Analysis of the CONRAD computational problems expressing only stochastic uncertainties: neutrons and protons

Within the scope of CONRAD (A Coordinated Action for Radiation Dosimetry) Work Package 4 on Computational Dosimetry jointly collaborated with the other research actions on internal dosimetry, complex mixed radiation fields at workplaces and medical staff dosimetry. Besides these collaborative actions, WP4 promoted an international comparison on eight problems with their associated experimental data. A first set of three problems, the results of which are herewith summarised, dealt only with the expression of the stochastic uncertainties of the results: the analysis of the response function of a proton recoil telescope detector, the study of a Bonner sphere neutron spectrometer and the analysis of the neutron spectrum and dosimetric quantity H(p)(10) in a thermal neutron facility operated by IRSN Cadarache (the SIGMA facility). A second paper will summarise the results of the other five problems which dealt with the full uncertainty budget estimate. A third paper will present the results of a comparison on in vivo measurements of the (241)Am bone-seeker nuclide distributed in the knee. All the detailed papers will be presented in the WP4 Final Workshop Proceedings.     

Quantification of margins and uncertainties: Conceptual and computational basis

In 2001, the National Nuclear Security Administration of the U.S. Department of Energy in conjunction with the national security laboratories (i.e., Los Alamos National Laboratory, Lawrence Livermore National Laboratory and Sandia National Laboratories) initiated development of a process designated Quantification of Margins and Uncertainties (QMU) for the use of risk assessment methodologies in the certification of the reliability and safety of the nation's nuclear weapons stockpile. This presentation discusses and illustrates the conceptual and computational basis of QMU in analyses that use computational models to predict the behavior of complex systems. The following topics are considered: (i) the role of aleatory and epistemic uncertainty in QMU, (ii) the representation of uncertainty with probability, (iii) the probabilistic representation of uncertainty in QMU analyses involving only epistemic uncertainty, and (iv) the probabilistic representation of uncertainty in QMU analyses involving aleatory and epistemic uncertainty.     

Reproducibility of TL measurements in a mixed field of thermal neutrons and photons

The reproducibility of measurements performed with GR-100 (LiF:Mg,Ti) from the Solid Dosimetric Detector and Method Laboratory (DML) China, GR-107 (7LiF:Mg,Ti, DML), TLD-700H (7LiF:Mg.Cu,P, Harshaw) and Al2O3:Mg,Y (Hungary) in photon and mixed photon-neutron fields was investigated. Mixed-field irradiations were performed in a thermal neutron field generated at a nuclear reactor. GR-100 sensitivity decreased after mixed-field irradiations, while no significant change was found for the other materials. Using GR-100 for the dosimetry of mixed and high-intensity fields requires careful procedures.     

Dose response of commercially available optically stimulated luminescent detector, Al2O3:C for megavoltage photons and electrons

This study examined the dose response of an optically stimulated luminescence dosemeter (OSLD) to megavoltage photon and electron beams. A nanoDot? dosemeter was used to measure the dose response of the OSLD. Photons of 6-15 MV and electrons of 9-20 MeV were delivered by a Varian 21iX machine (Varian Medical System, Inc. Milpitas, CA, USA). The energy dependency was <1 %. For the 6-MV photons, the dose was linear until 200 cGy. The superficial dose measurements revealed photon irradiation to have an angular dependency. The nanoDot? dosemeter has potential use as an in vivo dosimetric tool that is independent of the energy, has dose linearity and a rapid response compared with normal in vivo dosimetric tools, such as thermoluminescence detectors. However, the OSLD must be treated very carefully due to the high angular dependency of the photon beam.     

Agent-based project scheduling: computational study of large problems

We present in this paper the results of a computational study for project scheduling based on new ideas for project representation taken from digital circuit technology (Knotts et al. , 1998a) and a solution approach based on the artificial intelligence notion of agent technology. We experimented with projects with up to 10 000 stochastic duration activities which can be executed in a number of modes requiring renewable, nonrenewable, and periodically renewable resources. This study is about agent implementation in a project scheduling domain. It compares agent types and priority rules with respect to their impact on project schedule duration and computational performance. This work demonstrates: (i) that artificial intelligence concepts of agent technology can be successfully implemented for project scheduling; and (ii) in conducting project scheduling studies we can experiment successfully with large project networks. Both points made in this research are new.     

