Distribution Of Energy Deposition Research Articles

Geant4 used in medical physics and hadrontherapy technique

This article mainly presents the application of Geant4 package (Geant4.7.0, 2005), developed by CERN (the Center of European Research of Nucleus), in medical physics and a novel technique, namely, hadrontherapy. The distribution of energy deposition in a water model by different radiation beams was also given. The results show that the distributions for proton beams are completely different from those for other radiation beams. Charged particles like protons exhibit little scattering when they penetrate the matter and give the highest energy deposition near the end of their range just before they reach the resting state. These characteristics permit very precise control of the shape of the energy distribution inside the patient's body.

Energy deposition in helium-3-based nuclear-pumped gas lasers

The amount and distribution of energy deposition in the cylindrical cells of nuclear-pumped lasers excited by products of the 3He(n, p)3H nuclear reaction are investigated theoretically. Calculation results for three neutron energy distributions, as well as the energy deposition distributions inside the laser cell at helium-3 initial pressures of 1 and 2 atm, are presented.

Anisotropic imaging performance in indirect x-ray imaging detectors

We report on the variability in imaging system performance due to oblique x-ray incidence, and the associated transport of quanta (both x rays and optical photons) through the phosphor, in columnar indirect digital detectors. The analysis uses MANTIS, a combined x-ray, electron, and optical Monte Carlo transport code freely available. We describe the main features of the simulation method and provide some validation of the phosphor screen models considered in this work. We report x-ray and electron three-dimensional energy deposition distributions and point-response functions (PRFs), including optical spread in columnar phosphor screens of thickness 100 and 500 microm, for 19, 39, 59, and 79 keV monoenergetic x-ray beams incident at 0 degrees, 10 degrees, and 15 degrees. In addition, we present pulse-height spectra for the same phosphor thickness, x-ray energies, and angles of incidence. Our results suggest that the PRF due to the phosphor blur is highly nonsymmetrical, and that the resolution properties of a columnar screen in a tomographic, or tomosynthetic imaging system varies significantly with the angle of x-ray incidence. Moreover, we find that the noise due to the variability in the number of light photons detected per primary x-ray interaction, summarized in the information or Swank factor, is somewhat independent of thickness and incidence angle of the x-ray beam. Our results also suggest that the anisotropy in the PRF is not less in screens with absorptive backings, while the noise introduced by variations in the gain and optical transport is larger. Predictions from MANTIS, after additional validation, can provide the needed understanding of the extent of such variations, and eventually, lead to the incorporation of the changes in imaging performance with incidence angle into the reconstruction algorithms for volumetric x-ray imaging systems.

Numerical Simulation of Radial and Angular Distribution of γ-Ray's Energy Deposition in Scintillation Optical Fibre

Angular and radial distributions of the energy deposition of γ-ray radiation in scintillation optical fibres are simulated and analysed using the Geant4 system. The results show a linear relation between the energy deposition and the radius of the fibres. The deposition is roughly inversely proportional to sinθ with θ the incident angle relative to the fibre axis. The results could provide corrections to the measurements of the scintillation fibres used in monitoring the γ-ray radiation.

Microarray analysis of differentially expressed genes after exposure of normal human fibroblasts to ionizing radiation from an external source and from DNA-incorporated iodine-125 radionuclide

Exposure of cells to ionizing radiation (IR) produces changes in the expression level of a large number of genes. However, less is known of gene-expression changes caused by local radiation exposure from radionuclides within cells. We studied changes in the genome-wide gene expression induced by decay of 125I incorporated into DNA as [ 125I]-iododeoxyuridine ( 125I-IUdR) in normal IMR-90 human lung fibroblasts and compared them with the changes produced by external γ-radiation delivered at high (HDR) or low (LDR) dose rate. We found that more than 2000 genes were consistently up- or down-regulated following HDR and LDR γ-radiation. The profiles of differentially expressed genes following HDR and LDR shared about 64% (up) and 74% (down) genes in common, with many genes identified as radiation-responsive for the first time. In contrast, in all only 206 genes changed their expression level in the 125I-IUdR-treated cells, even though the total number of DNA double-strand breaks (DSB) produced by 125I-IUdR exceeded that produced by the γ-radiation. With few exceptions, the expression levels of 125I-IUdR-responsive genes were also altered following γ-irradiation. Therefore, nuclear DNA-localized decays of 125I produce 10 times fewer differentially expressed genes than whole-cell exposure to γ-radiation of comparable dose. These results suggest that the effect of IR on the changes in global gene expression depends on the distribution of energy depositions within the cell. In contrast to cell survival, DNA DSB may not be the major factor modulating changes in gene expression following irradiation.

