100 research outputs found
Study of Neutron-Induced Ionization in Helium and Argon Chamber Gases
Ion chambers used to monitor the secondary hadron and tertiary muon beam in
the NuMI neutrino beamline will be exposed to background particles, including
low energy neutrons produced in the beam dump. To understand these backgrounds,
we have studied Helium- and Argon-filled ionization chambers exposed to intense
neutron fluxes from PuBe neutron sources ( MeV). The sources emit
about 10 neutrons per second. The number of ion pairs in the chamber gas
volume per incident neutron is derived. While limited in precision because of a
large gamma ray background from the PuBe sources, our results are consistent
with the expectation that the neutrons interact purely elastically in the
chamber gas.Comment: accepted for publication in NIM
Structure and properties of a novel fulleride Sm6C60
A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory.Comment: 9 pages, 3 figures, submitted to Phys. Rev. B (March 12, 1999
Evidence for hadronic deconfinement in -p collisions at 1.8 TeV
We have measured deconfined hadronic volumes, fm,
produced by a one dimensional (1D) expansion. These volumes are directly
proportional to the charged particle pseudorapidity densities . The hadronization temperature is (syst)
MeV. Using Bjorken's 1D model,the hadronization energy density is (stat) GeV/fm corresponding to an excitation of (stat) quark-gluon degrees of freedom.Comment: 15 pages, 3 figures, 2 table
Electronic structure, exchange interactions and Curie temperature in diluted III-V magnetic semiconductors: (GaCr)As, (GaMn)As, (GaFe)As
We complete our earlier (Phys. Rev. B, {\bf 66}, 134435 (2002)) study of the
electronic structure, exchange interactions and Curie temperature in (GaMn)As
and extend the study to two other diluted magnetic semiconductors (GaCr)As and
(GaFe)As. Four concentrations of the 3d impurities are studied: 25%, 12.5%,
6.25%, 3.125%. (GaCr)As and (GaMn)As are found to possess a number of similar
features. Both are semi-metallic and ferromagnetic, with similar properties of
the interatomic exchange interactions and the same scale of the Curie
temperature. In both systems the presence of the charge carriers is crucial for
establishing the ferromagnetic order. An important difference between two
systems is in the character of the dependence on the variation of the number of
carriers. The ferromagnetism in (GaMn)As is found to be very sensitive to the
presence of the donor defects, like As antisites. On the other hand,
the Curie temperature of (GaCr)As depends rather weakly on the presence of this
type of defects but decreases strongly with decreasing number of electrons. We
find the exchange interactions between 3d atoms that make a major contribution
into the ferromagnetism of (GaCr)As and (GaMn)As and propose an exchange path
responsible for these interactions. The properties of (GaFe)As are found to
differ crucially from the properties of (GaCr)As and (GaMn)As. (GaFe)As does
not show a trend to ferromagnetism and is not half-metallic that makes this
system unsuitable for the use in spintronic semiconductor devices
Secondary Beam Monitors for the NuMI Facility at FNAL
The Neutrinos at the Main Injector (NuMI) facility is a conventional neutrino
beam which produces muon neutrinos by focusing a beam of mesons into a long
evacuated decay volume. We have built four arrays of ionization chambers to
monitor the position and intensity of the hadron and muon beams associated with
neutrino production at locations downstream of the decay volume. This article
describes the chambers' construction, calibration, and commissioning in the
beam.Comment: Accepted for publication in Nucl. Instr. Meth.
New directions in island biogeography
Aim: Much of our current understanding of ecological and evolutionary processes comes from island research. With the increasing availability of data on distributions and phylogenetic relationships and new analytical approaches to understanding the processes that shape species distributions and interactions, a re-evaluation of this ever-interesting topic is timely.
Location: Islands globally.
Methods: We start by arguing that the reasons why island research has achieved so much in the past also apply to the future. We then critically assess the current state of island biogeography, focusing on recent changes in emphasis, including research featured in this special issue of Global Ecology and Biogeography. Finally, we suggest promising themes for the future. We cover both ecological and evolutionary topics, although the greater emphasis on island ecology reflects our own backgrounds and interests.
