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Polarized Positron Beams via Intense Two-Color Laser Pulses
摘要： The generation of ultrarelativistic polarized positrons during the interaction of an ultrarelativistic electron beam with a counterpropagating two-color petawatt laser pulse is investigated theoretically. Our Monte Carlo simulation, based on a semiclassical model, incorporates photon emissions and pair productions, using spin-resolved quantum probabilities in the local constant field approximation, and describes the polarization of electrons and positrons for the pair production and photon emission processes, as well as the classical spin precession in between. The main reason for the polarization is shown to be the spin asymmetry of the pair production process in strong external fields, combined with the asymmetry of the two-color laser field. Employing a feasible scenario, we show that highly polarized positron beams, with a polarization degree of ζ ≈ 60%, can be produced in a femtosecond timescale, with a small angular divergence, ～74 mrad, and high density, ～1014 cm?3. The laser-driven polarized positron source raises hope for providing an alternative for high-energy physics studies.
关键词： polarized positron beams,ultrarelativistic electron beam,local constant field approximation,spin-resolved quantum probabilities,high-energy physics,Monte Carlo simulation,two-color laser pulses
更新于2025-09-11 14:15:04
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Monte Carlo simulation of a transversely excited solar-pumped fiber laser
摘要： A Monte Carlo ray-tracing simulation was developed for a transversely excited solar-pumped fiber laser without focusing optics. To make this possible, the fiber is immersed in a liquid sensitizer and sandwiched between highly reflective mirrors. The top reflector is dichroic to transmit solar radiation but reflect fluorescence to confine photons that match the absorption band of the active fiber. Simulation was used to evaluate the validity of the concept and optimize device performance. For comparison with the calculations, preliminary experiments were conducted by illuminating a 30 cm aperture, laser module with a solar simulator. The observed gains were in good agreement with the calculations for various conditions, such as sensitizer concentration and mirror reflectivity. Finally, we show that the predicted output power reaches 29 mW when the fiber length is optimized, and it will be enhanced to 150 mW with 0.21% solar-to-laser efficiency under the assumption of a reabsorption-free sensitizer.
关键词： unconcentrated,dichroic mirror,Monte Carlo simulation,sensitizer,solar-pumped fiber laser,transversely excited
更新于2025-09-11 14:15:04
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Species discrimination and individual tree detection for predicting main dendrometric characteristics in mixed temperate forests by use of airborne laser scanning and ultra-high-resolution imagery
摘要： This study aims to investigate the combined use of two types of remote sensing data — ALS derived and digital aerial photogrammetry data (based on imagery collected by airborne UAV sensors) — along with intensive field measurements for extracting and predicting tree and stand parameters in even-aged mixed forests. The study is located in South West Romania and analyzes data collected from mixed-species plots. The main tree species within each plot are Norway spruce (Picea abies L. Karst.) and Beech (Fagus sylvatica L.). The ALS data were used to extract the digital terrain model (DTM), digital surface model (DSM) and normalized canopy height model (CHM). Object-Based Image Analysis (OBIA) classification was performed to automatically detect and separate the main tree species. A local filtering algorithm with a canopy-height based variable window size was applied to identify the position, height and crown diameter of the main tree species within each plot. The filter was separately applied for each of the plots and for the areas covered with Norway spruce and beech trees, respectively (i.e. as resulted from OBIA classification). The dbh was predicted based on ALS data by statistical Monte Carlo simulations and a linear regression model that relates field dbh for each tree species with their corresponding ALS-derived tree height and crown diameter. The overall RMSE for each of the tree species within all the plots was 5.8 cm for the Norway spruce trees, respectively 5.9 cm for the beech trees. The results indicate a higher individual tree detection rate and subsequently a more precise estimation of dendrometric parameters for Norway spruce compared to beech trees located in spruce-beech even-aged mixed stands. Further investigations are required, particularly in the case of choosing the best method for individual tree detection of beech trees located in temperate even-aged mixed stands.
