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cases. Phase shift and time-delay services are now offered over a wide frequency range using existing equipment. Coaxial frequency calibrations have been extended to 18 GHz in some cases. New lower prices recently announced have resulted in a considerable increase in demand for calibrations in the area of power, voltage, and especially in attenuation. Immittance calibrations have also increased markedly with the advent of the new precision connectors.

Microwave Standards

International Intercomparisons.-Power standard intercomparisons are being made on a "round robin" basis with a circulating set of two standards each from the participating nations: Japan, Hungary, Russia, and the United States.

Low Power Standards.-A new microcalorimeter for the 18.026.5 GHz band was built and evaluated, and is now in regular use. Evaluation of a new microcalorimeter for the 26.5-40.0 GHz band is nearing completion.

Attenuation Measurements.-A technique has been implemented to allow measurement of small changes in the dissipative components of insertion loss, providing sensitivity of a few parts in 105. The measurement is independent of source amplitude fluctuations, and since it avoids stringent microwave power stability requirements, the associated instrumentation is considerably reduced. A new type of readout device has been developed for rotary vane attenuators. This readout eliminates gear eccentricity errors, and has greatly reduced errors from backlash.

Miniature Precision Coaxial Connectors.-The Bureau is fostering cooperative research in miniature precision coaxial connectors which is leading to an extension of the state-of-the-art. It is hoped that this effort will result in a coordinated development by manufacturers of miniature connectors, so that the resulting concepts and devices will be suitable for standardization.

Microwave Calibrations

Power Calibration System.-A microwave power calibration system was developed for the measurement of low-level CW power in coaxial-terminated power meters operating at a frequency of 9.0 GHz. This service makes use of an existing microwave power calibration system for WR90 waveguide and takes into account the errors resulting from the coaxial-to-waveguide transition. A microwave power calibration system was developed for the measurement of low-level CW power in WR187 waveguide (3.95–5.85 GHz) utilizing the impedance variation method of power measurement.

Frequency Calibration System.-A microwave frequency calibration system was developed for the measurement of frequency resonance in cavity wavemeters in WR12 waveguide (60-90 GHz). A calibration system was also developed for measurement of the FM components in the power spectrum of precision signal sources having nominal frequencies of 1.0, 2.5, 5.0, and 10 MHz. The system measures the magnitude of spectral lines relative to the power of the carrier frequency of the signal.

Reflection Coefficient Measurements.-A microwave calibration system was developed for the measurement of reflection coefficient magnitude in WR187 waveguide (3.95-5.85 GHz) using the reflectometer method of measurement. Waveguide interlaboratory standards having reflection coefficient magnitudes from 0.024 to 0.2 in this waveguide size can be determined.

Attenuation Calibration System.-A microwave attenuation calibration system was developed for the measurement in WR430 waveguide (1.70-2.60 GHz) of attenuation difference in variable attenuators, and insertion loss in fixed waveguide attenuators. The IF-substitution method of attenuation measurement is used, and the accuracy of measurement is reported as ±0.1 dB or ±1 percent (whichever is greater) for variable attenuators, and +0.2 dB or +1 percent, for fixed attenuators.

PHOTOMETRIC AND RADIOMETRIC QUANTITIES

Color-Temperature Measurements.-A high-precision photoelectric color-temperature comparator for incandescent lamps has been developed. It generates pulses of red and blue light by alternately positioning two colored filters on a rapidly rotating disk in front of a photo-multiplier tube. The red-blue ratio indications of the instrument are sufficiently sensitive that color-temperature comparisons of test and standard lamps can be made to a precision of 1 °K. This precision is an order of magnitude better than that obtained in visual comparisons.

Optical Wedge Error Calculated.-Optical wedges-filters having a linear variation in optical density along their length-are calibrated for use as density standards. The method of measurement used in calibration introduces a small but previously unquantified error. On the basis of rigorous mathematical analysis, a computer was programmed to calculate wedge corrections which could be applied to measured densities to find the density at the center of the slit. Such wedges can be measured and specified more accurately, and the method can now be included in the "USA Standard Methods for Measuring Optical Density."

Photoionization Studies.-Absolute actinometry in photoionization studies is now possible as a result of the development of special techniques. This advance is based on the results of studies which show that saturation currents can be measured, given proper cell design and suitable choice of parameter such as pressure, light intensity, and monochromaticity. Photoionization studies provide more accurate kinetic information on ion-molecule reactions than conventional high-energy radiation systems, because with monochromatic light of a selected wavelength, a few well characterized ions can be produced in the reaction mixture. This can be coupled with ionization efficiency measurements to provide quantitative information on the number of neutral excited molecules produced.

Detector Standard for the Vacuum Ultraviolet.-A highly successful comparison of two absolute radiant flux detectors has been accomplished in the spectral region 500 to 1000 A (50 to 100 nm). One is a thermopile, calibrated in the visible, whose blackness, photoelectric energy loss, and surface variation in sensitivity have all been taken into account; the other is a rare-gas ionization counter, constructed and operated to insure that each absorbed photon creates one and only one electron-ion pair. At several wavelengths in this region these two detectors have been compared; results show an agreement of a few tenths of a percent, and a standard deviation of repeatability on the order of 2 percent. This represents an important improvement in the reliability of calibrations for this spectral region. Additional compari

sons utilizing other absolute calibration techniques are planned before a detector calibration service is established.

