James A. Rathkopf

The response history Monte Carlo method for electron transport.

In this paper, a new method, response history Monte Carlo (RHMC), is developed for solving electron transport problems through homogeneous material, and it is more accurate than the conventional method for energies below a few hundred kilo-electron-volts. Since electrons can suffer thousands of collisions and lose only a fraction of their incident energy, analog Monte Carlo (single scatter) is extremely time-consuming. The conventional electron transport method avoids simulating single scattering events by modeling the effect of multiple collisions. This condensed history method requires assumptions that are invalid at lower energies to analytically determine probability distribution functions (pdfs) representing the electron state after multiple collisions. Like the condensed history method, the RHMC method uses an approximate random walk where each step represents the cumulative effect of many collisions. However, the RHMC method is more accurate than the condensed history method since the multiscattered electron state is sampled from pdfs predetermined by analog Monte Carlo calculations instead of approximate analytic solutions.

Single-scatter Monte Carlo compared to condensed history results for low energy electrons *

Abstract A Monte Carlo code has been developed to simulate individual electron interactions. The code has been instrumental in determining the range of validity for the widely used condensed history method. This task was accomplished by isolating and testing the condensed history assumptions. The results show that the condensed history method fails for low energy electron transport due to inaccuracies in energy loss and spatial positioning.

The finite element response matrix method for the solution of the neutron transport equation

The finite element response matrix method has been applied to the solution of the neutron transport equation. This method has previously been applied to the neutron diffusion equation for coarse mesh reactor analysis with excellent results. As with the diffusion equation implementation, the transport method employs a local-global projection technique to allow treatment of internal heterogeneities that would normally not be resolved by the coarse global mesh that is needed for computational efficiency. However, since the transport equation includes the angular domain, the local~lobal projection technique has been extended to angle as well as space. Since the response matrices do not depend on the multiplication factor, a conventional fission source iteration method is utilized for criticality problems. The method has been applied to the one-dimensional and two- dimensional neutron transport equations. For one-dimensional geometries, the local global projection method yields excellent results, indicating the potential of this approach as a viable coarse mesh transport method. Numerical results are presented for several one-dimensional configurations that were analyzed with varying choices of local and global meshes in the spatial domain. Preliminary results with two-dimensional applications indicate that computational times may be an order of magnitude faster than with the conventional finite element solution of the two-dimensional transport equation.

Science at LLNL with IBM Blue Gene/Q

Lawrence Livermore National Laboratory (LLNL) has a long history of working with IBM on Blue Gene® supercomputers. Beginning in November 2001 with the joint announcement of a partnership to expand the Blue Gene research project (including Blue Gene®/L and Blue Gene®/P), the collaboration extends to this day with LLNL planning for the installation of a 96-rack Blue Gene®/Q (called Sequoia) supercomputer. As with previous machines, we envision Blue Gene/Q will be used for a wide array of applications at LLNL, ranging from meeting programmatic requirements for certification to increasing our understanding of basic physical processes. We briefly describe a representative sample of mature codes that span this application space and scale well on Blue Gene hardware. Finally, we describe advances in multi-scale whole-organ modeling of the human heart as an example of breakthrough science that will be enabled with the Blue Gene/Q architecture.

The All Particle Monte Carlo Method: 1990 Status Report

Abstract Development of the All Particle Method, a project to simulate the transport of particles via the Monte Carlo method, has proceeded on two fronts: data collection and algorithm development. In this paper we report on the status of both these aspects. The data collection is nearly complete with the addition of electron and atomic data libraries and a newly revised photon library to the existing neutron and charged particle libraries. We describe the basic organization of the Monte Carlo computer code that will actually perform the simulation. Algorithm development has proceeded less quickly than the data portion of the project. The new Response Matrix Monte Carlo method is summarized and some numerical results presented.

Hardening optical coatings to the effects of x rays

Analyses of the effects of x rays on optical components consisting of a metal reflecting layer, a beryllium heat sink, and a fused silica substrate were done using the computer code xrth. These are compared with calculations for components lacking the beryllium layer. Our analyses show that the addition of the beryllium significantly hardens the system against x rays. The mechanisms for this increased hardness are elucidated by time‐dependent temperature and enthalpy plots. The relative effectiveness of beryllium and diamondlike carbon heat sinks is explored. Complex designs which incorporate reflection‐enhancing dielectric stacks are also discussed. Some comparisons with experiments are given.