Monoenergetic Photon Beams Research Articles

Experimental L x-ray fluorescence cross-sections for elements with 45 ≤Z ≤ 50 at 7 keV by synchrotron radiation photoionization

L x-ray fluorescence cross-sections for elements with 45 ≤ Z ≤ 50 were measured at 7 keV using synchrotron radiation photoionization. The experimental set-up provided a linearly polarized monoenergetic photon beam producing a low background and improving the signal-to-noise ratio. The data obtained for the L lines, Ll, Lα, LβI, LβII, LγI and LγII, were grouped considering the transition scheme, the energies of the emission lines and the detector resolution. Results for the experimental cross-sections obtained were compared with theoretical values using two different data tables. In general, it was found that the experimental fluorescence cross-sections are slightly higher (7–10%) than the theoretically calculated data and in some cases these differences are up to 40%. Copyright © 2005 John Wiley & Sons, Ltd.

Effective dose transmission of diagnostic X-rays through concrete and lead shields

When computing the amount of leakage from a diagnostic X-ray room, the transmission data of X-ray beams through the shielding material, which are used in the computation, must agree with the conditions of use of the X-ray equipment. Even if the tube potential is the same, the energy spectrum of generated X-rays depends on conditions such as high voltage rectification and total filtration, and transmission through the shielding material, too, is subject to change. In this paper, we propose a new method of calculation, which uses transmission data of mono-energetic photon beams computed by means of a Monte Carlo simulation, for obtaining effective dose transmission data through the shielding material of an X-ray beam with spectral distribution. We also present effective dose transmission data of primary X-ray beams and 90 degrees scattered X-ray beams through concrete and lead shields as determined by this method. This method, which can calculate the transmission data of X-ray beams with any spectral distribution, is useful in evaluating the leakage dose of diagnostic X-ray facilities.

Hf, Ta, W and Re experimental L X-ray fluorescence cross-sections at 12, 13 and 14 keV with synchrotron radiation

Experimental determinations of L X-ray fluorescence cross-sections for elements with 72≤Z≤75 at 12, 13 and 14 keV were carried out with synchrotron radiation. The experimental set-up provided a linearly polarized monoenergetic photon beam producing a very low background, which improved the signal-to-noise ratio and reduced the experimental uncertainties. Results of the experimental cross-sections obtained for the L lines were grouped considering the transitions scheme, the energies of the emission lines and the detector's resolution. The data obtained were compared with theoretical ones using two different data tables. A very good agreement was found between our experimental values and the theoretical values calculated using Puri's data (X-Ray Spectrom. 22 (1993) 358–361).

L X-ray fluorescence cross-sections for rare earths at 10 and 11 keV with synchrotron radiation

Experimental determinations of L X-ray fluorescence cross-sections for elements with 64⩽ Z⩽70 at 10 and 11 keV were carried out with synchrotron radiation. The experimental set-up provided a linearly polarized monoenergetic photon beam producing a very low background, which improved the signal-to-noise ratio and reduced the experimental uncertainties. Results of the experimental cross-sections obtained for the L lines were grouped considering the transitions scheme, the energies of the emission lines and the detector's resolution. The data obtained were compared with theoretical ones using two different data tables. A very good agreement was found between our experimental values and the theoretical values calculated using Puri's data. A new set of ω L1 ,f 12 and f 13 coefficients was also used to calculate theoretical cross-sections for Gd. These theoretical cross-sections and our experimental values are in excellent agreement.

Lantanides' Experimental L X-ray Fluorescence Cross Sections at 9 keV and 12 keV with Synchrotron Radiation

Experimental determinations of L x-ray fluorescence cross sections for elements with 64 ≤ Z ≤ 70 at 9 keV and 12 keV were carried out with synchrotron radiation. The experimental setup provided a linearly polarized monoenergetic photon beam producing a very low background, which improved the signal to noise ratio and reduced the experimental uncertainties. Results of the experimental cross sections obtained for the L lines were grouped considering the transitions scheme, the energies of the emission lines and the detector's resolution. The data obtained were compared with theoretical data using two different data tables. A very good agreement was found between our experimental values and the theoretical values calculated using Puri's data. A new set of ωL1, f12 and f13 coefficients for Gd were also used to calculate theoretical cross sections and the experimenal values are in excellent agreement with them.

