There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

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

R-matrix formalism is extended beyond compound nuclear (CN) resonant reactions to include parameterization of direct as well as doorway processes. Direct processes in the R-matrix exterior are parameterized by a unitary matrix that introduces mixing among wave function coefficients of the incoming and outgoing wave function components at the R-matrix channel surface. Doorway processes are parameterized by separating the Hilbert space of the interior R-matrix region into its doorway and CN subspaces, from which doorway state eigenenergies, reduced width amplitudes, and the strengths of their coupling to CN levels appear as new R-matrix parameters. Parameterization of generalized as well as the conventional Reich–Moore approximation for eliminated capture channels in the presence of direct, doorway, and CN processes is presented along with a complex-valued scattering length with contributions from direct, doorway, and CN capture processes. Derivation of Brune’s alternative R-matrix parameters is extended to include doorway states. This work suggests how R-matrix formalism could be extended further by adopting the concepts from related reaction formalisms.

/pdf/parameterization-of-direct-and-doorway-processes-in-r-matrix-vhyloapd.pdf

Parameterization of Direct and Doorway Processes in R-Matrix Formalism

Neutron Capture Measurements and Resonance Analysis of Dysprosium

The use of a Lead Slowing-Down Spectrometer (LSDS) is considered as a possible option for nondestructive assay of fissile material in used nuclear fuel. The primary objective is to quantify the 239 Pu and 235 U fissile content via a direct measurement distinguishing them through their characteristic fission spectra in the LSDS. Threshold fission chambers are employed to detect fission neutrons due to their insensitivity to lower energy interrogation neutrons and their favorable behavior in an irradiating environment. In the framework of the LSDS Collaboration for the Material Protection and Control Technology (MPACT) Campaign we performed several assay experiments at the Rensselaer Polytechnic Institute (RPI) to demonstrate the feasibility of such method and to provide benchmark experiments. A fresh UOX fuel rod from the RPI Criticality Research Facility and a PuBe source were assayed consecutively in the LSDS. The characteristic fission spectra were measured with 238 U and 232 Th threshold fission chambers. Despite the constant neutron and gamma background from the PuBe source, the LSDS system was able to measure a 239 Pu response. This is the first time that Pu was assayed in the US using a LSDS system. Both measurements were found to be in good agreement with calculations done with MCNP5. Experiments with a combination of a fuel pin and the PuBe source were performed to simulate to some extent burn-up conditions. The distance of the PuBe source with respect to the detectors was varied to simulate different Pu contents and several mixed 239 Pu and 235 U response spectra were measured and compared to calculations.

/pdf/towards-benchmark-measurements-for-used-nuclear-fuel-assay-5dmgxb7px7.pdf

Towards benchmark measurements for used nuclear fuel assay using a lead

Improved nondestructive assay of isotopic masses in used nuclear fuel would be valuable for nuclear safeguards operations associated with the transport, storage, and reprocessing of used nuclear fu...

/pdf/lead-slowing-down-spectrometry-analysis-of-data-from-17wk864shh.pdf

Lead Slowing-Down Spectrometry Analysis of Data from Measurements on Nuclear Fuel

A novel method was developed to separate the 238U fission contribution measured in quasi-differential time-of-flight scattering experiments in order to isolate the elastic and inelastic events. Pulse height distributions from in-beam measurements were used to generate response functions, which were used to reconstruct the 238U prompt fission neutron spectra. This method was validated by reconstructing the measured 252Cf spontaneous fission pulse height distribution. Monte Carlo calculations were used to model the experiment. Good agreement was observed between the measured and calculated 238U fission contribution.

/pdf/method-to-compare-fission-to-scattering-ratios-using-uranium-3513uqjk.pdf

Yaron Danon

Papers

Parameterization of Direct and Doorway Processes in R-Matrix Formalism

Neutron Capture Measurements and Resonance Analysis of Dysprosium

Towards benchmark measurements for used nuclear fuel assay using a lead

Lead Slowing-Down Spectrometry Analysis of Data from Measurements on Nuclear Fuel

Method to Compare Fission-to-Scattering Ratios using Uranium-238