Responses of a superheated superconducting tin granule to photons and neutrons at a temperature of 30mK

We measured responses of a 30µm tin SSG by irradiating 60keV photons at a temperature of 30mK. A spectrum of energy deposits is calculated by using the superconducting specific heat together with a global heating model. The resultant spectrum of energies absorbed by the tin granule has two peaks at 60keV or less and at about 30keV. The former peak is due to energy deposits by electrons which are excited mainly from the L-shell by either primary gamma-rays or 20% of secondary X-rays produced by the filling of the K vacancies. However, 80% of the secondary X-rays pass out of the grain carrying an energy of more than 25keV. The latter peak is due to excited K-shell electrons accompanied by such escaping secondary X-rays. The threshold of response energies was decreased down to 1keV. Therefore, an improvement in response energy was attained by three orders of magnitude in comparison with the result previously obtained in the -ray irradiation experiment. A plan of an experimental study on the response of tin SSGs to irradiations of neutron-rays is briefly described.     

Pitfalls and modelling inconsistencies in computational radiation dosimetry: lessons learnt from the QUADOS intercomparison. Part II: Photons, electrons and protons

'QUADOS', a concerted action of the European Commission, has promoted an intercomparison aimed at evaluating the use of computational codes for dosimetry in radiation protection and medical physics. This intercomparison was open to all users of radiation transport codes. Eight problems were selected for their relevance to the radiation dosimetry community, five of which involved photon and proton transport. This paper focuses on a discussion of lessons learned from the participation in solving the photon and charged particle problems. The lessons learned from the participation in solving the neutron problems are presented in a companion paper (in this issue).     

Characterization of the computational accuracy in surface crack problems

The paper focuses on the comparison of different methods for calculating stress intensity factors (K_I) in surface crack problems based on results of numerical analyses of elastic crack-tip fields. The computational accuracy is quantified by means of the so-called ‘averaged error estimation technique’ which is extended to the evaluation of local errors in the determination of stress intensity factors at characteristic points of the crack front. Numerical data involved in the present study are obtained from boundary-element calculations. Three values of the stress intensity factor, i.e. those defined from nodal tractions, displacements and energy-release rate, are provided. The highest error level is found for the displacement-based data, while the energy-release calculations yield the best accuracy. A considerable increase in the error value is noticed near the intersection of the crack front with a body surface where the conventional assumption on the square-root stress singularity is, in general, not applied. It is shown that the accuracy of stress intensity factor analysis can be improved by eliminating uncertainties associated with the local stress state along the crack front. © 1998 John Wiley Sons, Ltd.     

Pitfalls and modelling inconsistencies in computational radiation dosimetry: lessons learnt from the QUADOS intercomparison. Part I: Neutrons and uncertainties

The QUADOS EU cost shared action conducted an intercomparison on the usage of numerical methods in radiation protection and dosimetry. The eight problems proposed were intended to test the usage of Monte Carlo and deterministic methods by assessing the accuracy with which the codes are applied and also the methods used to evaluate uncertainty in the answer gained through these methods. The overall objective was to spread good practice through the community and give users information on how to assess the uncertainties associated with their calculated results.     

Suppression of the yield of hard photons generated by electrons with energies above 100 GeV in oriented crystals

The yield of hard gamma quanta generated by a beam of electrons with energies above 100 GeV propagating through an oriented crystal has been studied as a function of the crystal thickness and electron energy. The phenomenon of suppression of the yield of hard photons in an oriented crystal as compared to that in an amorphous target of the same thickness is predicted.     

Generalized probabilistic approach of uncertainties in computational dynamics using random matrices and polynomial chaos decompositions

A new generalized probabilistic approach of uncertainties is proposed for computational model in structural linear dynamics and can be extended without difficulty to computational linear vibroacoustics and to computational non‐linear structural dynamics. This method allows the prior probability model of each type of uncertainties (model‐parameter uncertainties and modeling errors) to be separately constructed and identified. The modeling errors are not taken into account with the usual output‐prediction‐error method, but with the nonparametric probabilistic approach of modeling errors recently introduced and based on the use of the random matrix theory. The theory, an identification procedure and a numerical validation are presented. Then a chaos decomposition with random coefficients is proposed to represent the prior probabilistic model of random responses. The random germ is related to the prior probability model of model‐parameter uncertainties. The random coefficients are related to the prior probability model of modeling errors and then depends on the random matrices introduced by the nonparametric probabilistic approach of modeling errors. A validation is presented. Finally, a future perspective is introduced when experimental data are available. The prior probability model of the random coefficients can be improved in constructing a posterior probability model using the Bayesian approach. Copyright © 2009 John Wiley & Sons, Ltd.     

