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At least 847 records · Page 47Linked to original sources
PubMed1983

Electron contamination of large cobalt 60 photon beams.

Experimental results are presented of the electron contamination of cobalt 60 gamma photon beams from the Mobaltron MS80, over a range of SSD's from 60 cm to 3.5 m. In the absence of an electron filter, the skin dose can exceed the dose at 4 mm depth by up to 1.6 times due to the high level of electron emission from the depleted uranium collimators. 4 mm of perspex effectively removes these electrons, and in the filtered beam, the skin dose cannot exceed the 4 mm dose. The electron contamination of a 6 MV X-ray beam is compared.

Cobalt Radioisotopes↗
PubMed1983

Exit dose studies in megavoltage photon therapy.

A coin-shaped ionisation chamber, orientated with its thin window facing away from the radiation source, was used to investigate the dose perturbations caused by the absence of back-scattering material near the exit surface of solid phantoms. Cobalt 60 and 4, 8 and 16 MV X-ray beams were used in the study. With no scattering material beyond the chamber window the ionisation was found to be as much as 17% less than the full scatter value. This was attributed to the absence of both back-scattered electrons and back-scattered photons. Full electron back-scattering could be restored by placing between 1.0 and 2.7 mm of unit density material beyond the chamber, depending on the primary beam energy. Under these circumstances the reduction in dose, now due to the absence of back-scattered photons only, was found to be small.

Cobalt Radioisotopes↗
PubMed1984

Conversion factors for calculating mass stopping power ratios.

In order to use an air-filled ionisation chamber for dosimetry of a photon or electron beam in a material, it is necessary to know the restricted (Spencer-Attix) mass stopping power ratio of the material, relative to air. A method is described in which a conversion factor, k, is derived by comparing unrestricted stopping power ratios of various materials (relative to air) with those for water (relative to air). Provided that the material does not differ too greatly in effective atomic number from that of water, this ratio is independent of energy and is a constant for any material both for unrestricted and restricted stopping powers. Values of k have been calculated for 31 materials and application of any one such factor to the restricted mass stopping power ratio of water relative to air allows the appropriate restricted mass stopping power of the material, relative to air, to be derived.

Air↗
PubMed1984

Dose-rate effect between 1 and 10 Gy/min in mammalian cell culture.

A dose-rate effect is observed between 1 and 10 Gy/min for cultured Chinese hamster V-79 fibroblast cells irradiated under pO2 levels of less than 10 ppm, 100 ppm, 300 ppm and 21%. The dose for a cellular surviving fraction of 0.1% increases by 10-15% as the dose rate is changed from 10 to 1 Gy/min. This observed dose-rate effect is consistent with the repair of sublethal damage taking place during the radiation delivery. A related finding is that acute hypoxia in cell culture does not inhibit sublethal damage repair.

Animals↗
PubMed1994

Space proton transport in one dimension.

An approximate evaluation procedure is derived for a second-order theory of coupled nucleon transport in one dimension. An analytical solution with a simplified interaction model is used to determine quadrature parameters to minimize truncation error. Effects of the improved method on transport solutions with the BRYNTRN data base are evaluated. Comparisons with Monte Carlo benchmarks are given. Using different shield materials, the computational procedure is used to study the physics of space protons. A transition effect occurs in tissue near the shield interface and is most important in shields of high atomic number.

Computer Simulation↗
PubMed1996

A Green's function method for high charge and energy ion transport.

A heavy-ion transport code using Green's function methods is developed. The low-order perturbation terms exhibiting the greatest energy variation are used as dominant energy-dependent terms, and the higher order collision terms are evaluated using nonperturbative methods. The recently revised NUCFRG database is used to evaluate the solution for comparison with experimental data for 625A MeV 20Ne and 517A MeV 40Ar ion beams. Improved agreements with the attenuation characteristics for neon ions are found, and reasonable agreement is obtained for the transport of argon ions in water.

Argon↗
PubMed1976

Muon-induced visual sensations.

The visual phenomena induced by the passage of a pulse of extremely relativistic muons through the vitreous humor have been studied at the Alternating Gradient Synchrotron at Brookhaven National Laboratory. The visual phenomena include flashes that range from small crescents of light in the peripheral field of view to large clouds of light that fill the entire field of view as well as bright flashes with dark centers. Three subjects have been exposed to date. Arguments are given to show that the physical mechanism behind these flashes is Cerenkov radiation. Standard psychophysical techniques are used to determine the threshold for muoninduced visual sensations for one subject. Comparison is made with his pion treshold measured under the same condition.

Elementary Particles↗
PubMed1977

X-ray photoelectron spectrometry of copper-thiouracil complexes.

X-ray photoelectron spectrometry was applied to evaluate the correct oxidation number of copper in complexes with 2-thiouracil, 6-amino-2-thiouracil and 6-methyl-2-thiouracil. Regardless of the mode of preparation exclusively Cu(I) was bound to the different thiouracil compounds, producing one homogeneous signal of the Cu2p3/2 electrons at 932.5 eV. Upon oxidation with H2O2, a typical Cu(II) satellite appeared in the main signal of the Cu2p3/2 level was shifted to higher binding energy values. The reaction of Cu(II) with thioracil yielded identical complexes as above, in which Cu had the formal oxidation state +I. During this reaction portions of the thiouracil were oxidized to 2,2'-dithiobis(4-pyrimidinol) [bis(4-hydroxy-2-pyrimidinyl)disulphide], as seen by the shifted sulphur 2p signals to 163.5 eV. After treating the Cu-thiouracil complexes with H2O2, the simultaneous presence of sulphur species having the oxidation states RStheta (161 eV), RSSR (163.5 eV) and RSOtheta3 (168.5 eV) is indicated by the ESCA signals monitored.

Copper↗
PubMed2004

Bipolaron in different configuration of quantum confinement.

The authors used Landau-Pekar variational method to investigate a strong-coupling singlet optical bipolaron in different configuration of quantum confinement. Numerical and analytical results showed that when configuration changes from quantum dot and wire to well, confinement shows different effect on the formation of a bipolaron. In contrast to a bipolaron in a quantum dot or wire, the binding energy of a bipolaron in a quantum well increases with increasing confinement, indicating that confinement favors bipolaron formation in a quantum well.

Algorithms↗
PubMed2003

High-energy neutron spectroscopy with thick silicon detectors.

The high-energy neutron component of the space radiation environment in thick structures such as the International Space Station contributes to the total radiation dose received by an astronaut. Detector design constraints such as size and mass have limited the energy range of neutron spectrum measurements in orbit to about 12 MeV in Space Shuttle studies. We present a new method for high-energy neutron spectroscopy using small silicon detectors that can extend these measurements to more than 500 MeV. The methodology is based on measurement of the detector response function for high-energy neutrons and inversion of this response function with measured deposition data to deduce neutron energy spectra. We also present the results of an initial shielding study performed with the thick silicon detector system for high-energy neutrons incident on polyethylene.

Elementary Particle Interactions↗