arXiv subjects

S. G. Mashnik

Publications and source records attributed to S. G. Mashnik.

65 records · Page 4Linked to original sources

arXiv2000

A Covariant Generalization of the Real-Time Green's Functions Method in the Theory of Kinetic Equations

A generalized quantum kinetic equation (RKE) of the Kadanoff-Baym type is obtained on the basis of the Heisenberg equations of motion where the time evolution and space translation are separated from each other by means of the covariant method. The same approach is used also for a covariant modification of the real-time Green's functions method based on the Wigner representation. The suggested approach does not contain arbitrariness' elements and uncertainties which often arise from derivation of RKE on the basis of the motion equations of the Kadanoff-Baym type for the correlation functions in the case of systems with inner degrees of freedom. Possibilities of the proposed method are demonstrated by examples of derivation of RKE of the Vlasov type and collision integrals of the Boltzmann-Uehling-Uhlenbeck (BUU) type in the frame of the $σω$ -version of quantum hadrodynamics, for the simplest case of spin saturated nuclear matter without antinuclear component. Here, the quasiparticle approximation in a covariant performance is used. A generalization of the method for the description of strong non-equilibrium states based on the non-equilibrium statistical operator is then proposed as well.

hep-ph↗

arXiv1999

Nuclear Data Requirements for the Production Of Medical Isotopes in Fission Reactors and Particle Accelerators

Through decades of effort in nuclear data development and simulations of reactor neutronics and accelerator transmutation, a collection of reaction data is continuing to evolve with the potential of direct applications to the production of medical isotopes. At Los Alamos the CINDER'90 code and library have been developed for nuclide inventory calculations using neutron-reaction (En < 20 MeV) and/or decay data for 3400 nuclides; coupled with the LAHET Code System (LCS), irradiations in neutron and proton environments below a few GeV are tractable; additional work with the European Activation File, the HMS-ALICE code and the reaction models of MCNPX (CEM95, BERTINI, or ISABEL with or without preequilibrium, evaporation and fission) have been used to produce evaluated reaction data for neutrons and protons to 1.7 GeV. At the Pacific Northwest National Laboratory, efforts have focused on production of medical isotopes and the identification of available neutron reaction data from results of integral measurements.

physics.med-ph↗

arXiv1999

Experimental and Computer Simulation Study of Radioactivity of Materials Irradiated by Intermediate Energy Protons

The results of measurements and computer simulations of radioactivities and dose rates as functions of decay time are presented for Pb-nat and Bi-209 irradiated by 1.5-GeV protons, Co-59, Cu-63, and Cu-65 irradiated by 0.13- and 1.2-GeV protons, and Th-232 and U-nat irradiated by 0.1- and 0.8-GeV protons. The activities and dose rates are measured by direct high-precision gamma spectrometry. The irradiations were made using external beams extracted from the ITEP U-10 proton synchrotron. Simulations made using the LCS and CINDER'90 code systems are compared with measurements.

nucl-ex↗

arXiv1999

Experimental and Computer Simulation Study of Radionuclide Production in Heavy Materials Irradiated by Intermediate Energy Protons

The results of measurements and computer simulations are presented for the yields of residual product nuclei in thin targets: U-nat irradiated by 0.1, 0.8, 1.2, and 1.6 GeV and Tc-99 irradiated by 0.2, 0.8, 1.0, 1.4, and 1.6 GeV protons. The yields were measured at ITEP by direct high-precision gamma spectrometry. About 820 cross sections are presented and used in comparison between measured yields and simulations by the LAHET, INUCL, CEM95, HETC, CASCADE, YIELDX, and ALICE codes.

nucl-ex↗

arXiv1999

Computer Study of Isotope Production in High Power Accelerators

Methods for radionuclide production calculation in a high power proton accelerator have been developed and applied to study production of 22 isotopes by high-energy protons and neutrons. These methods are readily applicable to accelerator, and reactor, environments other than the particular model we considered and to the production of other radioactive and stable isotopes. We have also developed methods for evaluating cross sections from a wide variety of sources into a single cross section set and have produced an evaluated library covering about a third of all natural elements. These methods also are applicable to an expanded set of reactions. A 684 page detailed report on this study, with 37 tables and 264 color figures is available on the Web at http://t2.lanl.gov/publications/publications.html, or, if not accessible, in hard copy from the authors.

nucl-th↗

arXiv1998

Production and Validation of Isotope Production Cross Section Libraries for Neutrons and Protons to 1.7 GeV

