Primary Photon Energy Research Articles

Constraints on the number density of evaporating primordial black holes for the chromospheric evaporation models

Cosmic gamma-ray bursts with primary-photon energies ≥ 10 GeV are sought in the data from the Andyrchy array obtained in the mode of detection of a single cosmic-ray component during a net observation period of 2005.4 d. The distribution of fluctuations of the detector count rate agrees with the expected cosmic-ray background, the only exception being an event with a deviation of 7.9σ. Constraints on the number density of evaporating primordial black holes in a local region of the Galaxy are obtained for the chromospheric evaporation models.

SU-FF-T-236: Experimental Validation of the Inner Shell Ionisation Model to Predict the Radiosensitisation Induced by the IDU and BrDU Halogenated Pyrimidine

Purpose — Halogenated pyrimidines (IDU and BrDU) are heavy atoms compounds used as radiosensitizers. For IUdR, it has been questionned if inner shell ionisations (ISI) and Auger cascades that are rare events using megavoltage beams could be at the origin of the radiosensitization. The purpose of this work is to validate experimentally the ISI model previously published by the authors. This model involves the photon spectrum degradation in the tissues and the ISI induced by secondary electron knock‐on. Material and Methods — The ISI model predicts that the primary photon energy, irradiation depth, and type of halogen influence the Sensitization Enhancement Ratio (SER). Radiobiological assay were used to evaluate cell survival of the radioresistant CHO cell line using 192 Ir or 60 Co irradiation at various depths in water phantoms (2, 5 and 10 cm). Cells were grown in exponential or in plateau phase to account for different ratios, and exposed 3 days before irradiation to 4 mol of IDU or BrDU. The experimental validation of the ISI model was realized by testing for each experimental condition the correlation between i/− the theoretical SER calculated using Monte Carlo simulation of the overdosage due to Auger cascades; and ii/− the SER calculated from the radiobiological assays. Results: — The experimental SER values are in the same range than the theoretical one, and they are significantly correlated (p=0.017). As predicted by the ISI model, there is an influence of depth and energy that is stronger for the IDU than the BrDU. Also, as predicted by the model, the largest SER is found using IDU and 192 Ir . Conclusions — The ISI model is robust in predicting the radiosensitisation for a given experimental situation. Based on a pure particle‐matter interaction modelisation, the ISI model could be extended to other particles type, including electrons, protons of heavy ions.

Radiation background and beam quality in secondary X-ray imaging for human angiography with Au as contrast agent

A calculation is carried out to assess the capabilities of Secondary X-ray Imaging (SXI). Human angiography is used as an example. An originally well focused primary photon pencil beam is rastered through the target (the human heart) in two dimensions. The signal consists of fluorescent photons from a contrast agent, registered by a wide angle (of order π steradians) detector. The primary photons may be significantly more energetic than the signal photons. The last two features result in clearer images and a reduction of shadowing by obstructions inside the body. Sharp imaging is compatible with locally quantitative measurements, and with pixel-by-pixel elemental analysis. The detector need not be position sensitive. Most primary photons will be (predominantly Compton) scattered before they reach the target, the scattered photons form a broad halo around the unscattered primary beam, which remains sharp and well focused. To discriminate against scattered background, the photons have to pass through a position/momentum selector, a W–Hf absorber shield, and a time window. The calculation gives the approximate energy spectrum for the scattered photons generated, for the photons passing through the position/momentum selector, and for those arriving at the far side of the absorber shield, the last two evaluated for various time windows in the 1000– 167 ps range. The surviving background will cause relative image intensity fluctuations of the order of a percent. The primary beam intensity required for SXI is comparable or less than the intensity needed for the usual K-edge subtraction (KES) imaging process. However, for SXI the primary photon energy spread may be one or two orders higher than needed for KES, thus relaxing the requirements on the primary photon source. If an undulator is used, monochromatization may not be needed. That further reduces cost and demands on the photon source, which may be a small low energy electron ring. To realize the full potential of the method (beyond that stated above), would require a new type of detector.

