Photon Energy Distribution Research Articles

Impact of prompt gamma emission of 44Sc on quantification in preclinical and clinical PET systems

44Sc is an increasingly investigated positron emitter for use in positron emission tomography (PET) imaging. However, 44Sc is a non-pure positron emitter, since prompt photons are co-emitted during the decay process. This study investigates coincidence energy spectra of 44Sc and its impact on PET quantification on a preclinical and clinical PET system in comparison with 18F. The raw data of the coincidence events revealed characteristic differences comparing the photon energy distribution of 44Sc and 18F. Due to prompt gamma emission of 44Sc, activity recovery is underestimated on PET systems. However, clinical PET imaging of 44Sc with acceptable quantitative accuracy appears feasible by using a single, constant correction factor.

Nuclear resonance fluorescence of $$^{208}$$Pb in heavy-ion colliders

In ultraperipheral collisions (UPC) of nuclei the impact of Lorentz-contracted electromagnetic fields of collision partners leads to their excitations. In case of heavy nuclei the emission of neutrons is a main deexcitation channel and forward neutrons emitted in UPC were detected at the Relativistic Heavy-Ion Collider (RHIC) and at the Large Hadron Collider (LHC) by means of Zero Degree Calorimeters. However, the excitation of low-lying discrete nuclear states is also possible in UPC below the neutron separation energy. In this work by means of the Weizsacker-Williams method the data on nuclear resonance fluorescence (NRF) induced by real photons in 208 Pb are used to model the excitations of discrete levels in colliding nuclei. Due to Lorentz boosts one can expect that deexcitation photons with energies up to 40 GeV and 300 GeV are emitted in very forward direction, respectively, at the LHC and at the Future Circular Collider (FCC-hh). Energy, rapidity and angular distributions of such photons are calculated in the laboratory system, which can be used for monitoring of collider luminosity or triggering particle production in UPC.

Effect of different target materials of LINAC head on photon spectrum

ABSTRACT The X-rays were examined, which consist of the effect of 6 MeV energized electrons on different thickness and material of targets, using the MC method. When the PDDs are examined, we can see that the curves obtained for the Target 4 and Target 8 containing only tungsten material are very close PDD values to each other. The highest TPR20,10 value was also obtained for Target 8. For Target 4 and Target 8, which are composed only of tungsten material, when we examine the mean photon energy distributions formed, we observe an increase in energy towards the edge of the field. We obtained the closest TPR20,10 value for Target 6 to the TPR20,10 value generated by the original LINAC target (Target 1). However, the mean photon energy distribution value obtained for Target 6 is higher than the mean photon energy distribution value obtained from the Target 1. The mean photon energy distribution generated by the selected material according to the desired X-ray penetration and the homogeneity of this energy should be considered. The results obtained in the study are of a guiding nature both in modeling studies and for companies that will make target production.

Theoretical lateral and axial sensitivity limits and choices of molecular reporters for Cherenkov-excited luminescence in tissue during x-ray beam scanning.

.Purpose: Unlike fluorescence imaging utilizing an external excitation source, Cherenkov emissions and Cherenkov-excited luminescence occur within a medium when irradiated with high-energy x-rays. Methods to improve the understanding of the lateral spread and axial depth distribution of these emissions are needed as an initial step to improve the overall system resolution.Methods: Monte Carlo simulations were developed to investigate the lateral spread of thin sheets of high-energy sources and compared to experimental measurements of similar sources in water. Additional simulations of a multilayer skin model were used to investigate the limits of detection using both 6- and 18-MV x-ray sources with fluorescence excitation for inclusion depths up to 1 cm.Results: Simulations comparing the lateral spread of high-energy sources show approximately higher optical yield from electrons than photons, although electrons showed a larger penumbra in both the simulations and experimental measurements. Cherenkov excitation has a roughly inverse wavelength squared dependence in intensity but is largely redshifted in excitation through any distance of tissue. The calculated emission spectra in tissue were convolved with a database of luminescent compounds to produce a computational ranking of potential Cherenkov-excited luminescence molecular contrast agents.Conclusions: Models of thin x-ray and electron sources were compared with experimental measurements, showing similar trends in energy and source type. Surface detection of Cherenkov-excited luminescence appears to be limited by the mean free path of the luminescence emission, where for the given simulation only 2% of the inclusion emissions reached the surface from a depth of 7 mm in a multilayer tissue model.