Results: Much ecological theory has been directly or indirectly influenced by research on island biotas. Currently, island biogeography is renascent, with research focusing on, among other things, patterns and processes underlying species interaction networks, species coexistence and the assembly of island communities through ecological and evolutionary time. Continuing island research should provide additional insight into biological invasions and other impacts of human activities, functional diversity and ecosystem functioning, extinction and diversification, species pools and more. Deeper understanding of the similarities and differences between island and mainland systems will aid transferability of island theory to continental regions.
Main conclusions: As research in biogeography and related fields expands in new directions, islands continue to provide opportunities for developing insights, both as natural laboratories for ecology and evolution and because of the exceptions islands often present to the usual ârulesâ of ecology. New data collection initiatives are needed on islands world-wide and should be directed towards filling gaps in our knowledge of within-island distributions of species, as well as the functional traits and phylogenetic relationships of island species
The progeroid phenotype of Ku80 deficiency Is dominant over DNA-PK CS deficiency
Ku80 and DNA-PKCS are both involved in the repair of double strand DNA breaks via the nonhomologous end joining (NHEJ) pathway. While ku80-/- mice exhibit a severely reduced lifespan and size, this phenotype is less pronounced in dna-pkcs -/- mice. However, these observations are based on independent studies with varying genetic backgrounds. Here, we generated ku80-/-, dna-pkcs -/- and double knock out mice in a C57Bl6/J*FVB F1 hybrid background and compared their lifespan, end of life pathology and mutation frequency in liver a
Search for the Rare Decay KL --> pi0 ee
The KTeV/E799 experiment at Fermilab has searched for the rare kaon decay
KL--> pi0ee. This mode is expected to have a significant CP violating
component. The measurement of its branching ratio could support the Standard
Model or could indicate the existence of new physics. This letter reports new
results from the 1999-2000 data set. One event is observed with an expected
background at 0.99 +/- 0.35 events. We set a limit on the branching ratio of
3.5 x 10^(-10) at the 90% confidence level. Combining the results with the
dataset taken in 1997 yields the final KTeV result: BR(KL --> pi0 ee) < 2.8 x
10^(-10) at 90% CL.Comment: 4 pages, three figure
Superconductivity in Fullerides
Experimental studies of superconductivity properties of fullerides are
briefly reviewed. Theoretical calculations of the electron-phonon coupling, in
particular for the intramolecular phonons, are discussed extensively. The
calculations are compared with coupling constants deduced from a number of
different experimental techniques. It is discussed why the A_3 C_60 are not
Mott-Hubbard insulators, in spite of the large Coulomb interaction. Estimates
of the Coulomb pseudopotential , describing the effect of the Coulomb
repulsion on the superconductivity, as well as possible electronic mechanisms
for the superconductivity are reviewed. The calculation of various properties
within the Migdal-Eliashberg theory and attempts to go beyond this theory are
described.Comment: 33 pages, latex2e, revtex using rmp style, 15 figures, submitted to
Review of Modern Physics, more information at
http://radix2.mpi-stuttgart.mpg.de/fullerene/fullerene.htm
Topological String Amplitudes, Complete Intersection Calabi-Yau Spaces and Threshold Corrections
We present the most complete list of mirror pairs of Calabi-Yau complete
intersections in toric ambient varieties and develop the methods to solve the
topological string and to calculate higher genus amplitudes on these compact
Calabi-Yau spaces. These symplectic invariants are used to remove redundancies
in examples. The construction of the B-model propagators leads to compatibility
conditions, which constrain multi-parameter mirror maps. For K3 fibered
Calabi-Yau spaces without reducible fibers we find closed formulas for all
genus contributions in the fiber direction from the geometry of the fibration.
If the heterotic dual to this geometry is known, the higher genus invariants
can be identified with the degeneracies of BPS states contributing to
gravitational threshold corrections and all genus checks on string duality in
the perturbative regime are accomplished. We find, however, that the BPS
degeneracies do not uniquely fix the non-perturbative completion of the
heterotic string. For these geometries we can write the topological partition
function in terms of the Donaldson-Thomas invariants and we perform a
non-trivial check of S-duality in topological strings. We further investigate
transitions via collapsing D5 del Pezzo surfaces and the occurrence of free Z2
quotients that lead to a new class of heterotic duals.Comment: 117 pages, 1 Postscript figur
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