Average Energy Deposition Research Articles

Modeling focused ultrasound exposure for the optimal control of thermal dose distribution.

Preclinical studies indicate that focused ultrasound at exposure conditions close to the threshold for thermal damage can increase drug delivery at the focal region. Although these results are promising, the optimal control of temperature still remains a challenge. To address this issue, computer-simulated ultrasound treatments have been performed. When the treatments are delivered without taking into account the cooling effect exerted by the blood flow, the resulting thermal dose is highly variable with regions of thermal damage, regions of underdosage close to the vessels, and areas in between these two extremes. When the power deposition is adjusted so that the peak thermal dose remains close to the threshold for thermal damage, the thermal dose is more uniformly distributed but under-dosage is still visible around the thermally significant vessels. The results of these simulations suggest that, for focused ultrasound, as for other delivery methods, the only way to control temperature is to adjust the average energy deposition to compensate for the presence of thermally significant vessels in the target area. By doing this, we have shown that it is possible to reduce the temperature heterogeneity observed in focused ultrasound thermal treatments.

X-ray interaction-induced signal and noise performances of edge-on silicon microstrip detectors for digital mammography

We are developing a silicon microstrip-array detector, which is operated in photon-counting mode, for line-scanned digital mammography. To enhance the x-ray interaction efficiency, the x-ray beam is oriented toward the edge of microstrips, known as the edge-on geometry. To predict the fundamental signal and noise performances induced by x-ray interactions, we performed Monte Carlo simulations. Absorbed energy distribtuions were obtained for various tilting angles (5 to 85 degrees) in the edge-on detector geometry for a wide range of incident energies from 1 to 50 keV. Based on the energy-moments theory with the obtained absorbed energy distributions, we estimated various physical performance parameters such as the quantum absorption efficiency, the average energy deposition per interaction, and the Swank noise factor. In addition, relative accuracy and imprecision in photon-energy measurements were estimated. These analyses were extended to the typical poly-energetic mammography x-ray spectra from various target materials such as molybdenium, rhodium and tungsten. Among performance parameters, the quantum absorption efficiency was gradually decreased as the tilting angle increases because of the reduction in pathlength where x-ray photons travel, while others were almost insensitive to the tilting angle. The best signal and noise performances in the edge-on silicon microstrip detector were obtained for the rhodium spectrum.

A Heterogeneous Coarse Mesh Method for Coupled Photon Electron Transport Problems

A hybrid Monte Carlo/deterministic coarse mesh transport method (COMET-PE) has been developed for pure photon or coupled photon and electron transport in heterogeneous problems. The accuracy of the method was evaluated in two highly stylized 2D benchmark problems: (1) a homogeneous rectangular water phantom and (2) a heterogeneous problem that is typical of a 2D vertical slice of lung. It was found that the method produces results that are very close to those of direct Monte Carlo with significantly higher computational efficiency (2–3 orders of magnitude). In both test problems, the maximum and the average energy deposition differences between COMET-PE and the reference Monte Carlo solutions for the pure photon transport case ranged from 2.0%–2.4% and 1.0%–1.2%, respectively. The corresponding differences for the coupled photon and electron transport case ranged from 1.1%–2.2% and 0.5%–0.9%, respectively. The COMET-PE computation time was more than 200 and 700 times shorter than EGSnrc in the homogeneous and the heterogeneous problems, respectively. Therefore, it is concluded that the new incident flux response expansion method is highly accurate and efficient for energy deposition calculation in heterogeneous tissues.

Calibration of CMS Electromagnetic Calorimeter at LHC startup

The first 7 TeV LHC collisions recorded with the CMS detector have been used to perform a channel-by-channel calibration of the electromagnetic calorimeter (ECAL). Decays of π° and η into two photons as well as the azimuthal symmetry of the average energy deposition at a given pseudorapidity are utilized to equalize the response of the individual channels. The ECAL comprises a central barrel section and two endcaps. Based on an integrated luminosity of ∼ 100 nb−1 a channel-by-channel in-situ calibration precision of 1.15% has been achieved in the barrel ECAL in the pseudorapidity region |η| < 0.8. The energy scale of the ECAL has been investigated and found to agree with the simulation to within 1% in the barrel and 3% in the endcaps.

