Effective Photon Energy Research Articles

Instantaneous measurement system of the effective photon energy for a single shot x-ray pulse radiation

For a short pulse x-ray radiation with extremely broad spectrum such as Bremsstrahlung, the effective photon energy cannot be determined by conventional x-ray detectors with only a single x-ray pulse shot. An instantaneous measurement system capable of determining the energy with a single shot has been developed. The system is made of a metal–oxide–semiconductor linear array image sensor, a scintillation phospher, and an absorber material, and a control system of the sensor. The energy of a hard x-ray pulse with a short pulse duration of 500 ns is determined with this system. It is shown that the system can determine the energy more quickly and accurately than with the conventional method.

Measured water transmission curves and calculated zero field size tumor maximum ratios for 4, 6, and 15 MV x-rays.

A multihole diverging Cerrobend plug for megavoltage energies was used to measure water transmission values at different locations in a 20 x 20 cm field at 100 cm source-to-axis distance (SAD) for 4, 6, and 15 MV therapy photon beams. The transmission curves in water were measured at 25 locations across the 20 x 20 field, and each location was separated by 5 cm at the isocenter. Each transmission value was made using a 0.3175 cm diameter (0.079 cm2 area) hole of 20 cm length at the central axis (CAX). The small field measured transmission curve in water was used to derive the zero field size tumor maximum ratio (TMR) and the primary photon exposure spectrum as a function of energy at depth. The exposure spectrum was used to find an effective photon energy and linear attenuation coefficient at depth and at different locations in the field. These values were found to vary with location in the field.

Liquid ionization chambers for absorbed dose measurements in water at low dose rates and intermediate photon energies.

Two new liquid ionization chamber (LIC) designs, consisting of cylindrical and plane-parallel configurations, are presented. They are designed to be suitable for high-precision measurements of absorbed dose-to-water at dose rates and photon energies typical for LDR intermediate photon energy brachytherapy sources. The chambers have a sensitive liquid layer thickness of 1 mm and sensitive volumes of 7 mm3 (plane-parallel) and 20 mm3 (cylindrical). The liquids used as sensitive media in the chambers are either isooctane (C8H18), tetramethylsilane (Si(CH3)4) or mixtures of these two liquids in the approximate proportions 2 to 1. A chamber filled with such a liquid mixture and with a polarizing voltage of 300 V, provides a volume sensitivity of about 10(-9)C Gy(-1) mm(-3) for absorbed dose measurements in water in an x-ray radiation field with an effective photon energy of 120 keV. In the interval 30 to 140 keV, the relative change in sensitivity is less than +/- 2.5%. The leakage current of the chambers is low and stable, which implies that absorbed dose measurements can be done with good reproducibility at dose-rates as low as 50 microGy min-1 (sigma < 3%). The long-term calibration stability was tested for a set of five chambers over a period of more than 1 year. No systematic change in their sensitivity could be observed. The general recombination at a polarizing voltage of 300 V is less than 2% for dose-rates up to about 100 mGy min-1. The temperature dependence at room temperature is 0.5% per degree C. The response is almost independent of the direction of the radiation for the plane-parallel LIC.

Reconstruction of megavoltage photon spectra by attenuation analysis

A simple analytical model for reconstructing photon spectra from the indirect measurement of transmission or depth-dose curves is presented, based on the Schiff expression for forward-directed bremsstrahlung differential in photon energy with added inherent filtration. Calculated transmission curves for 21 simulated and measured bremsstrahlung spectra with maximum photon energies in the range 4 to 30 MeV covering a broad range of filtration conditions were used to assess the model's ability to represent megavoltage spectra. Input data were reproduced to within 0.1 per cent in all cases, which is the same order of magnitude as the uncertainty involved in its calculation. Inclusion of a fourth parameter in the model is shown to allow the effective maximum photon energy present in the spectrum to be derived to an accuracy of approximately 3 per cent.

