Considerable effort was devoted to preparation for the implementation of an enhanced research program that will be made possible by a new laboratory building now nearing completion. New high-intensity sources, such as the 1.5 and 4-MeV direct accelerators and the 100-MeV linear accelerator, will enable the Bureau to satisfy many of the increased requirements. The compilation and evaluation of critical data began in the areas of electromagnetic and photonuclear interactions. Programs were initiated on the investigation of low-level radioactive contamination in materials and on the calculation of energy dissipation by fast electrons and associated gamma radiation for given source and medium configurations.

Dosimetry

Cavity Chambers. Instruments used as laboratory standards for x- and gamma-ray measurements by medical institutions, research laboratories, manufacturers, atomic energy installations, the Military, and Civil Defense are calibrated using the Bureau's x-ray and gamma-ray standards. The accuracy of the measurement of the beams used in these calibrations is presently better than 2 percent. During the past year, a toal of 102 calibrations was performed.

Effects of cavity-chamber geometry on the measurements of gamma-ray beams used to calibrate secondary standard exposure meters were studied to find the corrections required for conversion of cavity-chamber measurement data to exposure units.

A collimating system for the high-exposure-rate cobalt 60 source was constructed, and the rectangular beam obtained was calibrated. The system permits calibration of instruments of ranges higher than 25 roentgens, under controlled irradiation conditions. Leakage and stem effects in condenser R-meter chambers used as secondary standards were studied to develop calibration procedures. Further study is needed to reconcile the measured stem effects with the differences observed between calibrations for different irradiation conditions.

Photographic Dosimetry. Studies on enhancing x- and gamma-ray responses in several photographic dosimetry emulsions included hypersensitization with certain salt baths, latensification by means of chemical treatments or post-exposures to selected narrow bands of light, special development procedures, and combinations of these. The study of special development procedures for reducing the energy dependence of dosimeter-film response to x and gamma radiation in the range from 0.025 to 1.25 MeV continued. A reduction in energy dependence by over a factor of 2 was achieved at the expense of some loss in sensitivity.

Characteristics of commercial x-ray dosimetry films exposed to photons of widely different energies were compared for processing in phenidone-thiosulfate monobaths and in x-ray and phenidone developers. For routine dosimetry applications, monobath processing has the advantages of little dependence of response on processing time and temperature and, because of lower contrast, extended useful exposure range. Difficulties due to rate

dependence are more likely to occur, however, and film sensitivity is reduced. Processes leading to solarization and other rate-dependent photographic effects were investigated. The study of the total silver content of developed photographic emulsions, concentration of silver grains, and grain structure was completed.

Solid-State Dosimetry. Comparative measurements were made to determine the sensitivity, energy dependence, and voltage dependence of the response to x rays of diffused p-n junction, p-i-n junction, and surface-barriertype silicon radiation detector cells. Measurements were limited to energies of less than 250 keV to avoid radiation damage possibly produced in silicon by radiations of higher photon energy.

Photocurrents observed in p-i-n junction cells were larger and showed a smaller increase with increasing bias voltage than photocurrents measured in p-n junction and surface-barrier cells at the same exposure rate and with the same irradiated cell surface area. However, because of their smaller zerovoltage resistance, p-i-n junction cells have larger dark currents and may show a smaller photovoltaic sensitivity than cells of other types when operated with higher load resistances or under open-circuit conditions.

When either a forward or reverse voltage was applied to the commercialtype surface-barrier cells, most of them showed similar, nearly symmetrical current-voltage characteristics. This similarity apparently was due to the existence in such cells of two opposing voltage barriers which are alternately reduced or increased when a bias voltage is applied in these two different directions. Consequently, the photocurrent showed a reversal with reversing bias voltage and the photovoltaic sensitivity was negligibly small. Surfacebarrier cells showing such performance charactertistics can be used only as photodiodes.

The dependence of the photocurrent on the quality of the x rays (energy dependence) was found to be similar for the three cell types investigated, and changed only slightly when moderate bias voltages were applied.

Response of Hurst Neutron Dosimeter. Recently, the fast neutron dose distribution in water from a D(d,n) He source was measured. The energy response of the Hurst proportional counter dosimeter used in the measurements did not follow the predictions of the original paper describing the instrument. An investigation showed that an incorrect spectrum had been assumed for the recoil particles incident on the cavity. A simple corrected calculation led to much better agreement with the experimental measurements, and a more sophisticated calculation for the response of spherical and cylindical counters of this type in an isotropic field was completed.

