The paper describes a TurbidMC code that implements a perturbative Monte Carlo method to model temporal point spread functions and sensitivity functions for time-resolved fluorescence molecular tomography (FMT). The code is aimed at working with a particular FMT method published earlier (Ref. [22]) which defines the specificity of sensitivity function calculation. The method solves the inverse problem first for a generalized fluorescence parameter distribution function and then calculates separate distributions for the fluorophore absorption coefficient and the fluorescence lifetime. The proper operation of the code was verified through a comparison between fluorescence temporal point spread functions from test calculations and data from experiments where a phantom with a fluorophore was scanned with a three-channel probe in the mesoscopic reflectance regime. An example is given on the reconstruction of fluorescence parameter distributions. It shows that the sensitivity functions are calculated correctly.

The paper is devoted to an original method of time-resolved fluorescence molecular tomog-raphy based on asymptotic approximation to the fluorescence source function. It analyzes re-sults the authors have obtained during last 4 years in their testing the method in numerical and physical experiments. The method is inferred to be quite promising and directions of fur-ther research for its verification as a sub-millimeter resolution method are outlined. As a modernization of the method, the authors propose an iterative approach where the fluoro-phore absorption coefficient and fluorescence lifetime are reconstructed successively using a posteriori knowledge on the object from previous iterations.

A neutron-beam-shaping assembly consisting of a moderator, a reflector, and an absorber is used to form a therapeutic neutron beam for the boron neutron-capture therapy of malignant tumors at accelerator neutron sources. A new structure of the moderator and reflector is proposed in the present article, and the results of a numerical simulation of the neutron spectrum and of the absorbed dose in a modified Snyder head phantom are presented. The application of a compositemoderator and of a composite reflector and the implementation of neutron production at the proton energy of 2.3MeVare shown to permit obtaining a high-quality therapeutic neutron beam.

This review is devoted to the application of graphite and graphite composites in the science and technology. Structure and electrical properties, technological aspects of producing of high-strength artificial graphite and dynamics of its destruction are considered. These type of graphite are traditionally used in the nuclear industry, so author concentrates on actual problems of application and testing of graphite materials in modern science and technology. Translated from chapters 1 of monograph (by Zhmurikov E.I., Bubnenkov I.A., Pokrovsky A.S. et al. Graphite in Science and Nuclear Technique// eprint arXiv:1307.1869, 07/2013 (BC 2013arXiv1307.1869Z).

For more than thirty years the code PRIZMA has been used at RFNC-VNIITF for solving radiation transport problems with the Monte Carlo method. The code models the separate and coupled transport of neutrons, photons, electrons, positrons and ions in one-, two-, and three-dimensional geometry. For criticality calculations the code implements the method of generations with a constant number of fission sites in one generation. Now the code is extending its capabilities for nuclear reactor calculations. The paper describes the current status of the code and gives examples of its application to particle transport in nuclear reactors and other physical facilities.

A great breakthrough in proton therapy has happened in the new century: several tens of dedicated centers are now operated throughout the world and their number increases every year. An important component of proton therapy is a treatment planning system. To make calculations faster, these systems usually use analytical methods whose reliability and accuracy do not allow the advantages of this method of treatment to implement to the full extent. Predictions by the Monte Carlo (MC) method are a “gold” standard for the verification of calculations with these systems. At the Institute of Experimental and Theoretical Physics (ITEP) which is one of the eldest proton therapy centers in the world, an MC code is an integral part of their treatment planning system. This code which is called IThMC was developed by scientists from RFNC-VNIITF (Snezhinsk) under ISTC Project 3563.

