Pad Detectors Research Articles

Free Charge Carriers Trapping Properties in Neutron-Irradiated DOFZ Silicon Pad Detectors

Silicon pad-detectors fabricated from diffused oxygenated silicon were irradiated with reactor neutrons with fluences between 1 times 1014 neq/cm2 and 1 times 1015 neq/cm2. The transient current technique was used to measure the trapping probability for holes and electrons. The results obtained support the model of a linear increase with fluence. Also the temperature dependence of the trapping time constant for electrons is investigated. This temperature dependence was found independent on the irradiated fluence.

Development of a large area silicon pad detector for the identification of cosmic ions

A silicon sensor with 64 large area pads (“pad” of 1 cm 2 area) was developed to identify relativistic ions in direct measurements of the elemental composition of cosmic rays. A single-element discrimination can be achieved via an accurate measurement of the electric charge Z, taking advantage of the Z 2 dependence of specific ionization in silicon. Space-based or balloon-borne cosmic ray experiments of the next generation require the coverage of large sensitive areas with arrays of this kind of detectors. Preliminary results on the performance of the sensor are presented.

Characterization of the radiation-induced defects in Si detectors by carrier transport and decay transients

Temperature variations of the microwave probed photoconductivity (MW-PCD) transients have been shown to be a non-invasive tool for separation of recombination and trapping centers associated with radiation induced defects in Si detectors. Peaks in the asymptotic carrier decay lifetime temperature changes, attributed to carrier capture/recombination effects, are analyzed. The correlative investigations of the MW-PCD transients and kinetics of the carrier transit within detector are simultaneously performed in the proton irradiated Si pad detectors to clarify field redistribution effects. Carrier decay and transit regimes varying applied electric field are analyzed. The recombination and trapping lifetime variations with irradiation fluence are discussed.

Low-temperature TCT characterization of heavily proton irradiated p-type magnetic Czochralski silicon detectors

n +/p −/p + pad detectors processed at the Microelectronics Center of Helsinki University of Technology on boron-doped p-type high-resistivity magnetic Czochralski (MCz-Si) silicon substrates have been investigated by the transient current technique (TCT) measurements between 100 and 240 K. The detectors were irradiated by 9 MeV protons at the Accelerator Laboratory of University of Helsinki up to 1 MeV neutron equivalent fluence of 2×10 15 n/cm 2. In some of the detectors the thermal donors (TD) were introduced by intentional heat treatment at 430 °C. Hole trapping time constants and full depletion voltage values were extracted from the TCT data. We observed that hole trapping times in the order of 10 ns were found in heavily (above 1×10 15 n eq/cm 2) irradiated samples. These detectors could be fully depleted below 500 V in the temperature range of 140–180 K.

Charge collection measurements with p-type Magnetic Czochralski silicon single pad detectors

The charge collected from beta source particles in single pad detectors produced on p-type Magnetic Czochralski (MCz) silicon wafers has been measured before and after irradiation with 26 MeV protons. After a 1 MeV neutron equivalent fluence of 1 × 10 15 cm - 2 the collected charge is reduced to 77% at bias voltages below 900 V. This result is compared with previous results from charge collection measurements.

Diamond pad detector telescope for beam conditions and luminosity monitoring in ATLAS

Beam conditions and the potential detector damage resulting from their anomalies have pushed the LHC experiments to plan their own monitoring devices in addition to those provided by the machine. ATLAS decided to build a telescope composed of two stations with four diamond pad detector modules each, placed symmetrically around the interaction point at z = ± 183.8 cm and r ∼ 55 mm ( η ∼ 4.2 ). Equipped with fast electronics it allows time-of-flight separation of events resulting from beam anomalies from normally occurring p– p interactions. In addition it will provide a coarse measurement of the LHC luminosity in ATLAS. Ten detector modules have been assembled and subjected to tests, from characterization of bare diamonds to source and beam tests. Preliminary results of beam test in the CERN PS indicate a signal-to-noise ratio of 14 ± 2 .

