198 research outputs found
Performance and stability tests of bare high purity germanium detectors in liquid argon for the GERDA experiment
GERDA will search for neutrinoless double beta decay of 76Ge by using a novel approach of bare germanium detectors in liquid argon (LAr). Enriched germanium detectors from the previous Heidelberg-Moscow and IGEX experiments have been reprocessed and will be deployed in GERDA Phase-I. At the center of this thesis project is the study of the performance of bare germanium detectors in cryogenic liquids. Identical detector performance as in vacuum cryostats (2.2 keV FWHM at 1.3 MeV) was achieved in cryogenic liquids with a new low-mass detector assembly and contacts. One major result is the discovery of a radiation induced leakage current (LC) increase when operating bare detectors with standard passivation layers in LAr. Charge collection and build-up on the passivation layer were identified as the origin of the LC increase. It was found that diodes without passivation do not exhibit this feature. Three month-long stable operation in LAr at 5 pA LC under periodic gamma irradiation demonstrated the suitability of the modifed detector design. Based on these results, all Phase-I detectors were reprocessed without passivation layer and subsequently successfully characterized in LAr in the GERDA underground Detector Laboratory. The mass loss during the reprocessing was 300 g out of 17.9 kg and the exposure above ground 5 days. This results in a negligible cosmogenic background increase of 5x10-4 cts/(keV kg y) at 76Ge Q for 60Co and 68Ge
Picasso : portrait de la sensibilité des détecteurs à gouttelettes surchauffées à diverses formes de rayonnement
Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal
Heavy Superheated Droplet Detectors as a Probe of Spin-independent WIMP Dark Matter Existence
At present, application of Superheated Droplet Detectors (SDDs) in WIMP dark
matter searches has been limited to the spin-dependent sector, owing to the
general use of fluorinated refrigerants which have high spin sensitivity. Given
their recent demonstration of a significant constraint capability with
relatively small exposures and the relative economy of the technique, we
consider the potential impact of heavy versions of such devices on the
spin-independent sector. Limits obtainable from a -loaded SDD
are estimated on the basis of the radiopurity levels and backgrounds already
achieved by the SIMPLE and PICASSO experiments. With 34 kgd exposure,
equivalent to the current CDMS, such a device may already probe to below
10 pb in the spin-independent cross section.Comment: 9 pages, 4 figures, accepted Phys. Rev.
Aberrant Lipid Metabolism in the Forebrain Niche Suppresses Adult Neural Stem Cell Proliferation in an Animal Model of Alzheimer’s Disease
SummaryLipid metabolism is fundamental for brain development and function, but its roles in normal and pathological neural stem cell (NSC) regulation remain largely unexplored. Here, we uncover a fatty acid-mediated mechanism suppressing endogenous NSC activity in Alzheimer’s disease (AD). We found that postmortem AD brains and triple-transgenic Alzheimer’s disease (3xTg-AD) mice accumulate neutral lipids within ependymal cells, the main support cell of the forebrain NSC niche. Mass spectrometry and microarray analyses identified these lipids as oleic acid-enriched triglycerides that originate from niche-derived rather than peripheral lipid metabolism defects. In wild-type mice, locally increasing oleic acid was sufficient to recapitulate the AD-associated ependymal triglyceride phenotype and inhibit NSC proliferation. Moreover, inhibiting the rate-limiting enzyme of oleic acid synthesis rescued proliferative defects in both adult neurogenic niches of 3xTg-AD mice. These studies support a pathogenic mechanism whereby AD-induced perturbation of niche fatty acid metabolism suppresses the homeostatic and regenerative functions of NSCs
Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76
The GERDA experiment searches for the neutrinoless double beta decay of Ge-76
using high-purity germanium detectors enriched in Ge-76. The analysis of the
signal time structure provides a powerful tool to identify neutrinoless double
beta decay events and to discriminate them from gamma-ray induced backgrounds.
