News from the BIPM laboratories—2017

The Physical Metrology Department is significantly involved in actions required by the plans to move towards a new definition of the kilogram. The BIPM served as the pilot laboratory for the Consultative Committee for Mass and Related Quantities (CCM) Pilot Study of future realizations of the kilogram. The first objective of this study was to test the consistency of future realizations of the kilogram based on different Kibble balances and x-ray crystal density (XRCD) experiments. The second objective was to test the continuity between traceability to the present definition (the mass of the International Prototype of the Kilogram, IPK) and to the proposed future definition (the numerical value of the Planck constant). Participants were the LNE (France), the NIST (USA), the NRC (Canada), which used Kibble balances, and the NMIJ (Japan) and the PTB (Germany), which used spheres made of ²⁸Si from the International Avogadro Coordination. They calibrated 1 kg mass standards using their realization experiments and sent them to the BIPM for comparison. All measurements were carried out during 2016, the final report was published in June 2017 and a paper has been published in the Metrologia Focus Issue on the redefinition of the kilogram [1]. The results of four of the participants agreed within the standard uncertainties and one result agreed within the expanded uncertainty. The weighted mean of the five results has an uncertainty of 10 µg and is in good agreement with the calibration result traceable to the IPK.

The BIPM is participating in a comparison of calibrations of 1 kg stainless steel mass standards (EURAMET.M.M-K4) to link it to the comparison CCM.M-K4, which was organized from 2011 to 2012 by the BIPM. The measurements were carried out at the BIPM during November 2017, and the results are expected in the second half of 2018.

A new 1 kg Pt–Ir prototype has been delivered to the KRISS (Republic of Korea). Four national standards of Pt–Ir and eleven of stainless steel have been calibrated. After the redefinition of the kilogram, the BIPM will continue to provide mass calibrations, which will then be traceable to the reference value obtained through comparisons of primary realizations. The maintenance and dissemination of the kilogram following its redefinition is described in [2].

A new experimental set-up for the Kibble balance has been designed and assembled. It includes a more stable suspension facilitating the coil alignment and a stiffer mass loading and exchanger system, which will allow a mass of up to 1 kg to be used. The refined experiment is operational in both air and vacuum. It now works with a 1 kg mass and a velocity of 1 mm s⁻¹, both of which contribute to improving the signal-to-noise ratio. Two programmable Josephson voltage standards were successfully completed and are now routinely used to calibrate the DVMs. Preliminary measurements show a day-to-day repeatability of several parts in 10⁷. The type B uncertainty of a few parts in 10⁷ is still limited by the alignment and the voltage measurement. A detailed study was carried out to evaluate the effect of the coil-current on the magnetic field [3]. It has been demonstrated that this effect has to be taken into account at the present level of uncertainty of Kibble balances.

The ensemble of reference mass standards (ERMS) is now fully operational, with all mass standards stored in their specific environments: air, nitrogen, argon and vacuum. During the CCM Pilot Study, the mass standards were calibrated against the primary mass standards of the five participating National Metrology Institutes (NMIs).

An important achievement in the field of electrical metrology was the organization of two successful on-site comparisons of quantum Hall resistance (QHR) standards, with the CMI (Czech Republic) and the METAS (Switzerland). For these comparisons, the BIPM QHR equipment, including the cryostat with the magnet and the quantum Hall sample, a resistance comparison bridge and thermo-regulated resistors of 1 Ω, 100 Ω and 10 kΩ are transported to the participating institute. The measurands are the value of the 100 Ω resistor, measured using the BIPM and the NMI QHR standards and bridges, and the 1:100 ratios between the resistors, measured with the BIPM and NMI bridges. The comparison between the BIPM and the CMI showed a very good agreement with relative differences at the level of 1 part in 10⁹ and standard uncertainties of 2–3 parts in 10⁹ [4]. The results obtained with the METAS have not yet been fully analyzed but it is expected that they will show the same level of agreement.