Investigation of the tissue equivalence of gells used for NMR dosimetry

The transition of Fe2+ to Fe3+ in Fricke solution after irradiation results in a change of NMR proton relaxation times in agarose gels which can be used for the dosimetry of ionizing radiation. The main advantage of this system is the possibility of observing dose distributions in 3 dimensions in a medium which is supposed to be tissue equivalent. The aim of the study was to quantify parameters which determine the tissue equivalence of NMR dosimetry gels. Electron densities and effective atomic numbers were calculated for gels with varying iron, sulphur and agarose concentration. The Hounsfield CT numbers so derived agree well with the CT numbers measured on a clinical CT scanner (effective photon energy 70.7 keV).

Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry

There is a new radiochromic film, a highly uniform, thin (100-microns) detector whose sensitive layer (6 microns thick) changes from colorless to blue by dye polymerization without processing, upon exposure to ionizing radiation. Because the dose gradients around brachytherapy sources are steep, the high spatial resolution offered by film dosimetry is an advantage over other detectors such as thermoluminescent dosimeters (TLDs). This compares the photon energy dependence of the sensitivities of GafChromic film, silver halide verification film (Kodak X-Omat V Film), and lithium fluoride TLDs (Harshaw), over the photon energy range 28 keV to 1.7 MeV, which is of interest in brachytherapy. Sensitivity of the radiochromic film is observed to decrease by about 30% as effective photon energy decreases from 1710 keV (4-MV x rays) to 28 keV (60-kV x rays, 2-mm A1 filter). In contrast, the sensitivity of verification film increases by 980% and that of LiF TLDs increases by 41%. The variation of the sensitivity of radiochromic film with photon energy is considerably less than that for silver halide film and similar to that for LiF TLDs, but in the opposite direction. Radiochromic film, like LIF TLDs, does not exhibit the drastic sensitivity changes below 127 keV that silver halide film exhibits. Dose distribution in the immediate vicinity of a high activity (370 GBq) brachytherapy 192Ir source has been mapped using radiochromic film and is presented to illustrate the applicability of this new technology to brachytherapy dosimetry.

Energy Dependence of the CO33- Signal Intensity in ESR Dosimetry of Human Tooth Enamel.

Experiments were made to determine the energy dependence of CO3-3 signal intensity in ESR dosimetry of tooth enamel. Packages containing 100 mg of mixed tooth enamel of particle size of 500-850μm were irradiated by X- or gamma-rays of various effective energies up to an exposure of about 1290×10-4 C/kg (500 R) . The irradiated tooth enamel samples were then subject to the ESR dosimetry and the relative response was determined as a function of effective photon energy. The response curve has a maximum at about 35 keV and is almost flat above 200 keV.

Testing performance of the improved pinhole collimator and its application to small animals

Testing performance of the improved pinhole collimator of 1.5 mm aperture attached to the gamma camera was carried out with three kinds of radionuclides, 99mTc, 67Ga and 131I, using a hand-made phantom. The results obtained in this experiment were compared to those obtained with a human pinhole collimator of 4 mm aperture. Those were nearly understood by taking account of the effective aperture of a pinhole collimator for each effective photon energy. And all scintigrams obtained from rats with various scintigraphy showed high resolution images.

X- and gamma-ray response of the TLD badge based on CaSO4:Dy Teflon TLD discs.

A TLD badge based on CaSO4: Dy Teflon TLD discs has been designed for personnel monitoring of radiation workers encountering X-rays of effective energies above 20 keV. The badge had two Teflon TLD discs, one of which is used bare (without any filter) and other which is used with a metal filter. By taking into account the TL readout of both of these discs, the problems of photon energy dependence as well as directional dependence of the badge have been reduced to a minimum. The uncertainty due to photon energy dependence above 29 KeV (eff.) was found to be within a value of +/- 15% (at an angle of incidence of 45 degrees). The directional dependence of the badge is within +/- 30% in the energy range from 35 KeV to 1.25 MeV. The badge is designed so as to provide approximate information on the effective photon energy of the X-ray exposures.