Absolute Double Differential Cross Sections of Bremsstrahlung Produced from 4.0 keV Electrons Incident on Free Ar Atoms

New results are reported on the measurements of absolute double differential cross sections (DDCSs) of bremsstrahlung produced from 4.0 keV electrons incident on free Ar atoms in the angular detection range of 45°–120°. A significant reduction of the thick target bremsstrahlung (TTB) of the chamber wall and of the photon transmission windows has been achieved by modifying the experimental set-up used previously; a large reduction of TTB in the present experiments is supported by the results of our model calculations for the ratio of TTB background to the normal bremsstrahlung (NB) spectrum carried out for the employed geometry of the experimental set-up. The results of photon energy distribution measured at different angles and those of angular distributions of photons of a given energy are compared with theoretical predictions of Kissel–Quarles–Pratt (KQP) theory for ordinary bremsstrahlung and with predictions of total bremsstrahlung including polarization bremsstrahlung (PBS) of the stripping approximation (SA). A satisfactory agreement observed between experiment and predictions using SA theory for absolute DDCSs of bremsstrahlung provides evidence for an appreciable contribution of polarization bremsstrahlung at the considered impact energy of electrons on one hand, while on the other hand, it exhibits a large discrepancy (about a factor of 2) in DDCSs of bremsstrahlung photons obtained by experiment and by KQP theory for photon energy distributions at all detection angles measured in these experiments. In addition, present results of the angular dependence of photons of different energies show anisotropic distributions and they are found to be in reasonable agreement with both KQP and SA theories. The satisfactory agreement between experiment and theory for angular distributions is an indication of a significant reduction of the background produced from TTB photons.

Depth Dose Enhancement on Flattening-Filter-Free Photon Beam: A Monte Carlo Study in Nanoparticle-Enhanced Radiotherapy

The aim of this study is to investigate the variations of depth dose enhancement (DDE) on different nanoparticle (NP) variables, when using the flattening-filter-free (FFF) photon beam in nanoparticle-enhanced radiotherapy. Monte Carlo simulation under a macroscopic approach was used to determine the DDE ratio (DDER) with variables of NP material (gold (Au) and iron (III) oxide (Fe2O3)), NP concentration (3–40 mg/mL) and photon beam (10 MV flattening-filter (FF) and 10 MV FFF). It is found that Au NPs had a higher DDER than Fe2O3 NPs, when the depths were shallower than 6 and 8 cm for the 10 MV FF and 10 MV FFF photon beams, respectively. However, in a deeper depth range of 10–20 cm, DDER for the Au NPs was lower than Fe2O3 NPs mainly due to the beam attenuation and photon energy distribution. It is concluded that DDER for the Au NPs and Fe2O3 NPs decreased with an increase of depth in the range of 10–20 cm, with rate of decrease depending on the NP material, NP concentration and the use of FF in the photon beam.

ESTIMATION OF PHOTON ENERGY AND DIRECTION DISTRIBUTIONS AT JAPANESE NUCLEAR POWER PLANTS BASED ON LITERATURE SURVEY FOR J-EPISODE STUDY.

