Silicon Pixel Detector Research Articles

DC–DC powering for the CMS pixel upgrade

The CMS experiment plans to replace its silicon pixel detector with a new one with improved rate capability and an additional detection layer at the end of 2016. In order to cope with the increased number of detector modules the new pixel detector will be powered via DC–DC converters close to the sensitive detector volume. This paper reviews the DC–DC powering scheme and reports on the ongoing R&D program to develop converters for the pixel upgrade. Design choices are discussed and results from the electrical and thermal characterisation of converter prototypes are shown. An emphasis is put on system tests with up to 24 converters. The performance of pixel modules powered by DC–DC converters is compared to conventional powering. The integration of the DC–DC powering scheme into the pixel detector is described and system design issues are reviewed.

BrachyView, A novel inbody imaging system for HDR prostate brachytherapy: Design and Monte Carlo feasibility study

High dose rate (HDR) brachytherapy is a form of radiation therapy for treating prostate cancer whereby a high activity radiation source is moved between predefined positions inside applicators inserted within the treatment volume. Accurate positioning of the source is essential in delivering the desired dose to the target area while avoiding radiation injury to the surrounding tissue. In this paper, HDR BrachyView, a novel inbody dosimetric imaging system for real time monitoring and verification of the radioactive seed position in HDR prostate brachytherapy treatment is introduced. The current prototype consists of a 15 × 60 mm(2) silicon pixel detector with a multipinhole tungsten collimator placed 6.5 mm above the detector. Seven identical pinholes allow full imaging coverage of the entire treatment volume. The combined pinhole and pixel sensor arrangement is geometrically designed to be able to resolve the three-dimensional location of the source. The probe may be rotated to keep the whole prostate within the transverse plane. The purpose of this paper is to demonstrate the efficacy of the design through computer simulation, and to estimate the accuracy in resolving the source position (in detector plane and in 3D space) as part of the feasibility study for the BrachyView project. Monte Carlo simulations were performed using the GEANT4 radiation transport model, with a (192)Ir source placed in different locations within a prostate phantom. A geometrically accurate model of the detector and collimator were constructed. Simulations were conducted with a single pinhole to evaluate the pinhole design and the signal to background ratio obtained. Second, a pair of adjacent pinholes were simulated to evaluate the error in calculated source location. Simulation results show that accurate determination of the true source position is easily obtainable within the typical one second source dwell time. The maximum error in the estimated projection position was found to be 0.95 mm in the imaging (detector) plane, resulting in a maximum source positioning estimation error of 1.48 mm. HDR BrachyView is a feasible design for real-time source tracking in HDR prostate brachytherapy. It is capable of resolving the source position within a subsecond dwell time. In combination with anatomical information obtained from transrectal ultrasound imaging, HDR BrachyView adds a significant quality assurance capability to HDR brachytherapy treatment systems.

Silicon sensors for HL-LHC tracking detectors

It is foreseen to significantly increase the luminosity of the LHC by upgrading towards the HL-LHC (High Luminosity LHC) in 2021 in order to harvest the maximum physics potential. After the upgrade, unprecedented levels of radiation will require the experiments to upgrade their tracking detectors to withstand hadron fluences equivalent to over 1016 1MeV neutrons per cm2. Within the RD50 Collaboration, a massive R&D program is underway to develop silicon sensors with sufficient radiation tolerance. Recent defect characterization and Edge-TCT measurement results improved the understanding of irradiated detector performance. RD50 results show that sensors with n-side readout, easiest made with p-type silicon, have a superior radiation hardness due to the high overlap of electric and weighting field after irradiation, larger contribution of electrons to the total signal and finally due to charge multiplication which may enhance the collected charge at high bias voltages in this type of detector. A further area of activity is the development of advanced sensor types like 3D silicon and thin pixel detectors designed for the extreme radiation levels expected for the inner layers.