An experimental study of the microdosimetric response of a graphite walled proportional counter in low energy neutron fields

Abstract This study describes the measurement of lineal energy spectra carried out with a graphite walled proportional counter (GPC) simulating 1 μm tissue equivalent (TE) site diameter in low energy neutron fields generated by employing the McMaster University 1.25 MV double stage Tandetron accelerator. Experiments were carried out for neutron beams of mean energies extending from 34 to 354 keV in direct neutron beams from the 7 Li target using proton beam energies ranging from 1.89 to 2.56 MeV. We have observed a continuously increasing trend in microdosimetric averages, y ¯ F and y ¯ D , and the ICRP 60 based quality factor, Q , with an increase in neutron energy. It is interesting to note that standard tissue equivalent proportional counters (TEPCs) give rise to a similar trend for these microdosimetric quantities of interest in the same energy range. Quantitatively, these quantities are also comparable to those obtained with standard TEPCs. This implies that at the microdosimetric level in the neutron energy range we employed in this study, the pattern of average energy deposited by ‘starter’ and ‘insider’ proton recoil events in the gas is similar to those generated cumulatively by ‘crosser’ and ‘stopper’ events originating from the counter wall plus ‘starter’ and ‘insider’ recoil events originating in the sensitive volume of a TEPC. This suggests that the average energy deposition per event in the wall of a TEPC counter is the same as in the counting gas itself, which confirms the similarity of the composition and mass stopping power of two media, i.e., TE wall and TE gas, used in neutron dosimetric measurements.

Monte Carlo calculations of low-energy electron dose-point-kernels in water using different stopping power approximations

The spatial distribution of absorbed energy in irradiated matter can be conveniently described by dose-point-kernel (DPK) distributions that characterize the average energy deposition around single charged-particle tracks during their slowing down process. In the present work, electron DPKs in liquid water in the energy range from 100eV to 10keV are presented based on Monte Carlo simulation of electron transport in the continuous-slowing-down-approximation (csda). Elastic collisions are individually simulated using the screened Rutherford formula, whereas the energy loss from inelastic interactions is determined from stopping power (SP) theory. Along with the standard Bethe SP formula we examine different empirical expressions of general-use which are meant to improve the performance of the Bethe formula at low electron energies. Comparison is also made with a recent Bethe-type parametric expression obtained from a dielectric optical data model of liquid water. Our findings indicate that for electron energies below ∼1keV the discrepancies between the DPKs calculated by the general-purpose SP formulae become apparent. Moreover, the results obtained by the empirical expressions compare rather poorly with those from the dielectric model over the entire energy range examined.

Statistical analysis of the energetic ion and ENA data for the Titan environment

The MIMI experiment (Magnetosphere Imaging Instrument) onboard Cassini is dedicated to the study of energetic particles, with in particular LEMMS analyzing charged particles, or the INCA detector which can image the Energetic Neutral Atoms produced by charge exchange collisions between cold neutrals and energetic ions. The MIMI experiment is thus well adapted to the study of the interaction between the Titan nitrogen rich atmosphere and the energetic Saturnian magnetospheric plasma. We analyze here the energetic protons at the Titan orbit crossings before January 2008 (MIMI-LEMMS data; 27–255 keV), which are very dynamic, with tri-modal flux spectra and probably quasi-isotropic pitch angle distributions. We provide statistical parameters for the proton fluxes, leading to estimates of the average energy deposition into Titan's atmosphere, before we discuss the possible influence of Titan on the magnetopause. We then analyze the H ENA images (24–55 keV) during the Titan flybys before June 2006 to obtain a better diagnostic of the Titan interaction: the ENAs variability is mostly related to the magnetospheric variability (the exosphere being roughly stable) or the distance from the moon, the ENAs halo around Titan is very stable (corresponding to a lower limit for ENAs emission at the exobase), and strong asymmetries are observed, due to finite gyroradii effects for the parent ions.

