Electron Transport Code Research Articles

Mega-voltage versus kilo-voltage cone beam CT used in image guided radiation therapy: comparative study of microdosimetric properties

Mega-voltage computed tomography (MVCT) and kilo-voltage cone beam CT (CBCT) are routinely used in image-guided radiation therapy. In current practice, doses from MVCT and CBCT are reported with no correction for radiation quality. In this study, we compared microdosimetric properties as well as doses for MVCT and CBCT. Monte Carlo simulation codes BEAMnrc and DOSXYZnrc were used to simulate a Varian CBCT 125 kVp photon beam and primary electron spectra for CT sets of two patients. The revised Oak Ridge Electron transport Code (NOREC) was used to simulate electron tracks in liquid water. C++ code was developed to compute lineal energy in a sphere of 1 μm diameter. Dose-mean lineal energy-based quality factors were calculated for critical organs in-field. The estimated quality factor for CBCT is up to a factor of 1.3 times that of MV beams.

Hot electron transport modelling in fast ignition relevant targets with non-Spitzer resistivity

The simple Lee-More model for electrical resistivity is implemented in the hybrid fast electron transport code THOR. The model is shown to reproduce experimental data across a wide range of temperatures using a small number of parameters. The effect of this model on the heating of simple Al targets by a short-pulse laser is studied and compared to the predictions of the classical Spitzer-Härm resistivity. The model is then used in simulations of hot electron transport experiments using buried layer targets.

H Lymanαline in Jovian aurorae: electron transport and radiative transfer coupled modelling

Aims. The aim of this work is to characterize the impact of the solar UV flux and of the electron precipitation on Jupiter's atmosphere and to study the H Lyman α intensity and spectral profile in Jupiter's aurorae. In particular we characterize the sensitivity of the line to the relevant input parameters.Methods. We use a multi-stream electron transport code solving the 1D Boltzmann equation and a Feautrier technique radiative transfer code.Results. We calculate ionization rate profiles of the main species of Jupiter's ionosphere and the heating rate due to electron precipitation, as a function of the energy of the precipitation. We also calculate the emission rate of Lyman α photons under auroral electron precipitation and the ensuing line profile. The profile exhibits a centre reversal which could be used as a diagnostic of Jupiter's auroral low-energy electron precipitation.

Semantic Web-based PMCEPT Monte Carlo Code Simulations in Medical Physics

PMCEPT is a three-dimensional parallel Monte Carlo electron and photon transport code based on the standard message passing interface. It has been intensively validated for use in diverse medical physics applications such as radiotherapy, dosimetry, nuclear medicine, X-ray diagnostics, and radiation protection calculations. Despite extensive validation, the Monte Carlo code may not be an easy tool to pick up for a user working in medical areas because it requires some advanced knowledge of information technologies. To overcome this diculty, we developed a function web based on recent semantic web technology. Recent developments of the semantic web make it possible to understand and satisfy various requests of people and machines to intelligently use web contents or even control other machines. This paper describes a specific functional web (including shell scripts) to run the PMCEPT code through a network and verifies the overall dosimetric accuracy of PMCEPT physics algorithms with a 6-MV beam model of the Siemens Primus linear accelerator. A medical professional with relatively little information technology experience can take advantage of the Monte Carlo simulation via the functional web running on our dedicated server with his or her PC (client). We compare calculations with measurements in water and poly methyl methacrylate (PMMA) phantoms for 6-MV Siemens Primus photon beams. The target geometry of a PMMA head and neck phantom was derived from the CT data (DICOM format). Maximum discrepancies were lesser than 2 % in the tail parts of the lateral dose distributions. This study also suggests that our functional web may have a wide range of scientific applications because it is able to control any computer code running on physically separate machines via the Internet.

Altitude of Saturn's aurora and its implications for the characteristic energy of precipitated electrons

Images of Saturn's aurora at the limb have been collected with the Advanced Camera for Surveys on board the Hubble Space Telescope. They show that the peak of Saturn's nightside emission is generally located 900–1300 km above the 1‐bar level. On the other hand, methane and H2 columns overlying the aurora have been determined from the analysis of FUV and EUV spectra, respectively. Using a low‐latitude model, these columns place the emission layer at or above 610 km. One possibility to solve this apparent discrepancy between imaging and spectral observations is to assume that the thermospheric temperature in the auroral region sharply increases at a higher pressure level than in the low‐latitude regions. Using an electron transport code, we estimate the characteristic energy of the precipitated electrons derived from these observations to be in the range 1–5 keV using a low latitude model and 5–30 keV in case of the modified model.