Tracking performance of a single-crystal and a polycrystalline diamond pixel-detector

We present a comparative characterization of the performance of a single-crystal and a polycrystalline diamond pixel-detector employing the standard CMS pixel readout chips. Measurements were carried out at the Fermilab Test Beam Facility, FTBF, using protons of momentum 120 GeV/c tracked by a high-resolution pixel telescope. Particular attention was directed to the study of the charge-collection, the charge-sharing among adjacent pixels and the achievable position resolution. The performance of the single-crystal detector was excellent and comparable to the best available silicon pixel-detectors. The measured average detection-efficiency was near unity, ε = 0.99860±0.00006, and the position-resolution for shared hits was about 6 μm. On the other hand, the performance of the polycrystalline detector was hampered by its lower charge collection distance and the readout chip threshold. A new readout chip, capable of operating at much lower threshold (around 1 ke−), would be required to fully exploit the potential performance of the polycrystalline diamond pixel-detector.

Radiation damage in the diamond based beam condition monitors of the CMS experiment at the Large Hadron Collider (LHC) at CERN

The Beam Condition Monitor (BCM) of the CMS detector at the LHC is a protection device similar to the LHC Beam Loss Monitor system. While the electronics used is the same, poly-crystalline Chemical Vapor Deposition (pCVD) diamonds are used instead of ionization chambers as the BCM sensor material. The main purpose of the system is the protection of the silicon Pixel and Strip tracking detectors by inducing a beam dump, if the beam losses are too high in the CMS detector. By comparing the detector current with the instantaneous luminosity, the BCM detector efficiency can be monitored. The number of radiation-induced defects in the diamond, reduces the charge collection distance, and hence lowers the signal. The number of these induced defects can be simulated using the FLUKA Monte Carlo simulation. The cross-section for creating defects increases with decreasing energies of the impinging particles. This explains, why diamond sensors mounted close to heavy calorimeters experience more radiation damage, because of the high number of low energy neutrons in these regions. The signal decrease was stronger than expected from the number of simulated defects. Here polarization from trapped charge carriers in the defects is a likely candidate for explaining the difference, as suggested by Transient Current Technique (TCT) measurements. A single-crystalline (sCVD) diamond sensor shows a faster relative signal decrease than a pCVD sensor mounted at the same location. This is expected, since the relative increase in the number of defects is larger in sCVD than in pCVD sensors.

Performance of CMS 3D silicon pixel detectors before and after irradiation

Three-dimensional (3D) silicon detectors are emerging as one of the most promising technologies for the innermost layers of tracking devices for the foreseen upgrades of the LHC. 3D sensors compatible with the CMS readout, fabricated at FBK (Trento, Italy), were tested in the laboratory and with a 120GeV/c proton beam at the FNAL test beam facility, before and after irradiation up to a fluence of 3.5×1015neq/cm2. Preliminary results of the data analysis are presented.

Non-invasive monitoring of therapeutic carbon ion beams in a homogeneous phantom by tracking of secondary ions

Radiotherapy with narrow scanned carbon ion beams enables a highly accurate treatment of tumours while sparing the surrounding healthy tissue. Changes in the patient’s geometry can alter the actual ion range in tissue and result in unfavourable changes in the dose distribution. Consequently, it is desired to verify the actual beam delivery within the patient. Real-time and non-invasive measurement methods are preferable. Currently, the only technically feasible method to monitor the delivered dose distribution within the patient is based on tissue activation measurements by means of positron emission tomography (PET). An alternative monitoring method based on tracking of prompt secondary ions leaving a patient irradiated with carbon ion beams has been previously suggested. It is expected to help in overcoming the limitations of the PET-based technique like physiological washout of the beam induced activity, low signal and to allow for real-time measurements. In this paper, measurements of secondary charged particle tracks around a head-sized homogeneous PMMA phantom irradiated with pencil-like carbon ion beams are presented. The investigated energies and beam widths are within the therapeutically used range. The aim of the study is to deduce properties of the primary beam from the distribution of the secondary charged particles. Experiments were performed at the Heidelberg Ion Beam Therapy Center, Germany. The directions of secondary charged particles emerging from the PMMA phantom were measured using an arrangement of two parallel pixelated silicon detectors (Timepix). The distribution of the registered particle tracks was analysed to deduce its dependence on clinically important beam parameters: beam range, width and position. Distinct dependencies of the secondary particle tracks on the properties of the primary carbon ion beam were observed. In the particular experimental set-up used, beam range differences of 1.3 mm were detectable. In addition, variations in the beam width could be measured with a precision of 0.9 mm. Furthermore, shifts of the lateral beam position could be monitored with a sub-millimetre precision. The presented investigations demonstrate experimentally that the non-invasive measurement and analysis of secondary ion distributions around head-sized homogeneous objects provide information on the actual beam delivery. Beam range, width and position could be monitored with a precision attractive for therapeutic situations.