Investigation of Energy Deposited by Femtosecond Electron Transfer in Collisions Using Hydrated Ion Nanocalorimetry

Ion nanocalorimetry is used to investigate the internal energy deposited into M (2+)(H 2O) n , M = Mg ( n = 3-11) and Ca ( n = 3-33), upon 100 keV collisions with a Cs or Ne atom target gas. Dissociation occurs by loss of water molecules from the precursor (charge retention) or by capture of an electron to form a reduced precursor (charge reduction) that can dissociate either by loss of a H atom accompanied by water molecule loss or by exclusively loss of water molecules. Formation of bare CaOH (+) and Ca (+) by these two respective dissociation pathways occurs for clusters with n up to 33 and 17, respectively. From the threshold dissociation energies for the loss of water molecules from the reduced clusters, obtained from binding energies calculated using a discrete implementation of the Thomson liquid drop model and from quantum chemistry, estimates of the internal energy deposition can be obtained. These values can be used to establish a lower limit to the maximum and average energy deposition. Not taking into account effects of a kinetic shift, over 16 eV can be deposited into Ca (2+)(H 2O) 33, the minimum energy necessary to form bare CaOH (+) from the reduced precursor. The electron capture efficiency is at least a factor of 40 greater for collisions of Ca (2+)(H 2O) 9 with Cs than with Ne, reflecting the lower ionization energy of Cs (3.9 eV) compared to Ne (21.6 eV). The branching ratio of the two electron capture dissociation pathways differs significantly for these two target gases, but the distributions of water molecules lost from the reduced precursors are similar. These results suggest that the ionization energy of the target gas has a large effect on the electron capture efficiency, but relatively little effect on the internal energy deposited into the ion. However, the different branching ratios suggest that different electronic excited states may be accessed in the reduced precursor upon collisions with these two different target gases.

Two‐Dimensional Multiangle, Multigroup Neutrino Radiation‐Hydrodynamic Simulations of Postbounce Supernova Cores

We perform axisymmetric (2D) multiangle, multigroup neutrino radiation-hydrodynamic calculations of the postbounce phase of core-collapse supernovae using a genuinely 2D discrete-ordinate (Sn) method. We follow the long-term postbounce evolution of the cores of one nonrotating and one rapidly rotating 20 M☉ stellar model for ~400 milliseconds from 160 to ~550 ms after bounce. We present a multidimensional analysis of the multiangle neutrino radiation fields and compare in detail with counterpart simulations carried out in the 2D multigroup flux-limited diffusion (MGFLD) approximation to neutrino transport. We find that 2D multiangle transport is superior in capturing the global and local radiation-field variations associated with rotation-induced and SASI-induced aspherical hydrodynamic configurations. In the rotating model, multiangle transport predicts much larger asymptotic neutrino flux asymmetries with pole-to-equator ratios of up to ~2.5, while MGFLD tends to sphericize the radiation fields already in the optically semitransparent postshock regions. Along the poles, the multiangle calculation predicts a dramatic enhancement of the neutrino heating by up to a factor of 3, which alters the postbounce evolution and results in greater polar shock radii and an earlier onset of the initially rotationally weakened SASI. In the nonrotating model, differences between multiangle and MGFLD calculations remain small at early times when the postshock region does not depart significantly from spherical symmetry. At later times, however, the growing SASI leads to large-scale asymmetries and the multiangle calculation predicts up to 30% higher average integral neutrino energy deposition rates than MGFLD.

Reduction Energy of 1 M Aqueous Ruthenium(III) Hexaammine in the Gas Phase: A Route toward Establishing an Absolute Electrochemical Scale

The internal energy deposited into gas-phase Ru(NH3)6(H2O)n3+ when reduced by thermal electrons is investigated as a function of cluster size. For n ≥ 40, reduction results exclusively in the loss of water molecules from the reduced precursor ion; loss of water is accompanied by the loss of a single ammonia molecule for smaller clusters. The average number of ligands lost from the reduced precursor decreases with cluster size for n ≤ 31, presumably because of increased binding energy of the ligands to the smaller, doubly charged clusters. For Ru(NH3)6(H2O)553+, which corresponds to a concentration or activity of about 1 M, reduction results in a mean loss of 18.2 water molecules, from which an average and maximum energy deposition of 7.9 and ∼8.2 to 8.7 eV, respectively, is determined. To the extent that the dissociation is statistical, the internal energy deposited corresponds to the reduction energy of the hydrated precursor ion by a thermal electron in the gas phase. This measured value is combined with the electron affinity of water and the difference in solvation energies of the precursor and reduced cluster ions to provide an absolute value for the reduction energy for 1 M Ru(NH3)63+ by a solvated electron in bulk water of about −3.8 eV at 0 K. This route toward establishing absolute half-cell reduction potentials has the advantages that effects of counterions, solvent, and chemical form can be easily controlled and quantified, and redox reactions not readily observed in solution can be measured.