In order to reconstruct organ-absorbed dose from recorded dose for risk estimation in nuclear worker cohort, the preceding study of the International Agency for Research on Cancer (IARC) 15-Country Collaborative Study estimated the organ dose conversion factor from the recorded dose of Hp(10) under the assumption that on average, in the nuclear power plants (NPPs), 10% of the dose received by workers was due to photon energies ranging from 100 to 300 keV and 90% from photon energies ranging from 300 to 3000 keV, with the average geometry being 50% in the antero-posterior geometry and 50% in the isotropic geometry. Similar examination was conducted at the Japanese Epidemiological Study on Low-Dose Radiation Effects (J-EPISODE).Literature survey disclosed that Japanese electric power companies had jointly conducted the research on energy distribution and incidence direction distribution of gamma rays in working environments during periodical inspection and maintenance as well as during operation in the 1980s. The analysis of the survey results on photon energy and geometry distribution of Japanese NPPs demonstrated appropriateness in applying the IARC study assumption for nuclear workers in Japan and reconstructing organ-absorbed dose in the J-EPISODE. These results in Japan also provide strong evidence to support the robustness and generality of the IARC study assumption, which was estimated based on the judgment of experts at nuclear facilities around the world.

Assessment of Angular Spectral Distributions of Laser Accelerated Particles for Simulation of Radiation Dose Map in Target Normal Sheath Acceleration Regime of High Power Laser-Thin Solid Target Interaction—Comparison with Experiments

An adequate simulation model has been used for the calculation of angular and energy distributions of electrons, protons, and photons emitted during a high-power laser, 5-µm thick Ag target interaction. Their energy spectra and fluencies have been calculated between 0 and 360 degrees around the interaction point with a step angle of five degrees. Thus, the contribution of each ionizing species to the total fluency value has been established. Considering the geometry of the experimental set-up, a map of the radiation dose inside the target vacuum chamber has been simulated, using the Geant4 General Particle Source code, and further compared with the experimental one. Maximum values of the measured dose of the order of tens of mGy per laser shot have been obtained in the direction normal to the target at about 30 cm from the interaction point.

Simulation of XFEL induced fluorescence spectra of hollow ions and studies of dense plasma effects

X-ray free electron laser (XFEL) interaction with solids has been simulated to resolve simultaneously variable XFEL photon energy and x-ray spectral distribution of the target emission (2D-maps). It is discovered that the highly transient charge state distribution exhibits a characteristic target response due to the action of the sharply rising radiation field. Finally, we identify advantageous features for studies of dense plasma effects of two K-shell vacancy hollow ion x-ray emission excited via resonance excitation. These features and characteristics permit the global study of dense plasma effects via the simulation of the time-integrated joint distribution of pumped and fluorescence energies. It is shown that the simulation of these specific 2D-maps offers a global vision of the complex interplay between different processes or phenomena such as photoionization, resonance excitation, or ionization potential depression.

Monte Carlo and water calorimetric determination of kilovoltage beam radiotherapy ionization chamber correction factors

The in-phantom calibration method for radiotherapy kilovoltage x-ray beams requires ionization chamber correction factors. The overall ionization chamber correction factor accounts for changes in the chamber response due to the displacement of water by the chamber cavity and wall, the presence of the stem and the change in incident photon energy and angular distribution in the phantom to that in air. A waterproof sheath, if required, is accounted for in a sheath correction factor. The aim of this study is to determine chamber correction factors through Monte Carlo (MC) simulations and water calorimetry measurements. Correction factors are determined for the PTW TM30013, NE2571, IBA FC65-G, IBA FC65-P and Exradin A12 ionization chambers. They are compared to experimental values obtained at the German national metrology institute Physikalisch-Technische Bundesanstalt (PTB) with their water calorimetry-based absorbed dose to water primary standard and at other national metrological institutes. An uncertainty analysis considers the contributions to the uncertainty on the chamber correction factors from the field size, photon cross sections, photon fluence spectra and chamber wall and central electrode dimensions. The MC calculated chamber correction factors are within 2.2% of unity with a standard uncertainty of 0.3%. For the 50 kV and 100 - 140 kV radiation beam qualities, the calculated correction factors deviate from the measured correction factors (with a standard uncertainty of 1%) by up to 2.6%. The calculated chamber correction factors for the PTW TM30013 and Exradin A12 are consistent with those derived from the BIPM kilovoltage primary standard. The inconsistencies between the calculated and experimental chamber correction factors indicate the need to further investigate the accuracy of kilovoltage absorbed dose to water primary standards and the use of MC simulations to determine kilovoltage beam chamber correction factors.

