Monte Carlo comparison of the St Petersburg phantom with a BOMAB phantom in the HML's whole-body counter

Three sizes of the St Petersburg phantom have been compared to six sizes of BOMAB phantoms measured by a virtual whole-body counter similar to the one in use in the Human Monitoring Laboratory using Monte Carlo simulations. The previously published data comparing the St Petersburg Reference Man sized phantom with a similar sized Bottle Manikin Absorber Phantoms (BOMAB) phantom at 662 keV is supported; however, the simulations also show that the smaller sized St Petersburg phantoms do not agree well with smaller BOMAB phantoms. It is concluded that the St Petersburg phantoms are system dependent meaning that all sizes of the St Petersburg phantoms should be experimentally compared over a wide photon energy range against corresponding BOMAB phantoms to validate their use for calibrating whole-body counters.     

Monte Carlo calculations for efficiency calibration of a whole-body monitor using BOMAB phantoms of different sizes

Internal contamination due to high-energy photon (HEP) emitters is assessed using a scanning bed whole-body monitor housed in a steel room at the Bhabha Atomic Research Centre (BARC). The monitor consists of a (203 mm diameter × 102 mm thickness) NaI(Tl) detector and is calibrated using a Reference BOMAB phantom representative of an average Indian radiation worker. However, a series of different size physical phantoms are required to account for size variability in workers, which is both expensive and time consuming. Therefore, a theoretical approach based on Monte Carlo techniques has been employed to calibrate the system in scanning geometry with BOMAB phantoms of different sizes characterised by their weight (W) and height (H) for several radionuclides of interest ((131)I, (137)Cs, (60)Co and (40)K). A computer program developed for this purpose generates the detector response and the detection efficiencies (DEs) for the BARC Reference phantom (63 kg/168 cm), ICRP Reference male phantom (70 kg/170 cm) and several of its scaled versions. The results obtained for different size phantoms indicated a decreasing trend of DEs with the increase in W/H values of the phantoms. The computed DEs for uniform distribution of (137)Cs in BOMAB phantom varied from 3.52 × 10(-3) to 2.88 × 10(-3) counts per photon as the W/H values increased from 0.26 to 0.50. The theoretical results obtained for the BARC Reference phantom have been verified with experimental measurements. The Monte Carlo results from this study will be useful for in vivo assessment of HEP emitters in radiation workers of different physiques.     

Evaluation of counting efficiencies of a whole-body counter using Monte Carlo simulation with voxel phantoms

In the 2007 recommendations of the International Commission on Radiological Protection, it is mentioned that the reference voxel phantoms are used for calculation of effective dose. From the standpoint of internal dosimetry services, calibration methods of whole-body counters using the voxel phantoms are of considerable practical interest. In the present study, counting efficiencies of a whole-body counter at the Japan Atomic Energy Agency (JAEA) were evaluated using Monte Carlo simulations with two voxel phantoms, 'MAX06', which has organ masses corresponding to those of the reference male, and 'Otoko', which is a representation for average Japanese male. To validate the calculation methods of the present study, calculations for the bottle manikin absorption phantom were also performed and compared with experiments. Consequently, it was found that the Monte Carlo simulation with voxel phantoms is a significant tool for the calibration of the JAEA whole-body counter.     

Monte Carlo simulation of a mammographic test phantom

A test phantom, including a wide range of mammographic tissue equivalent materials and test details, was imaged on a digital mammographic system. In order to quantify the effect of scatter on the contrast obtained for the test details, calculations of the scatter-to-primary ratio (S/P) have been made using a Monte Carlo simulation of the digital mammographic imaging chain, grid and test phantom. The results show that the S/P values corresponding to the imaging conditions used were in the range 0.084-0.126. Calculated and measured pixel values in different regions of the image were compared as a validation of the model and showed excellent agreement. The results indicate the potential of Monte Carlo methods in the image quality-patient dose process optimisation, especially in the assessment of imaging conditions not available on standard mammographic units.     

A general Monte Carlo calculation for the geometrical efficiency of a detection system

A general Monte Carlo procedure is described for calculating the geometrical efficiency of a detection system with regard to a beam of particles impinging on a gaseous target or on a plane parallel target plate with finite thickness, while the detected particles are emitted with an isotropic angular distribution or one of the form [1−0.5P₂ (cos θ)], with P₂ (cos θ) the second Legendre polynomial. This method was successfully applied to the analysis of the ¹⁶O(γ, P₀)¹⁵N data.     

