CMOS multichannel front-end electronics suitable for Silicon Photomultiplier detectors has been developed, mainly intended for medical imaging applications. The architecture of the analog channel, DC coupled to the detector, is based on a full current-mode approach, which allows to achieve a wide input dynamic range of about 70pC while retaining very fast self-triggering capabilities. An 8-channel ASIC with on-chip ADC and a 32-channel prototype have been designed and manufactured, both featuring serial and sparse readout capabilities and a fast-OR circuit, able to generate an high-speed trigger signal from the discriminator outputs of the analog channels. Measurements obtained by coupling the 8-channel prototypes to an injection capacitance have been carried out for characterization purposes. The circuit has been also used to read-out SiPMs coupled to different kinds of light sources, such as a blue LED and a small LYSO scintillation crystal excited by gamma photons with different energies. The results obtained from these tests are presented and discussed, confirming the effectiveness of the proposed front-end architecture.

It has already been shown how the shape of the current pulse produced by a SiPM in response to an incident photon is sensibly affected by the characteristics of the front-end electronics (FEE) used to read out the detector. When the application requires to approach the best theoretical time performance of the detection system, the influence of all the parasitics associated to the coupling SiPM–FEE can play a relevant role and must be adequately modeled. In particular, it has been reported that the shape of the current pulse is affected by the parasitic inductance of the wiring connection between SiPM and FEE. In this contribution, we extend the validity of a previously presented SiPM model to account for the wiring inductance. Various combinations of the main performance parameters of the FEE (input resistance and bandwidth) have been simulated in order to evaluate their influence on the time accuracy of the detection system, when the time pick-off of each single event is extracted by means of a leading edge discriminator (LED) technique.

Many factors influence the timing accuracy and precision of a TOF-PET system. In particular, the light yield and the decay time constant of the scintillating crystal used, the depth of interaction of the γ-photon within the crystal, the statistical fluctuations of the photon emission process and the light propagation within the crystal appear to be the most relevant ones connected to the detector. When the overall timing performance of the system has to be assessed, it is important also to estimate the influence of the front-end (FE) electronics on it, to find out suitable criteria for the correct choice of the main specifications which affect the timing accuracy. In this contribution Monte Carlo simulations have preventively been employed to evaluate a number of realistic sets of arrival times for the first scintillating photons on a Silicon Photo Multiplier (SiPM), once the geometrical and physical characteristics of the system are known and taking into account the statistical dispersion of the photon emission process of the scintillator and the light propagation mechanisms within the crystal. Then, the elementary current pulses produced by the SiPM coupled with the FE electronics in response to each incident low energy photon have been summed up, according to the respective arrival times, resulting in a realistic set of waveforms for the overall output current pulse in response to a scintillation event. Preliminary lab experiments have been used to validate the equivalent circuit model of the photo-detector coupled to the FE electronics which has been exploited to evaluate its response to a single photon excitation. When the overall output current pulse, is applied to a leading edge current discriminator, the described procedure allows estimating how the performance parameters of the FE electronics affect the timing accuracy of the detection system.

Silicon Photomultipliers (SiPM) have shown to be excellent substitutes for more traditional and bulky Photomultiplier Tubes (PMT) in many photon detection applications, thanks essentially to their high quantum efficiency, low bias voltage and immunity to magnetic fields. However, they pose some challenging design constraints on the design of the electronic front-end (FE) due to their intrinsic high gain and speed. In particular, when changing from low to high light levels of exposition, they produce output signals whose amplitude may span over about three order of magnitude with rise times of less than 1 ns. This is of particular concern when developing integrated multichannel electronics in deep submicron technology. We report here on the realization of a 32-channel ASIC in CMOS technology, based on an innovative current-mode architecture. Besides presenting the experimental results of the ASIC characterization, some perspective indications are given concerning the work currently in progress.

