Characterisation of a megavoltage linear accelerator Bremsstrahlung by means of Monte Carlo simulations

This work presents a method for megavoltage X‐ray Bremsstrahlung assessment by means of Monte Carlo (MC) (PENELOPE) simulations. The proposed method provides suitable spectrum characterisation by means of a few parameters, namely the acceleration potential and incident angle of electron beam on the anode. The obtained spectra have been used as incident beam for in‐depth dose distribution determination with the aim of performing further comparisons with experimental data. This comparison procedure has been employed for Bremsstrahlung assessment optimisation. Once the Linac Bremsstrahlung is well established, it may be used as incident beam for further dosimetric purposes, such as spatial dose distribution calculations. The whole process has been applied for the characterisation of the megavoltage Bremsstrahlung from a typical Linac, namely the 10 MV Varian Clinac 18. The obtained results for in‐depth dose distributions, when compared with experimental measurements, show an overall good agreement, which may suggest the feasibility of the method. Copyright © 2010 John Wiley & Sons, Ltd.     

Simulation study using GEANT4 Monte Carlo code for a Gd-coated resistive plate chamber as a thermal neutron detector

The Resistive Plate Chamber (RPC) has been developed in many application areas ever since its introduction, from high energy physics experiments to positron emission tomography. Such detectors can be coated with a Gd layer that enables them to detect thermal neutrons. Consequently these RPCs can be utilized for industrial and medical purposes. Here, we present the configuration of a resistive plate chamber which is utilized to detect thermal neutrons by employing GEANT4 Monte Carlo code. The response of the RPC was evaluated as a function of neutron energy in the GEANT4 Monte Carlo code. The simulation results are taken for incident neutron energy in the energy range from 25 meV to 100 meV. The detection efficiency was found to be between 10% and 20%, depending on the detector configuration, for incident thermal neutrons of 25 meV energy.     

Monte Carlo simulations for 20 MV X-ray spectrum reconstruction of a linear induction accelerator

To study the spectrum reconstruction of the 20 MV X-ray generated by the Dragon-I linear induction accelerator, the Monte Carlo method is applied to simulate the attenuations of the X-ray in the attenuators of different thicknesses and thus provide the transmission data. As is known, the spectrum estimation from transmission data is an ill-conditioned problem. The method based on iterative perturbations is employed to derive the X-ray spectra, where initial guesses are used to start the process. This algorithm takes into account not only the minimization of the differences between the measured and the calculated transmissions but also the smoothness feature of the spectrum function. In this work, various filter materials are put to use as the attenuator, and the condition for an accurate and robust solution of the X-ray spectrum calculation is demonstrated. The influences of the scattering photons within different intervals of emergence angle on the X-ray spectrum reconstruction are also analyzed.     

Terahertz radiation source using a high-power industrial electron linear accelerator

High-power (～ 100 kW) industrial electron linear accelerators (linacs) are used for irradiations, e.g., for pasteurization of food products, disinfection of medical waste, etc. We propose that high-power electron beam from such an industrial linac can first pass through an undulator to generate useful terahertz (THz) radiation, and the spent electron beam coming out of the undulator can still be used for the intended industrial applications. This will enhance the utilization of a high-power industrial linac. We have performed calculation of spontaneous emission in the undulator to show that for typical parameters, continuous terahertz radiation having power of the order of μW can be produced, which may be useful for many scientific applications such as multispectral imaging of biological samples, chemical samples etc.     

基于GEANT4的CsI(Tl)闪烁体探测器的Monte Carlo模拟

对闪烁光在晶体内的传输以及光电子倍增过程进行了建模,基于GEANT4软件包对CsI(T1)闪烁体探测器进行了蒙特卡罗模拟, 得到了不同形状、尺寸和包装的CsI(Tl)晶体测量γ射线的能谱。对比模拟和测试结果,两者得到了很好的符合,从而验证了模拟参数的合理性和可靠性。该模拟程序的建立为闪烁体探测器的设计提供了更精确的开发工具。 With the modeling of the light transportation in crystal and electron multiplication in the PMT, a Monte Carlo simulation of CsI(Tl) scintillator detector has been implemented with Geant4 toolkit. The energy spectra simulated with different crystal shape, size and wrappings are obtained. And the reliability of the simulation parameters is confirmed by comparing with the measurements. The simulation code can be used as an reliable tool for the design of scintillator detectors.     

Monte Carlo simulation for bremsstrahlung and photoneutron yields in high-energy x-ray radiography

This paper reports on the results of calculations using a Monte Carlo code (MCNP5) to study the properties of photons, electrons and photoneutrons obtained in the converted target and their transportations in x-ray radiography. A comparison between measurements and calculations for bremsstrahlung and photoneutrons is presented. The radiographic rule and the effect of the collimator on the image are studied with the experimental model. The results provide exact parameters for the optimal design of radiographic layout and shielding systems.     

