News from the BIPM laboratories—2013

In December 2013 the Mass Department started a campaign of Extraordinary Calibrations against the international prototype of the kilogram (IPK). This campaign will meet one of the prerequisites for the redefinition of the kilogram requested by the Consultative Committee for Mass and Related Quantities (CCM). It will enable the mass standards used in the watt balance and x-ray crystal density (XRCD) experiments, which will contribute to fixing the numerical value of the Planck constant, and the BIPM reference and working standards, to be compared as directly as possible with the IPK. In the first phase, the BIPM working standards will be calibrated with respect to the IPK. Subsequently, the effect of cleaning and washing a subset of the 'temoins' ('witnesses') and finally the IPK itself will be studied. In the second phase, which will start in the second half of 2014, transfer standards from NMIs involved in determinations of the Planck constant and the Avogadro constant will be calibrated. This work has been planned in discussion with a number of members of the CCM meeting as a support group chaired by the Director of the BIPM.

Significant progress has been made with the BIPM Ensemble of Reference Mass Standards (ERMS) in 2013. Experimental improvements have been implemented in each of the three storage networks (gas, vacuum and air). As an example, a new vacuum network has been set up and is now operational and specific mass containers have been designed and fabricated to be compatible with the vacuum weighing facilities available in the Mass Department.

A new project related to surface analysis of the ERMS mass standards has started. Inside each container that will accommodate one of the standards, six samples of 1.5 mm thickness and of the same material as the standard will be stored. The samples will undergo periodic surface analysis which will be carried out through partnerships with selected NMIs.

A successful collaboration began in 2013 between the BIPM and the Laboratoire Commun de Métrologie (LCM) LNE-CNAM, St Denis, France, to measure the surface roughness of all the mass standards and two discs of each stack from the BIPM Ensemble. The collaboration will also measure the surface roughness of many of the samples. Surface roughness is a critical parameter that influences surface contamination and consequently mass changes. By the end of 2013 the LCM LNE-CNAM will have made measurements on six 1 kg standards, two discs of one stack and eight samples. This collaboration will continue in 2014.

The BIPM watt balance [1] was transferred to a new dedicated laboratory in early 2013. The new location provides better vibration isolation, improved temperature stability and enough space for a vacuum enclosure. In addition, some experimental improvements such as a reduction of the influence of polarization and frequency mixing between the interferometer arms and a mathematical treatment of the effects of coil accelerations on the force measured by the weighing cell have been carried out. As a consequence, the standard deviation of measurements of the Planck constant in air was reduced from 1.4 parts in 10⁶ to about 5 parts in 10⁷.

The definitive magnetic circuit for the watt balance has been fabricated and assembled. It is based on an original BIPM design which has recently been applied to the construction of magnets for several other watt balances. Most of the parts were pre-machined by the BIPM workshop. Fabrication of the remaining components and the ultra-high precision machining of all parts has been undertaken by a company in the USA. Assembly was carried out at the BIPM with micrometre accuracy. The magnet has now been installed in the vacuum chamber of the watt balance. This is the first step in the assembly of a new version of the BIPM watt balance which has several new components, the most important of which is a device based on piezo-elements for the accurate control of the coil position and angle.

The Mass Department has calibrated six platinum–iridium prototypes and eleven stainless steel mass standards for Member States as part of its ongoing work programme. One prototype was characterized for its vacuum-to-air transfer behaviour and several new prototypes are being fabricated. Since June 2011, the Mass Department has organized a CCM comparison (CCM.M-K4) of stainless steel mass standards with 16 NMIs. This comparison is now in the Draft A stage.

BIPM Circular T is published monthly throughout the year, giving traceability to the SI second via Coordinated Universal Time (UTC) to its local realizations in national laboratories. It represents the only key comparison on time CCTF-K001.UTC. 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 73 participants at the end of 2013. These participants together contribute data from more than 400 atomic clocks. The BIPM received full responsibility for the calculation and publication of results from the Consultative Committee for Time and Frequency (CCTF).