Enhanced pulse shape discrimination capabilities of "Broad Energy Germanium"
detectors with a small read-out electrode have been recently reported. This
paper describes the full simulation of the response of such a detector,
including the Monte Carlo modeling of radiation interaction and subsequent
signal shape calculation. A pulse shape discrimination method based on the
ratio between the maximum current signal amplitude and the event energy applied
to the simulated data shows quantitative agreement with the experimental data
acquired with calibration sources. The simulation has been used to study the
survival probabilities of the decays which occur inside the detector volume and
are difficult to assess experimentally. Such internal decay events are produced
by the cosmogenic radio-isotopes Ge-68 and Co-60 and the neutrinoless double
beta decay of Ge-76. Fixing the experimental acceptance of the double escape
peak of the 2.614 MeV photon to 90%, the estimated survival probabilities at
Qbb = 2.039 MeV are (86+-3)% for Ge-76 neutrinoless double beta decays,
(4.5+-0.3)% for the Ge-68 daughter Ga-68, and (0.9+0.4-0.2)% for Co-60 decays.Comment: 27 pages, 17 figures. v2: fixed typos and references. Submitted to
JINS
Status of the PICASSO Project
The Picasso project is a dark matter search experiment based on the
superheated droplet technique. Preliminary runs performed at the Picasso Lab in
Montreal have showed the suitability of this detection technique to the search
for weakly interacting cold dark matter particles. In July 2002, a new phase of
the project started. A batch of six 1-liter detectors with an active mass of
approximately 40g was installed in a gallery of the SNO observatory in Sudbury,
Ontario, Canada at a depth of 6,800 feet (2,070m). We give a status report on
the new experimental setup, data analysis, and preliminary limits on
spin-dependent neutralino interaction cross section.Comment: 3 pages, 2 figures. To appear in the Proceedings of the TAUP 2003
conference, 5-9 September, 2003, University of Washington, Seattle, US
Dark Matter Spin-Dependent Limits for WIMP Interactions on 19-F by PICASSO
The PICASSO experiment at SNOLAB reports new results for spin-dependent WIMP
interactions on F using the superheated droplet technique. A new
generation of detectors and new features which enable background discrimination
via the rejection of non-particle induced events are described. First results
are presented for a subset of two detectors with target masses of F of
65 g and 69 g respectively and a total exposure of 13.75 0.48 kgd. No
dark matter signal was found and for WIMP masses around 24 GeV/c new limits
have been obtained on the spin-dependent cross section on F of
= 13.9 pb (90% C.L.) which can be converted into cross section
limits on protons and neutrons of = 0.16 pb and = 2.60 pb
respectively (90% C.L). The obtained limits on protons restrict recent
interpretations of the DAMA/LIBRA annual modulations in terms of spin-dependent
interactions.Comment: Revised version, accepted for publication in Phys. Lett. B, 20 pages,
7 figure
Direction-sensitive dark matter search results in a surface laboratory
We developed a three-dimensional gaseous tracking device and performed a
direction-sensitive dark matter search in a surface laboratory. By using 150
Torr carbon-tetrafluoride (CF_4 gas), we obtained a sky map drawn with the
recoil directions of the carbon and fluorine nuclei, and set the first limit on
the spin-dependent WIMP (Weakly Interacting Massive Particles)-proton cross
section by a direction-sensitive method. Thus, we showed that a WIMP-search
experiment with a gaseous tracking device can actually set limits. Furthermore,
we demonstrated that this method will potentially play a certain role in
revealing the nature of dark matter when a low-background large-volume detector
is developed.Comment: 9 figures, accepted for publication in Phys. Lett.
Pulse shape discrimination studies with a Broad-Energy Germanium detector for signal identification and background suppression in the GERDA double beta decay experiment
First studies of event discrimination with a Broad-Energy Germanium (BEGe)
detector are presented. A novel pulse shape method, exploiting the
characteristic electrical field distribution inside BEGe detectors, allows to
identify efficiently single-site events and to reject multi-site events. The
first are typical for neutrinoless double beta decays (0-nu-2-beta) and the
latter for backgrounds from gamma-ray interactions. The obtained survival
probabilities of backgrounds at energies close to Q(76Ge) = 2039 keV are 0.93%
for events from 60Co, 21% from 226Ra and 40% from 228Th. This background
suppression is achieved with 89% acceptance of 228Th double escape events,
which are dominated by single site interactions. Approximately equal acceptance
is expected for 0-nu-2-beta-decay events. Collimated beam and Compton
coincidence measurements demonstrate that the discrimination is largely
independent of the interaction location inside the crystal and validate the
pulse-shape cut in the energy range of Q(76Ge). The application of BEGe
detectors in the GERDA and the Majorana double beta decay experiments is under
study.Comment: 22 pages, 16 figures, submitted to JINST: JINST_018P_080
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