Charge transfer in high velocity Cn+ + He collisions

Dissociative and non-dissociative charge transfer cross sections in high velocity (v = 2.6 au) collisions between ionic carbon clusters Cn+ (n = 2–10) and helium atoms have been measured. The sum of the cross sections has been found to increase significantly with n. Measurements of branching ratios for all fragmentation channels of excited Cn clusters are reported. The summed branching ratios associated with a given number of emitted fragments exhibit odd–even alternations reflecting the higher stability of the species having an odd number of atoms. From an analysis of the summed branching ratios within the statistical microcanonical metropolis Monte Carlo model, and knowing the temperature of the incident clusters, deposited energy distributions due to the charge transfer process are deduced (n = 5–9). These distributions, of similar characteristics whatever n, peak around 4–5 eV and exhibit a large percentage of superexcited states situated above the continuum.

Positron follow-up in liquid water: I. A new Monte Carlo track-structure code

When biological matter is irradiated by charged particles, a wide variety of interactions occur, which lead to a deep modification of the cellular environment. To understand the fine structure of the microscopic distribution of energy deposits, Monte Carlo event-by-event simulations are particularly suitable. However, the development of these track-structure codes needs accurate interaction cross sections for all the electronic processes: ionization, excitation, positronium formation and even elastic scattering. Under these conditions, we have recently developed a Monte Carlo code for positrons in water, the latter being commonly used to simulate the biological medium. All the processes are studied in detail via theoretical differential and total cross-section calculations performed by using partial wave methods. Comparisons with existing theoretical and experimental data in terms of stopping powers, mean energy transfers and ranges show very good agreements. Moreover, thanks to the theoretical description of positronium formation, we have access, for the first time, to the complete kinematics of the electron capture process. Then, the present Monte Carlo code is able to describe the detailed positronium history, which will provide useful information for medical imaging (like positron emission tomography) where improvements are needed to define with the best accuracy the tumoural volumes.

Energy deposition of γ-ray in different instances and its influence on energy detection

Abstract In this paper, we simulate γ ray energy deposition for different incident energies with four different models using the tool GEANT4 (Geant4.7.0, 2005) developed by CERN (the Center of European Research of Nucleus). The results we obtained indicate that there are different peak values for different incident energies. That is, we can differentiate the incident energy accurately if the detector can determine the peak value accurately. This is meaningful for the geometrical configuration of the detector to get the most probable distribution (MPD) of energy deposition for different incident energies. According to the simulation, we can insert certain slices with large absorption coefficient to obtain a better MPD of energy deposition which will not alter the shape of the energy deposition.

Hydrodynamic escape of nitrogen from Pluto

Hydrodynamic escape of nitrogen from Pluto is studied by solving time‐dependent hydrodynamic escape equations that treat the spatial distribution of EUV energy deposition realistically. Simulations show that the N2 hydrodynamic escape rate is ∼1 × 1028 molecules s−1 when Pluto is at 40 AU (its average orbital location) and solar activity level is at minimum. The N2 hydrodynamic escape rate is ∼2 × 1028 molecules s−1 when Pluto is at its perihelion (30 AU) and solar activity level is at maximum. Through hydrodynamic escape, Pluto may have lost ∼0.5% of its total mass over the age of the solar system. Comet‐like interactions may occur between Pluto's atmosphere and the solar wind flow, which may be observed by the New Horizon mission.

Internal Microdosimetry for Single Cells in Radioimmunotherapy of B-Cell Lymphoma

Patients with B-cell lymphoma may have disease manifestations ranging in size from more than a 1000 cm3 down to the volume of a single cell. If targeted radionuclide therapy is to become a curative treatment, all individual tumor cells must also be eliminated. Given the vast differences in particle energy of different electron- emitting radionuclides, one questions whether the mean absorbed dose is a relevant parameter for use in single-cell dosimetry and whether it would not be more accurate to adopt a stochastic approach to dosimetry. Monte Carlo simulations were performed of energy deposition from 1000, 300, 100, or 10 electrons uniformly distributed in a sphere with a radius of 7.7 microm. The simulated electrons were monoenergetic (18 keV, 28 keV, 141 keV, or 935 keV). The absorbed dose per emitted electron, the absorbed fraction, the fraction of the cellular volume in which energy is deposited, and the dose-volume histograms were calculated. Absorbed fractions varied between 0.60 (18 keV) and 0.001 (935 keV), and the absorbed dose to the cell per electron emitted varied by a factor of 10, from 0.898 mGy (18 keV) to 0.096 mGy (935 keV). The specific energy varied between 0 and 46 mGy for the case showing the best uniformity (1000 18-keV electrons). The nonuniformity of the absorbed dose to a cell increases with increasing electron energy and decreases with the number of decays inside the studied volume. The wide distribution of energy deposition should be taken into account when analyzing and designing trials for targeted radionuclide therapy.

