Ilaria Sacco

A low power front-end architecture for SiPM readout with integrated ADC and multiplexed readout

Silicon Photo-Multiplier (SiPM) detectors are becoming widely used for optical photon and, in conjunction with suited scintillators, for gamma detection in both medical imaging and particle physics experiments. The spatial resolution can be improved by using smaller SiPMs with a corresponding increase in front-end channels density. The timing resolution of the whole system is a function of the detector parameters and of the characteristics of the front-end electronics. We present a low power front-end readout architecture which allows reading out several SiPMs though a single line in order to maximize the number of SiPMs. The design offers good timing performance and includes a simple charge digitizer in every channel. Four different single-ended channel designs have been designed, submitted for fabrication and characterized electronically and with SiPMs. The timing performance is obtained by using a low input impedance, precise threshold setting of a leading edge discriminator and a programmable input dc potential to set the SiPM HV bias on a channel per channel basis. Programmable low- and high-pass filters should allow reducing baseline fluctuations and noise. A simple ADC is implemented by first integrating the signal current and then discharging it at a constant rate until the baseline is reached again. The current consumption of the single channel is typically less than 10 mA. The time and energy information are sent out on a single wire. In order to keep as low as possible the output cabling the signals from different channels can be multiplexed on the same cable. The processing of these signals (extraction of time, ADC amplitude determination and channel number decoding) is performed by an external FPGA. The overall architecture, the front-end designs, and measurements with SiPMs are presented.

PETA4: a multi-channel TDC/ADC ASIC for SiPM readout

The PETA4 ASIC is the latest member of a family of chips targeted mainly at the readout of Silicon Photomultipliers in PET, with possible use in other detector applications. PETA4 houses 36 channels on a 5 × 5mm2 die and is fabricated in the UMC 180nm technology. It uses bump bonds with a convenient pitch of ≈ 270μm to allow the construction of very compact modules at moderate substrate cost. The chip requires nearly no external components by integrating everything (PLL loop filter, bandgap reference, bias DACs,...) on chip. Power consumption is ≤ 40mW per channel, depending on digital speed and bias settings. Every channel has two independent frontends: an established differential amplifier which has shown to be insensitive to pickup in the target application of PET/MRI, and a single-ended frontend with very low input impedance (Zin ≈ 7Ω) for high channel count operation. A fast discriminator with tunable threshold and a noise of ≤ 300μV self-triggers time stamping with a bin width of 50ps as well as an integrator with programmable integration time. The amplitude signal is converted by a ≈ 9-bit SAR ADC. After conversion, events with sufficient amplitude are queued for serial readout. The previous chip version PETA3 has achieved a CRT time resolution of ≈ 200ps when reading out scintillation light from a 3 × 3×5mm3 LYSO crystal coupled at room temperature to a 3 × 3mm2 SiPM from FBK. Energy resolution for LYSO is ≈ 12.5%FWHM. LYSO crystals of 1.3mm size could be clearly identified with SiPMs of 4 × 4mm2 when using a light spreader. The architecture of PETA4 and its performance in the lab and with SiPMs will be presented.

Interpolating Silicon Photo-Multiplier: A novel position sensitive device with submillimeter spatial resolution and depth of interaction capability

Silicon Photo Multiplier are becoming widely used, coupled with suited scintillating crystals, as gamma-ray detectors in high energy physics and medical imaging applications. We present a novel device topology, able to reach sub-millimeter spatial resolution with very few readout channels. The basic idea consists to connect each cell of a SiPM to one of the several readout channels. A cluster of photons, for example from a scintillator, illuminates the cells and the signal is shared between the readout channels, in such a way that the amplitudes include the spatial information. Two prototype generations have already been produced and they both have shown very good results in terms of spatial resolution: they are able to identify crystal arrays down to 0.8 × 0.8mm2 crystals, with only four readout channels. The measurements with the prototyped devices and future outlook are presented.

Performance of the AMBFTK board for the FastTracker processor for the ATLAS detector upgrade

Modern experiments at hadron colliders search for extremely rare processes hidden in a very large background. As the experiment complexity and the accelerator backgrounds and luminosity increase we need increasingly complex and exclusive selections. The FastTracker (FTK) processor for the ATLAS experiment offers extremely powerful, very compact and low power consumption processing units for the future, which is essential for increased efficiency and purity in the Level 2 trigger selection through the intensive use of tracking. Pattern recognition is performed with Associative Memories (AM). The AMBFTK board and the AMchip04 integrated circuit have been designed specifically for this purpose. We report on the preliminary test results of the first prototypes of the AMBFTK board and of the AMchip04.

