infer bond energies from rate constants. Although the previously accepted values for C-H bond dissocia-tion energies were derived from experimental methodologies now known to be deficient, work in the 1960's led to results which appeared to support those values. However, in recent years, there has been a gradual accumulation of rate constant data which differed by 1 to 3 orders of magnitude from predictions based on the accepted values of the bond dissociation energies. Since these results are usually assessed individually, the discrepancies have usually been dismissed as the effects of unknown experimental artifacts. The NBS effort [1] involved a critical examination of these previously rejected results on the rate constants of alkane and alkyl radical decomposition processes. Although these data were in gross disagreement with the values which would be expected on the basis of the accepted thermodynamics of the processes and the wellestablished kinetics of the reverse combination reactions, it was found that none of the measurements could be rejected. The next step was to establish thermodynamic self-consistency as a requirement for the data base, in order that the second law of thermodynamics should not be violated. Using recent theoretical and experimental entropy data, the NBS effort then demonstrated that the extensive body of rate data did lead to internallyconsistent higher values for the bond dissociation energies. The proposed new values for the bond energies make possible the resolution of many controversies in the chemical literature. For example, use of the previously accepted bond energy values led to a prediction that there should be an energy barrier for certain di-radical recombination processes (for example, ring closure of a tetramethylene species to give cyclobutane), but theoreticians had never been able to reproduce such a barrier. The revised bond energy values remove the predicted. barrier. Using the new values, the rate data for alkane-akyl radical systems now show the systematic trends which would be predicted by any reasonable theoretical model. Therefore, the results of this critical data evaluation can be considered to be a validation of the kinetic methodologies for the determination of molecular stabilities. The NBS work demonstrates the contribution that data evaluation within a proper theoretical framework can make toward a fundamental understanding of physico-chemical problems. In particular, the work demonstrated the importance of distinguishing between errors of measurement and errors of interpretation. References [1] Tsang, W., The stability of alkyl radicals, J. Am. Chem. Soc. 107, 2872-2880 (1985). NBS RESEARCHER DEVELOPS PARA- Scientists who use mass spectrometers to perform medical, environmental, or industrial analyses at low concentrations-parts per million or belowshould find a new device helpful in improving their measurement systems. Called a parametric electrometer, the invention was designed to measure extremely low levels of electrical current through the computer-controlled data collection systems of mass spectrometers. The device is an improvement over existing models because of its ability to measure signals at levels close to the theoretical limit of noise. It features variable response times at the flick of a switch, allowing an operator to control response time and noise level, two important factors in performing a mass spectrometric analysis at low levels. Though the electrometer was designed for use with NBS-developed thermal ionization mass. spectrometers, it may be used with other similar types of instruments and measurement systems. For further information contact Ronald W. Shideler, National Bureau of Standards, Gaithersburg, MD 20899. FIRST NEUTRON OBSERVATION OF MAGNETISM IN A MULTILAYER MATERIAL NBS scientists in collaboration with colleagues from the University of Illinois, have performed the first neutron-scattering study [1] of the magnetic order in a epitaxial multilayer of the rare earth metals dysprosium (Dy) and yttrium (Y). Multilayers are a very new class of materials which are prepared by molecular beam epitaxy techniques and consist of single-crystal layers of magnetic Dy and non-magnetic Y, each only 40 angstroms thick and stacked 64 bilayers high. These novel materials, combined with the unique probe of magnetic neutron scattering, have enabled the determination of the remarkable result that the helical magnetic order in the Dy layers is propagated through the intervening Y layer and into the next Dy layer without loss of phase coherence. The neutron experiments allowed details of the helical magnetic order and its range, which was greater than 5 bilayers, to be examined directly in a manner not possible with conventional techniques. These results are a dramatic confirmation, not previously available for ordinary alloy systems, of the existence of a peak in the generalized susceptibility of metallic yttrium at a wavelength close to that of the helical ordering found in other rare earth metals. These results have significant implications on the tailoring of such layered structures for specific magnetic applications. References [1] Falamon, M. B.; S. Simha, J. J. Rhyne, J. E. Cunningham, R. W. Erwin, J. Borders, and C. P. Flynn, Long-range incommensurate magnetic order in a Dy-y multilayer, Phys. Rev. Ltrs., Vol. 56, 259-262 (1986). References [1] Myklebust, R. L.; R. B. Marinenko, D. E. Newbury, and D. S. Bright, Quantitative compositional mapping on an electron microprobe. Electron microscopy and analysis, 78, 219-222 (1985), Inst. of Phys., London. [2] Newbury, D. E.; D. S. Bright, D. Williams, C. M. Sung, T. Page, and J. Ness, Application of digital SIMS imaging to light element and trace element mapping. In: Secondary ion mass spectrometry, 5, Springer-Verlag, Berlin (1986). COMPOSITIONAL MAPPING: QUANTITATIVE ELECTRON MICROPROBE IMAGING DEVELOPED Techniques to produce the first fully quantitative compositional images with micrometer spatial detail by means of the electron probe microanalyzer have been developed [1,2]. The imaging techniques involve the acquisition of digital images based upon direct measurement of x-ray counts at each pixel in an image. The full quantitative analysis procedure which is normally employed for analysis at individual points is then applied at each pixel in the scan, including correction for detector deadtime, background, spectrometer, defocussing, standardization, and matrix correction. The resulting images consist of quantitative compositional values which have equivalent accuracy and precision to conventional single-point analyses. Techniques have also been developed for quantitative display of these compositional images. The most successful technique has been the use of color encoding based on the so-called "thermal color" scale, in which the sequence of colors assigned to the intensity scale is that which is observed when an object is heated: various shades of red, orange, yellow, and white. Such encoding provides an intuitive scale in which the position in the color sequence is proportional to the sensation which the human eye receives. With this scale, concentrations ranging from 0.1 to 10 weight percent have been successfully displayed in a single image. The first application of of the digital compositional mapping technique has been in the study of diffusion-induced grain boundary migration in polycrystalline binary alloys. In this application, zinc distributions with concentrations as low as 0.1 weight percent were mapped with micrometer spatial resolution at grain boundaries in polycrystalline copper. NON-TOXIC "NATURAL" BONE CEMENT DEVELOPED BY DENTAL SCIENTISTS A new calcium phosphate cement for the repair of teeth and bone that is wholly compatible with living tissues in the body has been developed by dental scientists at NBS. Researchers from the Paffenbarger Research Center (PRC) of the American Dental Health Foundation at the bureau have developed the cement as a part of the PRC research program to study calcium compounds for use in dental treatments. The new material, which can be described as a "natural cement" because of its biocompatibility, is based on a mixture of two calcium phosphates, tetracalcium phosphate and brushite [1]. When the two compounds are combined they form hydroxyapatite, the primary mineral in teeth. and bone. Laboratory studies by the American Dental Association (ADA) in Chicago confirm the cement's biocompatibility with soft and hard body tissues, and the ADA reports there is no toxic reaction. Laboratory research also shows that because of the new cement's setting properties, the material has significant potential use in a wide variety of dental treatments and in other health care applications. In dentistry, the cement may be used as a base for dental fillings, as a filler for root canals, as a desensitizing agent for the roots of exposed teeth, and as a filler in bone sockets after tooth extraction. Preliminary studies also indicate that the new material may have use as a bone cement to set prosthetic knee and hip implant devices. The scientists report it will be at least two years before clinical studies on some of the cement's dental applications are completed and the material is available to dentists. It probably will be five years before the cement is approved for use in medicine. References [1] Shern, R. J.; L. C. Chow, K. M. Couet, A. Kingman, and W. E. Brown, Effects of sequential calcium phosphate-fluoride rinses on dental plaque, staining, fluoride uptake, and caries in rates, J. Dental Res. 63, 1355-1359 (1984). International Comparisons of National Standards For further information contact William F. Koch, National Bureau of Standards, Gaithersburg, MD 20899. U.S. AGREES WITH ITALY AND AUSTRALIA TO RECOGNIZE THE EQUIVALENCE OF EACH OTHER'S NATIONAL STANDARDS FOR THE SIX BASE UNITS OF THE SI SYSTEM. In signing ceremonies in October 1985, the NBS Director, Dr. Ambler, ratified an agreement with the Istituto Elettrotecnico Nazionale “Galileo Ferraris" of Italy to recognize the calibrations in length and temperature done by either institution as valid in the other country. The agreement is to facilitate the sale of products between the two countries. In a similar ceremony in Gaithersburg, MD, NBS signed an agreement with the National Measurement Laboratory of the Australian Commonwealth Scientific and Industrial Research Organization (CSIRO) to recognize the equivalence of the national standards for the six base units of the SI system. Both Italy and Australia are signatories to the Convention of the Meter and determine, like the U.S., their basic units of measurements in accordance with the definitions for SI units adopted by the General Conference of Weights and Measures, an intergovernmental body that is the recognized world authority on physical quantities and the way they are measured. COLLABORATION IN ELECTROLYTIC CONDUCTANCE STANDARDS WITH HUNGARY Hungarian electrolytic conductance standards were recently compared with those maintained at NBS by the Inorganic Analytical Research Division. Mrs. Anna Tomek from the