Inhomogeneity effects on HPGe gamma spectrometry detection efficiency using Monte Carlo technique

This work deals with the inhomogeneity effects on the full energy peak efficiency determination with HPGe gamma spectrometry. The inhomogeneity of the sample was defined in this work as being the fraction of its volume, which does not contain gamma emitters. We applied the Monte Carlo technique based on the GEANT4 code of CERN to study these effects for soil samples for the case of high density (1.54 g/cm³) where the attenuation effects are expected to increase the errors on the activity measurement due to the inhomogeneity. The correction of the efficiency against these effects using Monte Carlo method has been applied in two cases. The first is between samples with different homogeneity values and a calibration standard, which is perfectly homogeneous, and the second is between a sample and a calibration standard with different imperfect homogeneities.     

Distributed Monte Carlo simulation of a dynamic expansion system

This paper presents a new approach to the implementation of the Monte Carlo (MC) method for the analysis of high vacuum metrological systems based on the principle of dynamic gas expansion. The main disadvantage of the Monte Carlo method, the very long time of computation, can be significantly reduced by using a distributed calculation schema. This paper describes the principles of the computation system and compares the computing times for some models of a chosen metrological system, performed using a network of Personal Computers.     

Assessing skull burdens of actinides using a mathematical phantom: a Monte Carlo approach

In vivo monitoring techniques are needed to estimate the amount of an actinide in the skeleton in addition to that in the lungs and liver. Skull counting with external detectors has been a standard procedure for this purpose. Realistic skull phantoms are normally used to calibrate the counting systems. However, the fabrication of realistic phantoms is extremely difficult and expensive. Therefore, a theoretical approach based on Monte Carlo methods in conjunction with a Cristy mathematical phantom has been examined for assessing skull burdens of actinides. A computer program that generates surface sources of actinides on the skull and simulates low-energy photon transport in the heterogeneous media of the head region of the Cristy phantom was developed for this purpose. The program determines the observable pulse height spectrum of the detector and the corresponding calibration factors for different counting geometries. The computer program was used to generate the pulse height spectra and the corresponding calibration factors of 20 cm and 15 cm diameter phoswich detectors, each positioned on the left and right sides and on the top of the head region of the Cristy phantom, whose skull surfaces were assumed to have a uniform distribution of 241Am. The computed calibration factor for a counting geometry consisting of three phoswich detectors (15 cm diameter each) surrounding the phantom's skull was found to be in excellent agreement with the experimental results obtained for the same geometry using a realistic physical skull phantom. This provided a validation of the realistic design of the skull in the Cristy phantom, implying that the results reported in this paper could be used for in vivo measurements of skull burdens of 241Am for the stated counting geometry.     

Tolerance-based process plan evaluation using Monte Carlo simulation

In the discrete part manufacturing industry, engineers develop process plans by selecting appropriate machining processes and production equipment to ensure the quality of finished components. The decisions in process planning are usually made based on personal experience and the verification of process plans is based on physical trial-and-error runs, which is costly and time-consuming. This paper proposes to verify process plans by predicting machining tolerances via Monte Carlo simulation. The basic idea is to use a set of discrete sample points to represent workpiece geometry. The changes of their spatial position are simulated and tracked as the workpiece undergoes a series of machining processes. Virtual inspections are then conducted to determine the dimensional and geometric tolerances of the machined component. Machining tolerance prediction is completed through: (1) manufacturing error synthesis, and (2) error propagation in multiple operations. In this way, engineers can quickly screen alternative process plans, spot the root error causes, and improve their decisions. Therefore, physical trial-and-error runs can be reduced, if not eliminated, resulting in significant savings in both time and costs.     