Investigating the inverse square law with the Timepix hybrid silicon pixel detector: a CERN@school demonstration experiment

The Timepix hybrid silicon pixel detector has been used to investigate the inverse square law of radiation from a point source as a demonstration of the CERN@school detector kit capabilities. The experiment described uses a Timepix detector to detect the gamma rays emitted by an 241Am radioactive source at a number of different distances. Datasets for each distance were collected, processed and analysed using the Pixelman software suite and CERN’s ROOT analysis software. The inverse square law approximation describes the data well, and so by following the experiment students can make the connection between the relevant elements of the physics curriculum and cutting-edge physics research. Additionally, an analytic model of the detector geometry is used to provide a comparable description of the data without the need for approximations. Such an exercise is typical of the kind of extension activity that can pave the way to students and teachers going beyond the physics curriculum and performing their own research with CERN@school.

Neural network based cluster reconstruction in the ATLAS silicon Pixel Detector

The hit signals read out from pixels on planar semi-conductor sensors are grouped into clusters, to reconstruct the location where a charged particle passed through. The spatial resolution of the pixel detector can be improved significantly using the information from the cluster of adjacent pixels. Such analogue cluster creation techniques have been used by the ATLAS experiment for many years giving an excellent performance. However, in dense environments, such as those inside high-energy jets, it is likely that the charge deposited by two or more close-by tracks merges into one single cluster. A clusterization algorithm based on neural network methods has been developed for the ATLAS Pixel Detector. This can identify the shared clusters, split them if necessary, and estimate the positions of all particles traversing the cluster. The algorithm significantly reduces ambiguities in the assignment of pixel detector measurements to tracks within jets, and improves the positional accuracy with respect to standard interpolation techniques, by the use of the 2-dimensional charge distribution information. The reconstruction using the neural network reduces strongly the number of hits shared by more than one track and improves the resolution of the impact parameter by about 15%.

Using pixel detectors in investigations of EC/EC decay

A new multi-detector spectrometer based on Timepix silicon pixel detectors is developed to investigate the 2νEC/EC decay of 106Cd. The detectors have high efficiency for recording two low-energy KX radiation quanta emitted during 2νEC/EC decay. The ability of pixel detector to identify α, β, and γ particles and precisely localize them allows us to suppress background radiation efficiently. Test measurements for an assembly of two pixel detectors in an underground laboratory were performed. They showed the possibility of using them in the search for the 2νEC/EC decay of 106Cd.

The Micro-Vertex-Detector for the P̄ANDA experiment

P̄ANDA is a fixed target experiment that will be carried out at the future FAIR facility. P̄ANDA will provide an excellent tool to address fundamental question in the field of hadronic physics, with a physic program that extends from the investigation of QCD (providing insight in the mechanisms of mass generation and confinement) to the test of fundamental symmetries.The Micro-Vertex-Detector located in the innermost part of the central tracking system will be composed by hybrid pixel and double-sided micro-strip silicon detectors. The Micro-Vertex-Detector will play an important role for the P̄ANDA physics goals. The possibility to reconstruct the secondary vertices and the applicability of a precise D meson tagging is essential for the spectroscopy in the open charm sector and the charmonium mass region. To this aim the Micro-Vertex-Detector features a spatial resolution better than 100μm, a time resolution better than 20ns, a limited material budget, and a high data rate capability in a triggerless environment. An overview of the Micro-Vertex-Detector related to the physics goals will be presented.