基于GEANT4的CsI(T1)闪烁体探测器的Monte Carlo模拟

对闪烁光在晶体内的传输以及光电子倍增过程进行了建模,基于GEANT4软件包对CsI(T1)闪烁体探测器进行了蒙特卡罗模拟,得到了不同形状、尺寸和包装的CsI(T1)晶体测量γ射线的能谱.对比模拟和测试结果,两者得到了很好的符合,从而验证了模拟参数的合理性和可靠性.该模拟程序的建立为闪烁体探测器的设计提供了更精确的开发工具.     

GEANT4 Monte Carlo simulation response of parallel plate avalanche counter for fast neutrons detection

In the present study, GEANT4 based Monte Carlo codes have been employed to evaluate parallel plate avalanche counter (PPAC) response for fast neutron detection. In order to detect fast neutrons, a thin polyethylene layer is coated on the surface of the electrode of the PPAC. Neutrons entering the converter produce protons which enter the counter and are detected. Fast neutrons in the energy range of 4.0 MeV–20.0 MeV have been transported onto the PPAC surface using GEANT4 MC code. The performance of the PPAC counter has been evaluated by means of simulation by employing QGSP_BERT_HP and QGSP_BIC_HP physics lists. The detection efficiencies of polyethylene-coated PPAC are 1.69 × 10^?2 and 1.86 × 10^?2 using converter thickness of 1 mm and 2 mm, respectively. The obtained results are discussed in detail.     

Absorbed dose estimations of 131I for critical organs using the GEANT4 Monte Carlo simulation code

The aim of this study is to compare the absorbed doses of critical organs of ¹³¹I using the MIRD (Medical Internal Radiation Dose) with the corresponding predictions made by GEANT4 simulations. S-values (mean absorbed dose rate per unit activity) and energy deposition per decay for critical organs of ¹³¹I for various ages, using standard cylindrical phantom comprising water and ICRP soft-tissue material, have also been estimated. In this study the effect of volume reduction of thyroid, during radiation therapy, on the calculation of absorbed dose is also being estimated using GEANT4. Photon specific energy deposition in the other organs of the neck, due to ¹³¹I decay in the thyroid organ, has also been estimated. The maximum relative difference of MIRD with the GEANT4 simulated results is 5.64% for an adult's critical organs of ¹³¹I. Excellent agreement was found between the results of water and ICRP soft tissue using the cylindrical model. S-values are tabulated for critical organs of ¹³¹I, using 1, 5, 10, 15 and 18 years (adults) individuals. S-values for a cylindrical thyroid of different sizes, having 3.07% relative differences of GEANT4 with Siegel &; Stabin results. Comparison of the experimentally measured values at 0.5 and 1 m away from neck of the ionization chamber with GEANT4 based Monte Carlo simulations results show good agreement. This study shows that GEANT4 code is an important tool for the internal dosimetry calculations.     

Absorbed dose estimations of 131I for critical organs using the GEANT4 Monte Carlo simulation code

The aim of this study is to compare the absorbed doses of critical organs of ¹³¹I using the MIRD (Medical Internal Radiation Dose) with the corresponding predictions made by GEANT4 simulations. S-values (mean absorbed dose rate per unit activity) and energy deposition per decay for critical organs of ¹³¹I for various ages, using standard cylindrical phantom comprising water and ICRP soft-tissue material, have also been estimated. In this study the effect of volume reduction of thyroid, during radiation therapy, on the calculation of absorbed dose is also being estimated using GEANT4. Photon specific energy deposition in the other organs of the neck, due to ¹³¹I decay in the thyroid organ, has also been estimated. The maximum relative difference of MIRD with the GEANT4 simulated results is 5.64% for an adult's critical organs of ¹³¹I. Excellent agreement was found between the results of water and ICRP soft tissue using the cylindrical model. S-values are tabulated for critical organs of ¹³¹I, using 1, 5, 10, 15 and 18 years (adults) individuals. S-values for a cylindrical thyroid of different sizes, having 3.07% relative differences of GEANT4 with Siegel & Stabin results. Comparison of the experimentally measured values at 0.5 and 1 m away from neck of the ionization chamber with GEANT4 based Monte Carlo simulations results show good agreement. This study shows that GEANT4 code is an important tool for the internal dosimetry calculations.     