Rare earth experimental L x‐ray fluorescence cross‐sections at 13 and 14 keV with synchrotron radiation

AbstractExperimental determinations of L x‐ray fluorescence cross‐sections for elements with 64 ≤ Z ≤ 70 at 13 and 14 keV were carried out with synchrotron radiation. The experimental set‐up provided a linearly polarized monoenergetic photon beam producing a very low background, which improved the signal‐to‐noise ratio and reduced the experimental uncertainties. Results of the experimental cross‐sections obtained for the L lines were grouped considering the transitions scheme, the energies of the emission lines and the detector's resolution. The data obtained were compared with theoretical data using two different data tables. Very good agreement was found between our experimental values and the theoretical values calculated using Puri et al.'s data. A new set of ω1, f12 and f13 coefficients for Gd were also used to calculate theoretical cross‐sections and the experimental values were in excellent agreement. Copyright © 2001 John Wiley & Sons, Ltd.

Determining clinical photon beam spectra from measured depth dose with the Cimmino algorithm

A method to determine the spectrum of a clinical photon beam from measured depth-dose data is described. At shallow depths, where the range of Compton-generated electrons increases rapidly with photon energy, the depth dose provides the information to discriminate the spectral contributions. To minimize the influence of contaminating electrons, small (6 × 6 cm2) fields were used. The measured depth dose is represented as a linear combination of basis functions, namely the depth doses of monoenergetic photon beams derived by Monte Carlo simulations. The weights of the basis functions were obtained with the Cimmino feasibility algorithm, which examines in each iteration the discrepancy between predicted and measured depth dose. For 6 and 15 MV photon beams of a clinical accelerator, the depth dose obtained from the derived spectral weights was within about 1% of the measured depth dose at all depths. Because the problem is ill conditioned, solutions for the spectrum can fluctuate with energy. Physically realistic smooth spectra for these photon beams appeared when a small margin (about ±1%) was attributed to the measured depth dose. The maximum energy of both derived spectra agreed with the measured energy of the electrons striking the target to within 1 MeV. The use of a feasibility method on minimally relaxed constraints provides realistic spectra quickly and interactively.

Synchrotron characterization of deep depletion epitaxial GaAs detectors

We report the results of a series of synchrotron characterizations of two epitaxial GaAs detectors of active areas 2.22 mm2 and thicknesses 40 and 400 μm. In spite of an order of magnitude difference in depletion depths, the detectors were found to have comparable performances at ∼−40 °C, with energy resolutions of ∼1 keV full width at half maximum (FWHM) at 7 keV rising to ∼2 keV FWHM at 200 keV and noise floors in the range of 1–1.5 keV. At the lower energies, the energy resolution was dominated by leakage current and electromagnetic pickup. At the highest energies, however, the measured resolutions appear to approach the expected Fano limit; e.g., ∼950 eV at 200 keV. Both detectors were remarkably linear, with average rms nonlinearities of 0.2% over the energy range of 10–60 keV. By raster scanning the active areas with 20×20 μm2 monoenergetic photon beams, it was found that the nonuniformity in the spatial response of both detectors was less than 1% and independent of energy. The material used to fabricate the detector is extremely pure. For example, low temperature photoluminescence measurements indicate that the density of the As antisite defect (EL2) is of the order of 1012 cm−3, which is ∼2–3 orders of magnitude lower than that generally reported. This indirect measurement of material purity is confirmed by Monte Carlo simulations of the detector x-ray response, which show that in order to reproduce the observed energy-loss spectra, electron and hole trapping cross-section/density products must be ≪1 cm−1.