WE‐C‐T‐617‐06: Spectral Reconstruction by Scatter Analysis

Purpose: To reconstruct accelerator photon beam energy spectra by scatter analysis. Method and Materials: The method consists in irradiating a plastic phantom at standard 100cm SSD and inferring primary beam spectral information based on the measurement with a standard Farmer chamber of scatter around the phantom at several specific scatter angles: a scatter curve is measured which is characteristic of the primary spectrum at hand. A Monte Carlo code is used to simulate the scatter measurement setup and predict the relative magnitude of scatter measurements for mono‐energetic primary beams. Based on mono‐energetic primary scatter data, measured scatter curves are analyzed and the spectrum unfolded as the sum of mono‐energetic individual energy bins using the Schiff bremsstrahlung model. In comparison with spectral reconstruction by attenuation analysis, scatter is shown to be a measurable quantity that varies steeper with primary photon energy than attenuation does, providing the potential for a better resolving of adjacent energy bins within the spectrum. Results: The method is applied to an Elekta/SL18 6MV photon beam. The reconstructed spectrum matches the Monte Carlo calculated spectrum for the same beam within 6.2% (average error when spectra are compared bin by bin). Depth dose values calculated for the reconstructed spectrum agree with physically measured depth dose data to within 1%. Conclusion: The method has potential as a practical spectral reconstruction tool requiring few measurements under standard 100cm SSD and feasible in any radiotherapy department equipped with a conventional plastic scattering phantom and Farmer chamber.

SU-FF-T-357: Modeling Thick-Target Bremsstrahlung Production by Compound-Poisson-Process Electron Multiple Scatter

Purpose: To develop an accurate analytic algorithm to model bremsstrahlung production in a linac target and to calculate the angular distribution of the primary photon energy spectrum. This provides an alternative to Monte Carlo calculation of the primary photon spectrum. Method and Materials: The Compound-Poisson-Process (CPP) method is used to calculate the directional distribution of electrons in the target at each depth, explicitly including multiple scatters at large angles. This CPP method is modified to include the electron loss of energy at depth in the thick target. The Schiff integral formula is used to compute the bremsstrahlung spectrum produced at each angle and depth, and the contributions are integrated over depth to yield the photon energy fluence spectrum as a function of polar angle. This formula makes no small-angle approximations, so it should be accurate at all angles. Results: The primary (and attenuated first-scatter) photon energy fluence distribution was computed for a 15 MeV electron beam incident on a thick lead target and compared to a measurement in the same geometry. Conclusion: It is hoped that an accurate analytic calculation of the bremsstrahlung energy fluence distribution emerging from a linear accelerator treatment head will provide a quick and useful alternative to Monte Carlo calculations of this same distribution.

Excitation photocapacitance study of EL2 in n-GaAs prepared by annealing under different arsenic vapor pressures

The excitation photocapacitance method was applied to n-GaAs:Te (n=4×1016cm−3) prepared by annealing under various excess arsenic vapor pressures. By changing the primary excitation photon energies, the emission spectra for the EL20→EL2+ and EL2+→EL2++ transitions were determined. Considering the electron capture processes of the valence band (VB)→EL2+ (0.67eV) and VB→EL2++ (0.47eV) at 77K, the Frank–Condon shifts (dFC) of the annealed GaAs crystals were determined for the EL2+ level on the basis of the configuration coordinate model. It was shown that the lattice relaxation around the EL2 level is larger under arsenic-poor conditions, and thus it is considered that arsenic vacancies are closely related with the atomic configuration of the EL2 defect, in combination with excess arsenic defects.

Compensation of CdZnTe signals using a twin shaping filter technique

The performance of high Z solid state detectors has been significantly improved over the last decade: one of the most promising materials for X and gamma ray detectors is CdTe/CdZnTe, which combines high detection efficiency, no requirement for cooling to cryogenic temperatures, good spectroscopic performances, good time response and imaging capabilities. Unfortunately some important features of semiconductor detectors (pulse height, energy resolution, photopeak efficiency) are affected by the charge collection efficiency: the low mobility of the charge carriers (particularly the holes) and trapping/detrapping phenomena can degrade the CdTe/CdZnTe detectors' response, depending on the distance between the charge formation position and the collecting electrodes. The deterioration of the spectroscopic performances can be reduced by using either hardware or software techniques. The biparametric method herein described is based on a hybrid HW and SW technique that utilizes a twin pulse shaping active filter (one slow and one fast) to analyze the same signal. This technique could be particularly suitable for implementation in a readout integrated circuit for multipixel detectors. The experimental results obtained with the application of this biparametric technique with CdZnTe detectors are presented in order to assess the reliability and the efficiency of this compensation method as a function of bias voltage, primary photon energy and shaping time pairs.

