15,334 research outputs found
An LED pulser for measuring photomultiplier linearity
A light-emitting diode (LED) pulser for testing the low-rate response of a
photomultiplier tube (PMT) to scintillator-like pulses has been designed,
developed, and implemented. This pulser is intended to simulate 80 ns full
width at half maximum photon pulses over the dynamic range of the PMT, in order
to precisely determine PMT linearity. This particular design has the advantage
that, unlike many LED test rigs, it does not require the use of multiple
calibrated LEDs, making it insensitive to LED gain drifts. Instead, a
finite-difference measurement is made using two LEDs which need not be
calibrated with respect to one another. These measurements give a better than
1% mapping of the response function, allowing for the testing and development
of particularly linear PMT bases.Comment: 5 pages, 5 figure
The Role of Cold Flows in the Assembly of Galaxy Disks
We use high resolution cosmological hydrodynamical simulations to demonstrate
that cold flow gas accretion, particularly along filaments, modifies the
standard picture of gas accretion and cooling onto galaxy disks. In the
standard picture, all gas is initially heated to the virial temperature of the
galaxy as it enters the virial radius. Low mass galaxies are instead dominated
by accretion of gas that stays well below the virial temperature, and even when
a hot halo is able to develop in more massive galaxies there exist dense
filaments that penetrate inside of the virial radius and deliver cold gas to
the central galaxy. For galaxies up to ~L*, this cold accretion gas is
responsible for the star formation in the disk at all times to the present.
Even for galaxies at higher masses, cold flows dominate the growth of the disk
at early times. Within this modified picture, galaxies are able to accrete a
large mass of cold gas, with lower initial gas temperatures leading to shorter
cooling times to reach the disk. Although star formation in the disk is
mitigated by supernovae feedback, the short cooling times allow for the growth
of stellar disks at higher redshifts than predicted by the standard model.Comment: accepted to Ap
A generalization of Gabriel's Galois covering functors II: 2-categorical Cohen-Montgomery duality
Given a group , we define suitable 2-categorical structures on the class
of all small categories with -actions and on the class of all small
-graded categories, and prove that 2-categorical extensions of the orbit
category construction and of the smash product construction turn out to be
2-equivalences (2-quasi-inverses to each other), which extends the
Cohen-Montgomery duality.Comment: 31 pages. I moved the Sec of G-GrCat into Sec 3, and added Lem 5.6. I
added more explanations in the proof of Cor 7.6 with (7.5). I added Def 7.7
and Lem 7.8 with the necessary additional assumptions in Props 7.9 and 7.11.
I added Lem 8.8 with a short proof, Rmk 8.9 and the proof of Lem 8.10. The
final publication is available at Springer via
http://dx.doi.org/10.1007/s10485-015-9416-
Real-world comparison of probe vehicle emissions and fuel consumption using diesel and 5 % biodiesel (B5) blend.
An instrumented EURO I Ford Mondeo was used to perform a real-world comparison of vehicle exhaust (carbon dioxide, carbon monoxide, hydrocarbons and oxides of nitrogen) emissions and fuel consumption for diesel and 5% biodiesel in diesel blend (B5) fuels. Data were collected on multiple replicates of three standardised on-road journeys: (1) A simple urban route; (2) A combined urban/inter-urban route; and, (3) An urban route subject to significant traffic management. At the total journey measurement level, data collected here indicate that replacing diesel with a B5 substitute could result in significant increases in both NOx emissions (8-13%) and fuel consumption (7-8%). However, statistical analysis of probe vehicle data demonstrated the limitations of comparisons based on such total journey measurements, i.e., methods analogous to those used in conventional dynamometer/drive cycle fuel comparison studies. Here, methods based on the comparison of speed/acceleration emissions and fuel consumption maps are presented. Significant variations across the speed/acceleration surface indicated that direct emission and fuel consumption impacts were highly dependent on the journey/drive cycle employed. The emission and fuel consumption maps were used both as descriptive tools to characterise impacts and predictive tools to estimate journey-specific emission and fuel consumption effects
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 850âeV 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 600âeV, 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
Resolving the Structure of Cold Dark Matter Halos
We examine the effects of mass resolution and force softening on the density
profiles of cold dark matter halos that form within cosmological N-body
simulations. As we increase the mass and force resolution, we resolve
progenitor halos that collapse at higher redshifts and have very high
densities. At our highest resolution we have nearly 3 million particles within
the virial radius, several orders of magnitude more than previously used and we
can resolve more than one thousand surviving dark matter halos within this
single virialised system. The halo profiles become steeper in the central
regions and we may not have achieved convergence to a unique slope within the
inner 10% of the virialised region. Results from two very high resolution halo
simulations yield steep inner density profiles, . The
abundance and properties of arcs formed within this potential will be different
from calculations based on lower resolution simulations. The kinematics of
disks within such a steep potential may prove problematic for the CDM model
when compared with the observed properties of halos on galactic scales.Comment: Final version, to be published in the ApJLetter
Strategic Outsourcing: Evidence from the British Companies
Outsourcing has become an increasingly popular option for many organisations. But they vary in terms of activities being outsourced, reasons for and benefits from outsourcing, and how the decision was made. This article presents an empirical research on fourteen companies. It found out, a) in most cases it was the âperipheralâ support activity being outsourced with cost reduction as the primary driver; b) outsourcing decision was being made early in the process without active involvement of the in-house provider; and c) there were problems in supplier selection and management. The research identified pre-outsourcing decision process and post-outsourcing management as the two key areas that gave cause for concern, and offered recommendations for improvement
- âŠ