Table of contents

The magnetostatics of cylindrical superconductors with parallel axes is formally analogous to the electrostatics of cylindrical normal conductors of the same geometry, provided that the London penetration depth can be neglected. This analogy when applied to the Thompson-Lampard theorem of electrostatics, which forms the basis of the calculable capacitor, leads to a method of deriving an SI standard of dc resistance, R₀, given by R₀ = μ₀c (π/log 4) (f_J/f_opt) (n_fr/n_fq), where μ₀c is the impedance of vacuum, f_J is the Josephson frequency of the voltage across the potential terminals of R₀, f_opt is the optical frequency of an interferometer light source, n_fr is an optical interferometer fringe count and n_fq is a SQUID flux-quantum count. This paper gives the theory of the method, and offers some further theoretical results that point to the practical feasibility of the standard.

To examine the stability of prototypes of the kilogram after cleaning, surface analysis techniques, including X-ray photoelectron spectroscopy (XPS), were applied to specimens cut from prototype material of Pt-10% Ir alloy. Traditional steam-jet cleaning (SJC) was compared with ultrasonic cleaning with solvents (UCS) in terms of cleaning effects and the progress of contamination. The results show that mass changes due to metallic factors such as oxidation, reduction and dissolution were negligibly small. In terms of carbon decrease and adsorption of water, UCS was superior to SJC. From analyses of peak area ratio by XPS, it was shown that the cleaner the surfaces, the higher the rate of contamination. But the contamination levels converged to a common value after a six-month exposure to air, irrespective of the cleaning method used. This finding is of great importance for the preservation of prototypes. The contaminating substances were found to be hydrocarbons from the ambient air. The mass gain for the surfaces of a prototype due to hydrocarbons after a six-month exposure to ambient air is estimated to be 4,3 μg for SJC and 16 μg for UCS.

Improvements to the NRC HeNe/CH₄ laser have produced a stable infrared source at 88 THz suitable for checking the stability of the CO₂ laser frequencies in the NRC phase-locked frequency chain. About 30 000 1 s average readings of the 88 THz laser frequency were obtained over the period 89-03-29 to 91-05-10. The Allan deviation σ_y(1 s) of each CO₂ laser frequency in the chain was determined to be less than 2,5 × 10^-12 (82 Hz), when a new NRC hydrogen maser was used as a reference. The standard deviation of the mean of samples containing between 592 and 1 390 1 s average readings of the HeNe/CH₄ frequency, obtained on a given day, ranged from a fractional value 7,1 × 10^-14 to 3,0 × 10^-13. The weighted standard deviation of 34 determinations of the HeNe/CH₄ laser frequency, obtained at various times over this period, was 73 Hz (8,3 × 10^-13), and the mean frequency relative to the frequency derived from the International Atomic Time scale (TAI) was 88 376 181 599 431(73)Hz. Other tests showed no significant systematic errors in the phase-locked CO₂ laser frequencies.

The Edlén equation for the refractive index of air, published in 1966, is still widely used. Subsequent to its formulation, however, improved data have become available on the density of air and the refractivity of water vapour. The practical temperature scale has also been revised and the carbon dioxide content of normal laboratory air has increased. These effects result in a discrepancy of typically 1 × 10^-7. The consequent revision of the equation brings the agreement between calculation and experiment within an experimental uncertainty of ± 3 × 10^-8.

We report a complete description of a frequency standard based on the ³P₁-³P₀²⁴Mg transition at 601 GHz; in particular the metastable atomic source, the Ramsey-type interaction region, the submillimetric frequency synthesizer and the frequency-control loop are described both theoretically and experimentally. The main experimental results concerning the operation of the atomic beam, the Ramsey signal intensity and width versus some important parameters and the signal-to-noise ratio, with and without optical pumping, are reported and compared with theoretical predictions. The clock transition frequency has been measured with respect to UTC (IEN) with an uncertainty of ± 1 × 10^-12; at present the frequency instability reaches the value of 2 × 10^-13 for counting times of 3 000 s and the repeatability the level of 3 × 10^-13. The accuracy budget is also reported and discussed as are possible improvements of the present prototype.

For about one year the time scales UTC (OCA) and UTC (TUG) were compared by means of GPS common-view and two-way satellite time transfer. At the end of the experiment, both links were independently "calibrated" by measuring the differential delays of the GPS receivers and the satellite Earth stations by transportation of one GPS receiver and one satellite terminal to the other site. The results obtained by the two methods differ by about 3 ns, but reveal a seasonal variation of about 8 ns which, most likely, is mainly the result of temperature-dependent delays in the GPS receiving equipment used.

An analysis of the hyperfine spectrum of the 62P(17-1) line of the ¹²⁷I₂ iodine molecule has been made from experimental data. The uncertainty in the frequency interval measurements between the hyperfine components, of order 20 kHz, allows determination of the differences between the constants of the four principal hyperfine interactions.

The workshop is organized by two Italian institutions, the Istituto di Metrologia "G Colonnetti" (IMGC) of the Consiglio Nazionale delle Ricerche, and the Institute of General Physics "A Avogadro" of the University of Torino. It will be held in Torino from 9 to 11 March 1994 at Villa Gualino, where accommodation for participants is available. The programme of the workshop includes plenary sessions of invited lectures and a final round table.

The main subject of the workshop will be the determination of the Avogadro constant through the measurement of the molar and atomic volume of single crystal silicon, but other methods will be analysed and discussed and experimental results compared with estimates obtained from the adjustment of the fundamental constants.

The aims of the workshop are:

• state-of-the-art assessment

• improved coordination of research work

• definition of expected progress

• discussion of the possibility of a new definition of the unit of mass.

The lecturers are eminent experts in their fields, which include X-ray and optical interferometry, measurement of density, molar mass and impurity content of silicon crystals. The workshop Proceedings will be published in a special issue of Metrologia.

Participation is free but the number of places is limited. Registration forms (to be returned no later than 20 December 1993) and requests for the detailed programme should be addressed to G Mana, Istituto di Metrologia "G Colonnetti", Strada delle Cacce 73, 1-10135 Torino, Italy. Tel. (011) 39771; Fax (011) 346 761; E-mail LIDIA at IMGC.TO.CNR.IT.

This meeting, organized by the Paul Scherrer Institute's Department of Applied Solid State Physics, will be held from 27–30 March 1994 at the Hotel Regina-Titlis, Engelberg, Switzerland. The aim is to bring together scientists from two important fields of current research and increasing industrial relevance. Optical metrology is a traditional discipline of applied optics which reached the nanometre scale a long time ago. Nanotechnology is setting new limits and represents a major challenge to metrology, as well as offering new opportunities to optics.

The meeting is intended to help define a common future for optical metrology and nanotechnology. Topics to be covered include:

• nanometre position control and measuring techniques

• ultrahigh precision interferometry

• scanning probe microscopy (AFM, SNOM, etc.)

• surface modification by scanning probe methods

• precision surface fabrication and characterization

• nanolithography

• micro-optics, diffractive optics components, including systems and applications

• subwavelength optical structures

• synthetic optical materials

• structures and technologies for X-ray optics.

For further information please contact: Jens Gobrecht (Secretary), Paul Scherrer Institute, CH-5232 Villigen-PSI, Switzerland. Tel. (41)56992529; Fax (41) 5698 2635.