National Office of Measures (OMH) in Budapest, Hungary, spent three weeks at the NBS Conductance Facility to confirm the compatibility of the two nations' measurement systems in this important field. Further intercomparisons will be conducted when NBS issues a Standard Reference Material for electrolytic conductivity. The collaboration between. NBS and OMH continues as research is conducted into improving the accuracy of electrolytic conductivity measurements and standards and this work complements the effort in standardization of pH measurements that was begun last year between NBS and OMH. INTERNATIONAL COMPARISON OF CLOCKS This past year NBS scientists in Boulder, CO succeeded in comparing two clocks thousands of miles apart with an accuracy that amounts to one second in the course of a million years. David W. Allan achieved the most accurate clock comparison across the Pacific with colleagues at the Radio Research Laboratory in Tokyo. They checked the U.S. primary standard in Boulder, CO against the Japanese primary standard. The intermediary was the U.S. Department of Defense's Global Positioning System (GPS) of navigation satellites with their on-board atomic clocks. This intercomparison brings Japan into the community of nations that contibute to the definition of the international atomic second, thus making International Atomic Time still more accurate. Besides the U.S., other nations that contribute to the definition of the second are Canada and the Federal Republic of Germany. As a result of this intercomparison, the National Aeronautics and Space Administration, and the scientific community will be able to obtain a better time reference for deep space probes and the investigation of pulsars, the still somewhat mysterious emitters of radio waves in the universe. Most industrial nations use laboratory clocks based on the vibrations of cesium atoms as their national standards of time. Prior to the availability of access to the GPS, international comparisons had to be done with portable clocks carried between national laboratories. Now the comparison can be done quickly by reviewing an electromagnetic signal from a satellite that is in common view of both time laboratories. For further information contact David W. Allan in Boulder, CO 80303. New Services From NBS BUREAU STARTS TELECOMMUNICATIONS TESTING SERVICES NBS is establishing a voluntary laboratory accreditation program (LAP) for laboratories that perform electromagnetic compatibility and telecommunications equipment testing. The program was requested by five commercial testing laboratories under new procedures of the National Voluntary Laboratory Accreditation Program. The companies believe accreditation will aid them in exporting their products to other countries. The LAP will provide national recognition to accredited laboratories that are capable of performing specific test methods for conducted emissions, radiated emissions, and terminal equipment compatibility in Federal Communications Commission (FCC) standards. For further information on the LAP, or for an application package that includes a list of test methods, fee schedule and handbook, contact Harvey Berger, National Bureau of Standards, Gaithersburg, MD 20899. NEW MAP FOR SPECTROPHOTOMETRIC NBS has established a new Measurement Assurance Program (MAP) for laboratories measuring spectral transmittance [1]. The program makes it possible to check the absolute calibration of spectrophotometers against the NBS scale over a transmittance range of 0.92 to 0.001, as well as the wavelength accuracy of the spectrophotometers. MAPS are special calibration programs developed by NBS that make it possible to evaluate the entire measurement system and procedures of a participating laboratory, rather than simply the instruments used by that laboratory. The cost of the spectrophotometric transmittance MAP is $1700. References [1] Eckerle, K., Transmittance MAP Service (SP 692), U.S. Government Printing Office, Washington, DC 20402. ($2 prepaid. Request stock no. 003-003-02655-7.) For further information on this MAP contact Kenneth Eckerle. NEW CALIBRATION SERVICES FOR USERS OF AIR NAVIGATION SYSTEMS An airplane approaching an airport may accurately measure its direction of flight relative to magnetic north using VOR (Very-high-frequency Omni Direction Range) signals from ground based transmitters. The world wide VOR navigational system which has been in use since 1946, consists of more than 1000 ground based transmitters located near airports which broadcast a standardized signal which can be decoded by bearing-angle indicators aboard individual aircraft. NBS has recently developed a calibration service for bearing angle indicators and for the audio generators which are used to modulate the VHF signals that are transmitted and provide the bearing angle information [1]. The new calibration service is based on two new instruments which have been designed and built at NBS; the first, a standard signal generator produces both a variable phase 30 Hz signal and a 9,960 Hz signal frequency modulated at 30 Hz which provides a reference phase. The estimated uncertainty of the phase difference (bearing angle) produced by this generator is 0.028°, approximately, a factor of 10 better than generators currently in use in operational VOR systems. The second instrument, a standard bearing angle indicator (phase meter) has the same estimated overall uncertainty, based upon tests made using instru ments. Bearing angle indicators may be calibrated at NBS using the standard signal generator. While in principle