BrachyView: Proof‐of‐principle of a novel in‐body gamma camera for low dose‐rate prostate brachytherapy

The conformity of the achieved dose distribution to the treatment plan strongly correlates with the accuracy of seed implantation in a prostate brachytherapy treatment procedure. Incorrect seed placement leads to both short and long term complications, including urethral and rectal toxicity. The authors present BrachyView, a novel concept of a fast intraoperative treatment planning system, to provide real-time seed placement information based on in-body gamma camera data. BrachyView combines the high spatial resolution of a pixellated silicon detector (Medipix2) with the volumetric information acquired by a transrectal ultrasound (TRUS). The two systems will be embedded in the same probe so as to provide anatomically correct seed positions for intraoperative planning and postimplant dosimetry. Dosimetric calculations are based on the TG-43 method using the real position of the seeds. The purpose of this paper is to demonstrate the feasibility of BrachyView using the Medipix2 pixel detector and a pinhole collimator to reconstruct the real-time 3D position of low dose-rate brachytherapy seeds in a phantom. BrachyView incorporates three Medipix2 detectors coupled to a multipinhole collimator. Three-dimensionally triangulated seed positions from multiple planar images are used to determine the seed placement in a PMMA prostate phantom in real time. MATLAB codes were used to test the reconstruction method and to optimize the device geometry. The results presented in this paper show a 3D position reconstruction accuracy of the seed in the range of 0.5-3 mm for a 10-60 mm seed-to-detector distance interval (Z direction), respectively. The BrachyView system also demonstrates a spatial resolution of 0.25 mm in the XY plane for sources at 10 mm distance from Medipix2 detector plane, comparable to the theoretical value calculated for an equivalent gamma camera arrangement. The authors successfully demonstrated the capability of BrachyView for real-time imaging (using a 3 s data acquisition time) of different brachytherapy seed configurations (with an activity of 0.05 U) throughout a 60 × 60 × 60 mm(3) Perspex prostate phantom. The newly developed miniature gamma camera component of BrachyView, with its high spatial resolution and real time capability, allows accurate 3D localization of seeds in a prostate phantom. Combination of the gamma camera with TRUS in a single probe will complete the BrachyView system.

The P̄ANDA MVD silicon strip detector

The P̄ANDA experiment at the future FAIR facility will study annihilation reactions of antiprotons. The Micro-Vertex-Detector (MVD) as part of the tracking system will permit precise tracking and detection of secondary vertices. It is made of silicon pixel detectors and double-sided silicon strip detectors.The unique data acquisition concept without a central trigger poses a challenge to all sub-detectors.Developments for the MVD strip detector cover the evaluation of prototype sensors as well as the readout chain, ranging from the front-end for the trigger-less readout over the Module Data Concentrator ASIC to the off-detector electronics.

Triggerless readout architecture for the silicon pixel detector of the PANDA experiment

The readout architecture for the silicon pixel detector of the PANDA MVD (Micro Vertex Detector) is presented. The pixel detector has to provide timing, position and energy information on a event-driven base, since no trigger signal is foreseen. The readout system is based on a custom ASIC, named ToPiX, directly connected to the GBT (GigaBit Transceiver) optical transceiver. A reduced size prototype with most of the main functionalities has been designed and tested. The ASIC has been bonded to a sensor based on the epitaxial technology and tested on a beam test. TID (Total Ionizing Dose) tests on the ToPiX prototype have been performed.