THERMAL QUANTITIES

Calorimeter for Heat Capacity and Enthalpy Measurements.— Taking cognizance of the increased importance of high temperature processes in both industry and defense, a calorimeter has been developed for measurements of heat capacity and enthalpy at temperatures between 900 and 2500 °C. It is believed that this calorimeter is the most accurate in its temperature range. Measurements have been made on pure aluminum oxide up to 2500 °C (200° above its melting point); this substance serves as a heat capacity standard throughout the world. Similar measurements have been made on pure tungsten. This substance will serve as a standard for heat capacity at the highest temperatures and the results will also be used for interpreting the measurements on liquid Al2O3.

IONIZING RADIATIONS

Radioactivity Standards Developments.-In fiscal year 1966, 505 radioactivity standards were sold and 46 calibrations were performed. One new radioactivity standard, cerium-praseodymium-144 in the form of a ẞ solution standard, was developed, and two new tests were established: strontium-89 for the 2-ß ionization chamber, and tin-113-indium-113m for the 4-у ionization chamber. Gamma-ray solution standards of radium–226 were resissued with an overall uncertainty of only 0.5 percent. Investigations were made of the zirconium-niobium decay-scheme parameters, and of the number of emitted 145-keV gamma rays per disintegration of cerium-141. Also, a new accurate method for calibration of gamma-emission rate by peak efficiencies was proposed.

X-ray and Gamma-ray Standards.-A calorimeter for measuring the total power output of gamma-ray sources was completed, and will be used to improve the accuracy of exposure measurements with cavity ionization chambers. Studies were made of the effect of humidity on ionization in free-air and air-filled cavity chambers; the correction previously applied was shown to be incorrect.

Improved Facilities for Calibrating Laboratory Standards.— Concurrent with the move to the new site, facilities for calibrating instruments used as laboratory standards have been greatly improved, modernized and expanded, and now provide an accuracy better than 2 percent. New facilities for the calibration of radium and other small radioactive sources have been installed and calibrations were carried out for 20 radium sources, 19 cobalt-60 sources, and three cesium-137 ranges. Five different types of radiation instruments were given evaluation tests.

Organic Dye Dosimetry Systems.-Organic dye systems, previously used at 105 to 108 rads in the form of thin films and gel blocks for dose distribution studies, have now been developed for the 102 to 105 rad range, using the liquid phase. The liquid may also be used to impregnate paper for use as a "go-no-go" indicator in a number of steps from 105 to 108 rads.

Radiographic Emulsions Response Improved.-The response of radiographic emulsion has been enhanced by the use of monochromatic light exposures before development, and by developing selectively the internal portion of the silver halide grains. Investigations have shown that the enhancement results primarily from a large increase in developed grain size.

Lithium Fluoride Thermoluminescence Dosimetry.-The total

thermoluminescence light emission of lithium fluoride was studied as a function of exposure and exposure rate, and as a function of photon energy. Cobalt-60 gamma rays and a broad spectrum of low-energy bremsstrahlung were employed. No rate dependence of the response was found over the entire range investigated, but the shape of the response-exposure curve was shown to change with photon energy, particularly in the region of superlinearity.

Beam Monitoring Activities.-Construction and assembly of the precision 100 kW Faraday Cup for the NBS linear accelerator was essentially completed, and final assembly of a secondary emission monitor was started. Initial experiments on these devices await the availability of the electron beam in the linac measurement room. Some preliminary experimental work on various current, position, and size monitoring devices was begun using the direct beams from the linac. Position-sensitive signals have

been observed in a mock-up nonintercepting pickup.

PHYSICAL PROPERTIES

Explosive growth of the scientific literature and increasing concern with scientific information are well-known national and world-wide phenomena. The Institute for Basic Standards, with its defined emphasis in the quantitative and measurable aspects of scientific information, has certain basic responsibilities to research establishments, to manufacturers and to users for providing reliable and accessible numerical information on the physical properties of substances. These responsibilities are expressed in all phases of the IBS program, including: (1) maintenance and application of measurement standards; (2) definition of physical quantities; (3) calibration services extending to others the capability for compatible measurement; and (4) measurement and dissemination of numerical values for properties which need be determined only once.

The IBS program in this area is very broad in terms of its coverage of the physical sciences. However, it obviously can and does cover only a small portion of the whole field of physical property measurement. The experimental portion of this program parallels the data compilation activities that are described under the National Standard Reference Data System.

The Institute is also engaged in a substantial amount of theoretical investigation. Most of it is aimed at augmenting measurement capabilities by developing methods of calculating physical properties. This is done where the properties are not directly observable or where it is more efficient to establish functional relationships which make it possible to extrapolate measurements

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