Monte Carlo Simulation of the Spatial Distribution of Energy Deposition for an Electron Microbeam

Dosimetry calculations characterizing the spatial variation of the energy deposited by the slowing and stopping of energetic electrons are reported and compared with experimental measurements from an electron microbeam facility. The computations involve event-by-event, detailed-histories Monte Carlo simulations of low-energy electrons interacting in water vapor. Simulations of electron tracks with starting energies from 30 to 80 keV are used to determine energy deposition distributions in thin cylindrical rings as a function of penetration and radial distance from a beam source. Experimental measurements of the spatial distribution of an electron microbeam in air show general agreement with the density-scaled simulation results for water vapor at these energies, yielding increased confidence in the predictions of Monte Carlo track-structure simulations for applications of the microbeam as a single-cell irradiator.

Spatial and temporal characteristics of energy deposition by protons and alpha particles in silicon

We present spatial and temporal distributions of energy deposition in silicon obtained from Monte Carlo simulations with Geant4. Detailed simulations are performed for protons and alphas over a wide energy range and include contributions from discrete /spl delta/-rays and nuclear reactions. The simulation technique we employ produces physically realistic complex track structures for which the plausibility is evaluated by comparison to the linear energy transfer, radial dose, and temporal characteristics described in previous works. The variability of events, including the distribution of energy deposition at varying distances from the ion track is discussed. Finally, we examine the temporal evolution of extreme events to investigate the physicality of the common assumption that energy deposition can be considered instantaneous.

Influence of multiple scattering and energy loss straggling on the absorbed dose distributions of therapeutic light ion beams: I. Analytical pencil beam model

The lateral and longitudinal distributions of absorbed dose of broad and narrow light ion beams in water are investigated. An analytical algorithm based on the generalized Fermi–Eyges theory is developed and used to calculate the effects of multiple scattering and range straggling on the dose distribution of light ion beams in water. A first-order Gaussian multiple scattering and energy loss straggling approach is generally sufficiently accurate for describing the lateral and longitudinal spread of the Bragg peak and the associated energy deposition distribution of therapeutic light ion beams at ranges of clinical interest. Nuclear reactions are not taken into account in this study. The analytical algorithm given in the present study allows an accurate description of the radial spread and the range straggling of light ions traversing matter. A verification of this approach by comparing with experimental data, Monte Carlo methods and other analytical techniques will be presented in a forthcoming paper.

Experimental and simulation results of gamma imaging with hybrid pixel detectors

We have tested the high-energy imaging capabilities of detectors made of a Medipix1 pixel photon-counting chip hybridized with a 300μm thick Si or a 200μm thick GaAs substrate. The results presented in this paper show that an important part of the signal is due to secondary particles coming from γ-ray diffusion in the detector surroundings. According to simulation studies realized with the Monte-Carlo code PENELOPE, the contribution of a 2mm thick Pyrex screen placed between the source (60Co) and the Si detector can reach 75% of the total number of counts. The main reason for this high relative sensitivity of the substrate to the secondary particles is its very poor detection efficiency at high γ-ray energies. We also present results for imaging with a double-cone pinhole collimator, through comprehensive comparisons between experiment and simulation.The first prototypes of a Medipix2 chip hybridized with a 1mm thick pixel CdTe substrate have been carried out and we present experimental tests done with γ-ray sources at high energies (up to 1.33MeV). We also present results obtained with a dedicated pseudo-Monte-Carlo 3D simulation code. This simulation code, whose first stage is the single-event energy-deposition distribution, processes the charge-carrier transport taking into account trapping and diffusion and, at the same time, registers the induced signal on pixels. The first confrontations with experimental measurements are shown.

