1,886 research outputs found
Radiation hardness of CMS pixel barrel modules
Pixel detectors are used in the innermost part of the multi purpose
experiments at LHC and are therefore exposed to the highest fluences of
ionising radiation, which in this part of the detectors consists mainly of
charged pions. The radiation hardness of all detector components has thoroughly
been tested up to the fluences expected at the LHC. In case of an LHC upgrade,
the fluence will be much higher and it is not yet clear how long the present
pixel modules will stay operative in such a harsh environment. The aim of this
study was to establish such a limit as a benchmark for other possible detector
concepts considered for the upgrade.
As the sensors and the readout chip are the parts most sensitive to radiation
damage, samples consisting of a small pixel sensor bump-bonded to a CMS-readout
chip (PSI46V2.1) have been irradiated with positive 200 MeV pions at PSI up to
6E14 Neq and with 21 GeV protons at CERN up to 5E15 Neq.
After irradiation the response of the system to beta particles from a Sr-90
source was measured to characterise the charge collection efficiency of the
sensor. Radiation induced changes in the readout chip were also measured. The
results show that the present pixel modules can be expected to be still
operational after a fluence of 2.8E15 Neq. Samples irradiated up to 5E15 Neq
still see the beta particles. However, further tests are needed to confirm
whether a stable operation with high particle detection efficiency is possible
after such a high fluence.Comment: Contribution to the 11th European Symposium on Semiconductor
Detectors June 7-11, 2009 Wildbad Kreuth, German
Human colonic intraepithelial lymphocytes regulate the cytokines produced by lamina propria mononuclear cells
Using an in vitro autologous human system, the immunomodulatory function of colonic intraepithelial lymphocytes (IEL) on cytokine production by lamina propria mononuclear cells (LPMNC) has been investigated. In contrast to LPMNC, colonic IEL produced only low amounts of IL-10, interferon-γ and interleukin-2. However, co-culture experiments (IEL + LPMNC) have shown that IEL can enhance the PHA-induced synthesis of IL-2 and interferon-γ, but not IL-10 by LPMNC. Using a transwell filter culture system apparatus, this effect was shown not to require a cell-to-cell interaction. Thus, IEL in vitro may modulate the cytokine synthesis of LPMNC, through the production of soluble factors. This may prove highly relevant in the in vivo immune activation of the gastrointestinal mucosa
Two mini-band model for self-sustained oscillations of the current through resonant tunneling semiconductor superlattices
A two miniband model for electron transport in semiconductor superlattices
that includes scattering and interminiband tunnelling is proposed. The model is
formulated in terms of Wigner functions in a basis spanned by Pauli matrices,
includes electron-electron scattering in the Hartree approximation and modified
Bhatnagar-Gross-Krook collision tems. For strong applied fields, balance
equations for the electric field and the miniband populations are derived using
a Chapman-Enskog perturbation technique. These equations are then solved
numerically for a dc voltage biased superlattice. Results include
self-sustained current oscillations due to repeated nucleation of electric
field pulses at the injecting contact region and their motion towards the
collector. Numerical reconstruction of the Wigner functions shows that the
miniband with higher energy is empty during most of the oscillation period: it
becomes populated only when the local electric field (corresponding to the
passing pulse) is sufficiently large to trigger resonant tunneling.Comment: 26 pages, 3 figures, to appear in Phys. Rev.
ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass functions of the pre-main sequence embedded cluster
We present the results of the first extensive mid-infrared (IR) imaging
survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera
on board the ISO satellite. The main molecular cloud L1688, as well as L1689N
and L1689S, have been completely surveyed for point sources at 6.7 and 14.3
micron. A total of 425 sources are detected including 16 Class I, 123 Class II,
and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR
sources coincide with near-IR sources, but a large proportion of them are
recognized for the first time as YSOs. Our dual-wavelength survey allows us to
identify essentially all the YSOs with IR excess in the embedded cluster down
to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II
YSOs and represents the most complete census to date of newly formed stars in
the rho Ophiuchi central region. The stellar luminosity function of the
complete sample of Class II YSOs is derived with a good accuracy down to L=
0.03 Lsun. A modeling of this lumino- sity function, using available pre-main
sequence tracks and plausible star for- mation histories, allows us to derive
the mass distribution of the Class II YSOs which arguably reflects the IMF of
the embedded cluster. We estimate that the IMF in rho Ophiuchi is well
described by a two-component power law with a low- mass index of -0.35+/-0.25,
a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35),
and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no
evidence for a low-mass cutoff down to at least 0.06 Msun.Comment: A&A Document Class -- version 5.01, 27 pages, 10 figures v2: typos
added including few changes in source numberin
ISOCAM observations of the L1551 star formation region
The results of a deep mid-IR ISOCAM survey of the L1551 dark molecular cloud
are presented. The aim of this survey is a search for new YSO (Young Stellar
Object) candidates, using two broad-band filters centred at 6.7 and 14.3
micron. Although two regions close to the centre of L1551 had to be avoided due
to saturation problems, 96 sources were detected in total (76 sources at 6.7
micron and 44 sources at 14.3 micron). Using the 24 sources detected in both
filters, 14 were found to have intrinsic mid-IR excess at 14.3 micron and were
therefore classified as YSO candidates. Using additional observations in B, V,
I, J, H and K obtained from the ground, most candidates detected at these
wavelengths were confirmed to have mid-IR excess at 6.7 micron as well, and
three additional YSO candidates were found. Prior to this survey only three
YSOs were known in the observed region (avoiding L1551 IRS5/NE and HL/XZ Tau).
This survey reveals 15 new YSO candidates, although several of these are
uncertain due to their extended nature either in the mid-IR or in the
optical/near-IR observations. Two of the sources with mid-IR excess are
previously known YSOs, one is a brown dwarf MHO 5 and the other is the well
known T Tauri star HH30, consisting of an outflow and an optically thick disk
seen edge on.Comment: 14 Pages, 8 Figure
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