Comparison between an event-by-event Monte Carlo code, NOREC, and ETRAN for electron scaled point kernels between 20 keV and 1 MeV

An event-by-event Monte Carlo code called NOREC, a substantially improved version of the Oak Ridge electron transport code (OREC), was released in 2003, after a number of modifications to OREC. In spite of some earlier work, the characteristics of the code have not been clearly shown so far, especially for a wide range of electron energies. Therefore, NOREC was used in this study to generate one of the popular dosimetric quantities, the scaled point kernel, for a number of electron energies between 0.02 and 1.0 MeV. Calculated kernels were compared with the most well-known published kernels based on a condensed history Monte Carlo code, ETRAN, to show not only general agreement between the codes for the electron energy range considered but also possible differences between an event-by-event code and a condensed history code. There was general agreement between the kernels within about 5% up to 0.7 r/r (0) for 100 keV and 1 MeV electrons. Note that r/r (0) denotes the scaled distance, where r is the radial distance from the source to the dose point and r (0) is the continuous slowing down approximation (CSDA) range of a mono-energetic electron. For the same range of scaled distances, the discrepancies for 20 and 500 keV electrons were up to 6 and 12%, respectively. Especially, there was more pronounced disagreement for 500 keV electrons than for 20 keV electrons. The degree of disagreement for 500 keV electrons decreased when NOREC results were compared with published EGS4/PRESTA results, producing similar agreement to other electron energies.

Electron track simulation using ETMICRO

A new Monte Carlo code ETMICRO (Electron Transport code for MICROdosimetry) is now available for tracing the electrons down to 10 eV in liquid water. ETMICRO employs much the same frame of track construction as the previous programs except that the plasmon excitation is left out of consideration and thus there is no delocalisation of energy deposition. The inverse mean free path data and the corresponding differential values (DIMFPs) were prepared by adopting the Drude model parameters that were derived from a new optical data set for liquid water. The potential of electrons causing direct damage on DNA has been estimated as functions of the energy of electrons and their distance from DNA in an arbitrary direction.

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.

SU‐FF‐T‐327: Monte Carlo Dose Calculations in Homogeneous and Heterogeneous Media: A Comparison Between the PMCEPT Code and the MCNP5, EGS4, DPM Codes and Measurements

The Monte Carlo method for high‐energy photon and charged particle transport is the most accurate means for predicting dose distributions in radiation treatment of patients. Owing to rapid development of computer hardware and network technologies the use of this method is not restricted only to big research centers any more. A new parallel Monte Carlo electron and photon transport (PMCEPT) code [Kum and Lee, J. Kor. Phys. Soc. 47 (2005) 716] for calculating electron and photon beam doses has been developed based on a three dimensional geometry defined by computerized tomography (CT) images and implemented on the Beowulf PC cluster. The PMCEPT code was validated on the homogeneous and multi‐layered targets for megavoltage electron beams by comparing with the results of experiments and calculations from conventional Monte Carlo codes of the MCNP5, EGS4, and DPM. The computing time of the PMCEPT code was approximately twenty times faster than that of MCNP5 on the IBM ThinkPad X40 (laptop) with 1.2‐GHz CPU and 512‐MB ram memory operated by RedHat Linux 9. The PMCEPT results in general agreed well with others in homogeneous and heterogeneous media within a maximum of 2–3 % error. At the conference, we will show various benchmark results for the PMCEPT. This work was supported, in part, by the SRC/ERC program of MOST/KOSEF (grant number: R11‐1999‐054).

The Cassini Campaign observations of the Jupiter aurora by the Ultraviolet Imaging Spectrograph and the Space Telescope Imaging Spectrograph

