Many experiments that aim at the direct detection of Dark Matter are able to distinguish a dominant background from the expected feeble signals, based on some measured discrimination parameter. We ...develop a statistical model for such experiments using the Profile Likelihood ratio as a test statistic in a frequentist approach. We take data from calibrations as control measurements for signal and background, and the method allows the inclusion of data from Monte Carlo simulations. Systematic detector uncertainties, such as uncertainties in the energy scale, as well as astrophysical uncertainties, are included in the model. The statistical model can be used to either set an exclusion limit or to make a discovery claim, and the results are derived with a proper treatment of statistical and systematic uncertainties. We apply the model to the first data release of the XENON100 experiment, which allows to extract additional information from the data, and place stronger limits on the spin-independent elastic WIMP-nucleon scattering cross-section. In particular, we derive a single limit, including all relevant systematic uncertainties, with a minimum of 2.4x10^-44 cm^2 for WIMPs with a mass of 50 GeV/c^2.
The XENON100 experiment, located at the Laboratori Nazionali del Gran Sasso (LNGS), aims to directly detect dark matter in the form of Weakly Interacting Massive Particles (WIMPs) via their elastic ...scattering off xenon nuclei. We present a comprehensive study of the predicted electronic recoil background coming from radioactive decays inside the detector and shield materials, and intrinsic contamination. Based on GEANT4 Monte Carlo simulations using a detailed geometry together with the measured radioactivity of all detector components, we predict an electronic recoil background in the WIMP-search energy range (0-100 keV) in the 30 kg fiducial mass of less than 10e-2 events/(kg-day-keV), consistent with the experiment's design goal. The predicted background spectrum is in very good agreement with the data taken during the commissioning of the detector, in Fall 2009.
The charge read out of a LXe detector via Proportional Scintillation in the liquid phase was first realized by the Waseda group 40 years ago, but at that time the technical challenges were ...overwhelming. Although the tests were successful, this method was finally discarded and eventually nearly forgotten. For present day large LXe Dark Matter detectors, this approach was not even considered. Instead the Dual Phase technology was selected despite many limitations and challenges. In two independent studies the groups from Columbia University and Shanghai Jiao Tong University reevaluated Proportional Scintillation in the liquid phase. Both established the merits for very large LXe detectors, but the Columbia group also encountered apparent limitations, namely the shadowing of the light by the anode wires and a dependence of the pulse shape on the drift path of the electrons in the anode region. The discrepancies between the two studies, however, are not intrinsic to the technique, but a direct consequence of the chosen geometry. Taking the geometrical differences into account the results match without ambiguity. They also agree with the original results from the Waseda group. With the technical problems solved, the path is now open to use this method in future large LXe TPCs.
We have measured the energy dependence of the liquid xenon (LXe) scintillation yield of electrons with energies between 2.1 and 120.2 keV, using the Compton coincidence technique. A LXe scintillation ...detector with a very high light detection efficiency was irradiated with super(137)Cs gamma rays, and the energy of the Compton-scattered gamma rays was measured with a high-purity germanium detector placed at different scattering angles. The excellent energy resolution of the high-purity germanium detector allows the selection of events with Compton electrons of known energy in the LXe detector. We find that the scintillation yield initially increases as the electron energy decreases from 120 to about 60 keV but then decreases by about 30% from 60 to 2 keV. The scintillation yield was also measured with conversion electrons from the 32.1 and 9.4 keV transitions of the super(83)mKr isomer, used as an internal calibration source. We find that the scintillation yield of the 32.1 keV transition is compatible with that obtained from the Compton coincidence measurement. On the other hand, the yield for the 9.4 keV transition is much higher than that measured for a Compton electron of the same energy. We interpret the enhancement in the scintillation yield as due to the enhanced recombination rate in the presence of Xe ions left from the 32.1 keV transition, which precedes the 9.4 keV one by 220 ns, on average.
The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is the first balloon-borne instrument developed to validate the concept of a monolithic detector with 3D imaging capability as a Compton ...telescope for MeV astrophysics. The geometrical area is about 350 cm
2, an order of magnitude smaller than that of COMPTEL and the thickness of sensitive LXe is 7 cm, of equivalent stopping power as COMPTEL D2 detector. The spectroscopic and imaging response of LXeGRIT has been fully characterized in calibration experiments on the ground and during balloon flight experiments. During its most successful flight campaign of 27 h from Ft Sumner, in Fall 2000, the LXeTPC was operated without any external shield. The γ-ray background, measured at float altitude in the 0.5–10 MeV energy band, is well explained by the known atmospheric γ-ray flux. Results on the LXeGRIT in-flight performance, effective area, minimum flux sensitivity and background level are presented in this paper.
A liquid xenon time projection chamber (LXeTPC) has been developed to image cosmic γ-rays in the energy band 0.2–20 MeV. Its performance as Gamma Ray Imaging Telescope (LXeGRIT instrument) has been ...tested during a high altitude balloon flight (Spring ’99, New Mexico). The detector, with 400 cm
2 area and 7 cm drift gap, is filled with 7 l high purity LXe. Both ionization and scintillation light signals are detected to measure the energy deposits and the three spatial coordinates of individual γ-ray interactions within the sensitive volume. During the pre-flight calibration experiments the LXeGRIT instrument was extensively tested with γ-ray sources in the laboratory: a 10% FWHM energy resolution at 1 MeV was determined, scaling with
1/
E
. The detector shows a linear response in the energy range 511 keV–4.4 MeV.
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