1,689 research outputs found
Anything You Can Do, You Can Do Better: Neural Substrates of Incentive-Based Performance Enhancement
Performance-based pay schemes in many organizations share the fundamental assumption that the performance level for a given task will increase as a function of the amount of incentive provided. Consistent with this notion, psychological studies have demonstrated that expectations of reward can improve performance on a plethora of different cognitive and physical tasks, ranging from problem solving to the voluntary regulation of heart rate. However, much less is understood about the neural mechanisms of incentivized performance enhancement. In particular, it is still an open question how brain areas that encode expectations about reward are able to translate incentives into improved performance across fundamentally different cognitive and physical task requirements
Understanding visual map formation through vortex dynamics of spin Hamiltonian models
The pattern formation in orientation and ocular dominance columns is one of
the most investigated problems in the brain. From a known cortical structure,
we build spin-like Hamiltonian models with long-range interactions of the
Mexican hat type. These Hamiltonian models allow a coherent interpretation of
the diverse phenomena in the visual map formation with the help of relaxation
dynamics of spin systems. In particular, we explain various phenomena of
self-organization in orientation and ocular dominance map formation including
the pinwheel annihilation and its dependency on the columnar wave vector and
boundary conditions.Comment: 4 pages, 15 figure
First measurement of the Head-Tail directional nuclear recoil signature at energies relevant to WIMP dark matter searches
We present first evidence for the so-called Head-Tail asymmetry signature of
neutron-induced nuclear recoil tracks at energies down to 1.5 keV/amu using the
1m^3 DRIFT-IIc dark matter detector. This regime is appropriate for recoils
induced by Weakly Interacting Massive Particle (WIMPs) but one where the
differential ionization is poorly understood. We show that the distribution of
recoil energies and directions induced here by Cf-252 neutrons matches well
that expected from massive WIMPs. The results open a powerful new means of
searching for a galactic signature from WIMPs.Comment: 4 pages, 6 figures, 1 tabl
Measurement of the quenching factor of Na recoils in NaI(Tl)
Measurements of the quenching factor for sodium recoils in a 5 cm diameter
NaI(Tl) crystal at room temperature have been made at a dedicated neutron
facility at the University of Sheffield. The crystal has been exposed to 2.45
MeV mono-energetic neutrons generated by a Sodern GENIE 16 neutron generator,
yielding nuclear recoils of energies between 10 and 100 keVnr. A cylindrical
BC501A detector has been used to tag neutrons that scatter off sodium nuclei in
the crystal. Cuts on pulse shape and time of flight have been performed on
pulses recorded by an Acqiris DC265 digitiser with a 2 ns sampling time.
Measured quenching factors of Na nuclei range from 19% to 26% in good agreement
with other experiments, and a value of 25.2 \pm 6.4% has been determined for 10
keV sodium recoils. From pulse shape analysis, the mean times of pulses from
electron and nuclear recoils have been compared down to 2 keVee. The
experimental results are compared to those predicted by Lindhard theory,
simulated by the SRIM Monte Carlo code, and a preliminary curve calculated by
Prof. Akira Hitachi.Comment: 21 pages, 13 figure
The DRIFT Dark Matter Experiments
The current status of the DRIFT (Directional Recoil Identification From
Tracks) experiment at Boulby Mine is presented, including the latest limits on
the WIMP spin-dependent cross-section from 1.5 kg days of running with a
mixture of CS2 and CF4. Planned upgrades to DRIFT IId are detailed, along with
ongoing work towards DRIFT III, which aims to be the world's first 10 m3-scale
directional Dark Matter detector.Comment: Proceedings of the 3rd International conference on Directional
Detection of Dark Matter (CYGNUS 2011), Aussois, France, 8-10 June 201
Low Energy Electron and Nuclear Recoil Thresholds in the DRIFT-II Negative Ion TPC for Dark Matter Searches
Understanding the ability to measure and discriminate particle events at the
lowest possible energy is an essential requirement in developing new
experiments to search for weakly interacting massive particle (WIMP) dark
matter. In this paper we detail an assessment of the potential sensitivity
below 10 keV in the 1 m^3 DRIFT-II directionally sensitive, low pressure,
negative ion time projection chamber (NITPC), based on event-by-event track
reconstruction and calorimetry in the multiwire proportional chamber (MWPC)
readout. By application of a digital smoothing polynomial it is shown that the
detector is sensitive to sulfur and carbon recoils down to 2.9 and 1.9 keV
respectively, and 1.2 keV for electron induced events. The energy sensitivity
is demonstrated through the 5.9 keV gamma spectrum of 55Fe, where the energy
resolution is sufficient to identify the escape peak. The effect a lower energy
sensitivity on the WIMP exclusion limit is demonstrated. In addition to recoil
direction reconstruction for WIMP searches this sensitivity suggests new
prospects for applications also in KK axion searches
A 2nd generation cosmic axion experiment
An experiment is described to detect dark matter axions trapped in the halo
of our galaxy. Galactic axions are converted into microwave photons via the
Primakoff effect in a static background field provided by a superconducting
magnet. The photons are collected in a high Q microwave cavity and detected by
a low noise receiver. The axion mass range accessible by this experiment is
1.3-13 micro-eV. The expected sensitivity will be roughly 50 times greater than
achieved by previous experiments in this mass range. The assembly of the
detector is well under way at LLNL and data taking will start in mid-1995.Comment: Postscript, 6 pages, 4 figures; submitted to proceedings of: XXXth
Recontres de Moriond, 'Dark Matter in Cosmology", Villars-sur-Ollon,
Switzerland, Jan 21-28, 199
A Multiscale Approach to Determination of Thermal Properties and Changes in Free Energy: Application to Reconstruction of Dislocations in Silicon
We introduce an approach to exploit the existence of multiple levels of
description of a physical system to radically accelerate the determination of
thermodynamic quantities. We first give a proof of principle of the method
using two empirical interatomic potential functions. We then apply the
technique to feed information from an interatomic potential into otherwise
inaccessible quantum mechanical tight-binding calculations of the
reconstruction of partial dislocations in silicon at finite temperature. With
this approach, comprehensive ab initio studies at finite temperature will now
be possible.Comment: 5 pages, 3 figure
Finite Sized Atomistic Simulations of Screw Dislocations
The interaction of screw dislocations with an applied stress is studied using
atomistic simulations in conjunction with a continuum treatment of the role
played by the far field boundary condition. A finite cell of atoms is used to
consider the response of dislocations to an applied stress and this introduces
an additional force on the dislocation due to the presence of the boundary.
Continuum mechanics is used to calculate the boundary force which is
subsequently accounted for in the equilibrium condition for the dislocation.
Using this formulation, the lattice resistance curve and the associated Peierls
stress are calculated for screw dislocations in several close packed metals. As
a concrete example of the boundary force method, we compute the bow out of a
pinned screw dislocation; the line-tension of the dislocation is calculated
from the results of the atomistic simulations using a variational principle
that explicitly accounts for the boundary force.Comment: LaTex, 20 pages, 11 figure
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