Excitation photocapacitance study of EL2 in n -GaAs and its relation to non-stoichiometry

The excitation photocapacitance measurements are performed on n-GaAs to obtain charge transition characteristics of EL2 defects in a full spectral region. It is shown that the threshold photon energy for EL2 ++ to EL2 + transition is changed as a function of the primary excitation photon energy. It is also shown that the Frank–Condon shifts ( d FC) changed. It is considered that the lattice relaxation around the EL2 defect is affected by the deviation from the stoichiometric composition.

Scattering and its role in radiography simulations

The last decade has witnessed a proliferation of various radiography simulation codes. Most of these codes emphasize on a good geometrical representation of the test object. Simulated image formation is another point emphasized by these simulators. Most of the simulators use a variety of ways to generate primary beam photon energy spectrum. Apart from these common points, the simulation codes address photon object interactions with different sophistication levels. There has not been much discussion on what constitutes adequate level of physics used in these simulators. This paper provides a brief survey of these radiography efforts and discusses approaches adapted by various simulation efforts. Along with the survey of the simulation efforts, it provides an algorithm for computing scattered photon flux and shows impact of various parameters, assumptions and approximations.

The use of the Monte Carlo simulation technique for the design of an H*(10) dosemeter based on TLD-100.

The thermoluminescence (TL) detector material LiF:Mg,Ti (TLD-100) and appropriate filter materials were combined in order to design a passive dosemeter measuring the operational quantity ambient dose equivalent, H*(10), for monitoring low-dose external photon radiation fields. Using the Monte Carlo simulation technique, optimisations of energy dependent conversion coefficients from air kerma free-in-air compared to ICRU and ISO proposed values. h*K(10), were performed by varying dosemeter detector positioning. geometrical arrangements, and filter materials. Deviations smaller than 5% compared to h*K(10) between 30 keV and 2.5 MeV of primary photon energies were achieved by a dosemeter design consisting of a 15 microm Sn metal layer and a 5 mm PMMA layer surrounding the LiF detector. Subsequently performed free-air verification experiments carried out in well defined standard photon radiation fields showed an obviously TL-specific effect. An underestimation up to -15% of the modelled data at low photon energies was observed.

Monte Carlo models for dosimetry in photon beam radiotherapy

On the basis of phase space (PS) data Monte Carlo (MC) beam models for a 6 and 15 MV photon beam have been developed. This PS information was used to develop simple point source beam models. Since these models are lacking in flexibility and show limitations in modeling the beam outside the geometric radiation field a multiple sourcemodel (MSM) was developed. Original PS data and those reproduced by the MSM are then compared in terms of fluences and mean energies, which all agree within 1%. MC-calculated primary photon energy spectra are verified by energy spectra derived from transmission measurements. Comparisons of MC-calculated depth doses and profiles, using original and reproduced PS data, agree within 1% and 1 mm. Deviations from measured doses are within 1.5% and 1 mm. Additionally, the MSM was supplemented with a model of an 80-leaf MLC. Using this supplemented MSM, dose distributions are calculated in water phantoms. Two test fields for the step-and-shoot technique and two realistic fields delivered with dynamic MLC are investigated. MC-calculated dose profiles are in excellent agreement with measured data from film dosimetry. To assess spectral changes caused by the MLC, energy spectra in portal planes are calculated for moving slits of different widths. The investigation shows an increase in mean photon energy of up to 16% for the 0.25 cm slit compared to the open beam for 6 MV and of up to 6% for 15 MV, respectively. The supplemented MSM has proven to be a powerful tool for investigational and benchmarking purposes and for IMRTtreatment planning.