For validation and development of codes and for modeling isotope production in high power accelerators and APT Materials studies, we have produced experimental, calculated, and evaluated activation libraries for interaction of nucleons with nuclides covering about a third of all natural elements. For targets considered here, our compilation of experimental data is the most complete we are aware of, since it contains all data available on the Web, in journal papers, laboratory reports, theses, and books, as well as all data included in the large compilation by Sobolevsky with co-authors (NUCLEX) published recently by Springer-Verlag in 4 volumes. Our evaluated library was produced using all available experimental cross sections together with calculations by the CEM95, LAHET, and HMS-ALICE codes and with the European Activation File EAF-97 and LANL Update II of the ECNAF Neutron Activation Cross-Section Library.

nucl-th↗

arXiv1998

MARS Code Developments

Recent developments in the physical model of 1 MeV to 100 TeV hadron and lepton interactions with nuclei and atoms are described. These include a new nuclear cross section library, a model for soft pion production, the cascade-exciton model, the dual parton model, deuteron-nucleus and neutrino-nucleus interaction models, detailed description of muon, pion, and antiproton absorption and a unified treatment of muon and charged hadron electromagnetic interactions with matter. New algorithms are implemented into the MARS13(98)} Monte Carlo code and benchmarked against experimental data. The code capabilities to simulate cascades and generate a variety of results in complex media have been also enhanced.

nucl-th↗

arXiv1998

Cascade-Exciton Model Analysis of Nucleon-Induced Fission Cross Sections of Lead and Bismuth at Energies from 45 to 500 MeV

An extended version of the Cascade-Exciton Model (CEM) of nuclear reactions is applied to analyze nucleon-induced fission cross sections for Bi-209 and Pb-208 nuclei in the 45-500 MeV energy range. The available data on linear momentum transfer are analyzed as well. The results are compared with analytical approximations resulting from a comparative critical analysis of all available experimental data. Systematic discrepancies between calculations and experimental data are revealed. A modification of the CEM is proposed, which significantly improves the model predictions for projectile energies above 100 MeV.

nucl-th↗

arXiv1997

Experimental and Computer Simulation Study of the Radionuclides Produced in Thin Bi-209 Targets by 130 MeV and 1.5 GeV Protons

The results of experimental and computer simulation studies of the yields of residual product nuclei in Bi-209 thin targets irradiated by 130 MeV and 1.5 GeV protons are presented. The yields were measured by direct high-precision gamma-spectrometry. The gamma-spectrometer resolution was 1.8 keV in the 1332 keV line. The gamma-spectra were processed by the ASPRO code. The gamma-lines were identified, and the cross sections defined, by the SIGMA code using the GDISP radioactive database. The process was monitored by the Al-27(p,x)Na-24 reaction. Results are presented for comparisons between the 209-Bi(p,x) reaction yields obtained experimentally and simulated by the HETC, GNASH, LAHET, INUCL, CEM95, CASCADE, and ALICE codes.

nucl-th↗

arXiv1997

Cascade-Exciton Model Analysis of Proton Spallation from 10 MeV to 5 GeV

We have used an extended version of the Cascade-Exciton Model (CEM) to analyze more than 600 excitation functions for proton induced reactions on 19 targets ranging from C-12 to Au-197, for incident energies ranging from 10 MeV to 5 GeV. We have compared the calculations to available data, to calculations using approximately two dozen other models, and to predictions of several phenomenological systematics. We present here our conclusions concerning the relative roles of different reaction mechanisms in the production of specific final nuclides. We comment on the strengths and weaknesses of the CEM and suggest possible further improvements to the CEM and to other models.

nucl-th↗

arXiv1994

Inclusive Pion Double Charge Exchange above 0.5 GeV

A cascade model has been developed to study pion induced multichannel reactions (quasielastic, SCX, DCX, absorption and pion production) at pion energies above 0.5 GeV. Special attention has been paid to pion double charge exchange. Pion production is a determinant feature in the high energy pion nucleus reactions, and the DCX signal not related to pion production is sizeable only at forward angles and for high energy outgoing pions. The contribution to DCX of the conventional mechanism, with two quasielastic SCX steps decreases very fast as a function of the energy and reaches very low values at energies above 0.7 GeV. This opens the opportunity of having sizeable contributions of exotic mechanisms that are negligible at the delta resonance energies.

nucl-th↗