6,852 research outputs found
Miniature spectrally selective dosimeter
A miniature spectrally selective dosimeter capable of measuring selected bandwidths of radiation exposure on small mobile areas is described. This is achieved by the combination of photovoltaic detectors, electrochemical integrators (E-cells) and filters in a small compact case which can be easily attached in close proximity to and substantially parallel to the surface being measured. In one embodiment two photovoltaic detectors, two E-cells, and three filters are packaged in a small case with attaching means consisting of a safety pin. In another embodiment, two detectors, one E-cell, three filters are packaged in a small case with attaching means consisting of a clip to clip over a side piece of an eye glass frame
Inducing strong density modulation with small energy dispersion in particle beams and the harmonic amplifier free electron laser
We present a possible method of inducing a periodic density modulation in a particle beam with little increase in the energy dispersion of the particles. The flow of particles in phase space does not obey Liouville's Theorem. The method relies upon the Kuramoto-like model of collective synchronism found in free electron generators of radiation, such as Cyclotron Resonance Masers and the Free Electron Laser. For the case of an FEL interaction, electrons initially begin to bunch and emit radiation energy with a correlated energy dispersion which is periodic with the FEL ponderomotive potential. The relative phase between potential and particles is then changed by approximately 180 degrees. The particles continue to bunch, however, there is now a correlated re-absorption of energy from the field. We show that, by repeating this relative phase change many times, a significant density modulation of the particles may be achieved with only relatively small energy dispersion. A similar method of repeated relative electron/radiation phase changes is used to demonstrate supression of the fundamental growth in a high gain FEL so that the FEL lases at the harmonic only
Poisson Solver with Floating Conductor Implementation in REMCOM XFDTD Software, Benchmark Calculation Examples
A W:B4C multilayer phase retarder for broadband polarization analysis of soft x-ray radiation \ud
A W:B4C multilayer phase retarder has been designed and characterized which shows a nearly constant phase retardance between 640 and 850 eV photon energies when operated near the Bragg condition. This freestanding transmission multilayer was used successfully to determine, for the first time, the full polarization vector at soft x-ray energies above 600 eV, which was not possible before due to the lack of suitable optical elements. Thus, quantitative polarimetry is now possible at the 2p edges of the magnetic substances Fe, Co, and Ni for the benefit of magnetic circular dichroism spectroscopy employing circularly polarized synchrotron radiatio
Dark ages reionization & galaxy formation simulation XII: Bubbles at dawn
Direct detection of regions of ionized hydrogen (HII) has been suggested as a
promising probe of cosmic reionization. Observing the redshifted 21-cm signal
of hydrogen from the epoch of reionization (EoR) is a key scientific driver
behind new-generation, low-frequency radio interferometers. We investigate the
feasibility of combining low-frequency observations with the Square Kilometre
Array and near infra-red survey data of the Wide-Field Infrared Survey
Telescope to detect cosmic reionization by imaging HII bubbles surrounding
massive galaxies during the cosmic dawn. While individual bubbles will be too
small to be detected, we find that by stacking redshifted 21-cm spectra centred
on known galaxies, it will be possible to directly detect the EoR at , and to place qualitative constraints on the evolution of the spin
temperature of the intergalactic medium (IGM) at . In particular,
given a detection of ionized bubbles using this technique, it is possible to
determine if the IGM surrounding them is typically in absorption or emission.
Determining the globally-averaged neutral fraction of the IGM using this method
will prove more difficult due to degeneracy with the average size of HII
regions.Comment: 14 pages, 11 figures, 2 tables, published in MNRAS. Updated to match
published version. Additional results and comments added from previous
version. All other results and conclusions remain unchange
Dark-ages Reionization and Galaxy Formation Simulation - XIV. Gas accretion, cooling and star formation in dwarf galaxies at high redshift
We study dwarf galaxy formation at high redshift () using a suite of
high- resolution, cosmological hydrodynamic simulations and a semi-analytic
model (SAM). We focus on gas accretion, cooling and star formation in this work
by isolating the relevant process from reionization and supernova feedback,
which will be further discussed in a companion paper. We apply the SAM to halo
merger trees constructed from a collisionless N-body simulation sharing
identical initial conditions to the hydrodynamic suite, and calibrate the free
parameters against the stellar mass function predicted by the hydrodynamic
simulations at z = 5. By making comparisons of the star formation history and
gas components calculated by the two modelling techniques, we find that
semi-analytic prescriptions that are commonly adopted in the literature of
low-redshift galaxy formation do not accurately represent dwarf galaxy
properties in the hydrodynamic simulation at earlier times. We propose 3
modifications to SAMs that will provide more accurate high-redshift
simulations. These include 1) the halo mass and baryon fraction which are
overestimated by collisionless N-body simulations; 2) the star formation
efficiency which follows a different cosmic evolutionary path from the
hydrodynamic simulation; and 3) the cooling rate which is not well defined for
dwarf galaxies at high redshift. Accurate semi-analytic modelling of dwarf
galaxy formation informed by detailed hydrodynamical modelling will facilitate
reliable semi-analytic predictions over the large volumes needed for the study
of reionization.Comment: 20 pages, 10 figures; Updated to match the published version. All
results and conclusions remain unchange
Dark-ages reionization and galaxy formation simulation - IX. Economics of reionizing galaxies
Using a series of high-resolution hydrodynamical simulations we show that
during the rapid growth of high-redshift (z > 5) galaxies, reserves of
molecular gas are consumed over a time-scale of 300Myr, almost independent of
feedback scheme. We find that there exists no such simple relation for the
total gas fractions of these galaxies, with little correlation between gas
fractions and specific star formation rates. The bottleneck or limiting factor
in the growth of early galaxies is in converting infalling gas to cold
star-forming gas. Thus, we find that the majority of high redshift dwarf
galaxies are effectively in recession, with demand (of star formation) never
rising to meet supply (of gas), irrespective of the baryonic feedback physics
modelled. We conclude that the basic assumption of self-regulation in galaxies
- that they can adjust total gas consumption within a Hubble time - does not
apply for the dwarf galaxies thought to be responsible for providing most UV
photons to reionize the high redshift Universe. We demonstrate how this rapid
molecular time-scale improves agreement between semi-analytic model predictions
of the early Universe and observed stellar mass functions.Comment: 17 pages, 27 figures, accepted for publication in MNRAS, minor
updates to align with final published versio
- …