Validation of Monte Carlo simulation of 6 MV photon beam produced by Varian Clinac 2100 linear accelerator using BEAMnrc code and DOSXYZnrc code

The Monte Carlo model for the photon-beam output from the Varian Clinac 2100 linear accelerator was validated to compare the calculated to measured PDD and beam dose profiles The Monte Carlo calculation method is considered to be the most accurate method for dose calculation in radiotherapy. The objective of this study is to build a Monte Carlo geometry of Varian Clinac 2100 linear accelerator as realistically as possible. The Monte Carlo codes used in this work were the BEAMnrc code to simulate the photons beam and the DOSXYZnrc code to examinate the absorbed dose in the water phantom. We have calculated percentage depth dose (PDD) and beam profiles of the 6 MV photon beam for the 6 × 6 cm², 10 × 10 cm² and 15 × 15 cm² field sizes. We have used the gamma index technique for the quantitative evaluation to compare the measured and calculated distributions. Good agreement was found between calculated PDD and beam profile compared to measured data. The comparison was evaluated using the gamma index method and the criterions were 3% for dose difference and 3 mm for distance to agreement. The gamma index acceptance rate was more than 97% of both distribution comparisons PDDs and dose profiles and our results were more developed and accurate. The Varian Clinac 2100 linear accelerator was accurately modeled using Monte Carlo codes: BEAMnrc and DOSXYZnrc codes package.     

Photon attenuation coefficients of different rock samples using MCNPX,Geant4 simulation codes and experimental results: a comparison study

ABSTRACT

The photon attenuation coefficients for the rocks (Feldspathic basalt, Compact basalt, Volcanic rock, Pink granite, Sandstone and Dolerite) have been investigated using MCNPX and Geant4 codes for photon energies 122, 356, 511, 662, 1170, 1275 and 1330 keV. Comparison of the simulation and experimental results of mass attenuation coefficients is presented. The exposure buildup factors also have been calculated with the help of the G-P fitting method. The comparison showed reasonable agreement between the simulated and experimental data for all rocks. The results show that sandstone has the highest mean free path as well as exposure buildup factor which signifies that sandstone requires a larger thickness for protection from photons.     

Neutronics studies of solid targets for spallation neutron source using Monte Carlo simulation

Neutronics studies for a solid target have been done with Monte Carlo high-energy particle transport code NMTC/JAM,when the proton beam with high kinetic energy bombards the target.The effect of the main parameters of the target on the neutron flux is discussed to optimize the target,which will be used for the concept design of the target of spallation neutron source.A target with its aspect ratio 1.5:1 or 2:1 gives the highest neutron flux.Tungsten is the most acceptable material from the technical and economical points of view.Beryllium as a moderating reflector can increase the neutron flux effectively.     

A hybrid Monte Carlo approach to molecular aggregation in a porous medium

A Monte Carlo simulation is used to study the segregation of liquid crystal molecules (short chains) in a heterogeneous matrix (of barrier concentration ) in an ordering field (E). Aggregation of molecules, pinned by the matrix barriers, is enhanced at lower temperature (T) and higher barrier concentrations via clusters of clustering growth. Variation of the radial distribution function with T and , rms displacement of molecules, and visual analysis of their distribution reveal that the size of the molecular aggregates is relatively larger and less dispersed at higher than that at a lower at T=0.2. The orientational molecular ordering is found to be lower at low temperature at higher . Molecules remain segregated at higher temperatures unless the porosity is reduced and the ordering field is increased considerably. Received 29 October 1999 and Received in final form 19 January 2000     

Measurement of photoneutron doses in and out of high-energy X-ray beam of a SATURNE-20 medical linear accelerator by ECE polycarbonate detectors

Photoneutron contaminations in and out of high energy X-ray beams of the medical linear accelerator SATURNE 20 (CGR) of the Radiotherapy Department of Omeed Hospital in Isfahan, Iran, have been determined using 250 μm polycarbonate (PC) dosimeters, in strips or in sheets, processed by electrochemical etching (ECE) using specially designed ECE chambers to etch larger sheets. A two dimensional or topographical distribution of neutron contamination was also determined in a full size beam. The neutron dose equivalents (Hn) in the beam of 18 MV X-rays at 80 cm FSD were determined to be linear functions of X-ray dose equivalents (Hx) up to 1400 cSv. The distribution of the Hn at different X-ray doses showed bell-shape profiles with maxima at the isocenter. The ratios of dose equivalents of neutrons to those of X-rays increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10×10, 20×20, 30×30, and 40×40 cm² respectively. Although such neutron dose equivalents can be corrected for patient treatment, it can cause radiation protection problems for workers where the design of the facility is not well planned.     