Photoproduction of charged pions on neutrons. II

A comprehensive theory is developed for photoproduction of charged pions on bound neutrons in deuterons taking into consideration the effects of off-energy-shell amplitudes, Fermi motion, the D state and the Pauli exclusion principle. The authors point out primarily that there are two basic gamma N to pi N amplitudes, generally at two different energies in the respective pi N centre-of-mass (CM) frames, which contribute to the reaction gamma d to pi +or-NN and that both these amplitudes are necessarily off the energy shell. Following the prescription generally used that the off-energy-shell photoproduction amplitudes could be taken to be the same as the on-energy-shell amplitudes at an effective photon energy different from the energy of the beam, the authors investigate how the differential cross section, the pi -/ pi + ratio, the photon polarisation asymmetry, the recoil proton polarisation and the target asymmetry are sensitive to the precise inclusion of the exchange amplitudes. The effect of the D-state contributions on the polarisation and the asymmetries are also studied. The nature of the approximations involved in the existing formulae in the literature are also clearly brought out by showing that they follow from these results on making various simplifying assumptions.

The measurement of effective photon energy and "linearity" in computerized tomography.

Two liquid systems have been developed for the measurement of effective photon energy (Eeff) and "linearity" in computerized tomography (CT). A group of sixteen "keV liquids" enables an Eeff to be rapidly determined, by comparing the CT numbers of the specially formulated organic liquid mixtures with similar data for water. The "linearity" of the CT scanner, relating CT nubers to linear attenuation coefficients, may be investigated using 14 selected organic "linearity liquids". Details of these systems, together with some typical results, are presented.

Measured x-ray spectra from 25 to 110 kVp for a typical diagnostic unit.

Diagnostic x-ray spectra were measured with a NaI(T1) spectrometer system. A digital computer was used to correct the observed spectrum and to find the true spectrum by solving a matrix equation. X-ray spectra for various diagnostic x-ray units were measured under different conditions of focal spot size, duration of exposure, tube current, and kVp. Half-value layer, effective photon energy, and mean energy were measured and compared for similar x-ray units. The principal variation in x-ray spectra was related to differences in kVp and tube filtration. Focal spot size and mAs variation did not significantly influence the measured x-ray spectra.

A ferrous sulphate dosemeter independent of photon energy in the range from 25 keV up to 50 MeV

A modified form of ferrous sulphate dosemeter is described, which retains the accepted advantages of the Fricke dosemeter and possesses the following additional properties: (a) it determines the dose in water; (b) the response is independent (to within +or-1.5%) of effective photon energy over the range 24 keV to 50 MeV and, therefore, independent of the shape of X-ray spectrum; (c) the low concentration of sulphuric acid reduces the possibility of introducing impurities from the source.

A Thermoluminescent Microdosimetry System for the Measurement of Photon Quality

Thermoluminescent lithium fluoride and calcium fluoride were studied to determine the feasibility of their use in measuring X- or gamma-ray spectral quality. Both of these materials were quantitatively packaged as microdosimeters, and exposure-versus-thermoluminescence response calibration curves were determined for several X- and gamma-ray effective energies. The ratio of calcium fluoride to lithium fluoride response was taken as the index of radiation quality, and a calibration curve for this ratio versus effective energy was also determined. The technique was found to be useful for effective photon energies between 50 and 300 keV. The calibrated dosimeters were exposed at various depths in a large water phantom. Curves were plotted of dose and effective energy as functions of depth in the phantom for several incident X-ray energies. These curves were compared, where possible, with published data.

DEPTH-DOSE TABLES FOR 6 MV X RAYS FOR USE WITH A CONSTANT SOURCE-LESION DISTANCE.

