A model for calculating the effects of small inhomogeneities on electron beam dose distributions.

Charged particles penetrating a medium are repeatedly deflected from their line of motion by coulomb interactions with target nuclei. This physical feature should be included in the calculation of dose distributions when the irradiated medium contains small inhomogeneities. We use the mean path taken by particles in homogeneous media to represent their motion through inhomogeneous materials. This procedure adapts wht small angle multiple scattering formalism to yield manageable, approximate solutions when a medium contains arbitrarily composed inhomogeneities. To promote the development of this procedure into an electron beam treatment planning program, we discuss the salient features which arise in connection with electron beam inhomogeneity calculations. We use a solid phantom containing an air cavity just below its surface as a particular example of a small inhomogeneity. We discuss qualitatively and quantitatively the dose distributions which result from their irradiation with an 11-MeV electron beam.

Ionization collection efficiencies of some ionization chambers in pulsed and continuous radiation beams.

The most commonly used method of calibrating high-energy photon or electron beams consists in converting cavity ionization to dose by the application of the appropriate Clambda or CE multipled by the 60Co correction factor. The correct interpretation of calibration data for pulsed photon or electron beams requires a knowledge of the charge collection efficiencies of the ionization chambers used. The results are presented of efficiency measurements for both pulsed and continuous beams made with these chambers: 0.6-cm3 Farmer, 0.5-cm3 Spokas, 3-cm3 Shonka, 1-cm3 PTW, and 1-cm3 Memorial pancake. The dependence of collection efficiency on collection voltage, dose rate, and dose per pulse is demonstrated. These results are shown to agree with Boag's formulas for collection efficiency. Attention is drawn to the fact that several kinds of dosimeters provide only minimal collection voltages for efficient collection of charge at high dose rates, especially in Linac electron beams. It is recommended to check the collection efficiency of chambers which are to be used at high dose rates, and a simple method for this purpose is described.

Examination of the factors Ac and Aeq for cylindrical ion chambers used in cobalt-60 beams.

The calibration of a cobalt-60 beam in a phantom with an ion chamber, which has been calibrated with respect to exposure, requires the use of a displacement correction factor which essentially corrects the photon fluence for the attenuation and scatter when the chamber with buildup cap is removed and replaced by phantom material. To determine the displacement factor, Ac, a special set of cylindrical ionization chambers with various volumes were constructed out of polystyrene. Tissue-air ratios were measured with these chambers for cobalt-60 gamma rays in a polystyrene phantom, and the ratio Ac/Aeq was experimentally determined. In order to calculate Ac from this ratio, Aeq was determined also. It was found that Ac depended on chamber diameter only, and not on field size or depth. A value of 0.990 for Aeq is recommended and a table of Ac for chambers of different outer diameters is included.

Energy dependence of the response of lithium fluoride TLD rods in high energy electron fields.

The energy dependence of lithium fluoride dosemeters is a complicated function of energy as well as of cavity size. In the application of TLD to charged particle dosimetry, a cavity perturbation effect may exist even though the ratios of the mass stopping powers are constant over the energies encountered. This effect was investigated for lithium fluoride rods in electron fields ranging in energy from 2-5 to 20 MeV. A 13% change of TL response per unit of absorbed dose was measured over that energy range. A semi-empirical theory was developed to account for the cavity effect, using Burlin cavity theory as a starting point. The agreement between theory and measurement is satisfactory.

A nutritional and taxonomic survey of Arthrobacter soil isolates.

One hundred and sixty Arthrobacter soil isolates, four groups of 40 from each of four soil types, and 17 Arthrobacter named strains were characterized on the basis of 203 different nutritional tests performed on each culture. One hundred and twenty-one compounds were examined as sole sources of carbon, 44 compounds as sole sources of nitrogen, 22 carbohydrates for the production of acid, and 16 tests were done for tolerance to various dyes. The four groups of soil arthrobacters differed markedly in acid production from carbohydrates, and in utilization of aliphatic amino acids and aromatic hydrocarbons as sole sources of carbon. The named strains did not exhibit as much nutritional diversity as any of the four groups of soil isolates. The results of the nutritional tests were subjected to a computerized taxonomic analysis. Eighty-four of the isolates were contained in nine separate clusters with 87 to 94% similarity; 39 isolates were contained in five mixed but recognizable clusters with 84 to 86% similarity; 60 isolates were randomly mixed with no recognizable clusters and similarity of 72 to 83%. All clusters were composed of nearly equal numbers of isolates from each of the four soils; the named strains were not found associated with any of the clusters of soil isolates. The results indicated that the diversity of the genus Arthrobacter is not represented by the named species already described, and the possibility of using a series of standardized tests to categorize soil arthrobacters into readily identifiable nutritional groups is discussed.

Ecology of soil arthrobacters in clarion-webster toposequences of iowa.

Toposequence variations in soil properties were characterized and related to variations in populations of total isolatable bacteria and arthrobacters. Increases in soil NO(3)-N, available phosphorous, NO(3)-N-producing power, Arthrobacter counts, and the percentage of the total counts represented by arthrobacters were correlated with decreases in soil acidity. The total bacterial counts were not correlated with soil acidity but were associated with percentage of soil organic matter and percentage of clay. The percentage of the total counts represented by arthrobacters was lowest at the summit position and increased downslope to the highest value in the toeslope position. Factor analysis of the data revealed that 67 to 81% of the total variance exhibited by all variables per site-sampling period could be accounted for by soil acidity, soil structure, soil fertility, soil moisture, and bacterial factors. A selective medium was developed for soil arthrobacters and tested on a wide variety of central Iowa soils to determine its potential as a medium for enumeration as well as isolation. The medium developed in this study was found to be superior to the other available direct-isolation media for soil arthrobacters.

Nile blue A as a fluorescent stain for poly-beta-hydroxybutyrate.

Poly-beta-hydroxybutyrate granules exhibited a strong orange fluorescence when stained with Nile blue A. Heat-fixed cells were treated with 1% Nile blue A for 10 min and were observed at an excitation wavelength of 460 nm. Glycogen and polyphosphate did not stain. Nile blue A appears to be a more specific stain for poly-beta-hydroxybutyrate than Sudan black B.

Elution and inactivation of bacteriophages on soil and cation-exchange resin.

A variety of elution schemes was tested to determine the most effective procedure for eluting Arthrobacter bacteriophages from soil. A buffer solution of pH 8.0 was found to be the most satisfactory eluent. Bacteriophages were adsorbed to cation-exchange sites on soils, clays, and Dowex-50 resin and eluted. Eluted bacteriophages were detected by passive hemagglutination and plaque assay. Although bacteriophage antigen was successfully eluted, most recovered bacteriophages were noninfective. Inactivation was greatest in bacteriophages of Bradley group B and was associated with the elution process. Inactivation is believed to be caused by physical damage to bacteriophage tails upon elution. The significance of this inactivation relative to bacteriophage models for vertebrate viruses is discussed.

Differentiation of Arthrobacter soil isolates and named strains from other bacteria by reations on dye-containing media.

Twenty-five gram-negative and 62 gram-positive bacterial cultures, of which 37 werearthrobacters, were tested, using a multipoint inoculation device, for responses on varying concentrations of 35 dyes. Both selective (growth vs. no growth) and differential (dye absorption) responses were obtained. It was possible to differentiate the arthrobacters from the other gram-positive bacteria tested as well as to distinguish separately each Arthrobacter-named strain. The results indicated that dyes have a greater potential for use in selective and differential media than has been presently realized; and the possibility of using dye-reactions as features in taxonomic schemes to differentiate rapidly many of the common genera of soil bacteria is discussed.