : SIR Efficiency Curve Pure emitters are relatively seldom. Generally the samples are decaying at the same time by 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 Slide 35 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 35 Slide 36 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 36 Slide 37 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 37 Slide 38 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 38 Slide 39 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 39 Slide 40 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 40 Dimensions of the radium sources : L = 12 mm = 1,5 mm 40 Slide 41 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 41 Slide 42 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 42 Slide 43 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 43 Slide 44 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 44 Slide 45 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 45 Slide 46 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 46 Slide 47 Bureau International des Poids et Mesures Practical operation of the SIR 47 Slide 48 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 48 Slide 49 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 49 Slide 50 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 50 Slide 51 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 51 Slide 52 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 52 Slide 53 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 53 Slide 54 Bureau International des Poids et Mesures The International Reference System (SIR) for activity measurements 54 N.B. The right-hand axis gives approximate relative values only 54 Slide 55 Bureau International des Poids et Mesures The BIPM and the International Reference System (SIR) for activity measurements 55 Thank you for your attention 55 Slide 56 Bureau International des Poids et Mesures The BIPM and the International Reference System (SIR) for activity measurements 56