Displacement tracking in single human trabecula with metal-plated micro-spheres using X-ray radiography imaging

This study presents an improved radiographic method for strain measurement in very small samples of a single trabeculae. X-ray micro-radiography was used to track the deformation behaviour of individual trabecula during mechanical loading. As the X-ray micro-radiography images of a single trabecula show no significant features applicable for digital image correlation (DIC) a random pattern of markers was created on the surfaces of the samples to improve the accuracy of tracking. Metal plated borosilicate glassmicro-spheres (mean diameter 10 μm) were used as the markers for trabecular displacement tracking.Two different X-ray imaging setups were used for this purpose. The specimens of isolated trabeculae were loaded by a micro-mechanical testing device developed with respect to radiographical observation. This compact device enables a high precision three-point bending measurement. The specimen was continuously irradiated during the loading procedure by a micro-focus X-ray source. The radiographs were acquired by a single-photon counting silicon pixel detector and s flat panel sensor with CsI flipped scintillator plate.Circular Hough transform was used to locate positions of the spherical markers in the sequence of acquired radiographs and to calculate the strain in the loaded sample. The gold-coated micro-spheres provide clearly visible features in the sequence of radiographs after beam hardening correction, which in conjunction with pattern recognition algorithm enables to substantially improve the accuracy of strain measurements.

Search for Long-Lived Particles and Lepton-Jets with the ATLAS detector

Many extensions of the Standard Model include neutral weakly-coupled particles that can be long- lived. These long-lived particles occur in many models, included gauge-mediated extensions of the Minimal Supersymmetric Model (MSSM), MSSM with R-parity violation, inelastic dark matter and the Hidden Valley scenario. Results are presented on the ATLAS searches at the LHC for possible rare Higgs boson decays to pair of neutral, long-lived hidden-sector particles that lead to final states containing collimated lepton jets or fermion anti-fermion pairs. No excess of events above the expected background has been observed on data collected in 2011 at a center of mass energy of 7 TeV and limits on the cross sections are set. of the leptons increases with d mass, but decreases with d. In this model, d = 0.0, 0.1, and 0.3, and d masses (m ) of 150, 300, and 500 MeV are used. For m = 150 MeV, the d is below the + threshold and can only de- cay to electrons. With m 300 MeV, the d decays to electron and muon pairs. Additionally, for m = 500 MeV, 20% of the decays produce pion pairs. This analysis con- siders LJs in three signatures: single muon-jets with four or more muons, pairs of muon-jets each with two or more muons, and pairs of electron-jets each with two or more electrons. Events containing electron-jets (EJs) were selected using single-electron triggers with an online pT threshold of 20 or 22 GeV. To ensure proper modelling of the trigger ac- ceptance, events were required to contain at least one re- constructed electron with pT > 35 GeV, above which the trigger e ciency is constant. The reconstructed electron was required to match an electron reconstructed above the pT threshold in the trigger system with a separation R less than 0.2. The EJ candidates were built from elec- tromagnetic (EM) clusters. At least two tracks from the primary vertex (PV) with pT > 10 GeV were required to have R < 0.1 of the cluster position in the second sam- pling layer of the calorimeter. Additional requirements were made on the number of hits along the track in the silicon pixel and microstrip detectors to suppress back- grounds from photon conversions. The analysis required two LJ candidates in each event, with one cluster match- ing the electron reconstructed in the trigger system. The invariant mass of the two highest-pT tracks associated with each EJ had to be less than 2 GeV. To reduce the back- ground coming from multi-jet events, the electron cluster concentration (R 2), defined as the ratio of total energy in 3 7 cells to the total energy in 7 7 cells in , in the second sampling layer of the EM calorimeter (ECAL) was used, together with the electron cluster lateral shower width and number of TRT high threshold hits. A scaled

Directional detection of fast neutrons by the Timepix pixel detector coupled to plastic scintillator with silicon photomultiplier array

