Timepix ASIC Research Articles

Measurement of the energy response function of a silicon pixel detector readout by a Timepix chip using synchrotron radiation

In view of applications in X-ray spectral computed tomography, we determine the energy response function of a 300 μm thick silicon pixel detector, read out by a Timepix ASIC, for a wide range of energies, by using synchrotron radiation in the range 5-32.5 keV.We employ a simple analytical model of the charge transport in the sensor to fit the data and to parametrize the energy response function. This allows to interpolate the function between the energies at which the measurements were performed and to extrapolate it outside the experimentally accessible range.The response function thus parametrized is used to predict how an incoming X-ray spectrum will be distorted during the detection process in the sensor. The comparison of the calculation with measurements shows good agreement.

3D particle track reconstruction in a single layer cadmium-telluride hybrid active pixel detector

In the past 20 years the search for neutrinoless double beta decay has driven many developements in all kind of detector technology. A new branch in this field are highly-pixelated semiconductor detectors - such as the CdTe-Timepix detectors. It compromises a cadmium-telluride sensor of 14 mm x 14 mm x 1 mm size with an ASIC which has 256 x 256 pixel of 55 \textmu m pixel pitch and can be used to obtain either spectroscopic or timing information in every pixel. In regular operation it can provide a 2D projection of particle trajectories; however, three dimensional trajectories are desirable for neutrinoless double beta decay and other applications. In this paper we present a method to obtain such trajectories. The method was developed and tested with simulations that assume some minor modifications to the Timepix ASIC. Also, we were able to test the method experimentally and in the best case achieved a position resolution of about 90 \textmu m with electrons of 4.4 GeV.

A fast embedded readout system for large-area Medipix and Timepix systems

In this work we present a novel readout electronics for an X-ray sensor based on a Si crystal bump-bonded to an array of 3 × 2 Medipix ASICs. The pixel size is 55 μm × 55 μm with a total number of ∼ 400k pixels and a sensitive area of 42 mm × 28 mm. The readout electronics operate Medipix-2 MXR or Timepix ASICs with a clock speed of 125 MHz. The data acquisition system is centered around an FPGA and each of the six ASICs has a dedicated I/O port for simultaneous data acquisition. The settings of the auxiliary devices (ADCs and DACs) are also processed in the FPGA. Moreover, a high-resolution timer operates the electronic shutter to select the exposure time from 8 ns to several milliseconds. A sophisticated trigger is available in hardware and software to synchronize the acquisition with external electro-mechanical motors. The system includes a diagnostic subsystem to check the sensor temperature and to control the cooling Peltier cells and a programmable high-voltage generator to bias the crystal. A network cable transfers the data, encapsulated into the UDP protocol and streamed at 1 Gb/s. Therefore most notebooks or personal computers are able to process the data and to program the system without a dedicated interface. The data readout software is compatible with the well-known Pixelman 2.x running both on Windows and GNU/Linux. Furthermore the open architecture encourages users to write their own applications. With a low-level interface library which implements all the basic features, a MATLAB or Python script can be implemented for special manipulations of the raw data. In this paper we present selected images taken with a microfocus X-ray tube to demonstrate the capability to collect the data at rates up to 120 fps corresponding to 0.76 Gb/s.

Optimization of Timepix count rate capabilities for the applications with a periodic input signal

The capability of a Timepix readout to detect both position (with 55 μm accuracy) and time (with up to 10 ns resolution) of multiple simultaneous events enables novel non-destructive testing methods. The first generation Timepix ASIC provides timing only for one event per pixel per frame. With fast parallel readouts capable of > 1 KHz frame rates up to ∼ 3 × 106 counts cm−2s−1 can be detected with 10% overlap probability. For some applications, where incoming fluxes exceed this rate the event overlaps distort the measured timing signal. In this case many particles arriving late in the frame are not registered as many pixels are already occupied by preceding counts.In this paper we demonstrate how the count rate capability of Timepix readout can be increased in the experiments where incoming flux has a periodic structure. It is shown how the timing characteristics of input flux can be accurately reconstructed even for ∼ 80% pixel occupancy, where the probability of detection is reduced for the later events of the acquisition frame. The implementation of the correction algorithm is demonstrated for a UV flux varying with a 60 Hz frequency. The post-measurement reconstruction method is implemented for each individual pixel of the acquired images and thus can be applied even for the experiments where the incoming flux has a strong variation across the active area.

GridPix as a candidate for the future of CAST

A central part of the CAST upgrade program is the further reduction of the background rates and the extension of the experiment's sensitivity to lower energies. Both aims could be reached with a highly pixelized readout of the detector as, for example, with an InGrid, the combination of a Timepix ASIC and a Micromegas grid. The high granularity and efficiency of such a device make it possible to detect and to distinguish single primary electrons. Therefore, we have studied the energy resolution by electron counting and the separation of photon events from tracks with an event shape analysis.