Determination of scatter factor parameters and electron disequilibrium for monoenergetic photon beams.

The tissue-phantom-ratio (TPR) is expressed as the product of the phantom scatter factor (SF), an electron disequilibrium factor, and an attenuation factor, equal to the zero-area TPR. The scatter factor, as a function of depth d and field size s, has been described by two parameters a and w, SF(d,s) = 1 + asd/(ws + d). We have determined the parameters a and w for 20 monoenergetic photon beams between 1 and 20 MeV. Pencil-beam energy-deposition kernels were obtained using Monte Carlo simulations. The kernels were used to calculate broad-beam depth-dose data, which were converted to TPR and fitted to the equation above using an iterative search over a-w space. The parameter a is nearly equal to the attenuation coefficient for all energies while the parameter w increases with energy. The resulting a and w compare favorably to values determined for clinical photon beams, as a function of the measured attenuation coefficient. With the scatter factor determined, we isolated the electron disequilibrium factor for each monoenergetic beam. It can be characterized as a quadratic function of the depth. The coefficients of the quadratic function can be related to the range of the most energetic secondary electron produced.

Aspects on the Optimal Photon Beam Energy for Radiation Therapy

The selection of optimal photon beam energy is investigated both for realistic clinical bremsstrahlung beams and for monoenergetic photon beams. The photon energies covered in this investigation range from 60

Sex-specific tissue weighting factors for effective dose equivalent calculations.

The effective dose equivalent was defined in the International Commission on Radiological Protection Publication 26 in 1977 and later adopted by the U.S. Nuclear Regulatory Commission. To calculate organ doses and effective dose equivalent for external exposures using Monte Carlo simulations, sex-specific anthropomorphic phantoms and sex-specific weighting factors are always employed. This paper presents detailed mathematical derivation of a set of sex-specific tissue weighting factors and the conditions which the weighting factors must satisfy. Results of effective dose equivalent calculations using female and male phantoms exposed to monoenergetic photon beams of 0.08, 0.3, and 1.0 MeV are provided and compared with results published by other authors using different sex-specific weighting factors and phantoms. The results indicate that females always receive higher effective dose equivalent than males for the photon energies and geometries considered and that some published data may be wrong due to mistakes in deriving the sex-specific weighting factors.

Response of calcium fluoride to 275–2550 eV photons

CaF 2:Dy (TLD-200) chips were irradiated with monoenergetic photon beams of synchroton radiation of 275, 400, 500, 600, 730, 900, 1200, 1500 and 2550 eV energy. The phosphor response as a function of total energy delivered is reported. Glow curves for He- and air-annealed dosimeters are presented, showing the adverse effect of air anneal. Supralinearity factors based on the integrated TL response and on the height of the 200°C peak were determined and are reported.

The Effect of Air on the Dose Equivalent at 10 mm Depth in the ICRU Sphere

The effect of the air on the calculation of the ambient dose equivalent and of the dose equivalent at 10 mm depth on the principal axis of a ICRU sphere has been investigated by the Monte Carlo code FLUKA, in the case of parallel beams of monoenergetic photons from 0.5 MeV to 10 GeV. The influence of the air on the values of the dose equivalent depends strongly on the geometry of the irradiation. This could be a serious limitation to the introduction of the air surrounding a phantom in the calculation of the conversion coefficients from physical to operational quantities. The ambient dose equivalent is confirmed to be inappropriate as an operational quantity at photon energies higher than about 3 MeV.