关键词： Monte Carlo simulation,ALS,Forest inventory,UAV,OBIA
更新于2025-09-11 14:15:04
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Scale Accuracy Evaluation of Image-Based 3D Reconstruction Strategies Using Laser Photogrammetry
摘要： Rapid developments in the field of underwater photogrammetry have given scientists the ability to produce accurate 3-dimensional (3D) models which are now increasingly used in the representation and study of local areas of interest. This paper addresses the lack of systematic analysis of 3D reconstruction and navigation fusion strategies, as well as associated error evaluation of models produced at larger scales in GPS-denied environments using a monocular camera (often in deep sea scenarios). Based on our prior work on automatic scale estimation of Structure from Motion (SfM)-based 3D models using laser scalers, an automatic scale accuracy framework is presented. The confidence level for each of the scale error estimates is independently assessed through the propagation of the uncertainties associated with image features and laser spot detections using a Monte Carlo simulation. The number of iterations used in the simulation was validated through the analysis of the final estimate behavior. To facilitate the detection and uncertainty estimation of even greatly attenuated laser beams, an automatic laser spot detection method was developed, with the main novelty of estimating the uncertainties based on the recovered characteristic shapes of laser spots with radially decreasing intensities. The effects of four different reconstruction strategies resulting from the combinations of Incremental/Global SfM, and the a priori and a posteriori use of navigation data were analyzed using two distinct survey scenarios captured during the SUBSAINTES 2017 cruise (doi: 10.17600/17001000). The study demonstrates that surveys with multiple overlaps of nonsequential images result in a nearly identical solution regardless of the strategy (SfM or navigation fusion), while surveys with weakly connected sequentially acquired images are prone to produce broad-scale deformation (doming effect) when navigation is not included in the optimization. Thus the scenarios with complex survey patterns substantially benefit from using multiobjective BA navigation fusion. The errors in models, produced by the most appropriate strategy, were estimated at around 1% in the central parts and always inferior to 5% on the extremities. The effects of combining data from multiple surveys were also evaluated. The introduction of additional vectors in the optimization of multisurvey problems successfully accounted for offset changes present in the underwater USBL-based navigation data, and thus minimize the effect of contradicting navigation priors. Our results also illustrate the importance of collecting a multitude of evaluation data at different locations and moments during the survey.
关键词： navigation fusion,laser scalers,uncertainty estimation,structure-from-motion,Monte Carlo simulation,laser spot detection,metrology,underwater 3D reconstruction,multiobjective BA,scale drift evaluation,photogrammetry
更新于2025-09-11 14:15:04
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Photoneutronic aspects of the molybdenum-99 production by means of electron linear accelerators
摘要： Monte Carlo simulations were used to study photonuclear production of Molybdenum-99 isotope by means of electron linear accelerators. Simulation showed that direct production in the cylindrical target provides higher values of specific activity compared to production using separate bremsstrahlung converter. For 50 MeV 100 kW electron beam we obtained the maximum value of 61 Ci/g specific activity of produced Molybdenum-99 at saturation. Calculations for the targets with optimized shape showed that even higher levels of specific activity are achievable: for optimized target the calculated value of Molybdenum-99 specific activity at saturation exceeds values of 145.89 Ci/g for setup with bremsstrahlung converter and 170.07 Ci/g without the converter.
关键词： Electron linear accelerator,Mo-99 production,Monte Carlo simulation,Medical isotopes,PHITS
更新于2025-09-11 14:15:04
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Precise EBT3 Gafchromic film dosimetry for Grid therapy
摘要： OBJECTIVES: Grid therapy is a radiation therapy technique for the treatment of bulky tumors. The high dose gradients and non-uniformity of dose distributions within the target lead to a challenge in the dosimetry of the Grid radiation fields. The aim of this study is to perform a precise EBT3 Gafchromic film dosimetry for Grid therapy fields using a commercially available ?atbed colour scanner. METHODS: In this project, samples of the EBT3 Gafchromic films are exposed to Grid radiation fields. The irradiated EBT3 ?lms were read using a flatbed Microtek scanner. The responses of these films (i.e. films from the same batch) as functions of the absorbed dose values are calibrated by irradiation under a fixed standard technique (i.e. 10x10 cm2 filed, 100 cm SSD, and depth of the maximum dose). These films are also read with the same scanner using the red, green, and blue channels. Four different approaches were used to evaluate film dosimetry for the Grid therapy applications: 1) single channel film dosimetry method (SCM), 2) dual channel film dosimetry method (DCM), 3) linearized dose-response curve method (LRCM), and 4) triple channel film dosimetry method (TCM). A dose of 20 Gy was delivered to the point along the central axis of the grid hole at the depth of maximum dose (dmax) for a 10 × 10 cm2 Grid field size. Beam profiles and percentage depth dose distributions of the Grid radiation have been measured in water-equivalent phantom material, using EBT3 films. The accuracy of the relative and the absolute dosimetry of the films were examined by comparison of the TLD measured data with the Monte Carlo simulated values. RESULTS: The results of these investigations show that for a gamma index criterion of 5%/3mm, the agreements between the MC calculations dose profiles and the SCM, DCM, LRCM, and TCM film dosimetry approaches the passing rates of 91%, 92%, 95%, and 95%, respectively. A much closer agreement was observed for using a linearized dose-response curve and triple-channel methods. CONCLUSIONS: Selection of an appropriate methodology in Gafchromic film dosimetry may lead to an accurate dose-response in a high dose gradient radiation field such as Grid therapy.