Modeling low-coherence enhanced backscattering using Monte Carlo simulation

Constructive interference between coherent waves traveling time-reversed paths in a random medium gives rise to the enhancement of light scattering observed in directions close to backscattering. This phenomenon is known as enhanced backscattering (EBS). According to diffusion theory, the angular width of an EBS cone is proportional to the ratio of the wavelength of light lambda to the transport mean-free-path length l(s)* of a random medium. In biological media a large l(s)* approximately 0.5-2 mm > lambda results in an extremely small (approximately 0.001 degrees ) angular width of the EBS cone, making the experimental observation of such narrow peaks difficult. Recently, the feasibility of observing EBS under low spatial coherence illumination (spatial coherence length Lsc < l(s)*) was demonstrated. Low spatial coherence behaves as a spatial filter rejecting longer path lengths and thus resulting in an increase of more than 100 times in the angular width of low coherence EBS (LEBS) cones. However, a conventional diffusion approximation-based model of EBS has not been able to explain such a dramatic increase in LEBS width. We present a photon random walk model of LEBS by using Monte Carlo simulation to elucidate the mechanism accounting for the unprecedented broadening of the LEBS peaks. Typically, the exit angles of the scattered photons are not considered in modeling EBS in the diffusion regime. We show that small exit angles are highly sensitive to low-order scattering, which is crucial for accurate modeling of LEBS. Our results show that the predictions of the model are in excellent agreement with the experimental data.     

Numerical simulation of point-to-plane corona discharge using a Monte Carlo method

Monte Carlo simulation techniques are used to study the dynamical properties of charged particles in point-to-plane corona discharge. The numerical model includes the release of electron-ion pairs by photoionization and secondary electron emission from cathode as well as the first Townsend ionization. The simulation results of negative corona discharge in nitrogen show that electron avalanche takes place in the region of high electric field near pin electrode and the photoionization is the essential mechanism to sustain the discharge as well as electron impact ionization.     

Prediction of micro-channel flows using direct simulation Monte Carlo

The direct simulation Monte Carlo (DSMC) method is a particle-based numerical modeling technique. It is recently used for simulating gaseous flow in micro-electro-mechanical-systems (MEMS) where micron-scale features become important. In this paper, numerical simulations of fluid flow in micro-channels are carried out using the DSMC method. The details in determining the parameters critical for DSMC applications in micro-channels are provided. Streamwise velocity distributions in the slip-flow regime are compared with the analytical solution based on the Navier–Stokes equations with slip velocity boundary condition. Satisfactory agreements have been achieved. Effects of the entrance and exit regions on simulation results are discussed. Simulations are then extended to transition flow regime (Kn>0.1) and compared with the analytical solution. It is shown that the results are distinguished with the analytical solutions, which fail to predict the flow due to the break down of continuum assumption. It is indicated that the gradient of the pressure along the channel direction dominates the motion of the fluid flow.     

Calibration of phoswich-based lung counting system using realistic chest phantom

A phoswich detector, housed inside a low background steel room, coupled with a state-of-art pulse shape discrimination (PSD) electronics is recently established at Radiological Safety Division of IGCAR for in vivo monitoring of actinides. The various parameters of PSD electronics were optimised to achieve efficient background reduction in low-energy regions. The PSD with optimised parameters has reduced steel room background from 9.5 to 0.28 cps in the 17 keV region and 5.8 to 0.3 cps in the 60 keV region. The Figure of Merit for the timing spectrum of the system is 3.0. The true signal loss due to PSD was found to be less than 2 %. The phoswich system was calibrated with Lawrence Livermore National Laboratory realistic chest phantom loaded with (241)Am activity tagged lung set. Calibration factors for varying chest wall composition and chest wall thickness in terms of muscle equivalent chest wall thickness were established. (241)Am activity in the JAERI phantom which was received as a part of IAEA inter-comparison exercise was estimated. This paper presents the optimisation of PSD electronics and the salient results of the calibration.     

Production rate of synchronous transfer lines using Monte Carlo simulation

In this paper, we consider unpaced synchronous transfer lines producing a single product. The transfer line stations are arranged in a series con?guration, have no buffers, and are subject to operation-dependent failures. Throughput is an important performance measure for transfer lines, and we have adopted that measure. Analytical methods for determining capacity of such transfer lines are available only for the simplest systems, but we show Monte Carlo simulation to be a fast, flexible, easy, and accurate method of estimating throughput in lines of any length and having a wide range of operating characteristics.     

Monte Carlo simulation of a transition radiation detector

This paper employs Monte Carlo simulations of the performance of a transition radiation detector (TRD). The program has been written for the TRD in the ZEUS spectrometer, which separates electrons from hadrons in the momentum range between 1 and 30 GeV/c. Both, total charge method and cluster counting method were simulated taking into account various experimental parameters. In particular, it was found that the cluster counting method relies on a quantitative understanding of the background originating from the production of δ-electrons by charged particles. The results of the Monte Carlo calculations are in agreement with experimental data obtained with prototypes within a systematic uncertainty of 20%. We applied our Monte Carlo program to studies in order to find an optimum layout for the TRD within available space in the ZEUS spectrometer. In this context, the performance of TRD layouts with different geometries and materials has been evaluated comprehensively. The geometry found by optimization promises an improvement on hadron suppression by a factor of about two for both methods compared with present results from test measurements. Applying algorithms for a detailed analysis of the energy and space distributions of the clusters in the TRD, hadrons in the momentum range from 1 to 30 GeV/c can be suppressed to a level of less than 2%. This method of cluster analysing improves the suppression of hadrons by a factor of about two compared to the total charge method.     