We have characterized the intrinsic charge and time resolution of Hamamatsu Photonics multi-pixel photon counters (MPPC) S10362-11-050P and S10362-33-050C, both having a cell size of 50×50 μm2, sensitive area of 1×1 mm2 (400 cells) and 3×3 mm2 (3600 cells), respectively. Measurements have been performed by means of 409 nm light from a pico-laser with σ = 20 ps pulse duration and r.m.s. jitter better than 3 ps. After-pulse effects have been damped out recording the pulse height instead of the charge. Excess noise factors of 1.005 and 1.03 have been measured for the 400 and 3600 cell MCCPs respectively. Results are relevant for the possible use of these photodetectors in ring imaging Cherenkov detectors.

BASIC32_ADC is the last version of a family of multichannel ASICs developed to read-out Silicon Photo-Multiplier detectors in medical imaging applications. With respect to the previous realizations, modifications of the analog channel structure and a different overall organization of the ASIC architecture have been adopted to solve problems which could arise when a continuous slab of scintillating crystal is read out by an array of photodetectors to extract the Depth Of Interaction (DOI) information. In fact, in this case, the detectors peripheral with respect to the center of interaction will receive a small amount of photons which do not pile up in effective way, thus producing probably a current pulse smaller than the threshold set to cut off the dark pulses. As a consequence, the related channels can be ignored in a sparse read-out acquisition, even though they are associated to a significant amount of charge. The self-triggered ASIC features 32 read-out channels, exhibits improved configuration flexibility and includes an 8-bit, two step subranging ADC. In this work we report on the design and the first characterization results of the ASIC.

The design of a Positron Emission Tomography detection module capable of working inside a Magnetic Resonant Imaging system is the main objective of the 4D-MPET project. Combining the two imaging technologies offers better soft tissue contrast and lower radiation doses by providing both functional and morphological information at the same time. The proposed detector will feature a three-dimensional architecture based on two tiles of Silicon Photomultipliers coupled to a single LYSO scintillator on both its faces. Silicon Photomultipliers are magneticfield compatible photo-detectors with a very small size enabling novel detector geometries that allow the measurement of the Depth of Interaction as well as a high detector packing fraction to maximize system sensitivity. Furthermore they can be fabricated using standard silicon technology, have a large gain in the order of 106 and are very fast thus allowing evaluating the Time of Flight. Among the other features of the proposed detection system, the architecture of the innovative readout electronics will be also described which plays a relevant role for the achievement of the desired performance and is based on custom integrated circuits. Simulation results of the whole system show good performance in terms of time and spatial resolution: a timestamp of 100 ps is the ultimate performance achievable with the use of a double threshold technique along with fast electronics. Time over threshold is exploited to provide the energy information with a bin size of 400 ps. Moreover, a z resolution of 1.4mm Full Width at Half Maximum can be achieved. The proposed detector can also be exploited in other tracking applications, such as High Energy Physics and Astrophysics.

A small Positron Emission Tomography demonstrator based on LYSO slabs and Silicon Photomultiplier matrices is under construction at the University and INFN of Pisa. In this paper we present the characterization results of the read-out electronics and of the detection system. Two SiPM matrices, composed by 8 x 8 SiPM pixels, 1.5 mm pitch, have been coupled one to one to a LYSO crystals array. Custom Front-End ASICs were used to read the 64 channels of each matrix. Data from each Front-End were multiplexed and sent to a DAQ board for the digital conversion; a motherboard collects the data and communicates with a host computer through a USB port. Specific tests were carried out on the system in order to assess its performance. Futhermore we have measured some of the most important parameters of the system for PET application.

Silicon Photomultipliers (SiPM) have shown to be excellent substitutes for more traditional and bulky Photomultiplier Tubes (PMT) in many photon detection applications, thanks essentially to their high quantum efficiency, low bias voltage and immunity to magnetic fields. However, they pose some challenging design constraints on the design of the electronic front-end (FE) due to their intrinsic high gain and speed. This is of particular concern when developing integrated multichannel electronics in deep submicron technologies. We report here on the realization of multi-channel ASICs in CMOS technology, based on a current-mode architecture, presenting also experimental results of the last prototype of the family.