The BIPM is the pilot laboratory for a comparison of 10 pF and 100 pF capacitance calibrations, CCEM-K4.2017. This is the first time a Consultative Committee for Electricity and Magnetism (CCEM) comparison has been organized using the 'star scheme': the seven participating NMIs calibrated a set of four of their own capacitors, sent them to the BIPM for comparison and verified the stability after the return of the standards. This scheme will allow the comparison to be completed much faster and it is more robust against potential transport problems. The comparison measurements at the BIPM are complete and all return measurements from the participants have been received.

The BIPM is supporting one of the first comparisons by GULFMET in the field of electricity; a comparison of Zener voltage standards, GULFMET.EM.BIPM-K11. The BIPM participates in the measurements to link this comparison to the BIPM series of comparisons BIPM.EM-K11, and determines the temperature and pressure correction coefficients of the travelling standards. Before the start of this comparison, the organizer, Dr Steven Yang from the SCL (Hong Kong (China)) was seconded to the BIPM for two months to share experience on the calibration of Zener standards using Josephson voltage standards. To underpin the uncertainties of the department's Zener voltage comparisons and calibrations the temperature and pressure coefficients of the BIPM standards have been re-evaluated for the first time in 15 years. To our knowledge this is the only long-term study of the evolution of these important correction coefficients. Changes of some of the temperature coefficients have been observed, whereas the pressure coefficients have remained stable.

The BIPM is preparing to extend the present comparison of dc Josephson voltage standards to ac signals. An experimental comparison of ac Josephson systems with the PTB has been carried out; the BIPM's programmable Josephson voltage standard is based on NIST technology. The results were encouraging with an uncertainty of better than 1 ppm at a frequency of 62.5 Hz. The uncertainty was limited by the ac source, requiring the development of a dedicated signal generator. This work will continue with the help of Dr Mun-Seog Kim, seconded from the KRISS for one year. Another secondee, Dr Norihiko Sakamoto from the NMIJ, will support the BIPM in finalizing the work on the calculable capacitor and the associated measurement systems.

In addition to its comparison programme for electrical quantities, the department has provided about 70 calibrations to NMIs in the field of resistance, capacitance and voltage.

A major objective for the Time Department in 2017 has been the improvement of the uncertainty of (UTC-UTC(k)), the result of the key comparison in time that gives traceability to the SI second to local realizations of UTC in the national institutes. This has been achieved by tackling different aspects of UTC computation that have an impact on the uncertainty: the statistical uncertainty of time transfer; the programme of repeated calibrations harmonized with the RMOs; and improvement of the algorithms.

Two time transfer techniques have been used for the time comparisons for UTC; all time laboratories operate equipment for tracking global positioning system (GPS) constellation satellites, a number of which also contribute with data from bi-directional time comparisons using telecommunications satellites (TWSTFT, see later). In addition GLONASS observations have been used depending on the availability of data. New satellite constellations provide partial satellite coverage, but can already be used in experiments, with the perspective of implementing multi-satellite system time transfer in the future (BeiDou in China and Galileo in Europe). The BIPM Time Department's staff will incorporate the new time transfer techniques, and develop the best methods for their use in time and frequency metrology.

The department's work on two-way time and frequency transfer (TWSTFT) has opened the possibility of incorporating software designed radio (SDR) receivers at most of the earth stations operated by 13 UTC contributing laboratories in North America, Europe and Asia. This is the result of the BIPM's coordination work within the consultative committee for time and frequency (CCTF) working group on two-way satellite time and frequency transfer (WGTWSTFT). The Time Department developed the strategy for the exploitation of the new data, and led a pilot experiment, which ran throughout the year and which concluded with the validation of the technique and the implementation of two links in UTC since November 2017. It was demonstrated that additional noise, with a diurnal signature, increasing the typical small noise of TW links can be significantly reduced by using the SDR. Investigations into the causes of the diurnal noise, as well as the development of an appropriate calibration strategy, will be the next step, in association with the LNE-SYRTE.