X- and Gamma-Ray Standards. Several years ago the National Bureau of Standards designed, constructed, and calibrated some ionization chamber transfer instruments for the Bureau International des Poids et Mesures (BIPM). Since that time, a number of large laboratories and clinics have expressed an interest in such instruments, and the BIPM has recommended that each national laboratory obtain its own transfer instru

ment so that all could be intercompared at BIPM. On the basis of this broader need, one instrument was redesigned in a cooperative program arranged with F. R. Shonka of St. Procopius College, Lisle, Ill., who contributed knowledge of conducting plastics and molding techniques.

One model of the new version, with an outer wall thickness of 2.5 mm, can be used to measure 60 to 250 kV x rays and cobalt 60 and cesium 137 gamma rays with a variation in calibration factor of about 42 percent. Another model, with a wall thickness of 0.25 mm, can be used to measure 60 to 250 kV x rays with a variation in calibration factor of about 3 percent. For repeated calibrations, the range of calibration factors for any one quality of radiation is about 0.3 percent.

Recently a need has arisen for the calibration of instruments used by medical institutions, research laboratories, and others for the measurement of very soft x rays produced by 10 to 15 kV. Accordingly, a free-air chamber standard for the measurement of x rays produced by 10 to 60 kV was constructed. Comparison with the NBS "low" energy standard, used for measurement of x rays generated by 20 to 100 kV, in their overlapping range gave an agreement, on the average, to 0.3 percent. The estimated maximum inaccuracy is about 1 percent.

Radioactive Sources

Standards of Radioactivity. The sum of standards sold and calibrations performed exceeded 650. Radioactivity standards developed and prepared for distribution included tritiated (hydrogen 3-labeled) toluene, strontium 89, cerium 141, cesium 137-barium 137m (gamma-ray point source for gamma-ray spectroscopy), and americium 241. International comparisons. were conducted of the radionuclides strontium-yttrium 90, americium 241, and cobalt 60 (solid sources), distributed by the Bureau International des Poids et Mesures; cesium 137-barium 137m, distributed by the International Atomic Energy Agency.

Cross section of NBS model of a portable ionization chamber for use in the international standardization of radiation measurements by the International Bureau of Weights and Measures. The instrument measures 60 to 250 kV x rays as well as cobalt-60 and cesium-127 gamma rays. (See page 97)

A new, precise method for diluting highly volatile liquids was used to prepare tritiated toluene which when compared with previously standardized tritiated water by liquid scintillation counting gave a standard deviation of 0.1 percent.

Four radiochemical procedures, suitable for the determination of actinides at the 0.1 MPC (maximum permissible concentration) level were evaluated under the low-level radioactivity program, supported partly by the Atomic Energy Commission. A new method of low-level radioisotope dilution analysis was developed and applied to cerium 144. Low-level intercomparisons of cerium 144 in spinach (sponsored by the International Atomic Energy Agency) and in radium 226 in water (sponsored by the Public Health Service) were carried out.

Spectrometer for Neutron Source Studies. The energy range of greatest uncertainty in the use of neutron sources as standards is below 2 MeV, where most spectrometers do not work or work poorly at best. A proton recoil spectrometer using a methane proportional counter as radiator and a silicon junction detector as the major energy detector is being developed. Preliminary results indicate that the instrument will be useful down to at least 0.5 MeV, and will exhibit the low gamma sensitivity required for spectral measurements of sources, such as the NBS standard Ra-Be (y,n) source, having high gamma emission rates.

Radiation Interactions with Nuclei

Fast Time-of-Flight Measurements of Neutron Cross Sections. After interaction with the nucleus under study, neutrons of differing energies are separated by measuring their flight times from target to detector. With one technique (Mobley system), the neutrons are produced in bursts and the time of detection relative to the incident neutron burst is measured; with the other, a charged particle produced at the same time as the neutron is used as the time reference. A Mobley time compression magnet system installed on the Van de Graaff accelerator produced 0.8 microsecond proton pulses of about 360 microamperes. The charged particle technique was refined by improving the time resolution to about 1 microsecond and developing n-y discrimination compatible with fast time of flight.

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Nuclear Size Determinations from Coherent Neutral Meson Photoproduction. An extensive series of measurements on nuclear matter distributions by coherent photoproduction were completed for lithium, beryllium, carbon, oxygen, magnesium, aluminum, silicon, sulfur, calcium, and copper. The nuclear matter radii were determined with an accuracy of about 0.2 × 10-13 cm and agree well with nuclear size determinations by other techniques, such as elastic electron scattering.

Nuclear Theory. A dynamic theory of the nuclear collective model was developed in which the rotation-vibration model and the hydrodynamical dipole-oscillation model are unified. A coupling between the dipole oscillations and the quadrupole vibrations is introduced in the adiabatic approximation. The dipole oscillations act as a "driving force" for the quadrupole vibrations and stabilize the nucleus in a non-axially symmetric equilibrium