Interface crack problems in graded orthotropic media: Analytical and computational approaches

Interface crack problems in graded orthotropic media are considered using analytical and computational techniques. In the analytical formulation an interface crack between a graded orthotropic coating and a homogeneous orthotropic substrate is considered. The principal axes of orthotropy are assumed to be parallel and perpendicular to the crack plane. Mechanical properties of the medium are assumed to be continuous with discontinuous derivatives at the interface. The problem is formulated in terms of the averaged constants of plane orthotropic elasticity and reduced to a pair of singular integral equations which are solved numerically to compute the mixed mode stress intensity factors and the energy release rate. In the second part of the study, enriched finite elements are formulated and implemented for graded orthotropic materials. Comparisons of the finite element and analytical results show that enriched finite element technique is capable of producing highly accurate results for crack problems in graded orthotropic media. Finally, periodic interface cracking and the four point bending test for graded orthotropic solids are modeled using enriched finite elements and the results are briefly discussed.     

Auger-free luminescence of the BaF2:Sr, BaF2:MgF2 and CsBr:LiBr crystals under excitation of VUV photons and high-energy electrons

The emission spectrum, the time dependence of the luminescence, the excitation spectrum and the reflectance spectrum have been measured for the pure BaF₂ crystal, the BaF₂:Sr crystals with Sr⁺⁺ concentrations of 2 and 5 mol%, the BaF₂:MgF₂ crystal with a mole mixing ratio of 1:2, and the pure CsBr crystal and the CsBr:LiBr crystal with a mole mixing ratio of 1:1. The measurements have been made by using synchrotron orbital radiation and high-energy electrons produced by gamma-rays. It is shown that the BaF₂, the BaF₂:Sr and the BaF₂:MgF₂ crystals have the Auger-free luminescence having decay times around 0.90 ns. The slow and fast luminescence components of the BaF₂ crystal are suppressed by introduction Sr⁺⁺ and Mg⁺⁺ ions. It is, however, clearly seen that the suppression of the slow component is more effectively occurring than that of the fast component, especially in the BaF₂:MgF₂ crystal. By analyzing the Auger-free luminescence spectrum, it is shown that the valence band widths are 2.7 ± 0.3 eV, 2.7 ± 0.3 eV and 4.2 ± 0.3 eV and the band-gap energies are 11.1 ± 0.3 eV, 11.1 ± 0.4 eV and 11.9 ± 0.4 eV for BaF₂, BaF₂:Sr and BaF₂:MgF₂ crystals, respectively. The introduction of LiBr into CsBr has resulted in a significant enhancement in the luminescence intensity, attaining 40 times larger intensity than that of CsBr crystal at room temperature, due to the suppression of the non-radiative relaxation of the outer-most core holes due to the absorption induced by the Urbach effect. The observations of the increases in the band gap-energy, the exciton energy and the decay time of the CsBr:LiBr crystal compared with those of the CsBr crystal support the argument given in the present study regarding the suppression of the Urbach effect in the CsBr:LiBr crystal. The decay times observed are 0.20 ± 0.05 ns and 0.90 ± 0.06 ns for CsBr and CsBr:LiBr crystals, respectively.     

Waiting times,probabilities and the Q factor of fluorescent photons

Photons in resonance fluorescence from a single atom are correlated, and this affects the photon detection statistics. We consider the time dependence of the variance for the number of counts in a time interval [0,?T], and we show that there exist Poisson points for certain values of the atomic parameters. At these points, the variance equals the average, like for a Poisson process. We also show that the conditional probability density for the detection of the nth photon after a detection of a photon at time zero can be obtained from the conditional probability density for the first photon, and we illustrate this by computing several conditional probability densities for a special case of interest. The unconditional probability density for the detection of the nth photon can be obtained from the conditional probability densities and the probability for detecting n photons in [0,?T] can subsequently be found from the unconditional probability densities.     