Angular Dependence Factors and Air Kerma to Dose Equivalent Conversion Coefficients for Cylindrical Phantoms Irradiated by Plane-Parallel Extended Monoenergetic Photon Beams

With the aid of the Monte Carlo method, conversion coefficients between the receptor-free air collision kerma and the collision kerma for 4-element ICRU tissue have been calculated at a depth of 0.007 g.cm −2 in circular cylinder phantoms irradiated by broad plane-parallel photon beams in a direction perpendicular to the cylinder main axis. The phantom material was ICRU tissue or PMMA; the phantom diameter 1.9 cm, 7.3 cm, or 30 cm; the phantom length 30 cm. The calculations were performed for monoenergetic photons in the energy range between 4 keV and 1.25 MeV. The conversion coefficients can be used in the field of extremity dosimetry to convert measured kerma or exposure values into personal dose equivalents: the results for the 7.3 cm diameter cylinder phantoms for the calibration of wrist or ankle dosemeters; those for the 1.9 cm diameter phantoms for the calibration of finger dosemeters. In addition to the conversion coefficients, their angular dependence factors in the middle plane of the phantoms perpendicular to the main axis were also studied. Such quantities for the ICRU tissue phantoms are necessary in the procedure for type testing extremity dosemeters on rod or pillar phantoms

A study of the angular dependence problem in effective dose equivalent assessment.

The newly revised American National Standard N13.11 (1993) includes measurements of angular response as part of personnel dosimeter performance testing. However, data on effective dose equivalent (HE), the principle limiting quantity defined in International Commission on Radiological Protection (ICRP) Publication 26 and later adopted by U.S. Nuclear Regulatory Commission (NRC), for radiation incident on the body from off-normal angles are little seen in the literature. The absence of scientific data has led to unnecessarily conservative approaches in radiation protection practices. This paper presents a new set of fluence-to-HE conversion factors as a function of radiation angles and sex for monoenergetic photon beams of 0.08, 0.3, and 1.0 MeV. A Monte Carlo transport code (MCNP) and sex-specific anthropomorphic phantoms were used in this study. Results indicate that Anterior-posterior (AP) exposure produces the highest HE per unit photon fluence in all cases. Posterior-anterior (PA) exposure produces the highest HE among beams incident from the rear half-plane of the body. HE decreases dramatically as one departs from the AP and PA orientations. The results also indicate that overestimations caused by using isotropic dosimeters in assessing effective dose equivalent from near-overhead and near-underfoot exposures are 550%, 390%, and 254% for 0.08, 0.3, and 1.0 MeV, respectively. Comparisons of the angular dependence of HE with those based on the secondary quantities defined in International Commission on Radiation Units and Measurements (ICRU) Reports 39, 43, and 47 show significant differences. This paper discusses why more accurate assessments of HE are necessary and possible. An empirical equation is proposed which can be used as the optimum dosimeter angular response function for radiation angles ranging from 0 degrees to 90 degrees for dosimeter calibration, performance testing, and design.

Convolution calculations of dose in the buildup regions for high energy photon beams obliquely incident

The central dose received from high energy photon beams that are obliquely incident on an absorber is markedly different from that absorbed when such beams are normally incident at the surface. In this work, we calculate the dose differences using a convolution of energy deposition kernels in a phantom presenting an oblique entrance surface. The dose distributions were calculated with oblique incident 1.25, 3, and 6 MeV monoenergetic and polyenergetic photon beams angled from 0 degrees to 80 degrees. In order to study the dose variations with surface obliquity, we introduced an obliquity factor. For each energy, the obliquity factor was calculated as a function of depth and field size. We found that, ignoring the electron contamination from the air and from the treatment machine head, the influence of obliquity can be described in terms of upstream and downstream contribution of the electrons set in motion by the primary photons. In actual beams, especially for large field sizes, the electron contamination becomes significant and tends to reduce the influence of surface obliquity. Results indicate that the obliquity factor is highly dependent on the beam energy and depth, and are in good agreement with our experimental results measured for 10 and 25 MV x-ray beams. In this paper a theoretical explanation of these dose variations due to oblique incidence of the beam is presented.

