Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Geant4 is a software toolkit for the simulation of the passage of particles through matter. It is used by a large number of experiments and projects in a variety of application domains, including high energy physics, astrophysics and space science, medical physics and radiation protection. Over the past several years, major changes have been made to the toolkit in order to accommodate the needs of these user communities, and to efficiently exploit the growth of computing power made available by advances in technology. The adaptation of Geant4 to multithreading, advances in physics, detector modeling and visualization, extensions to the toolkit, including biasing and reverse Monte Carlo, and tools for physics and release validation are discussed here.

/pdf/recent-developments-in-geant4-3pgbpf1ko3.pdf

Recent developments in GEANT4

ENDF/B-VII.1 Nuclear Data for Science and Technology: Cross Sections, Covariances, Fission Product Yields and Decay Data

https://digital.library.unt.edu/ark:/67531/metadc888955/m2/1/high_res_d/900147.pdf

ENDF/B-VII.0: Next Generation Evaluated Nuclear Data Library for Nuclear Science and Technology

The fourth version of the Japanese Evaluated Nuclear Data Library has been produced in cooperation with the Japanese Nuclear Data Committee. In the new library, much emphasis is placed on the impro...

https://www.tandfonline.com/doi/pdf/10.1080/18811248.2011.9711675

JENDL-4.0: A New Library for Nuclear Science and Engineering

A new set of measurements for the total and capture cross section determination of W isotopes was done using GELINA (GEel LINear Accelerator), a neutron Time-Of-Flight facility at the Institute for Reference Materials and Measurements (IRMM). Measuring stations at different flight path lengths were used in order to cover a broad neutron energy range with high resolution demands. Experimental techniques adopted for both transmission and capture measurements are well established using a (6)Li glass detector and C(6)D(6) scintillation arrays as detections systems respectively., As target samples highly enriched (182,183,184,186)W metallic discs were used.

Neutron Total and Capture Cross Section of Tungsten Isotopes

The Oak Ridge Electron Linear Accelerator (ORELA) was used to measure neutron total and capture cross sections of aluminium, natural chlorine and silicon in the energy range from 100 eV to ~600 keV. ORELA is the only high power white neutron source with excellent time resolution and ideally suited for these experiments still operating in the USA. These measurements were carried out to support the Nuclear Criticality Predictability Program. Concerns about the use of existing cross section data in the nuclear criticality calculations using Monte Carlo codes and benchmarks have been a prime motivator for the new cross section measurements. More accurate nuclear data are not only needed for these calculations but also serve as input parameters for s-process stellar models.

/pdf/neutron-cross-sections-measurements-for-light-elements-at-1twyjejfcf.pdf

Neutron Cross Sections Measurements for Light Elements at ORELA and their Application in Nuclear Criticality

Many older neutron cross‐section evaluations from libraries such as ENDF/B‐VI or JENDL‐3.2 exhibit deficiencies or do not cover energy ranges that are important for criticality safety applications. These deficiencies may occur in the resolved and unresolved‐resonance regions. Consequently, these evaluated data may not be adequate for nuclear criticality calculations where effects such as self‐shielding, multiple scattering, or Doppler broadening are important. To support the Nuclear Criticality Predictability Program, neutron cross‐section measurements have been initiated at the Oak Ridge Electron Linear Accelerator (ORELA). ORELA is the only high‐power white neutron source with excellent time resolution still operating in the United States. It is ideally suited to measure fission, neutron total, and capture cross sections in the energy range from 1 eV to ∼600 keV, which is important for many nuclear criticality safety applications.

New Neutron Cross-Section Measurements at ORELA for Improved Nuclear Data Calculations

Almost since the time it was formulated, the overwhelming consensus has been that random matrix theory (RMT) is in excellent agreement with neutron resonance data. However, over the past few years, we have obtained new neutron-width data at Oak Ridge and Los Alamos National Laboratories that are in stark disagreement with this theory. We also have reanalyzed neutron widths in the most famous data set, the nuclear data ensemble (NDE), and found that it is seriously flawed, and, when analyzed carefully, excludes RMT with high confidence. More recently, we carefully examined energy spacings for these same resonances in the NDE using the Delta_3 statistic. We conclude that the data can be found to either confirm or refute the theory depending on which nuclides and whether known or suspected p-wave resonances are included in the analysis, in essence confirming results of our neutron-width analysis of the NDE. We also have examined radiation widths resulting from our Oak Ridge and Los Alamos measurements, and find that in some cases they do not agree with RMT. Although these disagreements presently are not understood, they could have broad impact on basic and applied nuclear physics, from nuclear astrophysics to nuclear criticality safety.

Neutron Resonance Data Exclude Random Matrix Theory

We have made the first experimental determinations of the astrophysical rates for the 192, 194, 195, 196Pt(n, γ) reactions across the range of temperatures needed for astrophysical models of the s process. Both neutron capture and total cross sections were measured for 192, 194, 195, 196Pt at the Oak Ridge Electron Linear Accelerator (ORELA) from 10 eV to 500 keV. The data were fitted using the R-matrix code SAMMY to obtain resonance parameters. These parameters as well as the measured cross sections in the unresolved region were used to obtain the astrophysical reaction rates for all these isotopes. We compare our results to previous data and theoretical reaction rates and discuss their astrophysical impact. For example, a classical analysis of the s-process branching at 192Ir using our new rates yields a much lower neutron density than previous analyses of other branchings, in agreement with the long-sought freeze-out effect predicted by dynamical (e.g. stellar) models.

High-Resolution Neutron Capture and Total Cross Section Measurements for 192, 194, 195, 196Pt and the Astrophysical Rates for the 192, 194, 195, 196Pt(n, γ) Reactions

Klaus H Guber

Papers

Neutron Total and Capture Cross Section of Tungsten Isotopes

Neutron Cross Sections Measurements for Light Elements at ORELA and their Application in Nuclear Criticality

New Neutron Cross-Section Measurements at ORELA for Improved Nuclear Data Calculations

Neutron Resonance Data Exclude Random Matrix Theory

High-Resolution Neutron Capture and Total Cross Section Measurements for 192, 194, 195, 196Pt and the Astrophysical Rates for the 192, 194, 195, 196Pt(n, γ) Reactions