关键词： Gafchoromic film dosimetry,Grid therapy,Monte Carlo simulation
更新于2025-09-11 14:15:04
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Evaluation of Radiation Dose for Dual Energy CBCT Using Multi-Grid Device
摘要： The paper discusses radiation dose of dual energy CT on which copper modulation layer, is mounted in order to improve diagnostic performance of the dual energy CT. The radiation dose is estimated using MCNPX and its results are compared with that of the conventional dual energy CT system. CT X-ray spectra of 80 and 120 kVp, which are usually used for thorax, abdominal, head, and neck CT scans, were generated by the SPEC78 code and were used for the source specification ‘SDEF’ card for MCNPX dose modeling. The copper modulation layer was located 20 cm away from a source covering half of the X-ray window. The radiation dose was measured as changing its thickness from 0.5 to 2.0 mm at intervals of 0.5 mm. Since the MCNPX tally provides only normalized values to a single particle, the dose conversion coefficients of F6 tally for the modulation layer-based dual energy CBCT should be calculated for matching the modeling results into the actual dose. The dose conversion coefficient is 7.2*104 cGy/output that is obtained from dose calibration curve between F6 tally and experimental results in which GAFCHORMIC EBT3 films were exposed by an already known source. Consequently, the dose of the modulation layer-based dual energy cone beam CT is 33～40% less than that of the single energy CT system. On the basis of the results, it is considered that scattered dose produced by the copper modulation layer is very small. It shows that the modulation layer-based dual energy CBCT system can effectively reduce radiation dose, which is the major disadvantage of established dual energy CT.
关键词： Beam hardening effect,Modulation layer,Monte Carlo Simulation,Dual energy cone beam CT,Dose evaluation
更新于2025-09-11 14:15:04
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Analysis of Beam Hardening of Modulation Layers for Dual Energy Cone-beam CT
摘要： Dual energy cone-beam CT can distinguish two materials with different atomic compositions. The principle of dual energy cone-beam CT based on modulation layer is that higher energy spectrum can be acquired at blocked x-ray window. To evaluate the possibility of modulation layer based dual energy cone-beam CT, we analyzed x-ray spectrum for various thicknesses of modulation layers by Monte Carlo simulation. To compare with the results of simulation, the experiment was performed on prototype cone-beam CT for 50～100 kVp with CdTe XR-100T detector. As the result of comparing, the mean energy of energy spectrum for 80 kVp are well matched with that of simulation. The mean energy of energy spectrum for 80 and 120 kVp were increased as 1.67 and 1.52 times by 2.0 mm modulation layer, respectively. We realized that the virtual dual energy x-ray source can be generated by modulation layer.
关键词： Dual energy cone-beam CT,Monte Carlo simulation,Beam hardening effect,Modulation layer
更新于2025-09-11 14:15:04
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia, Spain (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Performance Analysis of Speckle Filtering on Single-Look Polsar Data for Land Cover Classification
摘要： The speckle ?ltering is an essential preprocessing step of polarimetric SAR (PolSAR) data processing. The PolSAR speckle ?lters perform generally better on multilook datasets. The multilooking is performed to reduce speckle and improve parameter estimation but it also degrades resolution of the datasets. The speckle ?ltering after multilooking further degrades the resolution. The analysis on suitability of speckle ?ltering algorithms for single-look data is absent in the literature. In this paper, state-of-the-art PolSAR speckle ?lters are analyzed for the single-look PolSAR data. The single-look Monte Carlo simulated and real RADARSAT-2 PolSAR data are used for performance analysis.
关键词： SAR,polarimetric,Monte Carlo simulation,?ltering,Speckle
更新于2025-09-10 09:29:36
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Performance evaluation of total variation (TV) denoising technique for dual-energy contrast-enhanced digital mammography (CEDM) with photon counting detector (PCD): Monte Carlo simulation study
摘要： The dual-energy contrast-enhanced digital mammography (CEDM) system based on a photon counting detector (PCD) is very useful providing functional information for breast cancer detection. In particular, this system can be used to solve the spectral overlap and high radiation dose problems. However, imaging noise is a big problem because of the degradation image performance and cancer detection ratio in the CEDM system. To address this problem, a total variation (TV)-based denoising technique approach has recently been studied. Thus, the aim of this study was to evaluate and confirm the image performance of our TV-based denoising technique with dual-energy CEDM with a PCD. For this purpose, we simulated a dual-energy CEDM with a PCD and breast phantom in Monte Carlo simulation using the Geant4 Application for Tomographic Emission (GATE) that is an essential open source program. We also designed a TV-based denoising technique based on the L1-norm estimation included correction and iteration steps for acquiring high edge preservation in X-ray images. To evaluate the image performance, we used evaluation parameters with a contrast-to-noise ratio (CNR) and coefficient of variation (COV) as a function of the absorbed dose levels (2.18, 1.53, 1.09, and 0.66 mGy). According to the results, the average of all iodine thicknesses and absorbed dose conditions for the CNR using our proposed TV-based denoising technique was 1.71, 1.39, and 1.13 times higher than that acquired for the noisy image, median filter and Wiener filter, respectively. We also acquired excellent COV results for the dual-energy CEDM with a PCD system (2.53 times higher than that of the noisy image). In conclusion, the results of this study suggested that a TV-based denoising technique can be achieved with an improved image performance and the effect and feasibility of the TV-based denoising technique for dual-energy CEDM with a PCD can be investigated.
关键词： Dual-energy imaging method,Monte Carlo simulation,Total variation (TV)-based denoising technique,Photon counting detector (PCD),Evaluation of image performance,Contrast-enhanced digital mammography (CEDM)
更新于2025-09-10 09:29:36