Ion beam transport in tissue-like media using the Monte Carlo code SHIELD-HIT

The development of the Monte Carlo code SHIELD-HIT (heavy ion transport) for the simulation of the transport of protons and heavier ions in tissue-like media is described. The code SHIELD-HIT, a spin-off of SHIELD (available as RSICC CCC-667), extends the transport of hadron cascades from standard targets to that of ions in arbitrary tissue-like materials, taking into account ionization energy-loss straggling and multiple Coulomb scattering effects. The consistency of the results obtained with SHIELD-HIT has been verified against experimental data and other existing Monte Carlo codes (PTRAN, PETRA), as well as with deterministic models for ion transport, comparing depth distributions of energy deposition by protons, 12C and 20Ne ions impinging on water. The SHIELD-HIT code yields distributions consistent with a proper treatment of nuclear inelastic collisions. Energy depositions up to and well beyond the Bragg peak due to nuclear fragmentations are well predicted. Satisfactory agreement is also found with experimental determinations of the number of fragments of a given type, as a function of depth in water, produced by 12C and 14N ions of 670 MeV u−1, although less favourable agreement is observed for heavier projectiles such as 16O ions of the same energy. The calculated neutron spectra differential in energy and angle produced in a mimic of a Martian rock by irradiation with 12C ions of 290 MeV u−1 also shows good agreement with experimental data. It is concluded that a careful analysis of stopping power data for different tissues is necessary for radiation therapy applications, since an incorrect estimation of the position of the Bragg peak might lead to a significant deviation from the prescribed dose in small target volumes. The results presented in this study indicate the usefulness of the SHIELD-HIT code for Monte Carlo simulations in the field of light ion radiation therapy.

Determining the Spatial Structure of a High-Power Electron Beam Using Optical Radiation Emitted by a Beam Collector

A method for determining the spatial structure of a high-power relativistic electron beam is described. Using an absolutely calibrated CCD collector, the thermal-radiation brightness distribution of the beam-collector surface is registered in the visible and near-IR spectral regions and then converted into a spatial distribution of the beam energy deposition in a target. This method is applied at a high specific beam-energy flux (∼1 kJ/cm2), one which causes noticeable erosion of the beam collector.

Fabrication of Nanowires Using High-Energy Ion Beams

Nanowire formation in Si-based polymer thin films using a heavy ion beam is discussed in terms of energy density deposition along ion tracks. Gelation of the polymer along the ion track results in cross-linking to produce nanowires with size and number density controllable by selecting appropriate ion beam characteristics and polymer materials. Ion bombardment of poly(carbosilane) (PCS), PCS−poly(vinylsilane) blend polymer, and poly(methylphenylsilane) produces nanowires with radii of 7−30 nm depending on the type of ion beam. The difference in size is shown to be related to the efficiency of the cross-linking reaction considering the deposited energy distribution along the ion tracks.

Influence of the spatial and temporal structure of the deposited-energy distribution in swift-ion-induced sputtering

The sputter processes occurring under swift-ion bombardment in the electronic-stopping regime are investigated by molecular-dynamics simulations performed for a Lennard-Jones solid (Ar). Two aspects of the dynamics of the excited electronic subsystem are included in the simulation and their influence on the sputter yield is studied. First, we assume the energy transfer from the electronic to the atomic system not to be instantaneous, but to last for a period of time \ensuremath{\tau}. For $\ensuremath{\tau}\ensuremath{\gtrsim}1\mathrm{ps},$ we find the sputter yield Y to become strongly nonlinear as a function of the stopping power $dE/dx.$ Second, we test the influence of a nonhomogeneous spatial distribution of the electronic excitations. It is shown that such a spatial distribution also leads to a strongly nonlinear dependence of Y on $dE/dx.$

Optimization of Poly-(Methylphenylsilylene) Specimens for Cathodoluminescence Measurement