We have analyzed the Cassini Ultraviolet Imaging Spectrometer (UVIS) observations of the Jupiter aurora with an auroral atmosphere two-stream electron transport code. The observations of Jupiter by UVIS took place during the Cassini Campaign. The Cassini Campaign included support spectral and imaging observations by the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS). A major result for the UVIS observations was the identification of a large color variation between the far ultraviolet (FUV: 1100–1700 Å) and extreme ultraviolet (EUV: 800–1100 Å) spectral regions. This change probably occurs because of a large variation in the ratio of the soft electron flux (10–3000 eV) responsible for the EUV aurora to the hard electron flux (∼15–22 keV) responsible for the FUV aurora. On the basis of this result a new color ratio for integrated intensities for EUV and FUV was defined ( 4 π I 1550 – 1620 Å / 4 π I 1030 – 1150 Å ) which varied by approximately a factor of 6. The FUV color ratio ( 4 π I 1550 – 1620 Å / 4 π I 1230 – 1300 Å ) was more stable with a variation of less than 50% for the observations studied. The medium resolution (0.9 Å FWHM, G140M grating) FUV observations (1295–1345 Å and 1495–1540 Å) by STIS on 13 January 2001, on the other hand, were analyzed by a spectral modeling technique using a recently developed high-spectral resolution model for the electron-excited H 2 rotational lines. The STIS FUV data were analyzed with a model that considered the Lyman band spectrum (B Σ u + 1 → X Σ g + 1 ) as composed of an allowed direct excitation component (X Σ g + 1 → B Σ u + 1 ) and an optically forbidden component (X Σ g + 1 → EF , GK , H H ¯ , … Σ g + 1 followed by the cascade transition Σ g + 1 → B Σ u + 1 ). The medium-resolution spectral regions for the Jupiter aurora were carefully chosen to emphasize the cascade component. The ratio of the two components is a direct measurement of the mean secondary electron energy of the aurora. The mean secondary electron energy of the aurora varies between 50 and 200 eV for the polar cap, limb and auroral oval observations. We examine a long time base of Galileo Ultraviolet Spectrometer color ratios from the standard mission (1996–1998) and compare them to Cassini UVIS, HST, and International Ultraviolet Explorer (IUE) observations.

SU-FF-T-283: Benchmarking of An Event-By-Event Monte Carlo Code, NOREC: Generation of Scaled Point Kernels in Water for Mono-Energetic Electrons Between 10 KeV and 1 MeV

Purpose: To compare scaled point kernels (SPK) in water for mono‐energetic electrons generated by an event‐by‐event Monte Carlo code, NOREC, with published kernels based on a condensed history Monte Carlo code. Method and Materials: An event‐by‐event Monte Carlo code called NOREC, which replaces the Oak Ridge electron transport code (OREC), was released in 2003, after a number of modifications to the OREC including the replacement of the original OREC elastic cross sections by a newer data set from the National Institute of Standards and Technology (NIST). Initial benchmark tests showed that, for a number of dosimetric quantities (e.g., electron depth dose distribution), NOREC was capable of producing better agreement with published results than OREC, which indicates the effectiveness of the modifications introduced to the code. For a more comprehensive benchmarking, NOREC was used in this study to generate scaled point kernels for a number of electron energies between 0.01 and 1.0 MeV. Calculated kernels were compared with published kernels (i.e., the so‐called Berger kernels) based on a condensed history Monte Carlo code, ETRAN. Results: Although qualitatively similar, scaled point kernels generated by NOREC generally showed more energy deposition in the first half of continuous slowing down approximation (CSDA) electron ranges than the Berger kernels. Calculated ranges of electrons were similar between the two kernels. Conclusion: NOREC can produce SPK comparable to those generated by ETRAN. The current results suggest that NOREC can be used for both micro‐ and macro‐dosimetry problems which require more detailed transport of electrons below 1 MeV.

The use of Monte Carlo calculations in the determination of a Ge detector efficiency curve

Precise measured data and an extensive set of Monte Carlo calculations have been combined to optimize the parameters for a 280 cm 3 n-type Ge semiconductor photon detector, with the outcome that the Monte Carlo calculations now provide a very precise detector efficiency curve. The detector position and its length were determined from a set of measurements that included an axial scan. Measurements of the relative efficiencies based on radionuclides with accurately known relative photon emission rates were combined with efficiencies calculated by a Monte Carlo photon and electron transport code to adjust the detector diameter, internal deadlayer thickness, and the effective external deadlayer thickness. Our results show that, in a well-studied situation, it is possible to use Monte Carlo calculations to aid in the determination of a Ge detector's efficiency with an accuracy of about 0.2% from 50 to 1400 keV.