the NBS bearing angle indicator can be used to calibrate VOR signal generators most commercial generators cannot be calibrated directly, since the waveforms generated are not compatible with the requirements imposed in developing the NBS standards. However, generators may be calibrated indirectly by obtaining a calibration of a bearing angle indicator which may then be used as a transfer standard with only minor loss of accuracy. References [1] Detailed information on the new calibration services may be obtained by contacting Neil T. Larson, Boulder, CO 80303. VOR Calibration Services (TN 1069), Stock Number 003-003-02652-2, the U.S. Government Printing Office, Washington, DC 20402 (price $6.50). NEW MEASUREMENT SERVICE ESTAB- NBS has established a measurement service with standard beams of 147Pm, 204Tz, and 90SR +90Y. These are beta-particle sources with maximum energies of 0.2, 0.8, and 2.3 MeV, respectively. The beams have been standardized at certain fixed distances in terms of absorbed dose to water or tissue. The facility will be employed to calibrate userowned beta-particle sources, to calibrate transfer instruments suitable for calibration of beta-particle sources, and to irradiate user-owned personnel dosimeters and other passive dosimeters to known levels of absorbed dose. Measurement-assurance tests are planned between NBS and other beta-particle calibration laboratories. For further information contact J. S. Pruitt or M. Ehrlich, National Bureau of Standards, Gaithersburg, MD 20899. New Standard Reference Materials* MATERIALS AIMED AT BOOSTING ACCURACY OF LEAD-IN-BLOOD TESTS To improve the reliability of blood tests for determining long-term exposure to lead, NBS has developed a Standard Reference Material (SRM) for calibrating the laboratory instruments that measure minute amounts of lead in blood. Wide-ranging results-some in error by as much as 200 percenthave been obtained in past studies when blood samples with known lead concentrations were sent to various clinical and analytical laboratories for analysis. Though analyses have improved in recent years due to refinements in technology and increased quality control, there is still a need for a lead-in-blood standard as a reference for checking instrument accuracy and analytical methods. The new SRM is issued to fill this need. It is available in units that each contain four bottles of varying lead concentations in porcine (pig) blood: 5.7, 30.5, 49.4, and 73.2 micrograms per deciliter. For technical information contact D. Reeder. STANDARD REFERENCE MATERIAL FOR BETTER BREATH AND BLOOD ALCOHOL MEASUREMENTS Ensuring accuracy in the instruments law enforcement agencies use to measure alcohol content in breath and blood is NBS' goal in producing a SRM that is now available. Called Ethanol-Water Solutions, the SRM was developed after law enforcement agencies requested an NBS alcohol/water reference material be produced that could be cited in court as a reliable standard for gauging breath or blood alcohol concentration. (Courts in some states and localities require validation of measurement techniques in drunk-driving cases.) The SRM can be used to calibrate and standardize alcohol-measuring equipment as well as to evaluate the daily laboratory reference solutions used in alcohol determinations [1,2,3]. SRM 1828 consists of five vials of ethyl alcohol (ethanol)/water solutions certified for these percentages by weight: 95.629 0.2992 (two vials are included at this concentration, and 0.1487 (two vials). References MADE-IN-SPACE POLYSTYRENE SPHERES ARE NEW NBS STANDARD REFERENCE MATERIAL A new size primary particle standard is available from NBS. The Standard Reference Material, designated SRM- 1960, is a suspension of 10 micrometer polystyrene spheres in water. Previously only .3 and 1 micrometer standards were available. The spheres are used for calibrating particle size measuring instruments and optical and electron microscopes. The 10 micrometer size is especially useful because blood cells are approximately this same size and the accurate measurement of blood cell size is the basis for many medical diagnostic instruments. The accurate measurement of particles of this size is also necessary to implement a new air pollution standard which defines 10 micrometer particles as a cut off. The U.S. Pharmacopia is also developing a standard for pharmaceutical materials based on 10 micrometer size particles. The spheres from SRM 1960 were grown aboard the space shuttle Challenger using a process developed for NASA by Lehigh University. It is very difficult to grow acceptable spherical particles larger than a few micrometers on earth. The process for growing spheres making use of the very low gravity during space flights results in more spherical and uniformly sized particles and much greater yield than particles of this size grown on earth. Because the process of growing the particles is time consuming, the time in space of five shuttle flights was required to obtain 10 micrometer spheres. The spheres are the first product manufactured in space. The particles were provided by NASA to NBS for measurement and distribution. The NBS interest and expertise in micro-dimensional metrology was used to measure the diameter of the particles by a new technique developed specifically for the task. The new measurement technique is called "center distance finding." It is an optical technique related to array sizing [1]. The diameter measurements were also checked using electron microscope technqiues developed by NBS for one micrometer spheres [2]. |