The Timepix Telescope for high performance particle tracking

The Timepix particle tracking telescope has been developed as part of the LHCb VELO Upgrade project, supported by the Medipix Collaboration and the AIDA framework. It is a primary piece of infrastructure for the VELO Upgrade project and is being used for the development of new sensors and front end technologies for several upcoming LHC trackers and vertexing systems. The telescope is designed around the dual capability of the Timepix ASICs to provide information about either the deposited charge or the timing information from tracks traversing the 14×14mm matrix of 55×55μm pixels. The rate of reconstructed tracks available is optimised by taking advantage of the shutter driver readout architecture of the Timepix chip, operated with existing readout systems. Results of tests conducted in the SPS North Area beam facility at CERN show that the telescope typically provides reconstructed track rates during the beam spills of between 3.5 and 7.5kHz, depending on beam conditions. The tracks are time stamped with 1ns resolution with an efficiency of above 98% and provide a pointing resolution at the centre of the telescope of ∼1.6μm. By dropping the time stamping requirement the rate can be increased to ∼15kHz, at the expense of a small increase in background. The telescope infrastructure provides CO2 cooling and a flexible mechanical interface to the device under test, and has been used for a wide range of measurements during the 2011–2012 data taking campaigns.

The Timepix telescope for charged particle tracking

The Timepix telescope has been developed as a general purpose tool for studying the performance of position sensitive charged particle detectors. Initiated as part of the infrastructure for the development of a new vertex detector for the LHCb experiment, the system was extended under the FP7 project AIDA to allow its use as an external facility by several groups within both the high energy and medical physics communities. Based at the CERN SPS, high track rates (up to 18kHz), precise spatial resolution at the device under test (down to 1.6μm), and a flexible integration method have all been demonstrated. The telescope is constructed using the Timepix ASIC, a hybrid pixel chip with an active area of 14×14mm2.

High Resolution Photon Counting With MCP-Timepix Quad Parallel Readout Operating at $> 1~{\rm KHz}$ Frame Rates

The unique capability of microchannel plates (MCPs) to convert a single photon/ electron/ ion/ neutron into a charge of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> electrons localized within 4-12 μm from the event position is widely used in event counting imaging detectors. The high spatial and timing resolution of MCP detectors have been demonstrated with different readout techniques. A compromise between the spatial and temporal resolution, the global/local counting rate and active area must always be made for each detector application. In this paper we present a 28 × 28 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> MCP detector with 2 × 2 Timepix ASICs for readout, capable of ~ 10 μm spatial resolution at event rates up to ~ 3 MHz, and in excess of 200 MHz with ~ 55 μm pixels. This detector has a unique capability to detect multiple simultaneous events, up to several thousand with ~ 10 μm resolution and > 25000 for the 55 μm mode. The latter is enabled by the new fast readout electronics capable of readout speeds of ~ 1200 frames/sec. Despite its limitations (relatively small active area, readout dead time of 300 μs) the MCP-Timepix detector can be very attractive for applications where high spatial resolution needs to be preserved for nearly simultaneous events, e.g., time of flight measurements with pulsed sources. The low noise of the Timepix readout enables operation at gains as low as 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> , which should extend the lifetime of the MCP detectors operating at high counting rates.

Initial results on charge and velocity discrimination for heavy ions using silicon-Timepix detectors

The Timepix ASIC is a version of the hybrid pixel detector technology developed by the Medipix2 Collaboration [1]. Within the 256 by 256 pixel matrix, the electronics for each of the 55μm individual pixels are contained in the footprint of that pixel. The Timepix has a charge-sensitive pre-amp and an associated discriminator attached to a logic unit capable of being employed in one of several different modes. For the present work, the Time-Over-Threshold mode was used to allow measurement of deposited energy in the silicon sensor layer. The general properties of a Timepix-based device with a silicon sensor have been described in [2,3], and [4].Ionization along heavy ion particle tracks in the silicon sensor results in the production of free charge carriers in the detector. The charge carrier motion under the influence of an applied bias voltage leads to charge collection at the Timepix-sensor interface in one or more pixels. Signatures within the pixel cluster patterns are currently being examined, and initial results indicate that such signatures, when coupled with stopping power information, provide enough discrimination capability to begin to resolve heavy ion charge and velocity. Here we present the salient characteristics that have been identified for heavy ion charge and velocity discrimination using Timepix Silicon detectors and discuss the application of this method for particle track characterization.

