2.1.7. RADIATION PHYSICS

The increasing application of atomic and nuclear technology to industrial, medical, and defense activities has resulted in an expansion of the demands placed upon radiation research. Industrial uses of radiation have brought about a growing need for improved standards and dosimetry at high dose levels and high energies. Medical users increasingly turn to higher energies and call for improved determination of both source output and absorbed dose. Research workers interested in the effects of radiation on various materials have need for more information in their field. The Bureau has attempted to meet these increasing demands with its radiation research program.

The research of the Bureau is directed toward obtaining basic experimental and theoretical data concerning the interactions of radiation with nuclei, atoms, and molecules, as well as with bulk matter; the investigation, development, and improvement of radiation sources and standards; and the development of improved techniques and instruments for the detection and measurement of these radiations.

Radioactivity Standards. A manganese 54 point-source standard, a scandium 46 gamma-ray solution standard, an iron 55 electron-capturing nuclide standard, and a promethium 147 beta-ray standard were developed during the year. Using the NBS internal compensated gas counters, the half-life of carbon 14 has been redetermined as 5,760±50 years, where the indicated uncertainty denotes an estimated overall probable error of the result.

The low-level counting facilities have been completed and the "white" room which was designed to exclude 99.9 percent of all dust particles greater in size than 1 micron has so far revealed none larger than 0.1 micron. This facility is for studying methods of measuring the amounts of radionuclides present at very low concentrations and in making international intercomparisons of radioactive samples at these concentrations. The program has also included an examination of low levels of contamination of materials and reagent chemicals by any radionuclide.

Radiation Theory. To provide information on the basic processes of radiation production, on the characteristics of the radiations, and on their absorption by, or other interactions with. matter the Bureau maintains a broad program in radiation theory. Work on photon and neutron penetration, together with application to structure shielding against radiations from nuclear weapons, has been partially supported by the Office of Civil and Defense Mobilization and the Defense Atomic Support Agency. Work on charged particle penetration and elementary cross sections has been partially supported by the Office of Naval Research.

Computer Programs. Major and sophisticated computer programs have been developed for calculations of electron and gamma-ray penetration. The Monte Carlo program of calculating the multiple scattering of charged particles has been so refined that it is now possible to analyze the differences

"White" room constructed for use as a low-level radioactivity laboratory and a sample-preparation room. This facility is used in studying methods for measuring very low concentrations of radionuclides and for international comparisons of radioactive samples at these concentrations (page 64).

between electrons and positrons in regard to phenomena such as backscattering or transmission by thick foils. The Monte Carlo program has also been applied to the analysis of proton range and stopping power experiments, with the aim of extracting the best value of the "mean excitation potential," which is a key parameter in the stopping power formula. A program for calculating neutron penetration distributions by moment methods has been nearly completed. This program should make it possible to study the physics of neutron penetration in detail, and should also make possible a much wider variety of deep penetration data than has ever been available before. Exploratory applications of this program are underway.

A nuclear optical model code was written to predict neutron elastic scattering cross sections. This code includes an estimate of the compound elastic scattering in addition to the shape elastic scattering which comes directly from the nuclear optical model. A report was prepared on the elastic scattering cross sections for calcium.

Data Collection. Data collection activity during the past year included a tabulation of X-ray spectra in uniformly contaminated media. This tabulation was also used for a detailed analysis of the errors that arise in the numerical solution of the X-ray degradation equation.

Results of the proton Monte Carlo work at 340 Mev and 660 Mev on range and range straggling has been analyzed and an estimate has been obtained of the small systematic error resulting from the particular Monte Carlo model that has been used. The sampled data was also used to obtain an estimate of the statistical distribution of the difference between path-length and depth of penetration for high-energy protons.

Civil-Defense Shielding Problems. Considerable work has been done toward the theoretical solution of shielding problems associated with the "fallout" of fission products from nuclear explosions. A monograph, “Structure Shielding Against Fallout Radiation from Nuclear Weapons," has been virtually completed. Work has been started to develop engineering data applicable to initial radiations, in analogy to the work completed for fallout radiations. Some work on Monte Carlo calculations for the analysis of simple structure geometries has continued but the emphasis has shifted to more complex geometries than slabs. The computer programs for calculating gammaray penetration have been revised and made more general, in order to make possible their use by other investigators; their use to produce more detailed information about flux angular distributions; and to make possible reliable calculations at high energies (<10 Mev).