Monte Carlo models for dosimetry in photon beam radiotherapy

On the basis of phase space (PS) data Monte Carlo (MC) beam models for a 6 and 15 MV photon beam have been developed. This PS information was used to develop simple point source beam models. Since these models are lacking in flexibility and show limitations in modeling the beam outside the geometric radiation field a multiple source model (MSM) was developed. Original PS data and those reproduced by the MSM are then compared in terms of fluences and mean energies, which all agree within 1%. MC-calculated primary photon energy spectra are verified by energy spectra derived from transmission measurements. Comparisons of MC-calculated depth doses and profiles, using original and reproduced PS data, agree within 1% and 1 mm. Deviations from measured doses are within 1.5% and 1 mm. Additionally, the MSM was supplemented with a model of an 80-leaf MLC. Using this supplemented MSM, dose distributions are calculated in water phantoms. Two test fields for the step-and-shoot technique and two realistic fields delivered with dynamic MLC are investigated. MC-calculated dose profiles are in excellent agreement with measured data from film dosimetry. To assess spectral changes caused by the MLC, energy spectra in portal planes are calculated for moving slits of different widths. The investigation shows an increase in mean photon energy of up to 16% for the 0.25 cm slit compared to the open beam for 6 MV and of up to 6% for 15 MV, respectively. The supplemented MSM has proven to be a powerful tool for investigational and benchmarking purposes and for IMRT treatment planning.

Excitation Photacapacitance Study of Ionized Levels in n‐Type GaAs Observed during Photoquenching

The excitation photocapacitance method is applied under a constant capacitance condition to study Te‐doped GaAs bulk crystals, prepared by annealing at 900°C for 67 h under extremely high arsenic vapor pressure (4817 Torr). Photo‐capacitance measurements after monochromatic light irradiation reveal two deep levels at 0.50 and 0.68 eV above the valence band only during photoquenching. By varying the primary excitation photon energy, the emission and the capture photon energies are strictly determined depending on the change of the charge states of the excess arsenic atom‐related levels.

Scattered radiation in scanning slot mammography

Monte Carlo simulations were used to quantify the amount of scattered radiation a scanning slot detector geometry designed for use in digital mammography. Ratios of the scatter to primary (S/P) x-ray photon energy absorbed in the detector were obtained for a Lucite phantom, and were investigated as a function of photon energy, phantom thickness, and slot detector width. Over a Lucite phantom thickness range of 2-6 cm, the S/P ratios range from about 0.10 to 0.17 for a 4 mm wide slot detector at the x-ray photon energies used in mammography. These ratios increased by a factor of approximately 1.8 when the slot width was increased to 10 mm. In general, 20 keV photons gave S/P ratios similar to those of a 30 kVp x-ray spectrum (Mo target + 30 microns Mo filtration). The use of a 3 cm air gap reduced the S/P ratios by a factor of between 2.5 and 3.4, depending on the phantom thickness. For a constant primary energy fluence, coherent scatter was reduced as photon energy increased, whereas Compton scatter increased with increasing photon energy. With no air gap, the contributions of coherent and Compton scatter were found to be equal at 25 keV, whereas the introduction of a 3 cm air gap resulted in equal contributions for the two scatter processes at 36 keV. A 10 mm wide slot detector consisting of a 36.7 mg/cm2 thick Gd2O2S:Tb phosphor screen was compared to an ideal detector absorbing all incident primary/scatter photons. Average differences in the S/P ratios for these two detectors were 7% with no air gap and approximately 4% with a 3 cm air gap. The results obtained in this study will assist in the design of an optimal slot detector for use in digital mammography.

Electron transport in photon and electron beam modeling

To address the deficiencies of currently available dose calculation algorithms for radiotherapy planning, two dose calculation methods have been devised. The first method incorporates Fermi–Eyges multiple scattering theory into the primary dose calculation of the superposition method for external x‐ray beam radiotherapy. The inclusion of scattering theory into the superposition technique accounts for the density distribution between the primary photon interaction and energy deposition sites. The second method developed is the super‐Monte Carlo (SMC) method. SMC calculates dose by a superposition of pregenerated Monte Carlo electron track kernels. For x‐ray beams, the primary dose is calculated by transporting pregenerated (in water) Monte Carlo electron tracks from each primary photon interaction site. The length of each electron step is scaled by the inverse of the density of the medium at the beginning of the step. The scatter dose is calculated by superposition. SMC can also be applied to electron beam dose calculation. Pregenerated electron tracks are transported through media of varying density and atomic number. The perturbation of the electron fluence due to each material encountered by the electrons is explicitly accounted for. Dose distributions in a variety of phantoms show good agreement with Monte Carlo results. SMC is an accurate, three‐dimensional unified photon/electron dose calculation algorithm.

