Neutron measurements and radiation damage calculations for fusion materials studies

Fusion reactors will generate intense neutron fields, especially at the inner surfaces of containment vessels. With a typical wall loading of 1 MW/m/sup 2/, the yearly neutron fluence will be about 10/sup 26/ n/m/sup 2/. In a material like stainless steel this irradiation will produce about 10 atomic displacements-per-atom (DPA), 100 appM helium, 500 appM hydrogen, and various other transmutations. The gas-to-DPA ratios are very high compared to fission reactors due to the 14 MeV neutrons from the d-t fusion reaction. No existing neutron source can produce both the high fluence and high gas rates needed to simulate fusion damage. Consequently, fusion material studies are underway in a variety of facilities including fission reactors and accelerator-based neutron sources. A Subtask Group has been created by DOE to characterize these diverse facilities in terms of neutron flux and energy spectrum and to calculate DPA and transmutation for specific irradiations. Material property changes can then be correlated between facilities and extrapolated to fusion reactor conditions

Recent developments in neutron dosimetry and radiation damage calculations for fusion-materials studies

This paper is intended as an overview of activities designed to characterize neutron irradiation facilities in terms of neutron flux and energy spectrum and to use these data to calculate atomic displacements, gas production, and transmutation during fusion materials irradiations. A new computerized data file, called DOSFILE, has recently been developed to record dosimetry and damage data from a wide variety of materials test facilities. At present data are included from 20 different irradiations at fast and mixed-spectrum reactors, T(d,n) 14 MeV neutron sources, Be(d,n) broad-spectrum sources, and spallation neutron sources. Each file entry includes activation data, adjusted neutron flux and spectral data, and calculated atomic displacements and gas production. Such data will be used by materials experimenters to determine the exposure of their samples during specific irradiations. This data base will play an important role in correlating property changes between different facilities and, eventually, in predicting materials performance in fusion reactors. All known uncertainties and covariances are listed for each data record and explicit references are given to nuclear decay data and cross sections

Production of long-lived activities in fusion materials

Neutron cross sections have been measured near 14 MeV for reactions leading to long-lived radioisotopes in fusion materials. Results are summarized for various reactions leading to /sup 26/Al (720,000 y), /sup 55/Fe (2.73 y), /sup 63/Ni (100 y), /sup 59/Ni (76,000 y), and /sup 94/Nb (20,300 y). New data are presented for the production of /sup 91/Nb (680 y) from /sup 92/Mo, and from /sup 93/Mo (3500 y) and /sup 93m/Nb (13.6 y) for /sup 94/Mo. Our measurements suggest that the half-life of /sup 91/Nb may be shorter (350 y) than previous estimates. Measured cross sections are used to predict the production of these isotopes for various fusion reactor designs. 20 refs., 4 tabs

SPECTER: neutron damage calculations for materials irradiations

Neutron displacement damage-energy cross sections have been calculated for 41 isotopes in the energy range from 10/sup -10/ to 20 MeV. Calculations were performed on a 100-point energy grid using nuclear cross sections from ENDF/B-V and the DISCS computer code. Elastic scattering is treated exactly including angular distributions from ENDF/B-V. Inelastic scattering calculations consider both discrete and continuous nuclear level distributions. Multiple (n,xn) reactions use a Monte Carlo technique to derive the recoil distributions. The (n,d) and (n,t) reactions are treated as (n,p) and (n,/sup 3/He) as (n,/sup 4/He). The (n,..gamma..) reaction and subsequent ..beta..-decay are also included, using a new treatment of ..gamma..-..gamma.. coincidences, angular correlations, ..beta..-neutrino correlations, and the incident neutron energy. The Lindhard model was used to compute the energy available for nuclear displacement at each recoil energy. The SPECTER computer code has been developed to simplify damage calculations. The user need only specify a neutron energy spectrum. SPECTER will then calculate spectral-averaged displacements, recoil spectra, gas production, and total damage energy (Kerma). The SPECTER computer code package is readily accessible to the fusion community via the National Magnetic Fusion Energy Computer Center (NMFECC) at Lawrence Livermore National laboratory