Work is under way to validate the use of BeiDou time transfer in UTC as part of a cooperative agreement between the BIPM and the NIM (China). The experiment involved the BIPM and seven institutes distributed throughout the world. It included studies over multiple-length baselines and the absolute calibration of BeiDou receivers. The knowledge gained from the pilot experiments will be used to develop calibration capacities at the BIPM and NIM. The pilot work is an important first step towards implementing multi-system time comparisons in UTC. A 30-day GPS-BeiDou comparison over the NIM-BIPM baseline found that time differences computed with BeiDou are consistent with those from GPS.

The calibration campaigns put in place in coordination with the RMOs have progressed as expected. The node laboratories in the regions (G1 laboratories) have been re-visited with BIPM calibration equipment as per the agreement to perform recalibrations every two years. A calibration uncertainty of 1.5 ns can be confirmed for these laboratories. The results of calibrations organized by the NMIs within the RMOs have been reported and integrated into the UTC system. Results of GPS calibrations are available on the BIPM website.

Further improvements in the uncertainty of (UTC-UTC(k)) will require changes to the underlying algorithm. The current uncertainty values, as published in Circular T, strongly depend on the time link uncertainties. All the time links connect each contributing laboratory to the PTB and the uncertainty of PTB is underestimated. At present, no correlations are taken into account in the uncertainty propagation algorithm. The algorithm for the calculation of the uncertainties of (UTC-UTC(k)) has been refined to take the correlations in uncertainty propagation into account. The algorithm was validated by the CCTF in June 2017 and the first step of its implementation is scheduled for early 2018.

BIPM Circular T continues to be published monthly, giving traceability to the SI second via UTC to its local realizations in national laboratories. It is the most frequent key comparison, with one evaluation of the key comparison reference value UTC and the degrees of equivalence (UTC-UTC(k)) every five days for the 77 participants that together contributed data from about 500 atomic clocks in 2017.

The algorithm for the computation of rapid UTC (UTCr) has been upgraded and rendered fully consistent with that of UTC. This upgrade contributed to making UTCr a more reliable tool to help institutes check the steering of their local UTC realizations as well as the steering of the Global Navigation Satellite Systems' times to local representations of UTC. Regular publication of rapid UTC (UTCr) continued in 2017.

The Time Department is organizing a training course within the framework of the BIPM CBKT programme on 'Effective participation in UTC'. The course is supported by METAS and is scheduled to take place in February 2018. About twenty institutes will participate and the course will be open to RMO technical chairpersons in the time and frequency field. The aim of the course is to transfer knowledge on how to work effectively with receivers and equipment during a calibration campaign.

The BIPM has established a new comparison service for primary standards for high-energy photon radiotherapy, in response to the growing use of accelerators for cancer therapy. The service has been set up at the DOSEO facility, a specialist radiotherapy and medical imaging research centre at Gif-sur-Yvette, France. The beam has been characterized in terms of absorbed dose rate to water using the BIPM primary standard and NMI standards of absorbed dose to water can now be compared with the BIPM standard at this facility (BIPM.RI(I)-K6). The first such comparison took place with the KRISS in November 2017 and the results are expected to be published in 2018.

These high-energy photon beams are used to treat cancers deep in the body; lower energy x-rays (100 kV–250 kV) are used to treat tumours located in the first few cm of tissue. A number of NMIs use water calorimeter standards to determine absorbed dose to water in this energy range and there is a growing need for comparisons of these standards. The BIPM has therefore developed an ionometric primary standard to provide a robust and stable long-term reference for x-rays in this energy range. The standard will be used to make indirect comparisons of NMI standards through the calibration of waterproof transfer chambers in a water phantom; the standard is based on the free-air standard for air kerma and the method uses transfer ionization chambers and detailed Monte Carlo simulations to convert from air kerma to absorbed dose to water. By using this method rather than existing protocols, the uncertainty of the reference absorbed dose rate in the clinic is reduced from 2%–3% to 0.7%.

A member of the Department's staff has co-authored a text book [5] to replace one of the major texts in the field of ionizing radiation dosimetry. This comprehensive book includes chapters on radiation quantities, Monte Carlo methods, theoretical dosimetry and primary measurement standards. It also addresses absorbed dose determination for radionuclide therapy.