Response calculation of a stacked CdZnTe detector for 16N γ-ray measurement

A stacked CdZnTe semiconductor detector, which is able to detect the 6.13 MeV γ-rays from 16N, was prototyped and the response calculation was carried out by a Monte-Carlo simulation. The detector response was simulated by using the electron and photon transport code, EGS4 and taking into account the carrier trapping phenomena. The results of the response calculation agreed with the experimental data from the checking source of 137Cs and 60Co. The prototype's response and sensitivity to 16N were calculated by simulating an incident energy of 6.13 MeV . The source spectra were unfolded with the detector's response function obtained by simulation, and it indicated that the incident γ-ray energy and its intensity ratio was identified. We have acquired the potential to measure the γ-ray energy in the high energy region about 6 MeV with the stacked CdZnTe detector.

Ohmic heating as evidence for strong field‐aligned currents in filamentary aurora

A large increase in electron temperature measured in filamentary aurora with the European incoherent scatter radar has been modeled with a one‐dimensional electron transport and ion chemistry code. To account for the observed changes in electron temperature, while also reproducing the measured E region electron density profiles, a source of electron heating is required in addition to local heating from energy degradation of the precipitating electrons. We show that ohmic heating in a strong field‐aligned current can account for the required heat source.

Voxel size effects in three-dimensional nuclear magnetic resonance microscopy performed for trabecular bone dosimetry.

An important problem in internal dosimetry is the assessment of energy deposition by beta particles within trabecular regions of the skeleton. Recent dosimetry methods for trabecular bone are based on Monte Carlo particle transport simulations within three-dimensional (3D) images of real human bone samples. Nuclear magnetic resonance (NMR) microscopy is a 3D imaging technique of choice due to the large signal differential between bone tissue and the water-filled marrow cavities. Image voxel sizes currently used in NMR microscopy are between 50 microm and 100 microm, but the images are time consuming to acquire and can only be performed at present for in vitro samples. It is therefore important to evaluate what resolution is best suitable in order to properly characterize the trabecular microstructure, to adequately predict the tissue dosimetry, and to minimize imaging time. In this work, a mathematical model of trabecular bone, composed of a distribution of spherical marrow cavities, was constructed. The mathematical model was subsequently voxelized with different voxel sizes (16 microm to 1,000 microm) to simulate 3D NMR images. For each image, voxels are assigned to either bone or marrow according to their enclosed marrow fraction. Next, the images are coupled to the EGS4 electron transport code and absorbed fractions to bone and marrow are calculated for a marrow source of monoenergetic electrons. Radionuclide S values are also determined for the voxelized images with results compared to data calculated for the pure mathematical sample. The comparison shows that for higher energy electrons (>400 keV), good convergence of the results is seen even within images of poor resolution. Above 400 keV, a voxel resolution as large as 300 microm results in dosimetry errors below 5%. For low-energy electrons and high-resolution images, the self-dose to marrow is also determined to within 5% accuracy. Nevertheless, increased voxelization of the image overestimates the surface area of the bone-marrow interface leading to errors in the cross-dose to bone as high as 25% for some low-energy beta emitters.

A Monte Carlo code for positive ion track simulation.

An ion interaction model has been described for simulating positive ion tracks in a variety of media with the capability of interfacing with several secondary electron transport codes. Data are presented for single-and double-differential cross-sections, binding energies, probability density distribution for delocalisation parameters for conductors and tissue, branching ratios and ionisation efficiencies for water vapour and liquid water.

Energy deposition of electrons in low-, medium- and high-Z material: Comparison of the Monte Carlo transport code EGS4 with experiment

The Monte Carlo electron transport code EGS4 was benchmark tested against early experimental results derived by Freyberger. These consist of absolute depth ionization and depth dose curves measured at a pencil beam with sharp energy definition of nominally 4, 10 and 20 MeV electrons extracted from a Betatron. The Freyberger precision measurements have been made with a wide plane-parallel ionization chamber in a slab phantom for the materials PMMA, C, Al, Cu, and Pb. The bremsstrahlung and, in some experiments, the forward and backward directed contributions had been determined separately. The pencil-beam/wide-chamber geometry is equivalent, in respect to the measurement of depth ionization and depth dose curves, to the more common wide-beam/point-detector geometry. However, it requires the simulation of merely one pencil beam position and practically all particle histories contribute to the ionization in the wide air cavity. Thus a considerable amount of computing time is saved. We applied a prototype of the new electron transport code PRESTA II. The results of our simulation generally agree very well, even in absolute terms, with experiment. Small deviations are found at lower energies and high-Z materials. For low energies they may in part be explained by contamination with bremsstrahlung from the beam guide system and an overestimation of bremsstrahlung-production in the experiment. The simulation of a gas-filled chamber within a high-Z absorber block seems to produce small errors in the calculation of ionization for electrons of approximately 4 MeV. Larger deviations for high-Z materials are attributed to the employment of screened Rutherford cross sections which lead to an underprediction of ionization from backscattered particles. Backscatter was found to be sufficiently accurate in the simulation of the PMMA absorber.

