38 research outputs found
Design, Commissioning and Performance of the PIBETA Detector at PSI
We describe the design, construction and performance of the PIBETA detector
built for the precise measurement of the branching ratio of pion beta decay,
pi+ -> pi0 e+ nu, at the Paul Scherrer Institute. The central part of the
detector is a 240-module spherical pure CsI calorimeter covering 3*pi sr solid
angle. The calorimeter is supplemented with an active collimator/beam degrader
system, an active segmented plastic target, a pair of low-mass cylindrical wire
chambers and a 20-element cylindrical plastic scintillator hodoscope. The whole
detector system is housed inside a temperature-controlled lead brick enclosure
which in turn is lined with cosmic muon plastic veto counters. Commissioning
and calibration data were taken during two three-month beam periods in
1999/2000 with pi+ stopping rates between 1.3*E3 pi+/s and 1.3*E6 pi+/s. We
examine the timing, energy and angular detector resolution for photons,
positrons and protons in the energy range of 5-150 MeV, as well as the response
of the detector to cosmic muons. We illustrate the detector signatures for the
assorted rare pion and muon decays and their associated backgrounds.Comment: 117 pages, 48 Postscript figures, 5 tables, Elsevier LaTeX, submitted
to Nucl. Instrum. Meth.
New perspectives on the ecology and evolution of siboglinid tubeworms
Siboglinids are tube-dweling annelids that are important members of deep-sea chemosynthetic communities, which include hydrothermal vents, cold seeps, whale falls and reduced sediments. As adults, they lack a functional digestive system and rely on microbial endosymbionts for their energetic needs. Recent years have seen a revolution in our understanding of these fascinating worms. Molecular systematic methods now place these animals, formerly known as the phyla Pogonophora and Vestimentifera, within the polychaete clade Siboglinidae. Furthermore, an entirely new radiation of siboglinids, Osedax, has just recently been discovered living on whale bones. The unique and intricate evolutionary association of siboglinids with both geology, in the formation of spreading centres and seeps, and biology with the evolution of large whales, offers opportunities for studies of vicariant evolution and the calibration of molecular clocks. Moreover, new advances in our knowledge of siboglinid anatomy coupled with molecular characterization of microbial symbiont communities are revolutionizing our knowledge of host-symbiont relationships in the Metazoa. Despite these advances, considerable debate persists concerning the evolutionary history of siboglinids. Here we review the morphological, molecular, ecological and fossil data in order to address when and how siboglinids evolved. We discuss the role of ecological conditions in the evolution of siboglinids and present possible scenarios of the evolutionary origin of the symbiotic relationships between siboglinids and their endosymbiotic bacteria
Effector T-cell trafficking between the leptomeninges and the cerebrospinal fluid
In multiple sclerosis, brain-reactive T cells invade the central
nervous system (CNS) and induce a self-destructive inflammatory
process. T-cell infiltrates are not only found within the parenchyma
and the meninges, but also in the cerebrospinal fluid (CSF) that
bathes the entire CNS tissue 1,2 . How the T cells reach the CSF,
their functionality, and whether they traffic between the CSF
and other CNS compartments remains hypothetical 3–6 . Here we
show that effector T cells enter the CSF from the leptomeninges
during Lewis rat experimental autoimmune encephalomyelitis
(EAE), a model of multiple sclerosis. While moving through the
three-dimensional leptomeningeal network of collagen fibres in a
random Brownian walk, T cells were flushed from the surface by
the flow of the CSF. The detached cells displayed significantly lower
activation levels compared to T cells from the leptomeninges and
CNS parenchyma. However, they did not represent a specialized
non-pathogenic cellular sub-fraction, as their gene expression
profile strongly resembled that of tissue-derived T cells and they
fully retained their encephalitogenic potential. T-cell detachment
from the leptomeninges was counteracted by integrins VLA-4 and
LFA-1 binding to their respective ligands produced by resident
macrophages. Chemokine signalling via CCR5/CXCR3 and
antigenic stimulation of T cells in contact with the leptomeningeal
macrophages enforced their adhesiveness. T cells floating in the
CSF were able to reattach to the leptomeninges through steps
reminiscent of vascular adhesion in CNS blood vessels, and invade
the parenchyma. The molecular/cellular conditions for T-cell
reattachment were the same as the requirements for detachment
from the leptomeningeal milieu. Our data indicate that the
leptomeninges represent a checkpoint at which activated T cells
are licensed to enter the CNS parenchyma and non-activated T cells
are preferentially released into the CSF, from where they can reach
areas of antigen availability and tissue damage