Monte Carlo simulation of the operational quantities at the realistic mixed neutron-photon radiation fields CANEL and SIGMA

The Institute for Radiological Protection and Nuclear Safety owns two facilities producing realistic mixed neutron-photon radiation fields, CANEL, an accelerator driven moderator modular device, and SIGMA, a graphite moderated americium-beryllium assembly. These fields are representative of some of those encountered at nuclear workplaces, and the corresponding facilities are designed and used for calibration of various instruments, such as survey meters, personal dosimeters or spectrometric devices. In the framework of the European project EVIDOS, irradiations of personal dosimeters were performed at CANEL and SIGMA. Monte Carlo calculations were performed to estimate the reference values of the personal dose equivalent at both facilities. The Hp(10) values were calculated for three different angular positions, 0 degrees, 45 degrees and 75 degrees, of an ICRU phantom located at the position of irradiation.     

Characterisation of the IRSN CANEL/T400 facility producing realistic neutron fields for calibration and test purposes

The new CANEL/T400 facility has been set-up at the Institute for Radiological Protection and Nuclear Safety (IRSN) to produce a realistic neutron field. The accurate characterisation of this neutron field is mandatory since this facility will be used as a reference neutron source. For this reason an international measuring campaign, involving four laboratories with extensive expertise in neutron metrology and spectrometry, was organised through a concerted EUROMET project. Measurements were performed with Bonner sphere (BS) systems to determine the energy distribution of the emitted neutrons over the whole energy range (from thermal energy up to a few MeV). Additional measurements were performed with proton recoil detectors to provide detailed information in the energy region above 90 keV. The results obtained by the four laboratories are in agreement with each other and are compared with a calculation performed with the MCNP4C Monte-Carlo code. As a conclusion of this exercise, a reliable characterisation of the CANEL/T400 neutron field is obtained.     

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 the radiation field at aircraft altitudes

Energy spectra of secondary cosmic rays are calculated for aircraft altitudes and a discrete set of solar modulation parameters and rigidity cut-off values covering all possible conditions. The calculations are based on the Monte Carlo code FLUKA and on the most recent information on the interstellar cosmic ray flux including a detailed model of solar modulation. Results are compared to a large variety of experimental data obtained on the ground and aboard aircraft and balloons, such as neutron, proton, and muon spectra and yields of charged particles. Furthermore, particle fluence is converted into ambient dose equivalent and effective dose and the dependence of these quantities on height above sea level, solar modulation, and geographical location is studied. Finally, calculated dose equivalent is compared to results of comprehensive measurements performed aboard aircraft.     

Monte Carlo simulation of the field back-scattered from rough surfaces

A novel approach for the simulation of the field back-scattered from a rough surface is presented. It takes into account polarization and multiple scattering events on the surface, as well as diffraction effects. The validity and usefulness of this simulation is demonstrated in the case of surface topology measurement.     

Monte Carlo simulation of beta radiation response function for semiconductor Si detector

This paper describes the modeling of the Passivated Implanted Planar Silicon (PIPS) detector for the beta particles response function simulations with the MCNP-5 code. The simulated and measured energy response functions were compared and a good agreement was found in the entire range of energies. The verified model of a PIPS detector was applied in a non-destructive method that determines the activity of beta emitters in the sample with a known geometry and atomic number densities. The procedure for the identification of beta emitters in the samples was also described. Finally, the application of the proposed method for the determination of ¹³⁷Cs and ⁹⁰Sr activity in water samples taken from the RA reactor spent fuel storage pools and from the paper filter taken from an air monitor operated during repackaging of spent fuel elements, was presented.     

Monte Carlo simulation of model spin systems

In this paper, applications of the Monte Carlo technique to estimate the static and dynamic properties of model spin systems are discussed. Finite-size effects and choice of boundary conditions in simulating different types of real systems are outlined. Various applications of the Monte Carlo simulations to one-, two- and three-dimensional Ising models and Heisenberg models are dealt with in some detail. Recent applications of the Monte Carlo method to spin glass systems and to estimate renormalisation group critical exponents are reviewed. Communication No. 19 from the Solid State Structural Chemistry Unit.     

Monte Carlo simulation of tolerance synthesis equations

This paper presents results on ongoing research aimed at developing an integrated computer-aided tolerancing tool. Starting with explicit tolerance analysis equations used to model the relationship between a part's functional elements and an assembly's functional requirements, the reverse synthesis equations are obtained using a simple Jacobian inversion scheme. The explicit nature of the resulting equations lend themselves to conventional Monte Carlo simulation techniques to determine the percentage of rejects being produced given some statistical distributions of the tolerances appearing in these equations. An example is presented that will provide insight on the merits of the developed method.     