Coordinated EISCAT/DMSP measurements of electron density and energetic electron precipitation

A campaign was recently undertaken to obtain simultaneous radar and satellite measurements that would provide insight into the temporal and spatial morphology of the energetic particle precipitation. It included coordinated field‐aligned measurements of the altitudinal profile of the E region electron density from the European Incoherent Scatter (EISCAT) radar, and measurements of the energetic particle precipitation from the SAMPEX and DMSP F10 and F12 satellites. The Strickland auroral electron transport code, with the recent addition of auroral proton transport, was employed to produce density profiles from the energetic particle precipitation measurements by the SSJ/4 sensors on the DMSP F10 and F12 spacecraft for comparison with the EISCAT measurements. We found good agreement between the EISCAT measurements and the model results for three of the four conjunctions including a very close conjunction (< 12 km) when the integral ion energy flux was greater than the integral electron energy flux by a factor of 3 to 4. All of the conjunctions for which the results of the comparison were good occurred within a stable diffuse aurora. Conductivity calculations using the model results varied less than 15% from calculations using the radar measurements. The comparison was good even when the separation between the satellite and the radar was greater than 200 km. The fourth conjunction, for which the agreement was poor, occurred within the discrete aurora during a very active period when the ionospheric conditions were dynamic both spatially and temporally.

A Microdosimetric Algorithm for Electron Point Kernel Data: 1. Monoenergetic-Electron Sources

A microdosimetric approach has been used to produce monoenergetic electron point kernel data in the form of probability density functions of specific energy z. These probability density functions are calculated using a 1 µm diameter sphere target to simulate microscopic size receptors. Electron transport is simulated using the Monte Carlo code OREC (Oak Ridge Electron-transport Code). Electron dose point kernel data produced in this study, f(z;r,E,N), are given as functions of source-to-target distance r, initial electron energy E, and the number of electron emissions N. Good agreement is observed between mean specific energies, z, from the probability density functions and absorbed dose values, D, from conventional point kernel data.

Observations of the neutral atmosphere between 100 and 200 km using ARIA rocket‐borne and ground‐based instruments

The atmospheric response in the aurora (ARIA) rocket was launched at 1406 UT on March 3, 1992, from Poker Flat, Alaska, into a pulsating diffuse aurora; rocket‐borne instruments included an eight‐channel photometer, a far ultraviolet spectrometer, a 130.4‐nm atomic oxygen resonance lamp, and two particle spectrometers covering the energy range of 1–400 eV and 10 eV to 20 keV. The photometer channels were isolated using narrow‐band interference filters and included measurements of the strong permitted auroral emissions N2 (337.1 nm), N2+ (391.4 nm), and O I (844.6 nm). A ground‐based photometer measured the permitted N2+ (427.8 nm), the forbidden O I (630.0 nm), and the permitted O I (844.6 nm) emissions. The ground‐based instrument was pointed in the magnetic zenith. Also, the rocket payload was pointed in the magnetic zenith from 100 to 200 km on the upleg. The data were analyzed using the Strickland electron transport code, and the rocket and ground‐based results were found to be in good agreement regarding the inferred characteristic energy (E0 ≈ 3 keV) of the precipitating auroral flux and the composition of the neutral atmosphere during the rocket flight. In particular, it was found that the O/N2 density ratio in the neutral atmosphere diminished during the auroral substorm, which started about 2 hours before the ARIA rocket flight. The data showed that there was about a 10‐min delay between the onset of the substorm and the decrease of the O/N2 density ratio. At the time of the ARIA flight this ratio had nearly returned to its presubstorm value. However, the data also showed that the O/N2 density ratio did not recover to its presubstorm value until nearly 30 min after the particle and joule heating had subsided. Both the photometer and oxygen resonance lamp data showed the presence of structure in the atomic oxygen densities in the region above 130 km. The observed auroral brightness ratio B337.1/B391.4 equaled 0.29 and was in agreement with other recent measurements. This ratio was also consistent with the greater than expected flux of secondary electrons measured by the onboard particle spectrometer between 40 and 10 eV.