In many detector systems in which it is required to reveal the presence of high energy y photons, Silicon PhotoMultipliers (SiPM) have gained a dominant role over classical Photo-Multiplier Tubes (PMT) thanks to their much lower bias voltage (30 - 60V vs > 700V of PMT's), compact size and immunity to magnetic fields. These characteristics make SiPM's suitable for modem medical imaging systems such as Positron Emission Tomography coupled to Magnetic Resonance (PET/MRI) ones. However, the range of application of these devices extends to many other fields such as, for example, security systems (luggage scanners and dirty bomb detectors) high energy physics (e.g. calorimeters), astrophysics and space science. To fully exploit the excellent performance of SiPM's in terms of quantum efficiency, high gain, low bias voltage, and timing accuracy, suitable integrated front-end solutions and smart read-out architectures have to be devised. Here we present a current mode CMOS integrated frontend solution which has been implemented in a 32-channel ASIC. The chip has been designed and manufactured in a standard 0.351lm CMOS technology and has been extensively characterized using an external injection capacitance, proving that the architecture of the analog channel allows to achieve very good performance in terms of dynamic range (around 70pC), bandwidth and timing accuracy (O"=:llSps). Moreover, the ASIC has been used in a self-triggered mode to read-out a SiPM from Hamamatsu, coupled to a small L YSO scintillator, and the resulting spectra obtained by exposing the detector to different radiation sources confirm the effectiveness of our design approach. Remarkable dynamic range and good linearity have been experimentally achieved in all the measurements performed.

The 4D-MPET project aims to design a positron emission tomography detection module capable of working inside a magnetic resonant imaging system. The proposed detector will feature a threedimensional architecture based on two tiles of silicon photomultipliers coupled to a single LYSO scintillator on both its faces. Silicon photomultipliers are magnetic-field compatible photo-detectors with a very small size enabling novel detector geometries that allow the measurement of the depth of interaction. Furthermore they can be fabricated using standard silicon technology, have a large gain in the order of 10E6 and are very fast thus allowing evaluating the time of flight. Based on custom integrated circuits, the readout electronics include an innovative current mode front-end coupled to a novel time to digital converter. The former, implemented in AMS 0.35 micron SiGe-BiCMOS technology, features a very low input impedance (17 Ohm) current buffer and a large bandwidth (1 GHz), which lead to a time resolution of 100 ps FWHM. The time to digital converter exploits the combination of a submicron technology (UMC 65 nm LLLVT) together with a systolic topology so as to work at a high frequency of 2.5 GHz. This yields to a nominal time resolution of 29 ps ( whereas the photon energy is evaluated with a bin size of 400 ps by using a time over threshold technique. Finally, the depth of interaction measurement is performed by an external FPGA with a simulated spatial resolution of 1.3 mm FWHM along the z coordinate.

It is generally recognized that the main factors affecting the timing accuracy of a typical γ-ray detection system based on SiPM's coupled to fast scintillators are related to the statistical properties of the scintillation light, to the parameters of the SiPM and to the performance of the front-end electronics used to read out the detectors. In our study, Geant4 Monte Carlo simulations have been exploited to reproduce the statistical characteristics of the photon emission from the scintillator in response to the photoelectric absorption of a γ-ray. Each Monte Carlo trial provides the incidence times of the emitted photons on the surface of the detector. A comprehensive electrical model of the SiPM coupled to the front-end electronics allows finding the waveform of the elementary current pulses produced by each single photon which triggers the avalanche breakdown in a micro-cell of the SiPM, taking into account also the interconnection parasitics. Combining the available information for each Monte Carlo trial, the overall pulse waveform, obtained in response to the photoelectric interaction of a y-ray in the scintillator, is reconstructed. In case a leading edge discriminator is used to generate the signal which marks the arrival of the event, it is possible to study the accuracy of this timing signal as a function of the threshold level, considering also the contribution from the electronic noise of the front-end. Using this procedure, it is possible to distinguish the contribution of each individual parameter affecting the timing accuracy of the detection system and, in particular, to obtain useful indications for an effective choice of the specifications of the front-end electronics.

