The EDELWEISS-II Collaboration has completed a direct search for WIMP dark matter with an array of ten 400-g cryogenic germanium detectors in operation at the Laboratoire Souterrain de Modane. The ...combined use of thermal phonon sensors and charge collection electrodes with an interleaved geometry enables the efficient rejection of γ-induced radioactivity as well as near-surface interactions. A total effective exposure of 384 kg d has been achieved, mostly coming from fourteen months of continuous operation. Five nuclear recoil candidates are observed above 20 keV, while the estimated background is 3.0 events. The result is interpreted in terms of limits on the cross-section of spin-independent interactions of WIMPs and nucleons. A cross-section of 4.4×10−8 pb is excluded at 90%CL for a WIMP mass of 85 GeV. New constraints are also set on models where the WIMP-nucleon scattering is inelastic.
Vertexing for the future International Linear Collider represents a challenging goal because of the high spatial resolution required with low material budget and high ionizing radiation tolerance. ...CMOS Monolithic Active Pixel Sensors (MAPS) represent a good potential solution for this purpose. Up to now many MAPS sensors have been developed. They are based on various architectures and manufactured in different processes. However, up so far, the sensor diode has not been the subject of extensive modelization and simulation. Published simulation studies of sensor-signal formation have been less numerous than measurements on real sensors. This is a cause for concern because such sensor is physically based on the partially depleted diode, in the vicinity of which the electric field collects the minority carriers generated by an incident MIP (minimum ionizing particle). Although the microscopic mechanisms are well known and modelized, the global physical mechanisms for signal formation are not very rigorously established. This is partly due to the presence of a predominant diffusion component in the charge transport. We present here simulations mainly based on the S-PISCES code, in which physical mechanisms affecting transport are taken into account. Diffusion, influence of residual carrier concentration due to the doping level in the sensitive volume, and more importantly charge trapping due to deep levels in the active (detecting) layer are studied together with geometric aspects. The effect of neutron irradiation is studied to assess the effects of deep traps. A comparison with available experimental data, obtained on processed MAPS before or after neutron irradiation will be introduced. Simulated reconstruction of the Minimum Ionizing Particle (MIP) point of impact in two dimensions is also investigated. For further steps, guidelines for process choices of next Monolithic Active Pixel Sensors are introduced.
In order to develop precision vertex detectors for the future linear collider, fast monolithic active pixel sensors are studied. A standard CMOS 0.25 mum digital process is used to design a test chip ...which includes different pixel types, column-level discriminators, and a fully programmable digital sequencer. In-pixel amplification is implemented together with double sampling. Different charge-to-voltage conversion factors were obtained using amplifiers with different gains or diode sizes. Pixel architectures with dc and ac coupling to charge sensing element were proposed. Hits from conversion of 55 Fe photons were recorded for the dc-coupled and ac-coupled pixel versions. Double sampling is functional and allows almost a complete cancellation of fixed pattern noise
We report on the performance of the MIMOSA8 (HiMAPS1) chip. The chip is a 128times32 pixels array where 24 columns have discriminated binary outputs and eight columns analog test outputs. Offset ...correction techniques are used extensively in this chip to overcome process related mismatches. The array is divided in four blocks of pixels with different conversion factors and is controlled by a serially programmable sequencer. MIMOSA8 is a representative of the CMOS sensors development option considered as a promising candidate for the Vertex Detector of the future International Linear Collider (ILC). The readout technique, implemented on the chip, combines high spatial resolution capabilities with high processing readout speed. Data acquisition, providing control of the chip and signal buffering and linked to a VME system, was made on the eight analog outputs. Analog data, without and with a 55 Fe X-ray source, were acquired and processed using off-line analysis software. From the reconstruction of pixel clusters, built around a central pixel, we deduce that the charge spread is limited to the closest 25 pixels and almost all the available charge is collected. The position of the total charge collection peak (and subsequently the charge-to-voltage conversion factor) stays unaffected when the clock frequency is increased even up to 150 MHz (13.6 mus readout time per frame). The discriminators, placed in the readout chain, have proved to be fully functional. Beam tests have been made with high energy electrons at DESY (Germany) to study detection efficiency. The results prove that MIMOSA8 is the first and fastest successful monolithic active pixel sensor with on-chip signal discrimination for detection of MIPs
A multi-tiered data structure, analysis toolkit and data processing management system has been constructed using ROOT and CouchDB. This system is well suited for experiments that acquire many ...computer files of raw data over the course of months or years, that are distributed to different computing centers and further reduced in size by several steps of data processing. Data handling for experiments searching for rare events extracted from digitized pulse traces typically fit this description. An implementation of this system has been constructed for the EDELWEISS-III experiment and is described here in some detail. This solution may also serve as a prototype system for the proposed EURECA experiment.
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