Y. Gornushkin

Development of monolithic active pixel sensors for charged particle tracking

Monolithic active pixel sensors introduce a detection technique, where the active detecting element is a thin, moderately doped, and undepleted silicon layer and the readout electronics is implanted on top of it. The built-in potential, resulting from differences in doping, screens both parts, as well as it confines the charge diffusing to the readout electrodes. The R&D was triggered by the increasing need of high performance flavour identification capabilities that should be provided by future vertex detectors. The viability of the technology and its high tracking performances were demonstrated with small-scale prototypes, made of small arrays of a few thousands of pixels and more recently with a first prototype of a serviceable size of one million pixels. This paper summarizes results from tests performed with relativistic charged particles on prototypes essentially fabricated with a classical 3-transistor pixel configuration. Within the last year, two novel ideas optimising the pixel design for a vertex detector have been developed. They are presented with test results assessing their suitability.

The OPERA experiment Target Tracker

The main task of the Target Tracker detector of the long baseline neutrino oscillation OPERA experiment is to locate in which of the target elementary constituents, the lead/emulsion bricks, the neutrino interactions have occurred and also to give calorimetric information about each event. The technology used consists in walls of two planes of plastic scintillator strips, one per transverse direction. Wavelength shifting fibres collect the light signal emitted by the scintillator strips and guide it to both ends where it is read by multi-anode photomultiplier tubes. All the elements used in the construction of this detector and its main characteristics are described.

Test results of monolithic active pixel sensors for charged particle tracking

Abstract A new generation of semi-conducting pixel sensors for detecting minimum ionising particles (m.i.p.) was designed and first prototypes of Monolithic Active Pixel Sensors (MAPS), called MIMOSA, 2 were fabricated in a standard CMOS technology. The performances of the first prototypes were evaluated with high energy π− beams and with an X-ray source in strong magnetic fields. The beam test results demonstrate that the sensors detect m.i.p.s with very high efficiency and signal-to-noise ratio and provide excellent spatial resolution. The influence of strong magnetic fields is observed to be modest.

Radiation hardness study of an APS CMOS particle tracker

The adequacy of Monolithic Active Pixel Sensors for charged particle tracking has been assessed, based on prototypes fabricated in 0.6, 0.35 and 0.25 /spl mu/m CMOS processes. First radiation hardness studies of these prototypes are presented. Measurements were performed using a 30 MeV/c proton beam, a fast neutron beam from a nuclear reactor and a 10 keV X-ray generator as irradiation sources. The losses in the collected charge were measured after 5/spl times/10/sup 11/ protons/cm/sup 2/ as well as after a total fluence of 10/sup 12/ neutrons/cm/sup 2/. Moderate doses (hundreds of kRads) of X-ray photons induces an important increase of a leakage current, showing also limited effect on the collected charge. Test results are reviewed and new charge collecting radiation tolerant structures are proposed.

NEWS: Nuclear Emulsions for WIMP Search

Nowadays there is compelling evidence for the existence of dark matter in the Universe. A general consensus has been expressed on the need for a directional sensitive detector to confirm, with a complementary approach, the candidates found in conventional searches and to finally extend their sensitivity beyond the limit of neutrino-induced background. We propose here the use of a detector based on nuclear emulsions to measure the direction of WIMP-induced nuclear recoils. The production of nuclear emulsion films with nanometric grains is established. Several measurement campaigns have demonstrated the capability of detecting sub-micrometric tracks left by low energy ions in such emulsion films. Innovative analysis technologies with fully automated optical microscopes have made it possible to achieve the track reconstruction for path lengths down to one hundred nanometers and there are good prospects to further exceed this limit. The detector concept we propose foresees the use of a bulk of nuclear emulsion films surrounded by a shield from environmental radioactivity, to be placed on an equatorial telescope in order to cancel out the effect of the Earth rotation, thus keeping the detector at a fixed orientation toward the expected direction of galactic WIMPs. We report the schedule and cost estimate for a one-kilogram mass pilot experiment, aiming at delivering the first results on the time scale of six years.

