75 research outputs found
A large area detector for neutrons between 2 and 100 MeV
A neutron detector sensitive from 2 to 100 MeV is described. The detector is designed for high altitude balloon flight to measure the flux, energy and direction of albedo neutrons from the earth and to search for solar neutrons. A neutron scatter from a proton is required in each of two liquid scintillator tanks spaced 1 meter apart. The energy of the recoil proton in the first tank is obtained from pulse height analysis of the scintillator output. The energy of the recoil neutron is obtained from its time of flight between the tanks. The detector has been calibrated with 15.3 MeV neutrons and mu mesons. The minimum detectable flux is 10(-4) neutron/sq cm/sec at a counting rate of one per minute; the energy resolution is 12% at 15 MeV and 30% at 100 MeV. The angle between the incoming neutron and the recoil neutron is measured to + or - 10 deg
Collection of Athermal Phonons into Doped Germanium Thermistors Using Quasiparticle Trapping
We have developed a low‐temperature particle detector that uses a novel quasiparticle trapping mechanism to funnel athermal phonon energy from an 80 mg Ge absorber into a 1.6 mg doped Ge thermistor via a superconducting Al film. We report on pulse height spectra obtained at 320 mK by scanning a 241Am alpha source along the device, and show that up to 20% of the energy deposited in the Ge absorber by a 5.5 MeV alpha particle interaction can be collected into a thermistor via quasiparticle trapping. We show that this device is sensitive to the position of an alpha particle interaction in the Ge absorber for interaction distances of up to 5 mm from a quasiparticle trap
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Experimental pub crawl from Rayleigh–Bénard to magnetostrophic convection
The interplay between convective, rotational and magnetic forces defines the dynamics within the electrically conducting regions of planets and stars. Yet their triadic effects are separated from one another in most studies, arguably due to the richness of each subset. In a single laboratory experiment, we apply a fixed heat flux, two different magnetic field strengths and one rotation rate, allowing us to chart a continuous path through Rayleigh-Bénard convection (RBC), two regimes of magnetoconvection, rotating convection and two regimes of rotating magnetoconvection, before finishing back at RBC. Dynamically rapid transitions are determined to exist between jump rope vortex states, thermoelectrically driven magnetoprecessional modes, mixed wall- and oscillatory-mode rotating convection and a novel magnetostrophic wall mode. Thus, our laboratory 'pub crawl' provides a coherent intercomparison of the broadly varying responses arising as a function of the magnetorotational forces imposed on a liquid-metal convection system
Response of an Excitatory-Inhibitory Neural Network to External Stimulation: An Application to Image Segmentation
Neural network models comprising elements which have exclusively excitatory
or inhibitory synapses are capable of a wide range of dynamic behavior,
including chaos. In this paper, a simple excitatory-inhibitory neural pair,
which forms the building block of larger networks, is subjected to external
stimulation. The response shows transition between various types of dynamics,
depending upon the magnitude of the stimulus. Coupling such pairs over a local
neighborhood in a two-dimensional plane, the resultant network can achieve a
satisfactory segmentation of an image into ``object'' and ``background''.
Results for synthetic and and ``real-life'' images are given.Comment: 8 pages, latex, 5 figure
Zero-temperature responses of a 3D spin glass in a field
We probe the energy landscape of the 3D Edwards-Anderson spin glass in a
magnetic field to test for a spin glass ordering. We find that the spin glass
susceptibility is anomalously large on the lattice sizes we can reach. Our data
suggest that a transition from the spin glass to the paramagnetic phase takes
place at B_c=0.65, though the possibility B_c=0 cannot be excluded. We also
discuss the question of the nature of the putative frozen phase.Comment: RevTex, 4 pages, 4 figures, clarifications and added reference
Ising Spin Glasses in a Magnetic Field
Ground states of the three dimensional Edwards-Anderson spin glass are
computed in the presence of an external magnetic field. Our algorithm is
sufficiently powerful for us to treat systems with up to 600 spins. We perform
a statistical analysis of how the ground state changes as the field is
increased, and reach the conclusion that the spin glass phase at zero
temperature does not survive in the presence of any finite field. This is in
agreement with the droplet model or scaling predictions, but in sharp
disagreement with the mean field picture. For comparison, we also investigate a
dilute mean field spin glass model where an Almeida-Thouless line is present.Comment: 4 pages, 4 figures, Revte
Short-Range Ising Spin Glass: Multifractal Properties
The multifractal properties of the Edwards-Anderson order parameter of the
short-range Ising spin glass model on d=3 diamond hierarchical lattices is
studied via an exact recursion procedure. The profiles of the local order
parameter are calculated and analysed within a range of temperatures close to
the critical point with four symmetric distributions of the coupling constants
(Gaussian, Bimodal, Uniform and Exponential). Unlike the pure case, the
multifractal analysis of these profiles reveals that a large spectrum of the
-H\"older exponent is required to describe the singularities of the
measure defined by the normalized local order parameter, at and below the
critical point. Minor changes in these spectra are observed for distinct
initial distributions of coupling constants, suggesting an universal spectra
behavior. For temperatures slightly above T_{c}, a dramatic change in the
function is found, signalizing the transition.Comment: 8 pages, LaTex, PostScript-figures included but also available upon
request. To be published in Physical Review E (01/March 97
Electronic correlation effects and the Coulomb gap at finite temperature
We have investigated the effect of the long-range Coulomb interaction on the
one-particle excitation spectrum of n-type Germanium, using tunneling
spectroscopy on mechanically controllable break junctions. The tunnel
conductance was measured as a function of energy and temperature. At low
temperatures, the spectra reveal a minimum at zero bias voltage due to the
Coulomb gap. In the temperature range above 1 K the Coulomb gap is filled by
thermal excitations. This behavior is reflected in the temperature dependence
of the variable-range hopping resitivity measured on the same samples: Up to a
few degrees Kelvin the Efros-Shkovskii ln law is obeyed,
whereas at higher temperatures deviations from this law are observed,
indicating a cross-over to Mott's ln law. The mechanism of
this cross-over is different from that considered previously in the literature.Comment: 3 pages, 3 figure
Glass-Like Heat Conduction in High-Mobility Crystalline Semiconductors
The thermal conductivity of polycrystalline semiconductors with type-I
clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr
and/or Eu) exhibit lattice thermal conductivities typical of amorphous
materials. Remarkably, this behavior occurs in spite of the well-defined
crystalline structure and relatively high electron mobility (). The dynamics of dopant ions and their interaction with the
polyhedral cages of the structure are a likely source of the strong phonon
scattering.Comment: 4 pages, 3 postscript figures, to be published, Phys. Rev. Let
From Linear to Nonlinear Response in Spin Glasses: Importance of Mean-Field-Theory Predictions
Deviations from spin-glass linear response in a single crystal Cu:Mn 1.5 at %
are studied for a wide range of changes in magnetic field, . Three
quantities, the difference , the effective waiting time,
, and the difference are examined in our
analysis. Three regimes of spin-glass behavior are observed as
increases. Lines in the plane, corresponding to ``weak'' and
``strong'' violations of linear response under a change in magnetic field, are
shown to have the same functional form as the de Almeida-Thouless critical
line. Our results demonstrate the existence of a fundamental link between
static and dynamic properties of spin glasses, predicted by the mean-field
theory of aging phenomena.Comment: 9 pages, 10 figure
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