Charge collection measurements in single-type column 3D sensors

We report on charge collection studies on 3D silicon detectors of single-type column n-diffusions in p-substrate, configured either as strip or as pad detectors. The charge is generated by penetrating beta particles from a 90Sr source which, together with a scintillation counter, serves as an electron telescope. The charge collection as a function of bias voltage is compared with the depletion thickness derived from the measured C– V characteristics.

Comparison of the radiation hardness of Magnetic Czochralski and Epitaxial silicon substrates after 26 MeV proton and reactor neutron irradiation

We report on the processing and characterization of microstrip sensors and pad detectors produced on n- and p-type Magnetic Czochralski (MCz), Epitaxial (EPI) and Float Zone (FZ) silicon within the SMART project to develop radiation-hard silicon position sensitive detectors for future colliders. Each wafer contains 10 microstrip sensors with different geometries, several diodes and test structures. The isolation in the strip detectors produced on p-type material has been achieved by means of a uniform p-spray implantation, with doping of 3×10 12 cm −2 (low-dose p-spray) and 5×10 12 cm −2 (high-dose p-spray). The samples have undergone irradiations with 26 MeV protons and reactor neutrons up to ∼10 16 cm −2 1 MeV equivalent neutrons (n eq/cm 2), and have been completely characterized before and after irradiation in terms of leakage current, depletion voltage and breakdown voltage. The current damage parameter α has been determined for all substrates. MCz diodes show less pronounced dependence of effective doping concentration N eff on the fluence when compared to standard FZ silicon, giving results comparable to diffusion oxygenated FZ devices for all irradiation sources. The observed increase of N eff with fluence can be interpreted in EPI material as a net donor introduction process, overcompensating the usual acceptor introduction process. This effect is stronger for 26 MeV proton irradiation than for neutron irradiation.

Magnetic Czochralski silicon as detector material

The Czochralski silicon (Cz-Si) has intrinsically high oxygen concentration. Therefore Cz-Si is considered as a promising material for the tracking systems in future very high luminosity colliders. In this contribution a brief overview of the Czochralski crystal growth is given. The fabrication process issues of Cz-Si are discussed and the formation of thermal donors is especially emphasized. N +/p −/p + and p +/n −/n + detectors have been processed on magnetic Czochralski (MCz-Si) wafers. We show measurement data of AC-coupled strip detectors and single pad detectors as well as experimental results of intentional TD doping. Data of spatial homogeneity of electrical properties, full depletion voltage and leakage current, is shown and n and p-type devices are compared. Our results show that it is possible to manufacture high quality n +/p −/p + and p +/n −/n + particle detectors from high-resistivity Cz-Si.

Timing in thick silicon detectors—An update

Thick silicon detectors are becoming widely used with reliable detectors available. They are often considered as a part of coincidence setup, where the timing resolution is of a crucial importance. Since over-biasing of thick detectors is sometimes unpractical, the timing resolution can be compromised in thick detectors. For this article the electric and Ramo fields in a 1.4 by 1.4 mm 2 pad size and 1 mm thick pad detector were calculated. GEANT4 was used to determine the tracks of the interaction electron produced in photon interactions, and the drift of ionized carriers in the detector was simulated. The signals were processed using a virtual preamplifier, a CR-RC shaper with a shaping time of 200 ns and a leading edge discriminator. The distributions of delay of the trigger after the event were compared to the measurements and a good agreement was found, allowing for additional noise in experimental setup. We proposed and evaluated an alternative readout strategy reading signals from nine nearest pads which greatly reduces the effects of inhomogeneous Ramo field on the timing resolution.

Noise and trigger efficiency characterization of cooled silicon pad detectors

Technical progress on silicon pad electron detectors, currently used in emission channelling experiments to study lattice location of radioactive dopants and impurities in single crystals, is reported. Noise and trigger efficiency improvements are achieved by using 500 μm and 1 mm thick detectors coupled to a cooled readout system. The static properties, noise, gamma ray and electron trigger efficiency and energy resolution for different temperatures under air and vacuum were measured. The advantages of the future implementation of 1 mm silicon pad detectors with cooled self-triggering readout chips are discussed.