Monte Carlo simulation of lower hybrid current drive in tokamaks

In this paper, we present a method for noninductive current drive studies based on three-dimensional simulation of test particle orbits. A Monte Carlo momentum diffusion operator is developed to model the wave-particle interaction. The scheme can be utilized in studies of current drive efficiency as well as in examining the current density profiles caused by waves with a finite parallel wave number spectrum and a nonuniform power deposition profile in a toroidal configuration space of arbitrary shape. Calculations performed with a uniform power deposition profile of lower hybrid waves for axisymmetric magnetic configurations having different aspect ratios and poloidal cross-section shape confirm the semianalytic estimates for the current drive efficiency based on the solutions of the flux surface averaged Fokker-Planck equation for configurations with circular poloidal cross section. The consequences of the combined effect of radial diffusion, magnetic trapping and radially nonhomogeneous power deposition and background plasma parameter profiles are investigated     

Possibility of determining microimpurities in materials using a activation measuring complex based on a photoneutron source

An activation measuring complex containing a W–Be photoneutron source of neutrons and a lowbackground gamma spectrometer is designed at the Russian Academy of Sciences Institute for Nuclear Research. The photoneutron source is mounted on the beam of a LUE-8 industrial electron linear accelerator with an electron energy of 7–8 MeV and contains a tungsten bremsstrahlung target, a photoneutron beryllium target, and a fast neutron moderator. The measuring component of the complex includes a detector made from high purity germanium, positioned inside a low-background chamber with active-passive protection. Neutron-activation analysis (NAA) is used to determine the content of microimpurities in a substance. A procedure for measuring ultralow contents of elements in aerosol filters is developed. Contents of Au, As, Sb, Mn, Fe, W, and other elements are measured in a variety of samples. The limits of detecting microimpurities in materials are estimated.     

Study of neutron response for two hybrid RPC setups using the GEANT4 MC simulation approach

The present article describes a detailed neutron simulation study in the energy range 10~(-10) MeV to 1.0 GeV for two different RPC configurations.The simulation studies were taken by using the GEANT4 MC code.Aluminum was utilized on the GND and readout strips for the (a) Bakelite-based and (b) glass-based RPCs.For the former type of RPC setup the neutron sensitivity for the isotropic source was S_n=2.702×10~(-2) at E_n=1.0 GeV, while for the latter type of RPC, the neutron sensitivity for the same source was evaluated as S_n=4.049×10~(-2) at E_n=1.0 geV.These results were further compared with the previous RPC configuration in which copper was used for ground and pickup pads.Additionally A1 was employed at (GND+strips) of the phosphate glass RPC setup and compared with the copper-based phosphate glass RPC.Good agreement with sensitivity values was obtained with the current and previous simulation results.     

Study of neutron response for two hybrid RPC setups using the GEANT4 MC simulation approach

The present article describes a detailed neutron simulation study in the energy range 10^-10 MeV to 1.0 GeV for two different RPC configurations. The simulation studies were taken by using the GEANT4 MC code. Aluminum was utilized on the GND and readout strips for the （a） Bakelite-based and （b） glass-based RPCs. For the former type of RPC setup the neutron sensitivity for the isotropic source was Sn = 2.702 × 10^-2 at En = 1.0 GeV, while for the latter type of RPC, the neutron sensitivity for the same source was evaluated as Sn = 4.049 × 10^-2 at En = 1.0 GeV. These results were further compared with the previous RPC configuration in which copper was used for ground and pickup pads. Additionally A1 was employed at （GND＋strips） of the phosphate glass RPC setup and compared with the copper-based phosphate glass RPC. Good agreement with sensitivity values was obtained with the current and previous simulation results.     

Evaluation of effective source position for local linear accelerator and inserts

All types of treatment planning systems need some input measured beam data. Such data differ in type and number depending on the model of electron beam algorithm used inside. In addition to the number of percentage depth dose (%DD) and cross beam profiles, the effective source surface distance parameter SSD_eff was also recommended to be measured and transferred to the planning system. Output measurements were carried out using 0.6cc cylindrical Farmer type ion chamber placed in water phantom at depth of dose maximum, in Radiat. Phys. Dep., Malmö Hospital, Lund University, Sweden. Results were collected for all available energies combined with both applicators and inserts on a Varian Clinac 2100C. SSD_eff was determined from the ionization measurements using equation given by Khan et al. (1991). Comparing the results with those obtained by Roback et al. (1995) on a similar Varian Clinac 2100C, it was found that the deviation of SSD_eff was 7.5% for combination of energies, field sizes, and inserts except at 6 MeV. The variations of SSD_eff than the nominal SSD reflect the importance of corrections against air gap present in irregular clinical situations. SSD_eff should be measured during commissioning of both accelerator or/and local treatment-planning computer. Inserts used in measurements should be from the same material and thickness as that are used in clinical work.