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 103 to 10a 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

has provided scientists with many new techniques for measuring “old” physical properties and has made accessible many new ones. With new techniques it has been possible to increase the precision of measurement, provide the scientific community with a greater range of reference data, and characterize certain processes more fully. Examples of these activities, drawn from current work, are given below.

Nuclear Properties

Solid-State Beta-Ray Spectrometer.-A spectrometer has been constructed for analyzing electron energy distributions with a very high efficiency and with a resolution comparable to the much lower efficiency magnetic lens spectrometers. The spectrometer employs two cooled lithium-drifted silicon detectors of better than 5 keV resolution which are operated in a 4 geometry or in a 2 geometry with a selection of events where all of the electron's energy is deposited in one detector. Time coincidence with associated gamma rays selects the electron spectrum of interest.

The Nuclear Three-Body Problem.-A calculation of the photodisintegration of 3H and He has been made using approximate groundstate wave functions based upon two-body effective range theory. The results are in fair agreement with experiment and considerably better than previous calibrations. A new calculation has been started using wave functions derived from a separable nucleon potential. These wave functions have been successfully applied to neutron-deuteron scattering.

Radioactivity Studies.-Residual activity studies of the monoisotopic nuclides bismuth and praseodymium after bombardment with the linac electron beam have resulted in measurements of multi-neutron deficient daughter products in which as many ten neutrons have been removed. The data can be represented by a simple power law relating daughter product yield to the number of nucleons emitted. This power law has been used to predict the yield of any specific daughter product, the total decay gamma-ray rate, or the total gamma-ray dose to be expected from any target material, including composite materials, for arbitrary bombardment time and arbitrary times after bombardment. Experimental tests of these predictions indicate that they are sufficiently accurate for radiation protection purposes. The calculations also indicate a number of previously unobserved radioactive nuclei with reasonable half-lives, far off the stable mass curve, which should be produced in significant quantities by electron bombardment.

Electron Scattering. The apparatus for elastic and inelastic highenergy electron scattering with the NBS linac has been brought into operation. An experiment to measure the inelastic scattering cross

sections for exciting nuclear levels in 12C at 4.4, 7.6, 9.6, 10.8, 15.1, and 16.1 MeV was performed at three angles and at momentum transfers up to 220 MeV/c. Analysis of this data should yield strengths and transition radii for both longitudinal and transverse excitations of these levels. A new multi-channel detector system containing twelve semiconductor detectors has been completed and is presently being installed. The new system should improve the experimental energy resolution by a factor of four (to 0.1 percent) with no loss of datataking speed.

Heavy-Ion Spectrometer.—A large magnetic spectrometer for the spectroscopy of charged particles from electron- and photon-induced nuclear disintegration reactions has been constructed. The spectrometer magnet is a double-focusing, 120° wedge with a 45.6-cm radius of curvature. The energy resolution of the magnet at the central momentum orbit is better than 0.2 percent and the useful solid angle is 0.006 sr. (maximum solid angle is 0.01 sr.). When fully implemented with fifty 0.2 x 0.2 x 6 cm semiconductor radiation counters in the focal plane, the spectrometer will have a total energy acceptance interval of approximately 20 percent. The focal plane counters are lithium-drifted gold-surface barrier counters especially developed at NBS for the spectrometer. In use they are cooled to 77 °K to ensure fast and uniform response.

Cryostat for Neutron Cross Section Measurements.—A He3 refrigerator with a perpendicularly mounted 55 kilogauss superconducting solenoid was completed and used to provide a large polarized holmium target. This equipment was shipped to the Proton Synchrocyclotron at the Atomic Energy Resarch Establishment, Harwell, England, for use in a joint project to determine the shape of the highly deformed holmium nucleus. These measurements were carried out using neutrons covering the entire energy range from 1 to 130 MeV. The low-energy neutron data are being analyzed in terms of the optical model of the nucleus, whereas the high-energy data are being compared to the black nucleus model.

Photonuclear Data Center.-The file of photonuclear data and its index have been maintained up to date with the current literature. On the average, new data are added to the files within less than three months of the publication date. New entries are being made to the file at the rate of about 140 per year. Data in the files were the basis for a series of lectures surveying a part of the field given at the Institut fur Theoretische Physik der Universitat Frankfurt/Main.

Atomic and Molecular Properties

Spectra of the Lanthanide Elements.-Knowledge of the electronic level structures of neutral promethium, cerium, thulium, and thorium has been expanded during the past year. Significant progress has also been made on the spectra of singly ionized holmium and thulium. An analysis is underway on the spectrum of neutral holmium, for which no levels are presently known.

Atomic Spectra in the Vacuum Ultraviolet Region.-Observation of the neutral silicon spectrum has been completed. One hundred and forty-one Si I wavelengths of high accuracy suitable for use as standards in the region 156-200 nm were obtained. Selected lines for neutral carbon, nitrogen and germanium and for singly ionized copper and silicon were measured. There are now about 500 wavelengths in the region below 200 nm with uncertainties of 0.0002 nm or less.

Lyman-Alpha Excitation.-The flux of Lyman-a photons (121.6 nm) excited by an electron beam of known intensity and energy has been measured in a high-vacuum apparatus. The excitation probability has been measured with a precision of better than one percent and an accuracy of the order of 2 percent. This process is the cause of one of the strongest solar radiations, but one observable only at rocket and satellite altitudes because of ionospheric absorption. Detailed interpretation of observations of solar radiation will be aided by knowledge of this inelastic atom-electron collision process.

Profiles of Resonances in Atomic Continua.-Using the NBS 180Mev electron synchrotron as a background source for atomic absorption spectroscopy, the absorption profiles of all the prominent resonances in the continua of the noble gases have now been obtained. The resonances are caused by short-lived high-energy states involving inner electron excitation, or the excitation of two electrons simultaneously. The data were obtained on an NBS-constructed high-resolution scanning monochromator of the grazing incidence type, and data col· lection and reduction have been automated. All of the prominent resonances in helium through xenon have been studied, and data are now being reduced and made available to theorists.

Theoretical Studies.-A simple quantum-mechanical model for a maser or laser was extended to include correlation between radiating atoms and the field, and also between different atoms. Problems arising in the solution of the resulting set of nine coupled differential equations have now been overcome. The results indicate the accuracy of the semi-classical approximation used in the simpler theory in a way distinct from that used previously. An unexpected result of the above analysis was the integrated noise behavior of the maser. This led to a