ATLAS diamond Beam Condition Monitor

The ATLAS experiment has chosen to use diamond for its Beam Condition Monitor (BCM) given its radiation hardness, low capacitance and short charge collection time. In addition, due to low leakage current diamonds do not require cooling. The ATLAS Beam Condition Monitoring system is based on single beam bunch crossing measurements rather than integrating the accumulated particle flux. Its fast electronics will allow separation of LHC collisions from background events such as beam gas interactions or beam accidents. There will be two stations placed symmetrically about the interaction point along the beam axis at z = ± 183.8 cm . Timing of signals from the two stations will provide almost ideal separation of beam–beam interactions and background events. The ATLAS BCM module consists of diamond pad detectors of 1 cm 2 area and 500 μ m thickness coupled to a two-stage RF current amplifier. The production of the final detector modules is almost done. A S / N ratio of 10:1 has been achieved with minimum ionizing particles (MIPs) in the test beam setup at KEK. Results from the test beams and bench measurements are presented.

A prototype of very high-resolution small animal PET scanner using silicon pad detectors

A very high-resolution small animal positron emission tomograph (PET), which can achieve sub-millimeter spatial resolution, is being developed using silicon pad detectors. The prototype PET for a single slice instrument consists of two 1 mm thick silicon pad detectors, each containing a 32×16 array of 1.4×1.4 mm pads readout with four VATAGP3 chips which have 128 channels low-noise self-triggering ASIC in each chip, coincidence units, a source turntable and tungsten slice collimator. The silicon detectors were located edgewise on opposite sides of a 4 cm field-of-view to maximize efficiency. Energy resolution is dominated by electronic noise, which is 0.98% (1.38 keV) FWHM at 140.5 keV. Coincidence timing resolution is 82.1 ns FWHM and coincidence efficiency was measured to be 1.04×10 −3% from two silicon detectors with annihilation photons of 18F source. Image data were acquired and reconstructed using conventional 2-D filtered-back projection (FBP) and a maximum likelihood expectation maximization (ML-EM) method. Image resolution of approximately 1.45 mm FWHM is obtained from 1-D profile of 1.1 mm diameter 18F line source image. Even better resolution can be obtained with smaller detector element sizes. While many challenges remain in scaling up the instrument to useful efficiency including densely packed detectors and significantly improved timing resolution, performance of the test setup in terms of easily achieving sub-millimeter resolution is compelling.

Mass characterization of MaPMT tubes for the LHCb scintillator pad detector

The final choice as the photomultiplier solution for both the LHCb Pre-Shower (PS) and the Scintillator Pad Detector (SPD) are the R7600-00-M64MOD Hamamatsu 64 channel photomultiplier tubes (MaPMT). A total of 220 units have been purchased to the manufacturer and around 100 units, the part corresponding to the SPD, have been characterized at the photon detection test bench facility in the University of Barcelona (UB) high energy physics group laboratory. There, the crucial features of the tubes such as linearity, gain, channel cross-talk and anode uniformity of response have been measured to ensure the compliance with the specifications agreed with the manufacturer.

LHCb Level-0 trigger detectors

The calorimeter and muon systems are essential components to provide a trigger for the LHCb experiment. The calorimeter system comprises a Scintillating Pad Detector and Pre-Shower, followed by electromagnetic and hadronic calorimeters. The calorimeter system allows photons, electrons and hadrons to be identified, and their energy to be measured. The muon system consists of five measuring stations equipped with Multi-Wire Proportional Chambers (MWPCs) and triple-Gas Electron Multiplier (GEM) detectors, separated by iron filters. It allows the muons identification and transverse momentum measurement. The status of the two systems and their expected performance is presented.

