Solar Radiation Measurements.-A program for developing and testing instrumentation which will measure the incoming ultraviolet solar irradiance on a horizontal surface has been completed. Two nearly identical photoelectric-filter spectroradiometers consisting of nine narrow-band- and one wide-band-pass interference filters covering the wavelength range 300 mm to 400 mm were constructed and simutaneous measurements were made at two locations, one having an atmosphere characteristic of a high degree of air pollution and the other having an atmosphere relatively free of air pollutants. A series of measurements at the two locations was made over a period of a few days during the four seasons of the year. The instrumentation will be turned over to the U.S. Public Health Service, which will continue the ultraviolet solar radiation measurements in connection with their overall air pollution program. Systematic Notation for Reflection and Transmission. The general concepts underlying measurements of the effects of objects on the propagation of light and other forms of radiant energy have been analyzed and systematized. The theory of the measurement of reflection and transmission characteristics has been unified and generalized to include combinations of these processes and such related processes as fluorescence. Concepts, terms, and symbols have been selected, with due regard to existing usage and standards. A systematic notation, based on established mathematical conventions, was developed to specify any of the infinite variety of geometrical arrangements and spectral conditions which might occur in practical applications. Eighteen classes of reflectance measurements have been identified and the mathematical interrelations among them have been published. This generalized treatment serves as a basis for unifying the concepts and terminology related to this kind of measurement in photometry, colorimetry, photography, and the graphic arts, where considerable divergence in usage has been apparent. Standard Symbols in Radiometry and Photometry.-Members of the NBS staff directed efforts that led to the publication of USA Standard for Letter Symbols for Illuminating Engineering. This Standard is of exceptional importance because it contains for the first time a set of symbols for basic terms and units in the fields of radiation and light that has received international acceptance. These symbols were developed and agreed upon by committees of the International Electrotechnical Commission, the International Commission on Illumination, the International Organization for Standardization, and the SUN Commission of the International Union of Pure and Applied Physics. Laser Source Integrating Sphere. An integrating sphere was built to measure the spectral, directional-hemispherical reflectance at sample temperatures up to 2500 °K. To improve the signal to noise ratio a He Ne laser was used as a source. A two-detector amplification system automatically corrected for power variations of the laser. An error analysis showed that the accuracy of the measurement is better than 1 percent. The error analysis was verified by an experimental investigation, using calibrated mirrors as standards. The instrument measures high-temperature reflectance data with an accuracy unobtainable with any instrument that measures emittance directly. Optical Test Patterns.-Modern methods of testing optical and photographic systems rely heavily on the analysis of results obtained by imaging sinusoidal patterns of varying spatial frequency. The production of high-quality sinusoidal patterns for test purposes is not easy, nor is the quality evaluation of test patterns. To meet a specific Government need for such test patterns, a series of photographic patterns was produced. The photographs vary sinusoidally in transmittance and produce the desired pattern when placed on a diffuse illuminator. The optical density was measured with an automated microdensitometer, and computerized reduction produced tabular and graphic data for each pattern. The uniformity, modulation contrast, mean density, and wave form were computed automatically and shown to be within close tolerance. New Refractive Index Technique.-A need arose for a precise measurement of the refractive index for both polarizations of a cuboid ruby crystal. Since the shape could not be altered to make a prism, the usual minimum-deviation method could not be used. The crystal was adhered to a prism of known index with a thin liquid film and light was passed through the crystal and prism. The crystal produced two wave fronts and by measurement and ray-tracing computations the two indexes were determined within 3×10-5. IONIZING RADIATIONS Linear Accelerator.—The beam-handling system for the NBS linac was accepted from the contractor on July 2, 1966, and direct connection of the linac and beam-handling system was completed in the fall of 1966. Routine scheduling of accelerator time for experiments was begun early in January 1967, and a total of approximately 700 hours of beam on-time for experiments was provided in the remainder of the fiscal year. During this period of time all experimental programs prepared to use the accelerator were brought into operation to the point where they are now taking, or are about to take, meaningful data. These include programs in electron scattering, photoneutron production, photoproton production, measurement of neutron total absorption cross sections, studies in beam monitoring, programs in activation analysis, and production of radioactive sources for materials studies using Mössbauer sources and for studies of photonuclear reactions. The NBS linac magnet room houses a system of magnets and collimators which refine the energy spread of the linac beam to less than 0.1 percent and steer it into one of three measurement rooms. Positron Source.