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The EGS4 electron-photon Monte Carlo code has been used to study the characteristics of the bremsstrahlung x rays generated from the interaction of circulating electrons with the residual gas in accelerator storage rings. Gas bremsstrahlung dose rates are given for various opening angles as a function of the electron beam energy ranging from 0.5-10 GeV. Photon and neutron dose rates, generated from various devices struck by gas bremsstrahlung in a synchrotron radiation beamline, are also presented along with the photon spectral and transmission results. The EGS4-predicted results are found to be in basic agreement with the measurements made at the Stanford Synchrotron Radiation Laboratory. Figures, equations, and a simple method useful for the photon-neutron shielding design for beamlines are provided.

The spectral energy-dependence of the radiation-induced ESR signal has been studied in ovine cortical bone. Crushed bone samples were irradiated using photon beams with effective energies in the range from 0.06 to 6 MeV, and electron beams with mean energies in the range from 2 to 10 MeV. The photon and electron data were normalized to a dose to bone of 50 Gy and the results are reported as response relative to the ESR signal for photon irradiation at 1.25 MeV (60Co). The photon irradiation results show that the ESR response is greatest at low energies with a relative value of 1.2 at 0.06 MeV. The relative response decreases, as the energy increases, to approximately 0.85 in the region of 2 to 3 MeV. These variations in the relative ESR responses are significantly less than the ESR energy-dependent responses reported in the literature for human tooth enamel and synthetic hydroxyapatite. An explanation for this difference is offered. For electron beam irradiations, the ESR signal is fairly constant with energy, and approximately equal to that at a photon energy of 1.25 MeV. Implications of these results are discussed.

A simple analytical model is presented for the transport of secondary electrons at a photon beam edge using the energy averaged solution of the Boltzmann equation, originally developed for beta-ray dosimetry at a plane interface. Dose at a point under a block is assumed to be due to secondary electrons and the scattered photons generated from the primary photon beam. The diffusion approximation is used for the secondary electron transport at a virtual plane interface created by the block. The dose from the scattered photon component is treated as decaying exponentially with distance from the beam edge. Comparisons made with the model and measurements are in general agreement for high energy accelerator beams.

Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of the accelerators. The error can be positive or negative. Although the error can exceed 20% for the first few monitor units, it is usually less than 5% when more than 10 monitor units are delivered. When low doses are required proper precautions should be taken for dosimetric accuracy including the beam energy, beam flatness and dose per monitor unit.

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin-window parallel-plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co-60 gamma rays to 24-MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel-plate ion chambers. It is observed that for energies greater than 10 MV most parallel-plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high-Z interfaces and lower energies, the dose measurements should be performed with ultrathin-window parallel-plate ion chambers or extrapolation chambers.

We have measured the replacement correction factors (Prepl) for a PTW/Markus parallel plate chamber at mean incident electron energies of 3.1, 4.4, 8.9, 13.0, 16.3, and 18.8 MeV. The factors are significantly different from unity at low energies.

A comparison has been made of central axis percent depth dose and absorbed dose in electron beams of 7.8 and 10.2 MeV, measured with devices of differing geometry and construction. Flat and cylindrical ionization chambers have been used as well as thin thermoluminescent dosimeters. The ionization chambers had walls of air equivalent or tissue equivalent plastic. Results indicate that central axis depth dose measurements are independent of measuring device. No significant difference was found among the various ionization chambers with air equivalent walls in the determination of absorbed dose. The dose determined by the tissue-equivalent wall chamber was about 3% higher than the dose determined by the other ionization chambers. Dose maximum on the central axis in water is about 4% greater than when this same quantity is calculated from data measured in polystyrene.