Thirteen dosimetry comparisons were undertaken by the Ionizing Radiation Department during 2017 and thirty-six calibrations of national secondary dosimetry standards were carried out for nine NMIs and the International Atomic Energy Agency (IAEA).

The cornerstone of the BIPM's work in radionuclide metrology is high-stability ionization chambers—the 'Système Internationale de Référence (SIR)', which was established in 1976 and now covers 68 radionuclides. Comparisons were carried out for six radionuclides during 2017 and updates to Key Comparison Reference Values for nine radionuclides were agreed by the Consultative Committee for Ionizing Radiation (CCRI). A transportable version of the SIR (SIRTI) was used for comparisons of short-lived radionuclides (^99mTc, ¹⁸F, ⁶⁴Cu and ¹¹C at the NRC and ^99mTc, ¹⁸F and ⁶⁴Cu at ANSTO (Australia)). These radionuclides are used for medical imaging; the comparison of ¹¹C was the first time that this radionuclide had been measured on the SIRTI.

The use of beta- and alpha-emitting radionuclides is expanding in radio-immunotherapy. Work has therefore continued on extending the SIR methodology to cover beta-emitting radionuclides; extensive validation studies were carried out on a possible technique (in collaboration with NIST and NIM (China)) but the results were inconclusive. This work will continue in 2018, with the establishment of a similar system for alpha-emitters to follow.

Twenty visiting scientists from NMIs (on secondment to the BIPM from between three to eighteen months) participated in the Chemistry Department Programme in 2017. Four comparisons were run by the department in this period, involving ninety three participations in these studies by NMIs. Five comparison reports were published as well as five papers in peer reviewed journals. The Capacity Building and Knowledge Transfer (CBKT) programmes for 'Metrology for Clean Air' and 'Safe Food and Feed' continued, attracting twelve visiting scientists from NMIs, spending between 3 months and 1 year at the BIPM.

Three visiting scientists undertook the Metrology for Clean Air Course (NPL (UK), LNE and NPLI (India)) on FTIR Measurements on Gas Standards (NO₂, HCHO, HNO₃, CO₂) in the BIPM laboratories, and were trained on the use of B-FOS software for use with FTIR in gas metrology applications, with the software being made available later for use within the participating NMIs.

The second meeting for the CBKT programme for 'Metrology for Safe Food and Feed', focusing on mycotoxin metrology and standards, was held at the BIPM in April 2017. The laboratory programme on Mycotoxin Standards was supported by three visiting scientist from NIM (China) in related structure impurity analysis and calibration solution characterization and from UME (Turkey) and INMETRO (Brazil) in qNMR analysis of pure mycotoxin materials. In addition, five visiting scientists from INMETRO, INTI (Argentina), KEBS (Kenya), NIMT (Thailand) and NMISA (South Africa) undertook three-month training secondments on mycotoxin calibration solution production, characterization and value assignment. The training programme was performed on zearalenone (ZEN). Pure materials for aflatoxin B1 and have been characterized, and are available for future training programmes and comparisons.

Within the BIPM's small organic primary calibrator programme, the final report of the CCQM-K55.d comparison on folic acid purity was approved by the CCQM Working Group on Organic Analysis (OAWG). Measurements for the CCQM-K78.a comparison on multi-component amino acid calibration solutions were completed, and the results presented to the working group, with the Draft A report in preparation. The BIPM submitted results to the CCQM-P150.b comparison on qNMR, as well as completing characterization of samples and their homogeneity and stability for the CCQM-K148.a comparison (bisphenol A calibrator purity).

The first reference data document on 'Internal Standards for qNMR' was completed for maleic acid, and will be published on the BIPM website. This was an output of the universal calibrator programme for qNMR at the BIPM, an activity initiated together with the NMIJ, and supported in 2017 by secondees from the NMIJ, and INMETRO (Brazil), with characterization of the performance of four standards in three different solvents.

