Transmission of bremsstrahlung through alkali-halide crystals for various beam collimations

Abstract

In irradiating specimens with bremsstrahlung in radiation-physics investigations, two groups of problems are encountered requiring a knowledge of the energy parameters of the beam at various depths of the absorber. These are, first, the determination of the energy absorbed at various depths of the object irradiated and, second, the selection of conditions under which the use of a radiation source is most effective, i. e. . when energy absorption at the point of interest is the largest. It is known that when bremsstrahlung is transmitted through absorbers, its spectral content is modified [1-3], so that the absorption coefficient changes in value as the path traversed by radiat~on in the absorber increases. The authors [3-5] showed that this variation of the absorption coefficient of bremsstrahlung in a lkal i halide crystals is described for the range of maximum energies 0.2-80 MeV by a curve with saturation. The absolute value of the absorptioncoefficient variation and of the thickness of alkal i-hal ide crystals at which the transition to saturation occurs are determined by the radiation energy and by the crystal's effective atomic number [3-5]. The investigation of bremsstrahiung-spectrum transformation with absorber thickness by plotting calculated or measured spectra for various absorber thicknesses gives an accurate representation of the variations which occur, but it is not always convenient. The comparison Of different absorbers or different radiation intensities from the spectrum variation is difficult. Therefore, the authors [3] suggested using the mean spectrum energy for investigating bremsstrahlungspectrum variations. This quantity is convenient for comparing spectral-content variations in various materials, and it provides a convenient picture of the direction and rate of spectrum transformation as a function of the parameter of interest. Continuing the investigation of bremsstrahhing transmission through a lkal i -hal ide crystals, we studied the variation of spectral content with thickness in crystals of lithium fluoride, sodium chloride, and potassium chloride, bromide, and iodide for various collimations of the radiation beams. Investigations were carried out theoretically by calculating the bremsstrahlung spectra for various crystal thicknesses, using the Monte Carlo method on an M-20 computer. From the spectra calculated we determined the mean energy and plotted the mean energy as a function of absorber thickness for various collimations of tile radiation beams incident on the crystals. Bremsstrahlung beams 5 to 500 mm in diameter were investigated. Investigations of the accumulation factor as a function of beam diameter showed that an increase of the irradiation-field diameter above 200 mm gave no appreciable increase inthe scattered-radiation contribution to the total flux for crystal thicknesses of up to 100 g /cm 2. Therefore, to all practical accuracy, beams with more than 200 mm transverse cross section can be considered as beams of infinite cross section for 10 MeV maximum bremsstrahhing energy. Theoretical results of the mean energy of the bremsstrahlung beam for various thicknesses of alkal ihalide crystals of potassium chloride, bromide, and iodide are shown in the table. Data are given for irradiation fields 1 cm in diameter and for an infinite-diameter irradiation field. As can be seen from values of the mean energy calculated for various crystal thicknesses, it markediy increases with increasing absorber thickness. Thus. in the case of potassium iodide, it varies by more than twice in passing through a crystal layer 80 g /cm 2 in thickness for an irradiation field 1 cm in diameter. The rate of increase of the mean energy with absorber thickness depends to a greater extent upon the irradiation field than upon the crystal's effective atomic number. Thus, the maximum mean-energy difference in passing from potassium chloride to potassium iodide for constant irradiation fields is 0.2-0.3 MeV, whereas a beam-diameter increase from 10 to 100 mm causes a mean-energy variation of more than 0.5 MeV for 100 g /cm z crystal-layer thickness. In passing from a bremsstrahlung beam 10 mm in diameter to an infinite-diameter beam, the mean-energy variation is more than 1 MeV. An increase of the irradiation beam transverse cross section gives rise to a relative increase of its secondary-radiation content, and therefore the mean energy decreases in passing to wide irradiation beams.