Radiation from the impact of broad-line region clouds onto AGN accretion disks

Context.Active galactic nuclei are supermassive black holes surrounded by an accretion disk, two populations of clouds, bipolar jets, and a dusty torus. The clouds move in Keplerian orbits at high velocities. In particular, the broad-line region (BLR) clouds have velocities ranging from 1000 to 10 000 km s−1. Given the extreme proximity of these clouds to the supermassive black hole, frequent collisions with the accretion disk should occur.Aims.The impact of BLR clouds onto the accretion disk can produce strong shock waves where particles might be accelerated. The goal of this work is to investigate the production of relativistic particles, and the associated non-thermal radiation in these events. In particular, we apply the model we develop to the Seyfert galaxyNGC 1068.Methods.We analyze the efficiency of diffusive shock acceleration in the shock of colliding clouds of the BLR with the accretion disk. We calculate the spectral energy distribution of photons generated by the relativistic particles and estimate the number of simultaneous impacts needed to explain the gamma radiation observed byFermiin Seyfert galaxies.Results.We find that is possible to understand the measured gamma emission in terms of the interaction of clouds with the disk if the hard X-ray emission of the source is at least obscured between 20% and 40%. The total number of clouds contained in the BLR region might be between 3 × 108and 6 × 108, which are values in good agreement with the observational evidence. The maximum energy achieved by the protons (∼PeV) in this context allows the production of neutrinos in the observing range of IceCube.

Determination of X-ray tubes radiation beam characteristics with semiconductor pixel detectors

X-ray tubes belong among the most important sources of ionizing radiation and are used in various applications and methods, i.e. investigation of materials, diagnostics in medicine, irradiation, etc. For such applications, the exact properties of X-ray sources or their X-ray beams need to be known, as well as when Monte Carlo simulations of experimental setups are considered with the aim of optimization or calibration of X-ray systems. These properties include X-ray spectra, beam profiles, divergence and homogeneity of beams. Properties of low power X-ray tubes with the maximum voltage up to 50 kV were investigated in this study. These devices included a small laboratory X-ray tube with collimated X-ray beam and a microfocus X-ray tube with polycapillary focusing optics. Beam shape features were reconstructed using a silicon pixel detector. The pixel detector was inserted into X-ray beams at different distances from X-ray sources. Beam 2D profiles were obtained and since the pixel detector worked in a single event detection mode, approximate energy of individual photons could be measured as well as their position. In this way, detailed information about the beam properties was obtained. • X-ray beam parameters are mapped by a pixel detector. • The beam shape and energy distribution of the emitted photons are assessed. • The size of the irradiated point and the quality of the collimators are determined.

Positron Emission Tomography of Not-Full-Ring Sensors Arrangement: Simulation and Verification for Internal Imaging of Hydraulic Cylinder

The research goal of this article is to imaging the oil distribution in landing gear hydraulic cylinder by Positron Emission Tomography(PET). Considering the flexibility and commercial cost for automation testing, two kinds of Not-Full-Ring sensors arrangement, Uniform-arrangement and Bilateral-arrangement, are proposed. The Full-Ring PET is used as the contrast template of imaging. Firstly, GATE 7.1 is used as the Monte Carlo simulation platform to simulate the sensors arrangement of the two Not-Full-Ring schemes. The results of scattering fraction, sensitivity and photon energy distribution were used to evaluate the feasibility of the two schemes. Then, a set of verification device with rotating platform and variable sensing diameter is constructed. The two kinds of Not-Full-Ring sensors arrangements were verified through the verification device. By imaging the hydraulic oil in the hydraulic cylinder, the results were evaluated and compared by using Signal Noise Ratio(SNR). According to the simulation and verification results, Not-Full-Ring scheme is feasible for imaging the liquid distribution in the hydraulic cavity. Uniform- arrangement has a more better imaging performance than Bilateral-arrangement. Such sensors arrangement scheme is not only suitable for this research, but also suitable for other PET imaging applications which need to consider the flexibility and commercial cost.