Proton scattering in some water and tissue equivalent phantom materials was measured to evaluate their simulation accuracy of water and respective human biological tissues. The measurements were performed on the medical facility of the ITEP synchrotron, proton energy was 219 MeV, a narrow beam was formed by a 3 mm collimator. A stack of plastic slabs was set closely to the collimator hole as a scatterer. Three types of Plastic Water (PW, PW LR and PW DT), lung, cortical bone, adipose and muscle plastics (CIRS Inc., USA) were used in the experiments as the substitutes under investigation and liquid water and PMMA slabs as reference materials. Dose (intensity) profiles were measured for each sample by two orthogonal strips of the Gafchromic EBT film. A total thickness of the plastic slab was from 4 to 16 cm depending on the material. The Gafchromic film response nonlinearity was taken into account by an additional calibration vs. absorbed dose in a wide proton beam, the temporal irradiation-to-scanning dependence was also accounted. The central part of each angular distribution was fitted by the Gaussian function and compared with the respective parameters calculated for the simulated medium by Monte Carlo technique with the IThMC code.

Tissue and water equivalence of some phantom materials originally developed for conventional radiation therapy was investigated on the ITEP medical proton beam facility. The proton CSDA range in three variants of Plastic Water, lung, adipose, muscle and compact bone substitute materials (CIRS Inc., USA) was measured by a silicon diode as well as the residual proton range in liquid water after passing a slab of each material under investigation. In addition, the proton range in five materials of known elemental composition was calculated by Monte Carlo technique. The obtained results were compared with reference data from ICRU report 49 for respective biological tissues and water. A total uncertainty of the proton range ratios was estimated to be from 0.9 to 1.5% (1SD). Within these uncertainties, Plastic Water, Plastic Water LR, Plastic Water DT, muscle and compact bone demonstrated a good agreement with the reference data. The range in adipose and lung substitutes is a few percents lower than that in the respective tissues.

The paper offers a hybrid model which combines molecular dynamics and Monte Carlo (MD+MC) methods to describe primary radiation damage in crystals, caused by particles whose energies are no higher than several tens of keV. The particles are tracked in accord with equations of motion with account for pair interaction. The model also considers particle interaction with the mean-field potential (MFP) of the crystal. Only particles involved in cascading are tracked. Equations of motion for these particles include dissipative forces which describe energy exchange between cascade particles and electrons. New particles – the atoms of the crystal in the cascade region – have stochastic parameters (phase coordinates); they are sampled by the Monte Carlo method from the distribution that describes the classic canonical ensemble of non-interacting particles subjected to the external MFP. The introduction of particle interaction with the MFP helps avoid difficulties related to crystal stability and the choice of an adequate interparticle interaction potential in the traditional MD methods. Our technique is many times as fast as the traditional MD methods because we consider only particles which are involved in cascading and apply special methods to speedup the calculation of forces by accounting for the short-range pair potential used.

The paper presents results of molecular dynamics (MD) simulations which were performed to investigate mobility of defects in the δ-PuGa alloy. The defects diffuse through thermal fluctuations and MD results provided parameters for the Arrhenius law describing defect diffusion versus temperature. On the basis of this information a model of radiation defect accumulation allowing for different types of defects and grain size was constructed.The annealing of the defects at elevated temperatures and the effect of accelerated ageing due to adding small quantities of Pu-238 upon defect accumulation were evaluated.

The paper describes calculations on the evolution of damage cascades in self-irradiated unalloyed and gallium-alloyed delta-Pu. The fast stage of the evolution was simulated by the Monte Carlo (MC) method. When the energies of cascade particles became close to the displacement energy, the cascade configuration was transferred to a molecular dynamics (MD) code which tracked the further evolution of the system to similar to 2 ns. The simulations showed that a cascade of damages from the U recoil nucleus caused a large energy release into a lattice subsystem within a local region about 10 nm in size where the material melted and then recrystallized. Preliminary estimates showed that the energy transferred to the lattice was enough to cause melting in a region whose characteristic size was similar to 15 nm (similar to 200,000 atoms). MD simulations showed heat conductivity to reduce the characteristic size of the melting region to similar to 8 nm (similar to 12,000 atoms) in a sample whose initial temperature was 300 K. The time of recrystallization was estimated to be similar to 1 ns. It was shown that most point defects created during the fast stage of the cascade were recovered in melting and recrystallizing. A number of calculations were also done for polycrystalline samples. (C) 2006 Elsevier B.V. All rights reserved.