Fast neutrons are conventionally detected by scintillators of large volume, low spatial resolution and poor, if any, directional sensitivity. In this paper we present a detection technique based on the tracking of protons recoiled by fast neutrons. In this approach we use the silicon pixel detector Timepix attached in contact planar geometry to a fast plastic scintillator. The protons recoiled by neutrons in the scintillator are detected by the pixel detector while scintillation light is sensed by a 4 × 4 array of silicon photomultipliers (SiPM). Each photomultiplier is equipped with an independent amplifier and discriminator providing a fast trigger signal to the pixel detector. Variable threshold level allows adjustment of the trigger sensitivity. Single events in the pixel detector can be tagged and triggered by the scintillating detector. Position and energy sensitivity of the scintillator together with the position and the energy sensitivity of the pixel detector allow obtaining information about the position and the spectrum of the neutron source. The Timepix detector is operated with the FITPix readout interface and the Pixelman software package providing control, DAQ and online visualization. The assembled prototype has been tested with fast neutrons from a laboratory radioactive source (AmBe) and a Van de Graaff accelerator (D-T reaction). The detector architecture, comprising the Timepix device, the scintillator and the segmented SiPM, allows stacking several such units for increased detection efficiency and enhanced directional sensitivity.

Algorithms for minimization of charge sharing effects in a hybrid pixel detector taking into account hardware limitations in deep submicron technology

Charge sharing is the main limitation of pixel detectors used in spectroscopic applications, noting that this applies to both time and amplitude/energy spectroscopy. Even though, charge sharing was the subject of many studies, there is still no ultimate solution which could be implemented in the hardware to suppress the negative effects of charge sharing. This is mainly because of strong demand on low power dissipation and small silicon area of a single pixel. The first solution of this problem was proposed by CERN and consequently it was implemented in the Medipix III chip. However, due to pixel-to-pixel threshold dispersions and some imperfections of the simplified algorithm, the hit allocation was not functioning properly. We are presenting novel algorithms which allow proper hit allocation even at the presence of charge sharing. They can be implemented in an integrated circuit using a deep submicron technology. In performed simulations, we assumed not only diffusive charge spread occurring in the course of charge drifting towards the electrodes but also limitations in the readout electronics, i.e. signal fluctuations due to noise and mismatch (gain and offsets). The simulations show that using, for example, a silicon pixel detector in the low X-ray energy range, we have been able to perform proper hit position identification and use the information from summing inter-pixel nodes for spectroscopy measurements.

Performance of the ALICE SPD cooling system

The new generation of silicon detectors for particle physics requires very reduced mass and high resistance to radiations with very limited access to the detector for maintenance. The Silicon Pixel Detector (SPD) is one of the 18 detectors of the ALICE (A Large Io Collider Experiment) experiment at the Large Hadron Collider (LHC) at CERN. It constitute the two innermost layers of the Inner Tracking System (ITS) and it is the closest detector to th interaction point.An evaporative cooling system, based on C4F10 evaporation at 1.9 bar, was chosen to extrac the 1.35 kW power dissipated by the on-detector electronics. The whole system wa extensively tested and commissioned before its installation inside the ALICE experimenta area. Since then we had to deal with a decrease of the flow in some lines of the system tha imposed severe restrictions on the detector operation. Recently, a test bench has been built in order to carry out a series of tests to reproduce the misbehaviour of the system and investigat proper actions to cure the problem.The performance of the systems and the most interesting results of the above mentioned test will be presented.

The pixel detector readout ASIC for the MicroVertex Detector of the PANDA experiment

The silicon pixel detector of the PANDA experiment is characterized by both high track density and the absence of a hardware trigger signal, thus leading to a huge amount of data to be acquired and transmitted to the DAQ. In order to cope with such challenging requirements, an ASIC based custom solution for the electronic readout has been chosen. The ASIC, named ToPiX, will provide the time position of each hit and a measure of the charge released with the Time over Threshold (ToT) technique. A reduced scale prototype in a CMOS 0.13μm technology has been designed and tested. The prototype includes four columns made of 128 pixel cells, four columns of 32 cells and the end of column readout with a 32 cells deep FIFO for each double column. Each cell embeds a charge amplifier with constant current feedback capacitor discharge, a comparator with per cell adjustable threshold, 12-bits leading and trailing edge register for time and ToT measurement and an 8 bits configuration register. All the readout logic has been SEU-hardened by design using either Hamming encoding or triple modular redundancy. The chip has been tested both electrically via a test pulse input and connected to a detector in a beam test.