Monolithic active pixel sensors for high resolution vertex detectors

A novel Monolithic Active Pixel Sensor (MAPS) for charged particle tracking is presented. The partially depleted thin epitaxial layer of a low-resistivity silicon wafer is used as a sensitive detector volume from which the charge liberated by ionising particles is collected by diffusion. The sensor is a photodiode within a special structure allowing the high detection efficiency required for tracking applications. Two prototypes have been designed and fabricated using standard 0.6 and 0.35micron CMOS processes. Results of the first prototype are presented, which is made of four arrays, each containing 64×64 pixels with a readout pitch of 20 microns in both directions. Extensive tests made with a soft X-ray source (Fe) and beams of minimum ionising particles (pions of 15 and 120 GeV/c) at CERN have demonstrated the predicted performance. The individual pixel noise of around 12 ENC leads to an extremely favourable signal to noise ratio for minimum ionising particles for which over 1000 electrons can be collected at the peak of the Landau distribution. The new circuit in the 0.35-micron process on a thinner epitaxial layer seems to have similar good performance. These new devices are extremely promising for future high precision vertex detectors as they provide solutions with a very low material budget and with high resolution at relatively low cost due to their fabrication in a standard submicron CMOS technology.

Locating the neutrino interaction vertex with the help of electronic detectors in the OPERA experiment

The OPERA experiment is designed for the direct observation of the appearance of ντ from νμ → ντ oscillation in a νμ beam. A description of the procedure of neutrino interaction vertex localization (Brick Finding) by electronic detectors of a hybrid OPERA setup is presented. The procedure includes muon track and hadronic shower axis reconstruction and a determination of the target bricks with the highest probability to contain the vertex.

Search for the νμ→ντ Oscillation with the OPERA Hybrid Detector

This work reviews the status of the international OPERA experiment aimed at searching for the νμ → ντ oscillation. Development of the emulsion techniques in the preceding DONUT and CHORUS experiments is described. A brief overview of the NOMAD experiment is given, in which properties of ντ interactions were thoroughly investigated. Characteristics of the CERN-Gran Sasso (CNGS) neutrino beam are given, the structure of the OPERA hybrid detector and the functions of its subsystems are considered, and the automatic emulsion scanning technique is briefly reviewed. Uncertainties in reconstruction of event parameters using the electronic and emulsion detectors are discussed and the procedure adopted in the experiment for identification of ντ interactions is described. The main sources of background in the search for νμ → ντ oscillation are considered. Topologies of the first two ντ candidates observed in the OPERA emulsion detector are presented. Possibilities of increasing the ντ interaction selection efficiency using multidimensional criteria are briefly discussed.

Present status of the opera experiment for the direct observation of neutrino oscillations in the νμ → ντ channel

The OPERA experiment (Oscillation Project with an Emulsion-Tracking Apparatus) for the direct observation of neutrino oscillations in the νμ → ντ channel has been underway at the underground laboratory in Gran Sasso since 2007. In the course of data collection, more than 3000 νμ interactions have been registered in emulsion detectors. Twenty charmed neutrino interaction candidates have been found. Multidimensional criteria have been developed in order to select ντ interactions against νμ interactions accompanied by charmed hadron production. The first results from automated PAVIKOM emulsion scanning have been obtained. We discuss the measurements of the muon charge ratio of μ+ to μ−.

Fe-scintillator hodoscopic hadron calorimeter

Abstract A Fe-scintillator hodoscopic hadron calorimeter is described. It consists of independent sections, each containing four total absorption counters 20×20 cm 2 in cross section. The energy resolution for hadrons in the energy range E = 4–40 GeV is σ E / E = 0.02 + 0.52/√ E and the mean space resolution is ∼ 2 cm at 40 GeV.