Nine primary frequency standards and one secondary representation of the second contributed to International Atomic Time (TAI) in 2013, with an average of three to four caesium fountains reporting measurements each month. The frequency stability of TAI is estimated to be 3 parts in 10¹⁶ for averaging times of one month, and its frequency accuracy is in the low 10⁻¹⁶ range. A long-interval estimation of the stability of TAI indicates that it will decrease to 1.8 parts in 10¹⁶ over the next few years. Additionally, the procedure for including secondary frequency standard data in the accuracy algorithm of TAI was developed in the Time Department, and since July 2013 the rubidium secondary standard from LNE-SYRTE, France, has been used for assessing the frequency of TAI.

Observations of Global Positioning System (GPS) and the Russian Global Navigation Satellite System (GLONASS) satellites together with the Two-Way Satellite Time and Frequency Transfer (TWSTFT) technique have been used regularly in the calculation of TAI. Combined links are used regularly in the calculation; at the end of 2013 the combinations GPS/GLONASS and TW/GPS PPP serve to calculate 28% of the links in TAI. The GPS Precise Positioning Technique (GPS PPP) alone or in combination with TWSTFT is in use for TAI clock comparisons in 55% of the links, where the statistical uncertainty of time transfer is well below the nanosecond; the best value is 0.3 ns for 46% of the time links.

There were several major achievements in the Time Department in 2013. Revision of the algorithm for the calculation of TAI/UTC was completed, studies on a new clock weighting procedure were concluded based on the principle that a good clock is a predictable clock, rather than a stable clock as in the current procedure. A change in the strategy for fixing the upper limit of the weight has been identified. The result is a better distribution among the clock weights, with a reduction of the predominance of the caesium clocks and a 40% increase of the H-masers at the maximum weight. Both short- and long-term stability of TAI will improve by 20%. The new algorithm will officially come into use at the beginning of 2014.

After the successful completion of a pilot experiment on the calculation of a rapid UTC (UTCr), the CCTF has approved a final report which demonstrated that UTCr reached the expected quality, providing a weekly solution that was consistently better than ±2 ns peak to peak with the final UTC published in Circular T. This publication impacts on the quality of the representations of UTC in national laboratories, and on the steering of the Global Navigation Satellite Systems times to UTC.

Development of optical fibre links has begun between several laboratories contributing to the computation of UTC, and a few are already computed on a regular basis. The Time Department conducted an experiment on the fibre link between representations of UTC in Poland and the newly developed BIPM calibration system. This exercise demonstrated excellent agreement between the GPS PPP link and the optical fibre link at the level of the GPS PPP uncertainty. The optical fibre link can also be used to assess the calibration of a 'traditional' UTC link as a result of the small (hundred picosecond) and stable calibration uncertainty; the experiment validated the BIPM GNSS calibrating system with u_B ∼ 200 ps. In the future, 100 ps uncertainty could be attainable with optical fibre time links for UTC.

International coordination is a major activity in the Time Department. A workshop that was jointly organized by the BIPM and the International Telecommunication Union (ITU) was held in Geneva, Switzerland, on 19–20 September 2013. The workshop was in preparation for the World Radiocommunication Conference 2015, where a decision is expected on the redefinition of UTC without leap second adjustments. The meeting provided a unique opportunity to obtain input from most of the relevant communities, among them the four GNSS providers, the telecommunications sector, time stamping authorities, astronomers and geodesists.

The Electricity Department work programme in 2013 focused on the comparison programme to validate national primary standards for fundamental electrical quantities (voltage, resistance and capacitance), conducting calibrations for the same quantities for the NMIs of Member States, and support for the BIPM watt balance.