Characteristics of Photon Radiation Fields in Iron for Photon Sources with Energies from 10 to 50 MeV

Based on the results of Monte Carlo calculations of the spatial energy distributions of photons in iron from point isotropic and planar mononirectional monoenergy sources with energies of 10-50 MeV, the air Kerma attenuation multiplicities and dose accumulation factors of the material under consideration are determined.. The calculations take into account the contribution of the fluorescence, annihilation radiation and bremsstrahlung. The independence of the accumulation factors from the angular distribution of the source radiation is shown, and the independence of the attenuation multiplicities from the angular distribution of the source radiation and the weak dependence on its energy in the energy range 30-50 MeV is also shown. The corrections for the barrier protection are determined and their independence from the thickness of the protection and the photon energy of the source is noted. The obtained information allows to reduce the errors of the results of calculations of the thickness of anti-radiation protection of electronic accelerators at high energies, using developed engineering methods of calculation. The obtained information can also be used in calculations of protection against brake radiation of electronic accelerators by engineering methods.

Optimizing the role of impact ionization in conventional insulators

A mechanism for multiple carrier generation through impact ionization (IA) proposed earlier for bulk systems of strongly correlated insulators is generalized to the case of conventional insulators that contain localized bands a few eV above and below the highest occupied band. Specifically, we study the case of hybridization of localized orbitals with more dispersive bands near the Fermi level, where the generated multiple carriers, which ultimately decay to the edges of the dispersive bands by means of IA processes, acquire lighter mass and this could allow their more efficient separation before recombination. We argue that this may be applicable to the case of halide perovskites and it could be one of the reasons for their observed photovoltaic efficiency. We discuss the criteria one should use to uncover the appropriate material in order to harvest the optimum effect of IA for the spectrum of the solar photon energy distribution.

Explaining GRB prompt emission with sub-photospheric dissipation and Comptonization

Abstract Even though the observed spectra for GRB prompt emission is well constrained, no single radiation mechanism can robustly explain its distinct non-thermal nature. Here we explore the radiation mechanism with the photospheric emission model using our Monte Carlo Radiative Transfer (MCRaT) code. We study the sub-photospheric Comptonization of fast cooled synchrotron photons while the Maxwellian electrons and mono-energetic protons are accelerated to relativistic energies by repeated dissipation events. Unlike previous simulations, we implement a realistic photon to electron number ratio Nγ/Ne ∼ 105 consistent with the observed radiative efficiency of a few percent. We show that it is necessary to have a critical number of episodic energy injection events Nrh, cr ∼ few 10s − 100 in the jet in addition to the electron-proton Coulomb coupling in order to inject sufficient energy Einj, cr ∼ 2500 − 4000 mec2 per electron and produce an output photon spectrum consistent with observations. The observed GRB spectrum can be generated when the electrons are repeatedly accelerated to highly relativistic energies γe, in ∼ few 10s − 100 in a jet with bulk Lorentz factor Γ ∼ 30 − 100, starting out from moderate optical depths τin ∼ 20 − 40. The shape of the photon spectrum is independent of the initial photon energy distribution and baryonic energy content of the jet and hence independent of the emission mechanism, as expected for photospheric emission.

X-Rays from e± Pair Halos

Abstract It was previously suggested that e ± pair halos could be observed in γ-rays. Searching for pair halos is a challenging topic in γ-ray astronomy that has not yet yielded a convincing detection for an individual object. Here we propose that X-ray observations could reveal the existence of e ± pair halos. To support our suggestion, we computed the observed energy and spatial distribution of X-ray photons from the pair halos and check the feasibility of pair halo detection by current and future X-ray observatories. The results show that current X-ray missions could register a positive signal for the pair halos created under some conditions. For a magnetic field (B) of ∼1 μG, the X-ray could be observable for a seed gamma-ray energy of TeV. If TeV, the observable range of B would be B ≳ 300 nG.

Energy Window and Contrast Optimization for Single-photon Emission Computed Tomography Bremsstrahlung Imaging with Yttrium-90.