Charged particle tracking with the Timepix ASIC

A prototype particle tracking telescope has been constructed using Timepix and Medipix ASIC hybrid pixel assemblies as the six sensing planes. Each telescope plane consisted of one 1.4 cm2 assembly, providing a 256x256 array of 55 micron square pixels. The telescope achieved a pointing resolution of 2.3 micron at the position of the device under test. During a beam test in 2009 the telescope was used to evaluate in detail the performance of two Timepix hybrid pixel assemblies; a standard planar 300 micron thick sensor, and 285 micron thick double sided 3D sensor. This paper describes a detailed charge calibration study of the pixel devices, which allows the true charge to be extracted, and reports on measurements of the charge collection characteristics and Landau distributions. The planar sensor achieved a best resolution of 4.0 micron for angled tracks, and resolutions of between 4.4 and 11 micron for perpendicular tracks, depending on the applied bias voltage. The double sided 3D sensor, which has significantly less charge sharing, was found to have an optimal resolution of 9.0 micron for angled tracks, and a resolution of 16.0 micron for perpendicular tracks. Based on these studies it is concluded that the Timepix ASIC shows an excellent performance when used as a device for charged particle tracking.

Precision scans of the Pixel cell response of double sided 3D Pixel detectors to pion and X-ray beams

Three-dimensional (3D) silicon sensors offer potential advantages over standard planar sensors for radiation hardness in future high energy physics experiments and reduced charge-sharing for X-ray applications, but may introduce inefficiencies due to the columnar electrodes. These inefficiencies are probed by studying variations in response across a unit pixel cell in a 55μm pitch double-sided 3D pixel sensor bump bonded to TimePix and Medipix2 readout ASICs. Two complementary characterisation techniques are discussed: the first uses a custom built telescope and a 120GeV pion beam from the Super Proton Synchrotron (SPS) at CERN; the second employs a novel technique to illuminate the sensor with a micro-focused synchrotron X-ray beam at the Diamond Light Source, UK. For a pion beam incident perpendicular to the sensor plane an overall pixel efficiency of 93.0±0.5% is measured. After a 10o rotation of the device the effect of the columnar region becomes negligible and the overall efficiency rises to 99.8±0.5%. The double-sided 3D sensor shows significantly reduced charge sharing to neighbouring pixels compared to the planar device. The charge sharing results obtained from the X-ray beam study of the 3D sensor are shown to agree with a simple simulation in which charge diffusion is neglected. The devices tested are found to be compatible with having a region in which no charge is collected centred on the electrode columns and of radius 7.6±0.6μm. Charge collection above and below the columnar electrodes in the double-sided 3D sensor is observed.

MCP detector read out with a bare quad Timepix at kilohertz framerates

The existing Berkeley neutron sensitiveMCP/Timepix hybrid detector has been very successful at demonstratingenergy resolved spatial imaging with a single Timepix ASIC read out ata ∼ 30 Hz frame rate where each neutron's positionand time (energy) is determined (X,Y,E). By increasing the detectorformat using a quad arrangement of Timepix readouts and increasing theframe rate to 1 kHz, we can increase our total event throughput by afactor of 120, thereby taking full advantage of the high fluxes ofmodern pulsed neutron sources (106 ncm−2 s−1). The key to thisconversion is a new design for the ASIC readout, called the BerkeleyQuad Timepix detector, consisting of 3 major subsystems. The first is aquad (2 × 2) bare Timepix ASIC board mounted directly behind the neutronsensitive MCPs in a hermetic vacuum enclosure with a sapphire window.The data from the Timepix ASICs flow to the second subsystem called theInterface board whose field programmable gate array (FPGA) rearrangesand converts the digital bit stream to LVDS logic levels before sendingdownstream to the third subsystem, the Roach board. The Roach board isalso FPGA based, and takes the data from all the ASICs and analyses theframes to extract information on the input events to pass on to thehost PC. This paper describes in detail the hardware and firmwaredesigns to accomplish this task.

Transmission Bragg edge spectroscopy measurements at ORNL Spallation Neutron Source

Results of neutron transmission Bragg edge spectroscopic experiments performed at the SNAP beamline of the Spallation Neutron Source are presented. A high resolution neutron counting detector with a neutron sensitive microchannel plate and Timepix ASIC readout is capable of energy resolved two dimensional mapping of neutron transmission with spatial accuracy of ∼55 μm, limited by the readout pixel size, and energy resolution limited by the duration of the initial neutron pulse. A two dimensional map of the Fe 110 Bragg edge position was obtained for a bent steel screw sample. Although the neutron pulse duration corresponded to ∼30 mÅ energy resolution for 15.3 m flight path, the accuracy of the Bragg edge position in our measurements was improved by analytical fitting to a few mÅ level. A two dimensional strain map was calculated from measured Bragg edge values with an accuracy of ∼few hundreds μistrain for 300s of data acquisition time.