Dr Stéphane Solve from the Electricity Department worked as a guest researcher with the Quantum Voltage Project group of the NIST in Boulder, USA, from July 2012 until July 2013. During this time he was trained in the use of superconductor–normal metal–superconductor (SNS) arrays in the field of synthesis of ac voltages and their applications, in preparation for a future BIPM comparison in this field. A travelling programmable Josephson voltage standard (PJVS) with compact electronics has been designed for the BIPM under a cooperative research and development agreement. This new system has been compared against a second PJVS system and in this work the smallest uncertainty achieved to date when comparing PJVS standards has been obtained. The difference was found to be 2.6 parts in 10¹¹, with an uncertainty of 3.4 parts in 10¹¹. Several error sources have been investigated during the course of this work. The dominant effect is due to leakage resistances, which allow small currents to flow from components of the measurement circuit directly to ground. The effect was studied by using a technique that had previously been developed and proven by the BIPM. This effect can lead to systematic errors at the level of 1 nV at 10 V, that is 1 part in 10¹⁰. The work also demonstrated that trapped magnetic flux can lead to small systematic and reproducible voltage errors, which are difficult to detect. Details are described in a joint BIPM–NIST publication [2].

The Consultative Committee for Electricity and Magnetism (CCEM) has requested the BIPM to resume the BIPM.EM-K12 on-site comparison of quantum Hall resistance standards. In November 2013 a team from the BIPM carried out the first of a new series of on-site comparisons at the PTB, Braunschweig, Germany. This comparison tested the equipment and techniques used to realize the ohm from the quantum Hall effect (QHE). The BIPM and the PTB measured the same 100 Ω resistor with completely independent measurement systems and QHE references, and obtained agreement to better than 1 nΩ/Ω. Subsequently, the scaling of resistance values across four decades was tested by measuring 1 Ω and 10 kΩ standards. Similarly, excellent agreement was found at 10 kΩ, whereas at 1 Ω the uncertainty of the comparison was several times larger due to non-ideal properties of the 1 Ω reference resistor, which is a known problem for low value standards. The results at all values are substantially better than could be obtained by exchanging travelling standards, where the relative uncertainties are at best 1 or 2 parts in 10⁸.

Advances in cryogenic current comparator (CCC) bridge technology since the last series of on-site QHE comparisons (five were carried out in the period 1993–1997) have allowed a better investigation of limiting factors, in particular the problems due to Peltier heating in 1 Ω standards. Beyond the main comparison result, bringing two systems together in the same laboratory has provided useful information to both laboratories and will contribute to the optimization of the protocol for future on-site comparisons. After this successful first exercise at the PTB, there will now be a series of comparisons at NMIs around the world over the next few years.

The Ionizing Radiation Department has coordinated the sixth comparison in the series BIPM.RI(I)-K6 for absorbed dose to water in high-energy photon beams, with the NPL in the 6 MV, 10 MV and 25 MV beams of the Elekta accelerator, from 16 September to 4 October and 18 to 22 November 2013. Reports of two previous comparisons in the series were published in 2013, with the NIST and the LNE–LNHB, France. The report of the comparison carried out at the ARPANSA, Australia, in 2012 is close to completion. The method for establishing degrees of equivalence for the BIPM.RI(I)-K6 series, based on the BIPM calorimetric determination in terms of the Tissue Phantom Ratio (TPR_20,10), was agreed at the meeting of the Consultative Committee for Ionizing Radiation (CCRI) in May 2013 and was published [3]. The four comparison results that have been registered in the KCDB so far agree to better than 0.8% within the expanded uncertainties (typically 1.2%, k = 2).

The project to develop an absorbed-dose standard for medium-energy x-rays has continued. It is based on the existing free-air chamber standard using a set of waterproof transfer standards of different wall materials and shape. Initial problems with leakage currents have been overcome by using a conductive graphite coating. The experiment to test the photon interaction cross-section data (μ-values) used by the simulation code PENELOPE continued with improved calculations in the context of absorbed-dose for medium energy x-rays. It is worth noting that the calculated ratios μ_a,c and μ_a,w agree with their measured values at the level of around 5 parts in 10³ (the statistical uncertainty), except at 100 kV where a difference of 1 to 2 parts in 10² is observed.