Purpose:In yttrium-90 (Y-90) single-photon emission computed tomography (SPECT) imaging, the choice of the acquisition energy window is not trivial, due to the continuous and broad energy distribution of the bremsstrahlung photons. In this work, we investigate the effects of the energy windows on the image contrast to noise ratio (CNR), in order to select the optimal energy window for Y-90 imaging.Materials and Methods:We used the Monte Carlo SIMIND code to simulate the Jaszczak phantom which consists of the six hot spheres filled with Y-90 and ranging from 9.5 to 31.8 mm in diameter. Siemens Symbia gamma camera fitted with a high-energy collimator was simulated. To evaluate the effect of the energy windows on the image contrast, five narrow and large energy windows were assessed.Results:The optimal energy window obtained for Y-90 bremsstrahlung SPECT imaging was 120–150 keV. Furthermore, the results obtained for CNR indicate that the high detection is only for the three large spheres.Conclusion:The optimization of energy window in Y-90 bremsstrahlung has the potential to improve the image quality.

Photon spectrum and angle distribution for photon scattering with relativistic Maxwellian electrons

Description of photon scattering with relativistic Maxwellian electrons is numerically complex, and computationally time consuming for the final photon energy and angle distribution. A Monte Carlo method is used to simulate photon scattering with relativistic Maxwellian electrons. The main idea of this method is to transform the interaction of photonmoving electrons in the laboratory coordinate system into that in a new coordinate system in which the electrons are at rest, then to use the exact Klein-Nishina formula to describe this interaction and obtain the outgoing photon energy and angle, finally, to transform it into the primary laboratory coordinate system. In sum, there are eight steps, i.e.two two-dimensional (2D) transforms and two three-dimensional (3D) transforms and two Lorentz transforms, and two sampling. Repeating this process, summarizing and averaging all computed energy values and angles, the distribution of scattered energy and angle can be obtained.</br>A Monte Carlo processor is developed to simulate a photon of any energy interacting with electrons at any temperature. Some typical cases are simulated. The computed results indicate that the photon spectrum is different from that of the photon scattering with rest electrons remarkably, especially for a low energy photon scattering with the high temperature electrons. The main phenomena are Doppler broading and blue shifting. The moving electron can extend the distribution of the outgoing photon energy, and for a low energy photon scattering with the high temperature electrons, the photon maybe obtains the energy from electrons with significant probability. The angle distribution is very complicated, and it is determined by the incident photon energy, the outgoing photon energy, and the electron temperature. This processor can calculate the energy scattering differential cross-sections or energy-angle scattering double differential cross-sections, and provide the data in a tabulated form for other transport methods.

Mucosal dosimetry on unflattened photon beams: a Monte Carlo phantom study

Objective: This study investigated the dependences of mucosal dose and photon energy distribution on the flattened and unflattened photon beams using Monte Carlo simulation. Methods: A heterogeneous mucosa phantom with a variation of mucosal thickness (0.5–3 mm) was irradiated by the 6 and 10 MV flattened and unflattened photon beams generated by a Varian TrueBeam linac. The photon energy distributions at the mucosa and depth doses at the bones and mucosa were calculated using the EGSnrc-based Monte Carlo code. Results: It is found that the 6 MV unflattened photon beam contained more particle fluence in the low-energy range (0–100 keV) than the 10 MV. However, mucosal doses for the unflattened photon beams were found lower than the flattened. This is different from the skin dose having dose enhancement on the unflattened beam compared to the flattened, though both mucosal and skin structure involve an air-soft tissue interface. The particle fluence of the 6 and 10 MV unflattened photon beams increased with an increase of the mucosal thickness. This agreed well with the relative depth doses that increased in the ranges of 3.4%–3.6% and 2.5%–3.0% at the upper mucosa, and 5.1%–5.7% and 2.9%–3.2% at the lower mucosa, with their thickness increasing from 0.5–3 mm for the 6 and 10 MV unflattened photon beams. Conclusion: It is concluded that the unflattened photon beam results in a lower mucosal dose than the flattened, and the 6 MV unflattened beam has a larger dependence of mucosal dose on its thickness when compared to the 10 MV.