Monte Carlo calculation of radiation dose in CT examinations using phantom and patient tomographic models

By using a voxel-based Monte Carlo simulation technique, we developed and validated a method to calculate radiation-absorbed dose in the computed tomography (CT) examinations from the images of phantoms and patients. The ionising radiation transport was simulated using the EGS4 code system. The geometry of the X-ray beam (focus-to-axis distance, field of view, collimation, and primary and beam-shaper filtration) and the X-ray spectral distribution (HiSpeed LX/i) were included in the simulation. Each axial CT image was reduced to a 256 x 256 matrix and stacked in a volume. The patient images were segmented before the simulation of radiation transport by using four categories of materials, such as air, lung, muscle and bone. To test the voxel-based method, the values of the radiation dose derived from a simulated CT exposure were calculated and compared with those obtained from the measurements performed within the dosimetry phantoms. To complete the scope of the work, series of CT scans of the trunk of an anthropomorphic phantom and patients were simulated to calculate the average dose in each 1-cm-wide transverse slice (ADS). The comparison between the simulated and measured dose data for the CT indices showed a difference of <5% in all the cases. The estimated mean values of ADS from the chest, abdomen and pelvis of the anthropomorphic phantom were approximately 1.7-2 times the weighted CT dose index (CTDI(w)) value, whereas the mean ADS values for these anatomical areas were 1.3-2 times the CTDI(w) of patients. The voxel-based simulation method provided a technique for estimating the individual patient doses in the CT examinations.     

First-order differential sensitivity analysis of a nuclear safety system by Monte Carlo simulation

In this paper we employ a Monte Carlo method to compute the first-order, differential sensitivity indexes of the basic events characterizing the reliability behavior of the containment spray injection system of a nuclear power plant. An exemplification is provided as to how the obtained sensitivity indexes can be used to drive improvements in the system design and operation.     

The robustness and efficiency of trimmed elemental estimation in regression analysis: a Monte Carlo simulation study

Mayo and Gray [Am Statist 51 (1997) 122] introduced the leverage-residual weighted elemental (LRWE) classification of regression estimators and proposed a new method of estimation called trimmed elemental estimation (TEE). In this article, we perform a simulation study of the efficiency of certain TEE estimators relative to ordinary least squares under normal errors and their robustness under various non-normal error distributions in the context of the simple linear regression model. Comparisons among these estimators are made on the basis of mean square error and percentiles of the absolute estimation errors in the simulations.     

Monte Carlo simulation of a control for cathodic sputtering

Subject is a set-up for gas pressure and discharge power regulation of a cathodic sputtering plant. It comprises two film thickness monitors and a control unit. The film thickness monitors measure the deposition rate at target and substrate level. By means of a computer the control unit calculates a quotient from the monitor data and compares it with a set value. Any change in gas pressure or discharge power leads to a deviation of the quotient from the set value. The control unit eliminates the deviation by a regulation of gas pressure or discharge power. The Monte Carlo method makes it possible to calculate the pressure dependence of the set-sup. It was not possible to determine a relationship between discharge power and the output signal of the control unit by the used theoretical model.     

Procedures of Monte Carlo transport simulation for applications in system engineering

Monte Carlo (MC) simulation is the most promising tool for performing realistic reliability and availability analysis of complex systems. Yet, the efficient use of MC simulation technique is not trivial in large scale applications.This paper considers the two commonly adopted approaches to MC simulation: the direct, component-based approach and the indirect, system-based approach. The mathematical details of the two approaches are worked out in detail, so as to show their probabilistic equivalence. The proper formulation for biasing the simulation is introduced, thus leading to the correct expressions for the statistical weights.Both approaches are applied, in an analog as well as in a biased scheme, to a simple system of the literature and comparisons are made with respect to the computing time and the goodness of the estimate, as measured by the variance of the results.