Two possibilities of photon collection from the luminescence centra can be applied at cathodoluminescence (CL) measurement of transparent specimens. The CL emission can be collected both from the side of excitation and substrate using a lens and a light guide, respectively. To choose the better alternative, one has to study optical properties and electron-interactive volumes of specimen materials. Based on this knowledge, optimized geometry, dimension and arrangement of the specimen can be designed. The object of our interest was the CL study of Poly-(Methylphenylsilylene), i.e. PMPhSi, which is an interesting material both from the application and the basic research point of view. The PMPhSi was prepared by the Wurtz coupling polymerization [1]. The low-molecular weight fractions were extracted with boiling diethyl ether. The layers for the CL measurements were prepared from a toluene solution by casting on quartz disk substrates. The optical transmittance of 500 nm thick PMPhSi layer was measured using the Varian Cary 5E spectrophotometer in the wavelength range from 200 to 1600 nm. The measurement (not corrected for Fresnel’s reflections) is plotted in Fig. 1. After correction, the absorption coefficient of approximately 0.2 :m -1 can be calculated in the region of the CL emission (360 nm). So, the optical selfabsorption of PMPhSi is high enough to require a thickness optimization at CL measurements. To look for the optimal thickness of the PMPhSi, the size and shape of the electron interaction volume must be known. Therefore, the Monte Carlo (MC) simulation of electron trajectories and energy deposition in the bulk PMPhSi has been executed using the single scattering algorithm with the screened Rutherford elastic cross-section and the Bethe continuous slowing-down energy loss[2]. At the primary electron energy of 10 keV, the deposited energy distribution in the PMPhSi (penetrated from the surface to the depth) has been obtained (Fig. 2). It can be taken from the plot the 75 % of energy is converted to photons below the depth of 2 :m, and only remaining energy is absorbed above 2 :m up to the terminal penetration of 3 :m. So, photon collection from the side of substrate using a light guide seems to be a better choice. To minimize the losses due to the photon selfabsorption, the PMPhSi should be optimized to the thickness of 2 :m for CL measurements at 10 keV excitation electron beam. The PMPhSi layer is non-conductive and hence it is necessary to provide its surface with a thin conductive coating. The coating must be thin enough not to absorb the energy of signal electrons and at the same time thick enough to be conductive and to show high optical reflection. Therefore, Al and Ag films have been investigated to find their optical reflection and e-beam energy losses at the sufficient electrical conductivity. Thickness dependance of the internal optical reflection at the PMPhSi-(Al|Ag) boundary (Fig. 3) has been obtained by matrix method [3] calculation. Although Ag film is known as an excellent optical mirror, its reflection in the region of the CL emission band (360 nm) is poor, and only Al can be considered as an optically suitable film. Using the same MC method mentioned above, the dependance of the thickness of Al and Ag films on the electron energy

Comparison and physical interpretation of MCNP and TART neutron and γ Monte Carlo shielding calculations for a heavy-ion ICF system

Inertial confinement fusion (ICF) aims to induce implosions of D–T pellets to obtain a extremely dense and hot plasma with lasers or heavy-ion beams. For heavy-ion fusion (HIF), recent research has focused on “liquid-protected” designs that allow highly compact target chambers. In the design of a reactor such as HYLIFE-II [Fus. Techol. 25 (1984); HYLIFE-II Progress Report, UCID-21816, 4.82-100], the liquid used is a molten salt made of F 10, Li 6, Li 7, Be 9 (called flibe). Flibe allows the final-focus magnets to be closer to the target, which helps to reduce the focus spot size and in turn the size of the driver, with a large reduction of the cost of HIF electricity. Consequently the superconducting coils of the magnets closer to the D–T neutron source will potentially suffer higher damage though they can stand only a certain amount of energy deposited before quenching. This work has been primarily focusing on verifying that total energy deposited by fusion neutrons and induced γ rays remain under such limit values and the final purpose is the optimization of the shielding of the magnetic lens system from the points of view of the geometrical configuration and of the physical nature of the materials adopted. The system is analyzed in terms of six geometrical models going from simplified up to much more realistic representations of a system of 192 beam lines, each focused by six magnets. A 3-D transport calculation of the radiation penetrating through ducts, that takes into account the complexity of the system, requires Monte Carlo methods. The technical nature of the design problem and the methodology followed were presented in a previous paper [Nucl. Instr. and Meth. A 464 (2001) 410] by summarizing briefly the results for the deposited energy distribution on the six focal magnets of a beam line. Now a comparison of the performances of the two codes TART98 [TART98: A Coupled Neutron-Photon 3-D Combinational Geometry Monte Carlo Transport Code, Lawrence Livermore National Laboratory, UCRL-ID-126455, Rev. 1, November, 1997] and MCNP4B [MCNP – A General Monte Carlo N-Particle Transport Code, Version 4B, La-12625-m, March 1997, Los Alamos National Laboratory] for two different configurations of the system is discussed, separating the n and γ contributions, in the light of the physical interpretation of the results in terms of first flight and of scattered neutron fluxes, of primary γ and of secondary γ generated by inelastically scattered or radiatively captured neutrons. The final conclusions indicate some guidelines and suggest possible improvements for the future neutronic shielding design for a HIF facility.