250 research outputs found
320g Ionization-Heat Cryogenic Detector for Dark Matter Search in the EDELWEISS Experiment
The EDELWEISS experiment used in 2001 a 320g heat-and-ionization cryogenic Ge
detector operated in a low-background environment in the Laboratoire Souterrain
de Modane for direct WIMP detection. This detector presents an increase of more
than 4 times the mass of previous detectors. Calibrations of this detector are
used to determine its energy resolution and fiducial volume, and to optimize
the detector design for the 1kg phase of the EDELWEISS-I experiment. Analysis
of the calibrations and characteristics of a first series of 320g-detectors are
presented.Comment: 4 pages, 3 figure
Credit Risk Mitigation through CDSs: Evidence from the French Credit Derivative Market
This thesis provides four methodologies for estimating risk-neutral default probabilities. First, by using the Hull-White (2000) approach relying on bond prices. Secondly, by bootstrapping hazard rates from CDS spreads through the JP Morgan (1999) model, whilst assuming a piecewise constant hazard rate function. Thirdly, by applying Hull and White’s (2003) framework, while supposing a piecewise constant credit event probability density function. Finally, the hypothesis of a piecewise linear default probability distribution is examined instead. Additionally, this study will apply the Hull-White (2000) spread formula for the valuation process of CDSs and will thus offer an additional application to a theoretical framework that suffers from a lack of tangible data examples. This is operated in the case of the payoff being contingent on default by one reference entity only and an absence of counterparty default risk.
All aforementioned models are tested with real market data on five firms belonging to the French stock market (the CAC40). Theoretical CDS spreads are calculated for various maturities (from 1-10 years), 4th July 2014 being the study’s starting point. Results are satisfactory: theoretical and market CDS spreads share comparable levels; yet, Hull and White’s framework tends to overestimate CDS spreads when compared with quoted ones in the short term. However, an inherent limit to the model remains: the latter is built on rather stringent assumptions, making it unable to adjust to some real-life situations where parameters deviate from these suppositions
Ultra low energy results and their impact to dark matter and low energy neutrino physics
We present ultra low energy results taken with the novel Spherical
Proportional Counter. The energy threshold has been pushed down to about 25 eV
and single electrons are clearly collected and detected. To reach such
performance low energy calibration systems have been successfully developed: -
A pulsed UV lamp extracting photoelectrons from the inner surface of the
detector - Various radioactive sources allowing low energy peaks through
fluorescence processes. The bench mark result is the observation of a well
resolved peak at 270 eV due to carbon fluorescence which is unique performance
for such large-massive detector. It opens a new window in dark matter and low
energy neutrino search and may allow detection of neutrinos from a nuclear
reactor or from supernova via neutrino-nucleus elastic scatteringComment: 14 pages,16 figure
Neutron spectroscopy with the Spherical Proportional Counter
A novel large volume spherical proportional counter, recently developed, is
used for neutron measurements. Gas mixtures of with and
pure are studied for thermal and fast neutron detection, providing a
new way for the neutron spectroscopy. The neutrons are detected via the
and reactions. Here we
provide studies of the optimum gas mixture, the gas pressure and the most
appropriate high voltage supply on the sensor of the detector in order to
achieve the maximum amplification and better resolution. The detector is tested
for thermal and fast neutrons detection with a and a
neutron source. The atmospheric neutrons are successfully
measured from thermal up to several MeV, well separated from the cosmic ray
background. A comparison of the spherical proportional counter with the current
available neutron counters is also given.Comment: 7 pages, 10 figure
Textbook Broke: Textbook Affordability as a Social Justice Issue
In light of rising textbook prices, open education resources (OER) have been shown to decrease non-tuition costs, while simultaneously increasing academic access, student performance, and time-to-graduation rates. Yet very little research to date has explored OER’s specific impact on those who are presumed to benefit most from this potential: historically underserved students. This reality has left a significant gap of understanding in the current body of literature, resulting in calls for more empirically-based examinations of OER through a social justice lens. For each of these reasons, this study explored the impact of OER and textbook pricing among racial/ethnic minority students, low-income students, and first-generation college students at a four-year Hispanic Serving Institution (HSI) in Southern California. Drawing upon more than 700 undergraduate surveys, our univariate, bivariate and multivariate results revealed textbook costs to be a substantial barrier for the vast majority of students. However, those barriers were even more significant among historically underserved college students; thus, confirming textbook affordability as a redistributive justice issue, and positing OER as a potential avenue for realizing a more socially just college experience
Background discrimination capabilities of a heat and ionization germanium cryogenic detector
The discrimination capabilities of a 70 g heat and ionization Ge bolometer
are studied. This first prototype has been used by the EDELWEISS Dark Matter
experiment, installed in the Laboratoire Souterrain de Modane, for direct
detection of WIMPs. Gamma and neutron calibrations demonstrate that this type
of detector is able to reject more than 99.6% of the background while retaining
95% of the signal, provided that the background events distribution is not
biased towards the surface of the Ge crystal. However, the 1.17 kg.day of data
taken in a relatively important radioactive environment show an extra
population slightly overlapping the signal. This background is likely due to
interactions of low energy photons or electrons near the surface of the
crystal, and is somewhat reduced by applying a higher charge-collecting inverse
bias voltage (-6 V instead of -2 V) to the Ge diode. Despite this
contamination, more than 98% of the background can be rejected while retaining
50% of the signal. This yields a conservative upper limit of 0.7
event.day^{-1}.kg^{-1}.keV^{-1}_{recoil} at 90% confidence level in the 15-45
keV recoil energy interval; the present sensitivity appears to be limited by
the fast ambient neutrons. Upgrades in progress on the installation are
summarized.Comment: Submitted to Astroparticle Physics, 14 page
First Results of the EDELWEISS WIMP Search using a 320 g Heat-and-Ionization Ge Detector
The EDELWEISS collaboration has performed a direct search for WIMP dark
matter using a 320 g heat-and-ionization cryogenic Ge detector operated in a
low-background environment in the Laboratoire Souterrain de Modane. No nuclear
recoils are observed in the fiducial volume in the 30-200 keV energy range
during an effective exposure of 4.53 kg.days. Limits for the cross-section for
the spin-independent interaction of WIMPs and nucleons are set in the framework
of the Minimal Supersymmetric Standard Model (MSSM). The central value of the
signal reported by the experiment DAMA is excluded at 90% CL.Comment: 14 pages, Latex, 4 figures. Submitted to Phys. Lett.
Measurement of the response of heat-and-ionization germanium detectors to nuclear recoils
The heat quenching factor Q' (the ratio of the heat signals produced by
nuclear and electron recoils of equal energy) of the heat-and-ionization
germanium bolometers used by the EDELWEISS collaboration has been measured. It
is explained how this factor affects the energy scale and the effective
quenching factor observed in calibrations with neutron sources. This effective
quenching effect is found to be equal to Q/Q', where Q is the quenching factor
of the ionization yield. To measure Q', a precise EDELWEISS measurement of Q/Q'
is combined with values of Q obtained from a review of all available
measurements of this quantity in tagged neutron beam experiments. The
systematic uncertainties associated with this method to evaluate Q' are
discussed in detail. For recoil energies between 20 and 100 keV, the resulting
heat quenching factor is Q' = 0.91+-0.03+-0.04, where the two errors are the
contributions from the Q and Q/Q' measurements, respectively. The present
compilation of Q values and evaluation of Q' represent one of the most precise
determinations of the absolute energy scale for any detector used in direct
searches for dark matter.Comment: 28 pages, 7 figures. Submitted to Phys. Rev.
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