Pion freeze-out time in Pb+Pb collisions at 158 AGeV/c studied via π-/π+ and K-/K+ ratios

The effect of the final state Coulomb interaction on particles produced in Pb+Pb collisions at 158 AGeV/c has been investigated in the WA98 experiment through the study of the π-/π+ and K-/K+ ratios measured as a function of mT-m0. While the ratio for kaons shows no significant mT dependence, the π-/π+ ratio is enhanced at small mT-m0 values with an enhancement that increases with centrality. A silicon pad detector located near the target is used to estimate the contribution of hyperon decays to the π-/π+ ratio. The comparison of results with predictions of the RQMD model in which the Coulomb interaction has been incorporated allows to place constraints on the time of the pion freeze-out.

Study of radiation damage induced by 24 GeV/ c and 26 MeV protons on heavily irradiated MCz and FZ silicon detectors

The aim of this work is the development of radiation hard detectors for very high luminosity colliders. A growing interest has been recently focused on Czochralski silicon as a potentially radiation-hard material. We report on the processing and characterization of micro-strip sensors and pad detectors produced by ITC-IRST on n- and p-type magnetic Czochralski and float zone silicon. Part of the samples has been irradiated using 24 GeV/ c protons (CERN-Geneva), while another part has been irradiated with 26 MeV protons (FZK-Karlsruhe) up to a fluence of 5×10 15 1 MeV-neutron-equivalent/cm 2. All the samples have been completely characterized before and after irradiation. Their radiation hardness as a function of the irradiation fluence has been established in terms of breakdown voltage, leakage current and evaluating the more relevant mini-sensor parameter variation. Moreover, the time evolution of depletion voltage, leakage current and inter-strip capacitance has been monitored in order to study their annealing behavior and space charge sign inversion effects.

X-ray irradiation of silicon detectors

Silicon pad detectors were irradiated with a 30 kVp X-ray energy spectrum and the change in capacity and dark current was measured as a function of radiation dose and restoring time. After irradiation of the detectors the parameters were monitored and the time to get back to the baseline before irradiation was measured. The relaxation process of the detector functionality (the recovery of the characteristics) was observed with bias voltage applied to the detectors.

Prediction of charge collection efficiency in hadron-irradiated pad and pixel silicon detectors

The Transient Current Technique (TCT) is used to measure pulse shapes of charge collection and to derive trapping times in irradiated silicon pad detectors in a fluence range up to 10 15 n eq cm - 2 . Simulations of electrical fields and charge collection mechanisms compared to the measurements of the TCT method allow to derive predictions of the charge collection efficiency. Independently, charge collection efficiencies have been determined in dedicated test beam data employing ATLAS pixel modules. Considering the geometry of pad and pixel structures the simulation for the tested fluence range can be verified and allows to extrapolate to larger fluences. This yields a useful input for the design of future silicon-based pixel detectors applicable in Super-LHC experiments.

Thermal donor generation in Czochralski silicon particle detectors

In this report, the processing of thermal donor (TD) compensated detectors is described. Heat treatment of Czochralski silicon (Cz-Si) wafers between 400 and 600 °C leads to aggregation of interstitial oxygen atoms resulting in electrically active shallow levels in the silicon band gap. This process is known as TD formation. It depends on the temperature, the oxygen concentration in the silicon material and the presence of hydrogen in device manufacturing process. The oxygen concentration in silicon wafers grown by Magnetic Czochralski (MCz) method is sufficiently high in order that the concentration of TDs is comparable with the initial phosphorous or boron doping of high-resistivity Cz-Si. The TD formation has been studied by monitoring the detector full depletion voltage with respect to the heating time at 430 °C. The TD formation has been verified by Deep Level Transient Spectroscopy (DLTS) measurements. In addition, the annealing behavior of the irradiated samples at different temperatures is discussed. The TD formation in n +/p −/p + pad detectors has been observed not to influence the leakage current of the devices. Thus, the full depletion voltage of the detectors processed on p-type MCz-Si wafers can be modified by this method.