A paper has been submitted to Physics in Medicine and Biology on the W_a value for accelerator photon beams, which incorporates data from the BIPM.RI(I)-K6 comparisons, Monte Carlo calculations and cavity volume measurements of ionization chambers constructed at the BIPM. The paper concludes that there is no evidence for an energy variation for W_a, an important finding that will reduce the uncertainty of high-energy clinical reference dosimetry. A by-product of this work is a new determination of the I_c -value for graphite. These results will be incorporated into a report on Key Data being prepared by the International Commission on Radiation Units and Measurements (ICRU). Four comparisons, 42 calibrations of national secondary standards and eleven comparison reports were produced, all of which were underpinned by a significant effort in equipment calibration and maintenance.

During 2013, as part of the radionuclide measurements programme, the BIPM received seven ampoules filled with seven different radionuclides from four laboratories, i.e. one ampoule each containing ⁵⁷Co (POLATOM RC), ⁵⁹Fe (LNE–LNHB), ¹⁰⁹Cd (LNE–LNHB), ¹³⁷Cs (POLATOM RC), ^166mHo (PTB), ¹⁷⁷Lu (IFIN-HH) and ²²²Rn (LNE–LNHB). All the submissions had been made to generate equivalence values in the associated ongoing BIPM key comparison BIPM.RI(II)-K1. Measurements of ²²²Rn, a radioactive gas with a short half-life (T_1/2 = 3.8235 d, u = 0.0003 d), were repeated to try to remove a systematic bias detected in the activity determination of the gas. The analysis is ongoing. Updated reports of three comparisons were published in the Metrologia Technical Supplement covering ⁶⁴Cu [4], ¹³⁴Cs [5] and ¹³⁷Cs [6] including the linked COOMET.RI(II)-K2.Cs-137 comparison.

For short-lived radionuclides, the BIPM report [7] describing the SIR Transfer Instrument (SIRTI) in detail and reporting the tests made during its development has been finalized and published. The BIPM.RI(II)-K4.Tc-99m (T_1/2 = 6.0 h) key comparison using the SIRTI is running at a rate of two comparisons per year: the LNMRI-IRD, Brazil, and the IFIN-HH, Romania, participated in 2013. Results of the comparisons in China, Argentina and Brazil have been published. The VNIIM, Russian Federation, is the next scheduled participant. The SIRTI has remained very stable since 2007 even though it is regularly transported around the world, showing a relative standard deviation of 6 × 10⁻⁵ for the counting rate of the ⁹⁴Nb reference source.

The extension of the SIRTI for measuring ¹⁸F (T_1/2 = 1.8 h) is in development. Stability and reproducibility tests were carried out successfully and Monte Carlo simulations of the SIRTI response to ¹⁸F agree with preliminary measurements within 4 × 10⁻³. A trial comparison of ¹⁸F at a NMI in Europe which has already participated in the international reference system (SIR) will be organized for the ENEA-INMRI (Italy), NIST (USA), NIM (China), ANSTO (Australia), CNEA (Argentina), IFIN-HH (Romania), LNMRI-IRD (Brazil) and the VNIIM (Russian Federation).

The trial exercise for the extension of the SIR to the measurement of pure beta emitters has continued. Results obtained for the submissions of the eight participating laboratories (ENEA (Italy), IRMM (EU), LNE–LNHB (France), NIST (USA), NMISA (South Africa), NPL (UK), PTB (Germany) and POLATOM RC (Poland)) with the BIPM TDCR system have been evaluated and compared with those obtained with the same technique at the LNE–LNHB. The results of these measurements were presented together with those obtained using the universal efficiency curves (UEC) during the CCRI(II) meeting held at the BIPM in May 2013 and at the ICRM 2013 Conference in Antwerp, Belgium. A larger scale exercise was approved at the CCRI(II) in May 2013 that will cover ³H, ¹⁴C, ⁵⁵Fe and ⁶³Ni, with 19 NMIs having expressed interest in participating.

The core areas of laboratory activity for the Chemistry Department are: international comparisons and equivalence of gas standards for air quality and climate change monitoring; and the international comparison programme on primary organic calibrators.