Nonergodicity in Electron Capture Dissociation Investigated Using Hydrated Ion Nanocalorimetry

Hydrated divalent magnesium and calcium clusters are used as nanocalorimeters to measure the internal energy deposited into size-selected clusters upon capture of a thermally generated electron. The infrared radiation emitted from the cell and vacuum chamber surfaces as well as from the heated cathode results in some activation of these clusters, but this activation is minimal. No measurable excitation due to inelastic collisions occurs with the low-energy electrons used under these conditions. Two different dissociation pathways are observed for the divalent clusters that capture an electron: loss of water molecules (Pathway I) and loss of an H atom and water molecules (Pathway II). For Ca(H(2)O)(n)(2+), Pathway I occurs exclusively for n >or= 30 whereas Pathway II occurs exclusively for n <or= 22 with a sharp transition in the branching ratio for these two processes that occurs for n approximately 24. The number of water molecules lost by both pathways increases with increasing cluster size reaching a broad maximum between n = 23 and 32, and then decreases for larger clusters. From the number of water molecules that are lost from the reduced cluster, the average and maximum possible internal energy is determined to be approximately 4.4 and 5.2 eV, respectively, for Ca(H(2)O)(30)(2+). This value is approximately the same as the calculated ionization energies of M(H(2)O)(n)(+), M = Mg and Ca, for large n indicating that the vast majority of the recombination energy is partitioned into internal modes of the ion and that the dissociation of these ions is statistical. For smaller clusters, estimates of the dissociation energies for the loss of H and of water molecules are obtained from theory. For Mg(H(2)O)(n)(2+), n = 4-6, the average internal energy deposition is estimated to be 4.2-4.6 eV. The maximum possible energy deposited into the n = 5 cluster is <7.1 eV, which is significantly less than the calculated recombination energy for this cluster. There does not appear to be a significant trend in the internal energy deposition with cluster size whereas the recombination energy is calculated to increase significantly for clusters with fewer than 10 water molecules. These, and other results, indicate that the dissociation of these smaller clusters is nonergodic.

Internal Energy Deposition in Electron Capture Dissociation Measured Using Hydrated Divalent Metal Ions as Nanocalorimeters

Extensively hydrated divalent metal ions are used as nanocalorimeters to measure the internal energy deposition resulting from electron capture. For M(H2O)322+, M = Mg, Ca, Sr, and Ba, two dissociation pathways are observed: loss of a water molecule from the precursor (∼6%) owing to activation from blackbody photons or collisions with residual background gas, and loss of between 9 and 11 water molecules from the reduced precursor formed by electron capture. The binding energy of a water molecule to the reduced precursor ions is estimated to be approximately 10 kcal/mol. From the distribution of water molecules lost, corrected for residual activation, the average and maximum internal energy deposited into these ions is determined to be ∼4.4 and ∼4.8−5.2 eV, respectively. The average internal energy deposition does not depend significantly on metal ion size (∼0.1 eV difference for Mg to Ba) despite the large (5.0 eV) difference in second ionization energies for the isolated atoms. Similar results were obtained for Ca(H2O)n2+, n = 30 and 32, suggesting that neither the water binding energy nor the recombination energy changes significantly for clusters of this size. The recombination energy is roughly estimated from theory to be about 4.5 eV. These results show that these ions are not significantly activated by inelastic non-capture collisions with electrons and that the vast majority of the recombination energy resulting from electron capture is converted into internal energy of the reduced precursor ions, indicating that these ions dissociate statistically.