Analysis of long-lived isotopes by liquid scintillation spectrometry

Neutron production cross sections are reported for reactions leading to long-lived isotopes in fusion reactor materials. Pure elements and separated isotopes were irradiated with 14.6 to 14.8 MeV neutron fluences up to 10 Y n/cmS. Undesired activities were chemically separated and the long-lived activities were measured using both liquid scintillation and x-ray spectrometry. Results are presented for the reactions VWFe(n,2n)VVFe (2.73 y), WUNi(n,2n)WTNi (100 y), WTCu(n,P)WTNi, and WNi(n,2n)VZNi (76,000 y)

Comparison of differential and integral cross section measurements

A program has been undertaken to simultaneously measure integral and differential cross sections in order to establish the degree of consistency between integral and differentially derived spectra. An assessment is then made concerning cross section limitations in deriving high energy neutron spectra by the foil-activation spectral-unfolding technique. (SDF

Template-stripped gold surfaces with 0.4 nm rms roughness suitable for force measurements. Application to the Casimir force in the 20-100 nm range

Using a template-stripping method, macroscopic gold surfaces with root-mean-square (rms) roughness less than 0.4 nm have been prepared, making them useful for studies of surface interactions in the nanometer range. The utility of such substrates is demonstrated by measurements of the Casimir force at surface separations between 20 and 100 nm, resulting in good agreement with theory. The significance and quantification of this agreement is addressed, as well as some methodological aspects regarding the measurement of the Casimir force with high accuracy.Comment: 7 figure

Scale-free memory model for multiagent reinforcement learning. Mean field approximation and rock-paper-scissors dynamics

A continuous time model for multiagent systems governed by reinforcement learning with scale-free memory is developed. The agents are assumed to act independently of one another in optimizing their choice of possible actions via trial-and-error search. To gain awareness about the action value the agents accumulate in their memory the rewards obtained from taking a specific action at each moment of time. The contribution of the rewards in the past to the agent current perception of action value is described by an integral operator with a power-law kernel. Finally a fractional differential equation governing the system dynamics is obtained. The agents are considered to interact with one another implicitly via the reward of one agent depending on the choice of the other agents. The pairwise interaction model is adopted to describe this effect. As a specific example of systems with non-transitive interactions, a two agent and three agent systems of the rock-paper-scissors type are analyzed in detail, including the stability analysis and numerical simulation. Scale-free memory is demonstrated to cause complex dynamics of the systems at hand. In particular, it is shown that there can be simultaneously two modes of the system instability undergoing subcritical and supercritical bifurcation, with the latter one exhibiting anomalous oscillations with the amplitude and period growing with time. Besides, the instability onset via this supercritical mode may be regarded as "altruism self-organization". For the three agent system the instability dynamics is found to be rather irregular and can be composed of alternate fragments of oscillations different in their properties.Comment: 17 pages, 7 figur

Search for direct production of charginos and neutralinos in events with three leptons and missing transverse momentum in √s = 7 TeV pp collisions with the ATLAS detector

A search for the direct production of charginos and neutralinos in final states with three electrons or muons and missing transverse momentum is presented. The analysis is based on 4.7 fb−1 of proton–proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. Observations are consistent with Standard Model expectations in three signal regions that are either depleted or enriched in Z-boson decays. Upper limits at 95% confidence level are set in R-parity conserving phenomenological minimal supersymmetric models and in simplified models, significantly extending previous results

Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC

Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the anti-kt algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp. Jet production is found to be suppressed by approximately a factor of two in the 10% most central collisions relative to peripheral collisions. Rcp varies smoothly with centrality as characterized by the number of participating nucleons. The observed suppression is only weakly dependent on jet radius and transverse momentum. These results provide the first direct measurement of inclusive jet suppression in heavy ion collisions and complement previous measurements of dijet transverse energy imbalance at the LHC.Comment: 15 pages plus author list (30 pages total), 8 figures, 2 tables, submitted to Physics Letters B. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2011-02