Wedge factor constituents of high-energy photon beams: head and phantom scatter dose components

The head and phantom scatter contribution to the output of a treatment machine have been determined for open and wedged 60Co γ-ray beams and 4, 8, 16 and 25 MV X-ray beams, using an extended and a small-sized phantom. The wedge factor variation with field size and phantom depth have been analysed as a function of both scatter components. For the wedged beams a stronger increase of the head scatter contribution with field size, i.e. 4–9% for field sizes increasing from 5 cm × 5 cm to 20 cm × 20 cm, has been observed compared with open beams. This result indicates that the wedge factor variation with field size is related to a change of the primary photon fluence. Our study shows that the ratio of the head and phantom scatter contribution for the wedged and open beams remains unchanged for all beams except the 4 and 25 MV X-ray beam. This implies that, except for these latter energies, the variation of the wedge factor with phantom depth is determined by the wedge-induced change of the primary photon energy fluence. For the 4 and 25 MV X-ray beam it is shown that the wedge factor is also influenced by a change of the phantom scatter contribution. The wedge factor for the 25 MV X-ray beam is strongly influenced by the electron contamination for phantom depths up to 6 cm. For the 60Co and the 4 MV photon beam it is shown that the wedge factor decreases slightly with increasing source-to-skin distance due to a reduced contribution to the total dose from photons scattered in the wedge. For clinical use, an algorithm is given to calculate the wedge factor variation with field size and phantom depth.

Photon beam convolution using polyenergetic energy deposition kernels

In photon beam convolution calculations where polyenergetic energy deposition kernels (EDKs) are used, the primary photon energy spectrum should be correctly accounted for in Monte Carlo generation of EDKs. This requires the probability of interaction, determined by the linear attenuation coefficient, mu , to be taken into account when primary photon interactions are forced to occur at the EDK origin. The use of primary and scattered EDKs generated with a fixed photon spectrum can give rise to an error in the dose calculation due to neglecting the effects of beam hardening with depth. The proportion of primary photon energy that is transferred to secondary electrons increases with depth of interaction, due to the increase in the ratio ( mu ab/ mu as the beam hardens. Convolution depth-dose curves calculated using polyenergetic EDKs generated for the primary photon spectra which exist at depths of 0, 20 and 40 cm in water, show a fall-off which is too steep when compared with EGS4 Monte Carlo results. A beam hardening correction factor applied to primary and scattered 0 cm EDKs, based on the ratio of kerma to terma at each depth, gives primary, scattered and total dose in good agreement with Monte Carlo results.

Fluorescence, Coster-Kronig and Auger yields of the 62Sm L subshells measured with the synchrotron photoionization method

Using monochromatized synchrotron radiation a sample of thin samarium and titanium foils was excited, and the induced Sm L X-rays and Ti K X-rays were detected by a Si(Li) detector. When the primary photon energy is tuned across the Sm L2 and L1 edges, the intensities of the Sm L X-ray lines normalized to the Ti K alpha intensity exhibit abrupt changes. From these intensity changes the Coster-Kronig factors were determined: f23=0.14+or-0.03, f13=0.18+or-0.03 and f12=0.19+or-0.03. The value of f13 is substantially smaller than predicted by theory and extrapolated from heavier elements. Furthermore, from the total intensities of all lines originating from the same Sm L subshell the ratios of the fluorescence yields omega 1/ omega 3=0.48+or-0.07 and omega 2/3=1.05+or-0.05 were obtained.

Direct determination of molecular orbital symmetry of H2S using polarized x-ray emission.

X-ray emission from the molecule ${\mathrm{H}}_{2}$S is strongly polarized following excitation of a sulfur K-shell electron to an unoccupied subthreshold molecular orbital with a polarized x-ray beam. Changes in the polarization of the emission spectrum are observed as the incident beam's energy is swept across the subthreshold absorption resonance. The previously unresolved absorption resonance is shown experimentally to be primarily associated with a molecular orbital of ${\mathit{b}}_{2}$ symmetry, but with a high-excitation-energy component due to an orbital with ${\mathit{a}}_{1}$ symmetry. Satellite emission intensity is shown to depend on the primary photon energy and is therefore associated with multivacancy effects and not with contamination, as previously suggested.

Scatter detection in SPECT imaging: dependence on source depth, energy, and energy window

The authors examine the dependence of the scatter fraction on source depth, the energy window width and the emitted photon energy, Monte Carlo calculations are presented of scatter fractions for a line source of activity at eleven depths in a water-filled cylinder acquired in seven energy window widths for each of five primary photon energies corresponding to radionuclides commonly used in nuclear medicine studies.