-Final beam tests were made on a converter target which utilizes the pair production cross section to convert a high-intensity electron beam into a source of positrons. The converter is designed to be used with the maximum average current obtainable from the NBS linac. It was successfully operated with average beam currents up to 0.17 mA, the highest presently available from the linac. The 1.5-MeV Dynamitron.-The dynamitron has been successfully employed for experiments requiring both high and low current electron beams over its entire energy range from 0.2 to 1.5 MeV. For example, in nuclear excitation experiments where a high bremsstrahlung flux is required, a gold target was exposed to beam currents up to 5 milliamperes; however, in the measurements of the response of silicon solid state detectors to monoenergetic electrons where a low flux of electrons is desired, beam currents of 10-9 amperes were elastically scattered from a very thin gold target. The energy of the machine has been calibrated to better than ±2 percent. 4-MeV Accelerator.-A momentum analysis system has been installed on the NBS 4-MeV Van de Graaff accelerator. This system pro ་ས ་་་པ vides a momentum analyzed beam of electrons with a resolution of approximately 0.2 percent at 3.0 MeV. In addition, a beam deflection magnet has also been installed to provide more flexibility in setting up experiments. There are now nine separate ports which may be used for experimental setups. At the present time the voltage regulating circuits are being updated to provide voltage stability compatible with the high resolution now obtainable with the magnet system mentioned above. X- and Gamma-Ray Calibrations.-The new facilities for cobalt 60 and cesium 137 gamma-ray calibrations have been completed and provide exposure-rate ranges of from several thousand R/hr down to a few mR/hr. These gamma-beam facilities allow calibrations to be performed on a wide range of radiation instruments. The gamma-ray and X-ray facilities were used for the calibration and evaluation of about 70 instruments. In addition to the instrument calibrations, 37 small gamma-ray sources were calibrated, as well as several large cesium 137 calibrator units. Measurement of Electron Beam Energy and Current.-A simple apparatus has been developed which permits the simultaneous measurement of electron energy and current of an accelerator electron beam. This measurement is accomplished with negligible interference of the beam during the continuous operation of the accelerator and applies to electron energies greater than 50 keV. The apparatus consists of a thin aluminum foil which intercepts the beam with a negligible energy loss (less than 1 kilovolt), and two cylindrical aluminum electrodes which are positioned respectively on the incident and exit sides of the foil with axes normal to the foil surface and coincident with the beam direction. When an electron beam passes through this system, the electron energy can be determined from the ratio of the incident to the exit electrode currents, and the incident electron current can be determined from the incident electrode current. Radiochromic Dye Dosimetry.-New work with radiochromic dyes has shown these systems to be very promising for the accurate measurement of large radiation doses. The dye formation is measurable photometrically with precision limits of the order of one percent. The systems are highly versatile in that they can be used in liquid, vapor, powder, gel, or thin-film form. Current research for the Atomic Energy Commission is focused on developing liquid cells as measurement standards for measuring x- and gamma-ray exposures from 10 to 10% roentgens. Another project for NASA has provided a convenient device for beam profile evaluation. This electromagnetic isotope separator was recently installed in the NBS Radiation Physics Laboratory for rapid preparation of highly pure, thin, massidentified sources of stable and radioactive isotopes. Photographic Dosimetry.-New techniques were developed for changing and improving the energy dependence of the response of radiographic emulsions. These techniques employ special chemical treatments and photographic processing methods involving physical and localized development of the exposed silver halide emulsions. The energy dependence of some emulsions has been reduced as much as threefold. Radioactivity Standards.-In fiscal year 1967, 498 radioactivity standards were sold, and 39 calibrations performed. Two new radioactivity standards were produced: cobalt 60 in the form of a gammaray point-source, and mercury 197 in solution form. The following eight radioactivity standards were reissued with the same or better accuracies than previous issues: polonium 210; americum 241; solution standards of sodium 22 and iodine 125; point-source standards of yttrium 88; and point-source and solution standards of niobium 95 (with an accuracy of ±1.5 percent and ±1.2 percent, respectively). A special system for de-emanation of liquid samples containing radium 226 and subsequent counting of liberated radon has been installed. PHYSICAL PROPERTIES Approximately 50 percent of the activity within the Institute for Basic Standards involves the characterization of physical properties of well-defined substances. The rapid growth of the physical sciences |