Activities to support comparability of primary standards for peptide and protein analysis have continued. The final reports of the key comparison and associated pilot study on C-peptide purity (CCQM-K115/P55.2) were published in Metrologia and the BIPM KCDB. A paper on the evolving calibration hierarchies for C-peptide measurements was published in Clinical Chemistry [6]. Preparative work for the next comparison of peptide calibrant value assignment facilities, focusing on oxytocin (CCQM-K155.b) in collaboration with NIM (China) has continued with the secondment of a visiting scientist from NIM. Methods for pure peptide characterization to be used in future comparisons have been studied during secondments by visiting scientists from the LGC (UK), studying BNP calibrators, and from the NIBSC (UK) studying short-peptide tryptic digest calibrators. The methods used to characterize pure peptide calibrators were published in Analytical and Bioanalytical Chemistry [7], Journal of Chemical Metrology [8] and Trends in Analytical Chemistry [9].

In the area of air quality measurement standards, the BIPM continued to contribute to the CCQM-GAWG Ozone Cross-Section Task Group, organizing the review of input data by the group and drafting of the first version of the paper summarizing the recommended best value and uncertainty for the ozone cross-section to be used in the key comparison BIPM.QM-K1. Six NMIs: VSL (the Netherlands), NPL, ISCII (Spain), INE (Mexico), NIST and CHMI (Czech Republic)) sent their ozone standards to the BIPM and participated in BIPM.QM-K1, with two reports of the comparison published in Metrologia and the BIPM KCDB. Collaboration with the NIST on the upgrade of the electronic module for the Ozone SRP continued, with a prototype electronics module successfully constructed and tested at the BIPM, and the components and design for the final version agreed. The final report of the CCQM-K90 comparison on formaldehyde in nitrogen standards at 2 µmol mol⁻¹ was completed and published in Metrologia and the BIPM KCDB. Measurements on 24 standards from NMIs submitted to the BIPM as part of CCQM-K137 (NO in N₂ at 30 µmol mol⁻¹ and 70 µmol mol⁻¹) were completed, with standards returned to participating NMIs for stability assessment. The protocols for the comparison of NO₂ in N₂ standards at 10 µmol mol⁻¹ (CCQM-K74.2018) and the pilot study of HNO₃ measurements in such standards (CCQM-P172) were agreed with the CCQM Working Group on Gas Analysis (GAWG), with 14 NMIs electing to participate in the comparisons.

In the area of greenhouse gas standards, measurements on 46 standards of CO₂ in air submitted for the CCQM-K120 comparison were completed at the BIPM, including Fourier transform infrared spectroscopy (FTIR), isotope ratio infrared spectrometer (IRIS) and gas chromatography with a flame ionization detector GC-FID methods for mole fraction and isotope ratio value assignment' and the Draft A report of the comparison prepared. The method developed for measuring isotopic abundances in CO₂ with optically based instruments was published in Analytical Chemistry [10], and presented to the WMO-IAEA GGMT experts meeting in Switzerland in September 2017. This demonstrated the measurement standards and methods that could be used for such instruments in the future. Development and validation of a manometric system for CO₂ measurements, in support of a future planned ongoing comparison of CO₂ standards (BIPM.QM-K2), has progressed with a second secondment from the NIST. The first all-glass prototype was replaced with a coated stainless steel version, with much improved mechanical stability. The measurement protocol was optimized, including the identification and elimination of biases caused by trace gases within the system: new results are expected in early 2018. Preparation for a comparison on CO₂ isotope ratio standards, coordinated jointly by the BIPM and the IAEA continued, with an IRIS system for isotope ratio measurements integrated into the SIRM-GEN facility and the first blending experiments for CO₂ gases have been carried out. Validation work on optical tuneable diode laser spectroscopy (TDLS) and gas chromatography with electron capture detection (GC-ECD) systems in preparation for CCQM-K68.2019 (N₂O in air, ambient level) were undertaken during a 3 month secondment by a visiting scientist from KRISS, with linearity and measurement uncertainty of the optical instrument verified.