Many physical standards are determined through fundamental constants--except the kilogram, calculated the "old way": this is the mass of a platinum/iridium (Pt/Ir) cylinder manufactured more than a hundred years ago exactly for that purpose. Today physicists and chemists are out to determine a mass unit on the basis of a Si-28 isotope. Taking part in this work are also Russian scientists of the Institute of High Purity Chemistry named after Devyatykh (IHPC) in Nizhni Novgorod. More, in an interview of Dr. Andrei Bulanov, deputy director for science, to the newspaper Poisk (Search).

The Mendeleyev Periodic Table lists only twenty-two monoisotopic elements, while the greater part of others are a mix of 2 to 10 stable elements. Their physical and chemical characteristics are essentially different, and studying each one is of major interest to basic research and the materials science.

To see how the mass of an isotope affects its characteristics, we should achieve all-out grade purity of substances under study. By now the lowest concentration of impurities has been attained for silicon (Si) and Germanium (Ge), the elementary semiconductors. Many physical and physical-chemical properties of these native elements in high-grade monocrystals (single crystals) have been studied in much detail. Therefore we sought to get high-quality Si and Ge isotopes. Native silicon (Si) is a mix of three stable isotopes having atom masses 28, 29 and 30, their concentrations, 92.23, 4.67 and 3.10 percent. Germanium (Ge) has five isotopes, with their masses and concentrations 70 (20.84%), 72 (27.54%), 73 (7.73%), 74 (36.28%) and 76 (7.61%).

Pt-lr standard.

Si and Ge isotopes have found a variety of applications, namely in X-ray monochromators, in ionizing radiation detectors, and also in registration of double neutrino-free β-decay. Yet another spin-off for silicon-new semiconductors, including what we call super-lattices composed of isotopes of 28 and 29, or 29 and 30 atomic masses.

Getting substances with low concentrations of impurities is a multistage process that includes separation of isotopes in the form of a volatile compound, high-degree purification of the substance thus enriched, and isolation of a monoisotopic element from it, to be followed by its extra cleansing and growing a single crystal from it. Rosatom enterprises have a good record of isotope separation in gas centrifuges, with volatile-fluorides used for better purity. To isolate the element from a compound like that we need either extra chemical reagents (contaminating the product) or special physical conditions (plasma, hard laser radiation, electric arc).

We are making use of an upgraded hydride method that takes in the stages of synthesis, high purification and thermal decomposition of a monoisotopic hydride at t ~800 °C. For instance, the synthesis of Si hydrides, the monosilanes, is predicated on reaction of Si tetrafluoride interacting with Ca hydride. Monoisotopic silanes thus obtained are separated from admixtures by low-temperature filtration and distillation. Next, the purified substances are subjected to pyrolysis (high-temperature breakdown), with a polycrystal being precipitated on a special growth rod made from a corresponding monoisotope.

Our institute began the work of getting large specimens of Si-28 late in the 1990s. Our first single crystal contained as much as 99.89 percent of the stock isotope. Then we brought up the concentration to 99.93 and 99.98 percent. The final stage of this work concurred with a period when our research center joined the Avogadro Project launched in 2002 by the International Board of CIPM Directors (Eurocommission) involved with mass measurements. The aim was as follows: calculate a physical constant, the Avogadro* number, with very high accuracy. At that time 1 kg native silicon spheres were being used for that. European Union research centers making mass and volume measurements determined their molar mass. Proceeding from these data, it became possible to obtain the most exact value of the Avogadro constant. The error margin was only a three decimillionth. The principal cause in the way of better accuracy was pinpointed, and that was isotopic inhomogeneity of native silicon.

Further accuracy of this value was attained by using a highly enriched, chemically pure and structurally perfect Silicon-28 single crystal. To handle this objective research institutes of European Union and Russia joined hands. Our country was to shoulder the job of getting the stock material from Silicon-28 in the form of a polycrystal. An isotope-enriched variety of silicon was obtained by the Tsentrotekh-SP(b) company. We, in our turn, grew a 5,983 g crystal in the shape of a rod

* The Avogadro number (constant), the number of molecules in a mole of any substance or the number of atoms in a gram-atom of any chemically elementary substance; named after Amedeo Avogadro, an Italian physicist and chemist.--Ed.

61.5 mm across. The concentration of the basic isotope was nearly hundred percent: 99.99382±0.00240 percent. Meanwhile das IKZ Institut in Berlin, Germany, specializing in crystal growing had our crystal grown into the desired Silicon-28 monocrystal by the method of noncrucible zone smelting. Then the Australian Center of Precision Optics (ACPO) manufactured two identical spheres from it, 96.3 mm in diameter and 1 kg in mass. By measuring their volume, density, atomic lattice and molar mass, it became possible to bring down the error of the calculated Avogadro number to a three hundred-millionth fraction, or almost what was wanted.

In their chemical and isotopic purity our monocrystals are superior to specimens grown before. Such silicon-derived single crystals differ essentially in many characteristics both amongst themselves and from isotopic samples of native Si, specifically in heat conductivity, low-temperature photoluminescence intensity, fine spectral structure, refraction index and other significant characteristics. Our high-grade monocrystals offer good prospects for quantum computers, spin electronics, fiber optics wave guides in the near infrared range, for a new generation of detectors of nuclear particles and ionizing radiation. The novel Si-28 technology has also been used for growing single crystals of other rare Si isotopes with a concentration index above 99.9 percent.

Obtaining high-grade germanium isotopes is another line of our activity. Teaming up with an electrochemical plant in Zelenogorsk, Krasnoyarsk Territory, we have devised a method of separating Ge isotopes with a hydride of this element, the monogermanium, as a working medium. Taking native monogermanium, we isolated Ge isotopes of atomic masses 76, 74, 73 and 72 by centrifugation. Monocrystals were grown from them after purification. The Ge-76 monocrystal contains above 88 percent of the basic isotope, and the Ge-74 polycrystal, as much as 99.93 percent.

Pooling with Nizhni Novgorod University and the RAS Fiber Optics Research Center, we have measured the optical and thermal characteristics of Si and Ge monocrystals. The results of our studies attest to a significant effect of their composition on such characteristics as heat capacity and conductivity as well as luminescence and light absorption. There are plans to get Ge-73 and Ge-72 isotopes enriched to 99.9 percent and look into their characteristics. Turning to international projects, Dr. Bulanov said that in 2012 a Kilogram Project had been launched by das RTB Institut of Physics and Technology in Germany so as to beat the results of the Avogadro Project. The one kilogram spheres obtained earlier from monoisotopic Si-28 deviated from sphericity by 99 nm. And thus the main "input" to the error in the Avogadro constant came from the nonsphericity factor (error in determining the sphere volume), and not from the isotopic composition. Joining the new project, the Nizhni Novgorod Institute was to grow two polycrystals, each about 6,000 g in mass. The stock material for one of the crystals--silicon fluoride enriched to 99.997 percent--is already available at the Zelenograd electrochemical works and handed to our Nizhni Novgorod Institute, Dr. Bulanov said. Work is underway to obtain a source silicon hydride, the monosilane, and to handle the job of its further purification.

Das Berlin IKZ Institut will grow a single crystal from which das RTB Institut will excise two 1 kg spheres diverging from sphericity by <30 nm. So, using four spheres from the monoisotopic Si-28, scientists hope to fulfill the purpose of the Avogadro Project-calculate the Avogadro constant with an error not above a two-centimillionth. This fundamental objective, we are hoping, is within reach.

V. Yanchilin, No Extras.--Newspaper "Poisk", No. 45, 2013

Illustrations supplied by A. Bulanov

Prepared by Sergei MAKAROV