Electron stopping power and inelastic mean free path in amino acids and protein over the energy range of 20–20,000 eV

Systematic calculations of stopping power (SPs) and inelastic mean free path (IMFP) values for 20-20,000 eV electrons in a group of 15 amino acids and a simple protein have been performed. The calculations are based on the dielectric response model and take into account the exchange effect between the incident electron and target electrons. The optical energy-loss functions for the 15 investigated amino acids and the protein are evaluated by using an empirical approach, because of the lack of experimental optical data. For all the considered materials, the calculated mean ionization potentials are in good agreement with those given by Bragg's rule, and the evaluated SP values at 20 keV converge well to the Bethe-Bloch predictions. The data shown represent the first results of SP and IMFP, for these 15 amino acids and the protein in the energy range below 20 keV, and might be useful for studies of various radiation effects in these materials. In addition, the average energy deposited by inelastic scattering of the electrons on this group of 15 amino acids, on the protein, on Formvar and on DNA, respectively, has been estimated for energies below 20 keV. The dependences of the average energy deposition on the electron energy are given. These results are important for any detailed studies of radiation-induced inactivation of proteins and the DNA.

Assessment of induction of secondary tumours due to various radiotherapy modalities

One of the objectives of the European Sixth Framework integrated project MAESTRO is to perform an assessment of risk due to various radiotherapy modalities, regarding secondary tumour induction. Initially, the study will focus on cancer of the prostate and the present work represents the first step towards that goal. One of the intended tools, to be used in the assessment, is the Monte Carlo radiation transport code ORANGE. A validation of the ORANGE code's capability to tally dose on a grid superimposed on an existing MCNP geometry is given. Preliminary results on the dose distribution due to conventional radiotherapy treatment of prostate cancer are discussed. Two mathematical models of the patient are proposed and the clinical relevance of the ADAM phantom is investigated. A problem in comparing average doses provided by commercial treatment planning systems and those calculated with Monte Carlo is noticed. The two proposed models are shown to receive a lower dose and average energy deposition than a 'real' patient.

Measurements of fast-neutron-induced signals in silicon pad detectors

The aim of this work was to understand possible fast neutron contribution to background signals in the Semiconductor Tracker (SCT) of the ATLAS detector which is built on LHC at CERN. We measured current pulses induced by fast-neutron reactions in silicon pad detectors. Neutrons were produced at the Rez cyclotron by means of the (p,n) reaction on a thick beryllium target. Current pulses were stored by a fast digital oscilloscope for off-line analysis. Pulse-height spectra, corresponding to neutron interactions in the detector mainly due to (n,p) and (n,α) reactions, were obtained by integration of the pulses. The sensitivity of the detector against neutrons was estimated from analysis of the distribution of time intervals between two subsequent pulses. The neutron-induced signals correspond to an average energy deposition of several MeV and arise in the silicon detector of 1 cm 2 area with an average time spacing of about 575 μs. Results are compared with Monte-Carlo simulations.

Derivation of Geometry Factors for Internal Gamma Dose Calculations for a Cylindrical Radioactive Waste Package

A general methodology was developed to estimate geometry factors for internal gamma dose rate calculations within a cylindrical radioactive waste container. In particular, an average geometry factor is needed to calculate the average energy deposition rate within the container for determination of the internal gas generation rate. Such a calculation is required in order to assess the potential for radioactive waste packages to radiolytically generate combustible gases.This work therefore provides a method for estimating the point and average geometry factors for internal dose assessment for a cylindrical geometry. This analysis is compared to other results where it is shown that the classical work of Hine and Brownell do not correspond to the average geometry factors for a cylindrical body but rather to values at the center of its top or bottom end. The current treatment was further developed into a prototype computer code (PC-CAGE) that calculates the geometry factors numerically for a cylindrical body of any size and material, accounting both for gamma absorption and buildup effects.