Photoionized plasmas created in the interaction of short linearly polarized x-ray pulses with a gaseous medium are characterized and discussed in the context of experiments with femtosecond FEL pulses. The electron distribution function in these plasmas remains strongly anisotropic due to unique plasma atomic and kinetic processes until such time that electron-electron collisions are able to thermalize the distribution. The relaxation of nonequilibrium photoionized plasmas is studied by using Monte-Carlo simulations which account for photoabsorption, elastic electron collisions, impact ionization of atoms, and atomic excitations. The dispersion properties and instabilities of a fully ionized x-ray-produced plasma is described. The photoionized plasma is found to be subject to a two-stream type of instability which has a growth rate comparable in magnitude to the plasma frequency and is therefore expected to have a dramatic effect on the evolution of the plasma. We performed 3D PIC simulation of this photo-ionization two-stream (PITS) instability. A model for electromagnetic field generation and emission resulting from the interaction of a short x-ray laser pulse with a gas jet is proposed. This is used to explore the effect of the thermo-EMF at the edge of a plasma characterized by an anisotropic electron energy distribution which results from the photoionization of the gas. Terahertz pulse generation from the plasma is predicted.

The characterization of a photoionized plasma created through the interaction of a short, 50–100fs, linearly polarized X-ray pulse with an He gas jet is presented by using Monte Carlo (MC) simulations. A kinetic description, which takes into account electron production due to photo-effect, elastic electron scattering on atoms, ionization and excitation of atoms by the secondary electrons, and electron transport across the X-ray beam, is introduced to study relaxation of the electron distribution function (EDF) and evolution of the electron density and mean energy. It is shown that initially an anisotropic monoenergetic EDF forms. Then it relaxes to a monoenergetic isotropic EDF for the case of low-energy X-ray quanta or to a quasi-multi-monoenergetic isotropic EDF for a pulse of high-energy X-ray quanta. This nonequilibrium electron energy distribution remains long after the X-ray pulse terminates and disappears on a ps-time-scale. The electron density distribution in the plane across the X-ray beam is characterized by considerable asymmetry along and across the polarization direction even after the vanishing of the electron energy anisotropy. The results obtained are discussed in the context of future design of experiments on self-Thomson scattering and Thomson scattering of a probe laser beam in a plasma produced by femtosecond FEL pulses.

We describe a method of determining the neutron flux of an NG-12I neutron generator, using Al and F activation detectors. The numerical computation data used were obtained by the Monte Carlo method.

The paper considers designs of moderators where fast positron stopping medium consists of very fine tungsten strips separated by vacuum gaps and the strips are arranged into Venetian blinds- or honeycomb-type structures. Moderator efficiency is evaluated through Monte-Carlo simulations. According to the maximal estimate, the efficiency of conversion of fast positrons into slow ones in the Venetian blinds and honeycomb-type moderators is ∼5×10−3 for the reasonable thickness of the tungsten foil. If such moderator is used, the intensity of slow positron source on the hard synchrotron of SPring-8 storage ring can reach the level of ∼5×1010e+/s.

The paper provides results of numeric simulations of in-target positron production process, processes of moderation, thermalization, diffusion, and reemission of positrons in high-efficiency multi-wire moderator made of tungsten monocrystalline wire with regular wire spacing. The paper looks into dynamics of slow positrons in the moderator's vacuum gaps taking into account of external fields. The feasibility of using multi-wire moderator with non-regular structure - multi-layer “wire felt” moderator is discussed. According to maximal estimate the multi-wire moderators can reach very high efficiency of fast-slow positron transformation ∼10−2. Using such moderator the intensity of slow positron source on hard synchrotron radiation of SPring-8 can reach the level of ∼1011e+/s.