IN view of the desirable depth dose characteristics of high-energy radiations for treating deep-seated tumors, many supervoltage radiation therapy machines of different designs, constructions, and energies have appeared in the market. One of the most recent additions is the Varian Associates 6 Mev linear electron accelerator for x-ray and electron therapy. This accelerator is designed with a can-tilevered treatment couch so that rotational-, multiple-, or single-port treatments can be administered without moving the patient. Of course, in such a system the source-skin distance will change with changes in the direction of the beam. To our knowledge, constant SLD dose tables for 6 Mv x rays are not available. In fact, relatively little depth-dose information has been published, and this has been for only one source-skin distance. We have prepared a set of tables2 of central axis depth doses for use with 6 Mv x rays at a constant source-lesion distance of 100 cm. Published data for 4 and 8 Mv x rays were used as a starting point in preparing the tables (1). Interpolations were made from the 4 and 8 Mv data to obtain a set of depth-dose values for 6 Mv for 100 em source-skin distance. These values compared favorably with our measurements and with reported data for 6 Mv x rays obtained by Haimson and Karzmark, by Chamberlain and Baily, and by Morgan (2–4). From these data other SSD values were obtained by assuming an inverse-square variation of intensity with distance. Calculated data for other SSDs also were in close agreement with our measured values. Methods and Results The radiation source was a 6 Mev linear electron accelerator developed by Varian Associates for supervoltage x-ray therapy. This accelerator utilizes a 1.5 mm thick fixed transmission-type gold target to generate the bremsstrahlung x rays. Factors are a stainless-steel beam-flattening filter of 12.3 mm axial thickness, a 1.5 mrn lead electron filter, a thin nickel vacuum chamber window, a glass mirror, and an ionization chamber in the beam in addition to the gold target, which is bombarded by the 6 Mev electrons at an oblique angle. We estimate that the inherent filtration in this unit is equivalent to about 1.3 em of lead. The emergent x-ray beam has an effective photon energy of 1.9 Mev with a h.v.l, of 18.0 mm Cu or 12.5 mm Pb. Our copper value agrees reasonably well with the 17.4 mrn reported by Chamberlain and Baily (3). The experimental values were based on measurements taken with a Victoreen thimble ionization chamber and roentgen ratemeter in a 12 X 18 X 24-in. water phantom. The front wall of the phantom tank was a 1 em thick sheet of Lucite of density 1.17 g/cm2. With the front surface of this phantom positioned at the treatment distance, maximum build-up occurred at a position equivalent to 1.5 g/cm2 in water.

Radiochemical study of the competition between neutron emission and fission in U 238 at discrete excitations near the binding energy

Photoneutron emission and photofission in U 238 near the neutron separation energy were investigated radiochemically. The source of the photoexcitation was gamma radiation produced by thermal-neutron capture in selected metals placed in the thermal column of a 2-MW nuclear reactor. By varying the choice of the neutron-capturing source, the effective photon energy was changed so that the competing photonuclear reactions could be studied at different discrete excitations in U 238. Within the accuracy of the measurements, no definite change in the ratio of the neutron-emission-to-fission widths in U 238 was noticeable in the energy region from 6 to 9 MeV.

THE FREQUENCY OF DICENTRICS IN HUMAN LEUKOCYTES IRRADIATED IN VIVO AND IN VITRO.

The authors present a comparison of the frequency of dicentrics in human leukocytes irradiated in whole blood in vitro and in vivo. The results indicate that the frequency induced in vivo can be a factor of two higher or lower than the frequency in vitro for the same radiation dose. Whole blood was heparinized and irradiated in plastic bottles at 37° C. immediately after drawing. The oxygen tension in the blood during irradiation was essentially that of normal venous blood. Irradiations were carried out with the x-ray beam from a 6 Mev Linac (effective photon energy about 1.9 Mev) at dose rates of 200 rad/min. and 10 rad/min. The leukocytes were separated from the whole blood and incubated for seventy-two or ninety hours; chromosome preparations were then made in standard fashion. Every metaphase figure was scanned for chromosome aberrations, but tetraploid cells were excluded from the scoring. Data are presented herein for the class of aberrations known as dicentrics only, because these can be readily detected and can be scored unambiguously. Figure 1 illustrates, as an example, a dicentric in a leukocyte from a radiation worker. At the bottom of the figure is the cell as it appears under the microscope. At the top, the chromosomes are arranged in pairs, revealing that the dicentric was formed from the fusion of chromosomes numbered one and eleven. Figure 2 shows the results obtained in vitro. Data procured at the two dose rates and the two incubation times were not significantly different and therefore are not separately indicated on the graph. Also included are two points obtained with 100-kvp x-ray and one point obtained with 250-kvp x-ray. None of the three points is significantly different from the results obtained with the Linac, but effects of x-ray energy of as much as 20 per cent are not excluded. The figure clearly reveals that from a dose of 25 rads to one of 1,200 rads the yield of dicentrics is directly proportional to the square of the dose. The proportionality constant is 2.7 ± 0.14 × 10−6 dicentrics per cell per rad-squared. This figure is almost exactly half that reported by Bender and Gooch (1); we assume that one roentgen in their work corresponds to 0.93 rads. The reasons for the difference in results are not known, but do not appear to be a matter of x-ray energy, dose rate, or oxygen tension. They may be due to an accumulation of small errors; a discrepancy in absorbed dose of 10 per cent, for example, leads to a discrepancy of 20 per cent in dicentric frequency. Or they may be due to differences in the incubation conditions during the short-term culture of the leukocytes. At the highest dose, 1,200 rads, the yield is 4.2 dicentrics per cell. The failure of the yield to depart from a dose-squared dependence at this dose shows that the number of sites per cell at which chromosome exchanges can occur (2) must be large, probably in excess of ten sites per cell.