Monte Carlo simulation of low density flows

The necessity for adopting a kinetic-theoretical approach to obtain aerodynamic characteristics in low density flow past space vehicles is highlighted in this paper; it is shown how long-standing difficulties in theoretically handling such flows can be circumvented by adopting a Monte Carlo technique. The principles underlying the technique are briefly described, and are first illustrated by applying the technique to the evaluation of the drag of cylinders and cones in collisionless flow. The Markoff process underlying the Monte Carlo simulation of the full Boltzmann equation with collisions is then described in detail. Instead of the time-counter strategy of Bird, a theoretically sounder ‘Random Collision Number’ (RCN) strategy has been adopted in the present simulation. In this strategy the number of collisions in each time-step in the computation is a random number drawn from an appropriate distribution. Computer programs using this strategy have been developed for calculating aerodynamic characteristics like drag and heat transfer for a cone in the transition regime between free molecule and continuum flow. The results obtained from these programs show that both time-counter and RCN strategies require almost the same computer time.     

Workplace monitoring of mixed neutron-photon radiation fields and its contribution to external dosimetry

Workplace monitoring is a common procedure for determining measures for routine radiation protection in a particular working environment. For mixed radiation fields consisting of neutrons and photons, it is of increased importance because it contributes to the improved accuracy of individual monitoring. An example is the determination of field-specific correction factors, which can be applied to the readings of personal dosemeters. This paper explains the general problems associated with individual dosimetry of neutron radiation, and describes the various options for workplace monitoring. These options cover a range from the elaborate field characterisation using transport calculations or spectrometers to the simpler approach using area monitors. Examples are given for workplaces in nuclear industry, at particle accelerators and at flight altitudes.     

Monte Carlo simulations of the electric field close to the body in realistic environments for application in personal radiofrequency dosimetry

Personal dosemeters can play an important role in epidemiological studies and in radiofrequency safety programmes. In this study, a Monte Carlo approach is used in conjunction with the finite difference time domain method to obtain distributions of the electric field strength close to a human body model in simulated realistic environments. The field is a proxy for the response of an ideal body-worn electric field dosemeter. A set of eight environments were modelled based on the statistics of Rayleigh, Rice and log-normal fading to simulate outdoor and indoor multipath exposures at 450, 900 and 2100 MHz. Results indicate that a dosemeter mounted randomly within 10-50 mm of the adult or child body model (torso region) will on average underestimate the spatially averaged value of the incident electric field strength by a factor of 0.52 to 0.74 over the frequencies of 450, 900 and 2100 MHz. The uncertainty in results, assessed at the 95 % confidence level (between the 2.5th and 97.5th percentiles) was largest at 2100 MHz and smallest at 450 MHz.     

PVD薄膜生长的Monte Carlo模拟

蒙特卡罗(Monte Carlo,MC)和分子动力学是模拟薄膜生长的两种基本方法.本文结合各种PVD技术的工艺特点,介绍了薄膜生长的Monte Carlo模拟.详尽地总结了载能粒子沉积薄膜生长的Kinetic Monte Carlo模型和算法.最后指出建立精细的模型、算法并适当应用分子动力学是提高模拟可靠性的方法.     

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.     

Monte Carlo simulation of DNA damage by low LET radiation using inhomogeneous higher order DNA targets

To test possible effects of the heterogeneous nature of the cell nucleus on simulation results of radiation-induced DNA damage, inhomogeneous targets have been implemented in the biophysical code PARTRAC. The geometry of the DNA and the histones was defined by spheres around the constituent atoms. Electron cross sections in liquid water were scaled according to the mass density of the different materials, whereas photon cross sections were derived from the sum of the cross sections for the constituent atoms. In the case of higher energy electrons the simulations show an increase of energy deposition in the DNA proportional to its high mass density. For photons with energies in the range of the carbon and the oxygen K-shell (0.28-0.53 keV), cross sections of DNA are larger than those of water, leading to an increased yield of strand breaks per average absorbed dose in the cell nucleus.     

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.     

SiC电子输运的蒙特卡罗模拟

采用了Monte Carlo方法研究了2H-、4H-和6H-SiC的电子输运特性.在模拟中,采用动量弛豫率近似的方法确定散射角,显著压缩散射次数,并用高效的新查表法确定自由飞行时间,相对于阶梯值的自散射方法,完全消除了自散射,大量节省cpu时间.     

A kinetic Monte Carlo study of mixed 1D/3D defect migration

Defect clusters form readily in collision cascades in metals, and some of the self-interstitial atom clusters form as crowdion clusters that diffuse by one-dimensional migration along a close-packed direction. Defect interactions and thermal fluctuations can cause the direction of one-dimensional migration to change, resulting in a mixed one-dimensional/ three-dimensional migration. Kinetic Monte Carlo computer simulations applied to model systems are used to investigate the effects of one-dimensional, three-dimensional and mixed one-dimensional/ three-dimensional migration on defect reaction kinetics. The functional relationships between the sink strength, the size of sinks and the average distance between direction changes during mixed one-dimensional/three-dimensional migration are explored. This revised version was published online in July 2006 with corrections to the Cover Date.     

Analysis of light-emitting diodes by Monte Carlo photon simulation

A Monte Carlo photon simulation method, which is based on statistical tracing of photons inside the chip, has been developed for analysis of LED's in quantitative terms. Also included in the analysis is practical modeling of textured surfaces, which are often employed for enhanced light output. The method with its unique versatility is applicable to virtually any chip geometry and measures various important parameters such as photon-output-coupling efficiency, detailed photon flight statistics, and photon-output distribution patterns. It is speculated that the method can easily be extended to development of LED lamps and packages.