Linear Electron Accelerator. The Bureau has been engaged for some time in the design of a new linear electron accelerator to be housed in its new Gaithersburg facility. This accelerator will produce a 100-Mev electron beam with 40 kilowatts in the beam. The design of the accelerator has been completed and the machine is now under construction. It is expected to be completed in November 1962. During the past year considerable effort has been devoted to completing the design of the laboratory in which the accelerator will be housed and the design of a beam-handling and analysis system. Since the accelerator will provide a beam with an energy spread of less than 2 percent, it will be possible to utilize a large fraction of the electron beam even after energy analysis by the beam-handling and analysis system of magnets, which will provide energy resolutions as small as 0.05 percent. The magneto-optical properties of systems of magnets were studied during the course of the design of the linac beam-handling system. A convenient matrix method was developed and applied to combine by a firstorder procedure analyzing and quadrupole magnets in order to predict the focusing and dispersing properties of combinations of magnets. This method

has now been published in a paper that demonstrates the general applicability of the matrix techniques to general deflection magnets. The method has also been used to predict by a second-order theory the properties of precision particle spectrometers. It has now been demonstrated that an energy resolution better than 0.05 percent is obtainable for reasonable target sizes with particles having momenta up to 250 Mev/C and with solid angles in the range from 0.005 to 0.01 steradians.

High-Energy Radiation. Research utilizing the betatron and synchrotron included two studies partially supported by the Atomic Energy Commission. One of these dealt with the measurement of neutral meson production for carbon, aluminum, copper, cadmium, and lead. The other involved a measurement of the photoneutron yield and photon-scattering cross sections for the highly deformed nuclei of holmium and erbium.

Angular distributions of neutral mesons photoproduced by 170 Mev bremsstrahlung from the synchrotron have been measured. The experimental distributions have been compared to Monte Carlo predictions and thereby have provided a measurement of the root mean square radii of nucleon center distributions having an estimated uncertainty of three percent. These results are about 0.2 fermi lower than the electron scattering measurements of the rms radii of charge distributions obtained at Stanford University. About one-half of this difference is understandable on the basis of the proton size. The remaining difference is approximately equal to the present uncertainties in the experiment and, therefore, may or may not be real.

The observed X-ray scattering cross sections for holmium and erbium were large compared to those calculated from the observed absorption (photoneutron) cross sections by means of the optical theorem and the dispersion relations. The absorption and scattering cross section are consistent if it is assumed that these nuclei have a large intrinsic tensor polarizability. The data also suggest that in the energy region between 10 and 23 Mev as much as 10 to 20 percent of the integrated absorption cross section is not associated with the tensor polarizability.

A third nuclear physics program that was actively investigated during the past year was the measurement of total nuclear cross sections. This work has required the development and improvement of the energy resolution of high-energy scintillation X-ray spectrometers. The best results were provided by a sodium-iodide spectrometer with a resolution of 2.5 percent at 17.6 Mev.

Total nuclear absorption cross sections are being measured with this. spectrometer by examining the X-ray spectrum of 90 Mev bremsstrahlung transmitted by long absorbers in a good-geometry experiment. The use of the spectrometer combined with the high-intensity NBS synchrotron beam made it possible to attenuate the primary X-rays by a factor of over 10,000 with a resulting enhancement in the transmitted spectrum of the effect of small changes in the attenuation coefficient. The transmitted spectra show clearly the giant resonance nuclear cross section, as well as fine structure

Model of the linear electron accelerator complex which will be built as part of the NBS Radiation Physics Laboratory at Gaithersburg, Maryland. Dotted line on the model indicates the ground level (page 66).

in such absorbers as oxygen, carbon, and magnesium. The detailed evaluation of the total nuclear and the total interaction cross sections in a wide range of elements will be completed at the end of the present series of experiments.

X- and Gamma-Ray Dosimetry. It is often assumed that cobalt 60 beams used for instrument calibration and radiation treatment contain only gamma rays with energies of 1.17 and 1.33 Mev. Though it is widely recognized that this assumption is not strictly valid because of the energy degradation due to scattering, adequate experimental data was not previously available to show the magnitude of the discrepancy. Therefore, experiments were conducted to determine the intensity and energy of scattered radiation from multicurie cobalt 60 sources and its variation with source and collimator geometry.

Photographic Dosimetry. The Bureau has investigated the effects of exposure of X-ray film to two successive types of radiation. The results of this study show that the shapes of the density-versus-exposure curves resulting from such dual exposures are essentially the same as those of the curves resulting from the second exposure alone. This work, supported by the Atomic Energy Commission, may lead to a better understanding of the nature of the photographic latent image and may also be of some interest in industrial and military applications of photographic dosimetry of Xand gamma-radiation.