A revised technic for film dosimetry at Oak Ridge National Laboratory.

The Oak Ridge National Laboratory film-badge meter developed by Messrs. D. M. Davis and J. C. Hart (1) adequately fulfils the purpose for which it was designed, i.e., determination of (a) the dosage and energy of radiation when exposure is to roentgen rays only, (b) dosage when exposure is to gamma rays only, (c) approximate dose for exposure to beta rays only, and (d) approximate dose for exposure to mixed beta and gamma rays. Among the suggested modifications of this badge is a system of filters which would permit simplification and increased accuracy in determination of exposures to mixed photon and beta radiations. Filters have been selected and a method devised whereby the dosage from such mixed radiations may be more accurately obtained and with no increase in time and training beyond that required for the present method of determining the probable total reading (PTR). Description of Experiments Du Pont 553 film packets, containing film types 502, 510, and 606, were exposed in badges so constructed that various filters could be included. Photon exposures were made with the use of a constant-potential x-ray machine and a radium calibration set-up for films. Dosages were measured with a Victoreen Condenser r-Meter. The effective energies were taken from data of H. H. Hubbell, Jr. (2). The effective energy, as determined by absorption experiments, is the energy of monochromatic photons which would have an absorption coefficient equal to that experimentally determined for the photons used for these exposures. Photon data were extended to 1,200 kve (kilovolts effective) by reference to previous exposures with cobalt 60. For beta (beta particle) exposures, disks of normal uranium were used, and films were exposed to combinations of photon and beta radiation in order to determine the effect of various filters on the densities produced. The data obtained and the method to be discussed apply only to effective photon energies from 21 to 1,200 kve, to beta particles from normal uranium, and to Du Pont film type 502. Filters A. Photon Filtration: In the absence of uniform energy response, i.e., the same blackening of the film for the same dose of radiation, the ideal filter would be one which modified the radiation so that a uniform response for all energies is obtained. Because of the nature of the unfiltered response of the film and the high coefficients of absorption of most metals for photons of low energies, uniform response below 30 kv is not readily obtainable. For photon energies above 200 kv, uniform response can be obtained with little difficulty. Therefore, the best substitute for the ideal filter would be one which “flattened” the response from 30 to 200 kv. In addition to modifying the film response, filters may be used for determining energies of the radiation. For this purpose, for example, two filters which modify the film response so that the ratio of the densities produced is a single-valued function of the energy would be useful.

Low-Energy Photofission Yields forU238

The pattern of mass number dependence of fission yield has been determined in the photofission of natural uranium (${\mathrm{U}}^{238}$) by 16-Mev electrons from the linear accelerator. The effective photon energies were principally 9-14 Mev. Yields were determined by quantitative radiochemical isolation and close approximation to absolute $\ensuremath{\beta}$ counting for 19 chains. The peak-to-valley ratio is 110, the highest found yet outside of low-energy neutron studies. A spike in fission yields was found near mass number 133. Yields were also obtained for a number of chains in photofission with 10-Mev electrons.

Effective Photon Energies of High Energy Photo-Nuclear Reactions

An attempt has been made to use shower theory to evaluate the effective energies of the photo-nuclear reactions measured by Strauch. It seems that these energies can be determined most accurately from the area under the transition curve, the so-called "track length." A theoretical formula for the track length is discussed. The shape of the transition curve at small thicknesses can also be calculated quite accurately and serves as a rough check on the effective energies as derived from the track length. A comparison with experiment of the theoretical shape of the whole transition curve is given; and, as one would expect, the agreement is not very good.