Rotational Raman scattering in 127I

Elastic and Raman scattering of photons by 181Ta and 127I are studied using the 11.4 MeV mono-energetic photon beam installed at the Mol BR2 reactor. For the strongly deformed prolate nucleus 181Ta the simple rotator model is re-investigated and found to be valid. The intrinsic quadrupole moment Q 0 derived from spectroscopic data by assuming a strong particle-surface coupling was found to be in agreement with the splitting of the giant dipole resonance, thus removing previous inconsistencies. When applying the same model to the oblate nucleus 127I a severe discrepancy between experimental and predicted Raman cross sections was observed; this discrepancy is discussed in terms of nuclear-structure properties.

Primary dose in photon beams with lateral electron disequilibrium

It is shown that in narrow monoenergetic photon beams under conditions of lateral electron disequilibrium, primary absorbed dose P(r) is a simple function of beam radius r: P(r) = P lambda.(1 - e(-gamma.r)), where P lambda is the primary dose in broad beams for which complete lateral electron equilibrium exists, and gamma depends on photon energy and absorbing medium. This formula was evaluated using Monte-Carlo-generated data for the primary dose in water from monoenergetic photons in the energy range from 2 to 8 MeV. The primary dose was studied in beams of radii 0.006 cm less than r less than 5.0 cm and within the depth interval 0.5 cm less than or equal to d less than or equal to 24 cm. It was concluded that the saturation equation above provides an accurate description of the primary dose from monoenergetic photon beams, as judged by comparison with Monte Carlo results.

Bragg-Gray theory and ion chamber dosimetry for photon beams

The validity of the Bragg-Gray cavity theory in photon radiation dosimetry for photon energies from 10 keV to 10 MeV has been investigated quantitatively. A crucial assumption for Bragg-Gray cavity theory to be valid is that the dose from photon interactions in the detector material is negligible. The ratio, Fair, of the absorbed dose resulting from photon interactions in an air cavity, DairPA, to that in air under the condition of charged-particle equilibrium, DairCPE, has been used as a parameter to determine if the air cavity can be classified as a Bragg-Gray cavity. Monte Carlo calculated results show that, for monoenergetic photon beams of energies above about 220 keV, the dose ratio, Fair, is smaller than 0.05 for an air cavity of 6 mm thickness and 6 mm diameter in vacuum. Furthermore, it is shown that the Burlin general cavity theory seriously overestimates the departure from Bragg-Gray behaviour. For clinical photon beams the dose ratio, Fair, is 0.29 for a 150 kVp beam and 0.27 for a 240 kVp beam compared to 0.006 for a 60Co beam if the cavity is placed at a depth of 5 cm in water. This study confirms that typical air-filled ionization chambers cannot be considered to be Bragg-Gray cavities for low- and medium-energy photon radiation.

Photofission and photoneutron cross sections and photofission neutron multiplicities for 233U, 234U, 237Np, and 239Pu.

The photonuclear cross sections for $^{233}\mathrm{U}$, $^{234}\mathrm{U}$, $^{237}\mathrm{Np}$, and $^{239}\mathrm{Pu}$ have been measured from threshold up to 18 MeV. The source of radiation was the monoenergetic photon beam from the annihilation in flight of fast positrons. The branching among the neutron-producing reaction channels was determined by measuring the photofission prompt neutron multiplicities \ensuremath{\nu}${\ifmmode\bar\else\textasciimacron\fi{}}_{p}$. One interesting result is the complete absence of any (\ensuremath{\gamma},2n) cross section for $^{233}\mathrm{U}$ and $^{234}\mathrm{U}$. The values of \ensuremath{\nu}${\ifmmode\bar\else\textasciimacron\fi{}}_{p}$(E) for $^{234}\mathrm{U}$ agree with those measured with neutrons incident on $^{233}\mathrm{U}$. The parameters of the giant dipole resonance deduced from the total photonuclear cross sections show that these nuclei have large static deformations, as expected. The integrated photofission cross sections are large (as are the absolute fission probabilities), and account for 60% to 80% of the total photonuclear absorption strength.