A sub-millimeter resolution detector module for small-animal PET applications

We present a gamma detection module optimized for very high resolution PET applications, able to resolve arrays of scintillating crystals with sub-millimeter pitch. The detector is composed of a single ceramic substrate (LTCC): it hosts four flip-chip mounted PETA5 ASICs on the bottom side and an array of SiPM sensors on the top surface, fabricated in HD-RGB technology by FBK. Each chip has 36 channels, for a maximum of 144 readout channels on a sensitive area of about 32 mm × 32 mm. The module is MR-compatible. The thermal decoupling of the readout electronics from the photon sensors is obtained with an efficient internal liquid channel, integrated within the ceramic substrate. Two modules have been designed, based on different SiPM topologies: • Light spreader-based: an array of 12 × 12 SiPMs, with an overall pitch of 2.5 mm, is coupled with a scintillators array using a 1 mm thick glass plate. The light from one crystal is spread over a group of SiPMs, which are read out in parallel using PETA5 internal neighbor logic.• Interpolating SiPM-based: ISiPMs are intrinsic position-sensitive sensors. The photon diodes in the array are connected to one of the four available outputs so that the center of gravity of any bunch of detected photons can be reconstructed using a proper weight function of the read out amplitudes. An array of ISiPMs, each 7.5 mm× 5 mm sized, is directly coupled with the scintillating crystals. Both modules can clearly resolve LYSO arrays with a pitch of only 0.833 mm. The detector can be adjusted for clinical PET, where it has already shown ToF resolution of about 230 ps CRT at FWHM. The module designs, their features and results are described.

SPAD array chips with full frame readout for crystal characterization.

We present single photon sensitive 2D camera chips containing 88x88 avalanche photo diodes which can be read out in full frame mode with up to 400.000 frames per second. The sensors have an imaging area of ~5mm x 5mm covered by square pixels of ~56µm x 56µm with a ~55% fill factor in the latest chip generation. The chips contain a self triggering logic with selectable (column) multiplicities of up to >=4 hits within an adjustable coincidence time window. The photon accumulation time window is programmable as well. First prototypes have demonstrated low dark count rates of <50kHz/mm2 (SPAD area) at 10 degree C for 10% masked pixels. One chip version contains an automated readout of the photon cluster position. The readout of the detailed photon distribution for single events allows the characterization of light sharing, optical crosstalk etc., in crystals or crystal arrays as they are used in PET instrumentation. This knowledge could lead to improvements in spatial or temporal resolution.

A new position-sensitive silicon photomultiplier with submillimeter spatial resolution for photon-cluster identification

We present a new Silicon Photo-Multiplier topology (Interpolating SiPM, ISiPM), able to reach sub-millimeter spatial resolution in photon cluster identification. SiPMs are becoming widely used in different applications, from medical imaging to high energy physics, as γ-ray detectors, coupled with suited scintillating crystals. The intrinsic spatial resolution in such application is limited by the crystal size: smaller crystals can be identified only if many SiPMs (and then many readout channels) are read out. In the ISiPM the avalanche photodiodes are connected to few different readout channels, instead of the common output used in standard device. The cells are assigned to one of the available electrodes in such a way that the center of gravity of any arbitrary shaped group of cells can be reconstructed from a weighted function of the outputs. The devices have been fabricated with standard technologies, use a very limited number of readout channels, have reduced noise and can easily identified sub-millimeter crystals. The energy resolution achieved from the ISiPMs is still in an acceptable range for many applications. We present the prototyped sensor design and the results we obtained coupling it with different LYSO scintillators. Due to its very good results and the reduced number of readout channels, we consider the ISiPM a very promising alternative for photon-cluster detection systems.

An integrated circuit readout for TOF-PET detectors for PET/MRI (Conference Presentation)

The new generation positron emission tomography (PET) detectors for combined PET/magnetic resonance imaging (MRI) must achieve extremely good timing resolution while also being capable of simultaneous data acquisition in a high power, dynamic electromagnetic environment. SiPM-based detectors are a natural choice for this application, given the high gain and fast response of the SiPMs and their insensitivity to magnetic fields. We present the highly-integrated readout circuit “PETA”, specifically designed for SiPM readout and optimal timing resolution required for high performance TOF-PET. PETA is a bump-bonded chip, with pitch between bumps of 272 µm, suitable for compact detector design. It includes 36 readout channels, available with both single-ended and differential-ended front-ends. The event arrival time and energy are measured and digitized on chip with TDC/ADC circuits. The readout architecture is optimized for coping with high hit rate per channel. Two very compact MR-compatible TOF-PET detector modules based on PETA readout have been constructed and tested. Each is built on a single layer (ceramic and PCB substrate respectively), hosting the SiPM array and the readout electronics on opposite surfaces, and thermally decoupled with efficient integrated cooling. Coincidence measurements have shown coincidence time resolution of 230 ps FWHM using arrays of 2.25x2.25x10 mm3 LYSO crystals coupled to matching SiPMs.

A compact, water-cooled, 144-channel photo sensor module for γ detection in PET

We present a very compact photo sensor module with an area of 32.8 × 32.0mm2 containing 12 × 12 Silicon Photomultipliers (SiPMs) in a pitch of (2.5mm)2. The readout of the 144 channels is done by highly specialized PETA5 ASICs (Position Energy Timing Asic) which offer self triggered hit detection (including neighbor logic and fast veto mechanisms), time stamping with 50ps bin width and digital amplitude measurement. The module uses a substrate which is cooled by a fluid running in internal channels. The SiPMs are glued and wire bonded in a regular pattern on the top side, the ASICs are flip chip mounted at the bottom side. Several modules will be plugged into a control and data readout PCB. The full height of these parts is below 1cm. The module can be used to read scintillator crystal arrays with a pitch of only 2.5mm in a 1 : 1 coupling, or smaller crystals with light spreaders. We present the module design and the first results obtained with 1 : 1 coupled LYSO crystal arrays.