In the field of Gas Metrology, the BIPM has continued to coordinate comparisons of standards for greenhouse gases and air quality monitoring. In the area of greenhouse gases and their precursors, measurements for CCQM-K82 on methane at ambient levels were completed, and the Draft A report presented to the CCQM Working Group on Gas Analysis (GAWG). The results show an important improvement compared to a similar exercise organized in 2003, with the standard deviation of submitted results reduced by more than a factor of ten, reaching a value lower than 2 nmol mol⁻¹, equivalent to the Data Quality Objectives of the Global Atmosphere Watch programme run by the World Meteorological Organization (WMO).

Preparatory work for the coordination of CCQM-K90 on formaldehyde (HCHO) in nitrogen standards continued, with successful completion of stability and purity tests performed during 2013, and the standards to be used for the comparison in 2014 are in preparation. Of the two sets of standards tested, which differed in preparation procedure and cylinder surface treatment, the set that was considered suitable demonstrated a linear loss of formaldehyde concentration of less than 0.2% per month, while the other set showed increases of up to 2% per month.

The BIPM and the International Atomic Energy Agency (IAEA) organized a workshop, which was hosted by the VSL, the Netherlands, on 4 June 2012. Its aim was to understand the current status of CO₂ and CH₄ isotope ratio standards and their importance for accurate concentration measurements of these important greenhouse gases. The conclusions of the workshop were presented at the 30th Meeting of the CCQM-GAWG (November 2013), and the technical proposals for a key comparison (CCQM-K120) on CO₂ in air standards, to be coordinated by the BIPM and the NIST, were accepted. Initial work to develop measurement methods at the BIPM and validation standards for the comparison has started.

In the area of air quality gas standards, the ongoing Surface Ozone reference standard comparison (BIPM.QM-K1) has continued with five laboratories participating in 2013. The WMO guidelines [8] for continuous measurements of ozone in the troposphere were published in 2013 and now include guidance on traceability with references to the BIPM.QM-K1 comparison. Work on new ozone absorption cross-section measurements is nearing completion with the assistance of a visiting scientist from the KRISS, Republic of Korea, for three months in 2013. The final report of the CCQM-P110.B2 comparison for nitrogen dioxide standards has been published, and a paper describing accurate FTIR measurements of NO₂ and nitric acid by calibrations with synthetic spectra was published [9]. A relative uncertainty of 3.4% for the measurements of NO₂ concentrations was demonstrated with this method and attributed mainly to the uncertainty in the line strength values (HITRAN 2004) and the knowledge of the optical path length of the FTIR gas cell, to be compared with a 0.4% relative uncertainty when the FTIR was calibrated with standard gas mixtures.

The BIPM's organic programme forms an essential part of the CCQM Working Group on Organic Analysis (OAWG) strategy for Core Competency comparisons, and a paper describing the implementation at the BIPM of the mass balance method for determining the mass fraction of the main component of a high purity organic material was published [10]. The final report of the CCQM-K55.c [(L)-Valine purity] key comparison and the parallel CCQM-P117.c pilot study are in preparation following approval of the key comparison reference value. Thirty submissions were received from twenty-five participating institutes which used either mass balance or quantitative nuclear magnetic resonance (QNMR) approaches, or the combination of data from both approaches, to assign the valine content of the material. The KCRV for the valine content is 992 mg/g with an associated standard uncertainty of 0.6 mg/g. Preparative work on samples for CCQM-K55.d [folic acid purity] has started, with homogeneity and stability measurements under way. The aim is to establish the suitability of the material for the comparison by demonstrating that the between-unit relative homogeneity and stability of impurities present in the candidate material are below 2% and 1% respectively for any component present at levels above 2 mg/g. Dependent on the successful outcome of these studies, it is planned that a key comparison and a parallel pilot study will start towards the end of 2014. Activities in large organic molecule purity have made substantial progress with the completion of measurements on pure Angiotensin I, as part of a BIPM–NIST collaboration in preparation for future key comparisons in the area, and with publication of a paper on the methods developed [11]. Work on purity studies of insulin is also nearing completion, and the first CCQM key comparison on peptide purity (CCQM-K115) will be coordinated by the BIPM in collaboration with the NIM, China. Dr Ming Li from the NIM has joined the BIPM as a visiting scientist to work on development of the method and characterization of the study material, in preparation for this key comparison.