Comparison of Monte Carlo calculated electron slowing-down spectra generated by 60Co γ-rays, electrons, protons and light ions

When analysing the factors affecting the relative biological effectiveness (RBE) of different radiation qualities, it is essential to consider particularly the low-energy slowing-down electrons (around 100 eV to 1 keV) since they have the potential of inflicting severe damage to the DNA. We present a modified and extended version of the Monte Carlo code PENELOPE that enables scoring of slowing-down spectra, mean local energy imparted spectra and average intra-track nearest-neighbour energy deposition distances of the secondary electrons generated by different radiation qualities, such as electrons, photons, protons and light ions in general. The resulting spectra show that the low-linear energy transfer (LET) beams, 60Co γ-rays and electrons with initial energies of 0.1 MeV and higher, have as expected approximately the same electron slowing-down fluence per unit dose in the biologically important low-energy interval. Consistent with the general behaviour of the RBE of low-energy electrons, protons and light ions, the low-energy electron slowing-down fluence per unit dose is larger than for low-LET beams, and it increases with decreasing initial projectile energy.

BISTRO: an outer-volume suppression method that tolerates RF field inhomogeneity.

A technique is described for performing frequency-selective signal suppression with a high degree of tolerance to RF field inhomogeneity. The method is called B1-insensitive train to obliterate signal (BISTRO). BISTRO consists of multiple amplitude- and frequency-modulated (FM) pulses interleaved with spoiler gradients. BISTRO was developed for the purpose of accomplishing band-selective signal removal, as in water suppression and outer-volume suppression (OVS), in applications requiring the use of an inhomogeneous RF transmitter, such as a surface coil. In the present work, Bloch simulations were used to illustrate the principles and theoretical performance of BISTRO. Its performance for OVS was evaluated experimentally using MRI and spectroscopic imaging of phantoms and in vivo animal and human brain. By using FM pulses featuring offset-independent adiabaticity, BISTRO permitted high-quality, broadband suppression with one (or two) discrete borders demarcating the edge(s) of the suppression band. Simulations and experiments demonstrated the ability to operate BISTRO with reasonably attainable peak RF power levels and with average RF energy deposition similar to other multipulse OVS techniques.

High‐latitude currents and their energy exchange with the ionosphere‐thermosphere system

The transfer of electrical energy at high latitudes between the magnetosphere and ionosphere‐thermosphere (IT) system is contained within the resulting currents and electric fields between the two systems. As such, the electrodynamic state of the IT system can influence how much of the electrical energy is transferred at any one time by modulating the currents that flow in the region. For this study, 1557 hours of detailed E region measurements by the Sondrestrom incoherent‐scatter radar from 1993 through 1998 were collected, of which 95 hours were used to determine the local electromagnetic energy transfer rates within the IT system and evaluate the role of the IT system in modifying this energy transfer. The measurements have been organized into 59 periods of enhanced electrical energy transfer and cover all universal times but are local to the region above Sondrestrom at 74.2° magnetic latitude. In many of the events studied it was found that the IT system acted to reduce the net electromagnetic energy transferred from the magnetosphere owing to the presence of neutral winds. The dawn sector proved to have on average greater passive energy deposition rates than in the dusk sector. The reduction of this rate by neutral winds also proved stronger in the dawn sector. This enhanced modulation caused the dawn‐dusk asymmetry in energy transfer to be less pronounced but still apparent in the electromagnetic energy transfer rate.

Chemical modifications of ion irradiated polystyrene probed by optical absorption measurements

Polystyrene (PS) films are irradiated at room temperature with 3 MeV silicon ions and 1.4 GeV argon ions to fluences ranging from 1×1010 to 3×1014cm−2. Fourier transform infrared (FTIR) specula reflection spectra and ultraviolet/visible (UV/Vis) transmission spectra are measured to investigate the radiation-induced chemical modifications. It is found that the intrinsic absorption bands show reduction in intensity with increasing fluence and electronic energy loss, indicating the degradation of the polymeric structure. For Si ion implanted specimens, significant chemical modifications occur above about 3×1013cm−2, corresponding to an average energy deposition of about 45 MGy. For Ar ion irradiated specimens, the critical electronic energy loss is around 0.77 keV/nm at the fluence of 5.5×1012cm−2, corresponding to an energy deposition of about 6.4 MGy. The increase in absorbance in the UV/Vis range and a shift of the absorption edge toward the visible is attributed to carbonization of the material. The results are discussed on the basis of ion fluence and energy loss.