A review of computational phononics: the bulk,interfaces, and surfaces

Broad-based interest in microscale heat transport in novel materials, engineered phononic materials, metamaterials, and their relevant systems has created significant demand for computational approaches to aid in investigation and design of materials that support phonons. This review describes the significant improvements that have been made and new needs that have emerged for capabilities associated with the computability of phonons. The technical scope encompasses issues, especially relevant to bulk, interface, and surface effects. Traditional approaches such as molecular dynamics, lattice dynamics, and Boltzmann transport equation continue to advance the field but are frequently extended to the limits of their physical or numerical validity. New materials beyond traditional group-IV, III–V, and II–VI semiconductors, phenomena that critically depend on scattering, such as in low-dimensional nanostructures, materials with interior surfaces and defects, and in high-temperature environments, continue to push these limits. The basis for the traditional calculation methods shares their origins with the earliest theories for thermal transport, acoustic waves in solids, spectroscopy and dynamical crystal lattices. These will remain in wide use in the future. But computing methods and the accompanying advances in microprocessor technologies have enabled growth of phonon computing models and methods in sophistication, accuracy, fidelity and complexity that will lead to fundamental impacts beyond the classic types of problems for which they were developed. With their increasingly integrated use for design and research, the myriad developments that presently exist must be understood for their suitability for certain applications and their ability to aid in the pursuit of new technologies.     

Boundary integral equations for the computational modeling of three-dimensional groundwater flow problems

Boundary integral equations for the computational modeling of three-dimensional groundwater flow problems are derived. They follow from appropriate volume and surface source-type integral representations for the pressure and the flow velocity. The numerical handling of the integral equations is discussed in some detail, especially as far as the evaluation of singular integrals is concerned. Arbitrary anisotropy in the resistivity of the fluid-saturated soil is taken into account.     

Neumann exterior wave propagation problems: computational aspects of 3D energetic Galerkin BEM

In this work, we consider as model problem an exterior 3D wave propagation Neumann problem reformulated in terms of a space–time hypersingular boundary integral equation with retarded potentials. This latter is set in the so-called energetic weak form, recently proposed in Aimi et al. (Int J Numer Methods Eng 80:1196–1240, 2009; CMES 58:185–219, 2010), regularized as in Frangi (Int J Numer Methods Eng 45:721–740, 1999) and then approximated by the Galerkin boundary element method. Details on the discretization phase and, in particular, on the computation of integrals, double in time and double in space, constituting the elements of the final linear system matrix are given and analyzed. Various numerical results and simulations are presented and discussed.     

A suitable computational strategy for the parametric analysis of problems with multiple contact

The aim of the present work is to develop an application of the LArge Time INcrement (LATIN) approach for the parametric analysis of static problems with multiple contacts. The methodology adopted was originally introduced to solve viscoplastic and large‐transformation problems. Here, the applications concern elastic, quasi‐static structural assemblies with local non‐linearities such as unilateral contact with friction. Our approach is based on a decomposition of the assembly into substructures and interfaces. The interfaces play the vital role of enabling the local non‐linearities, such as contact and friction, to be modelled easily and accurately. The problem on each substructure is solved by the finite element method and an iterative scheme based on the LATIN method is used for the global resolution. More specifically, the objective is to calculate a large number of design configurations. Each design configuration corresponds to a set of values of all the variable parameters (friction coefficients, prestress) which are introduced into the mechanical analysis. A full computation is needed for each set of parameters. Here we propose, as an alternative to carrying out these full computations, to use the capability of the LATIN method to re‐use the solution to a given problem (for one set of parameters) in order to solve similar problems (for the other sets of parameters). Copyright © 2003 John Wiley & Sons, Ltd.     

协同制造链的系统不确定性分析与建模

针对协同制造链(CMC)中存在大量不确定性的问题,分析了CMC运行过程中出现的不确定性及其来源,建立了不确定性数学模型和传播模型.通过对系统不确定性评价与预测过程的分析,提出了基于故障树分析法进行系统不确定度评定的贝叶斯模型,给出了具体评价过程,实现了CMC的不确定性评价与预测,并采用案例验证了该方法的可行性.CMC的...     

Determinants of governance structure in alliances: the role of strategic, task and partner uncertainties

Research in transaction cost economics and structural sociologyhas emphasized the role of opportunism and trust in the choicebetween equity and non-equity governance forms in alliances.This paper suggests that the uncertainty surrounding partnercooperation is not straightforwardly predictive of governancestructure in alliances. Instead, task uncertainty and strategicuncertainty associated with the activities performed withinalliances induce coordination and adaptation requirements thatare important determinants of alliance governance, independentlyof partner uncertainty. Support for this view is provided basedon a 12-year panel of alliances formed in the US telecommunications,entertainment and computer industries.