A PET imaging system demonstrator based on LYSO crystal arrays coupled to SiPM matrices is under construction at the University and INFN of Pisa. Two SiPM matrices, composed of 8×8 SiPM pixels, and 1,5 mm pitch, have been coupled one to one to a LYSO crystals array and read out by a custom electronics system. front-end ASICs were used to read 8 channels of each matrix. Data from each front-end were multiplexed and sent to a DAQ board for the digital conversion; a motherboard collects the data and communicates with a host computer through a USB port for the storage and off-line data processing. In this paper we show the first preliminary tomographic image of a point-like radioactive source acquired with part of the two detection heads in time coincidence.

Next generation PET scanners should fulfill very high requirements in terms of spatial, energy and timing resolution. Modern scanner performances are inherently limited by the use of standard photomultiplier tubes. The use of Silicon Photomultiplier (SiPM) matrices is proposed for the construction of a 4D PET module based on LSO continuous crystals, which is envisaged to replace the standard PET block detector. The expected spatial resolution of the module for the photon hit position is below 1 mm, and it will perform at the same time, the Depth Of Interaction (DOI) calculation and the Time Of Flight (TOF) measurement. The use of large area multi-pixel Silicon Photomultiplier (SiPM) detectors requires the development of a multichannel Digital Acquisition system (DAQ) as well as of a dedicated front-end in order not to degrade the intrinsic detector performances. We have developed a flexible and modular DAQ system for the read-out of two modules in time coincidence for Positron Emission Tomography (PET) applications. The DAQ system is based on a previously developed custom front-end ASIC chip (BASIC) which allows to read-out SiPM matrices preserving their spectroscopy and timing capabilities. Here we describe the acquisition system architecture and its characterization measurements.

A current-mode approach is often used for the front-end electronics for Silicon Photomultipliers, to cope with the peculiar features of these detectors. Very low input resistance Rin and large bandwidth BW are classic design choices for the preamplifier, since it is assumed that the timing accuracy is optimized with this strategy. Here we show that this design approach leads to non-optimal results, due to the presence of parasitic interconnection inductance between the detector and the front-end. An approximate model, able to reproduce the behavior of the circuit during the fast rising edge of the output pulse, has been employed to analyze the influence of the interconnection inductance on the slope of the signal. Thus, considering a typical current-mode preamplifier, the existence of Rin-BW pairs which optimize the timing accuracy of the system has been demonstrated.

We are investigating the performances of a data acquisition system for Time of Flight PET, based on LYSO crystal slabs and 64 channels Silicon Photomultipliers matrices(1.2cm2 of active area each). Measurements have been performed to test the timing capability of the detection system (SiPM matices coupled to a LYSO slab and the read-out electronics)with both test signal and radio active source.

The design of front-end electronics for SiPM detectors poses some peculiar constraints which call for dedicated architectures that may remarkably differ from the commonly used ones for other solid state detectors. In particular, present day SiPM’s may have a number of microcells as high as 90,000. This may pose serious problems in terms of dynamic range of the output signal, especially when low voltage technologies are employed to implement the FE. Besides that, SiPM’s are inherently high speed devices characterized by large equivalent capacitance CDET. All the above characteristics indicate a current mode architecture as the most suitable one to interface this kind of devices. In the past several different topologies have been proposed in the literature most of them aiming at realizing the lowest possible value of input resistance, Rin, in the perspective of limiting the time constant in = RinCDET associated to the input node of the amplifier, which is usually deemed to be the dominant parameter affecting the dynamic performance of the FE. In this contribution we show that this is not the case in real situations and that there is an optimum range of values for the input resistance in dependence of the particular SiPM and of the interconnection parasitics, thus avoiding the need of realizing arbitrarily low values of input resistance that would also imply unnecessary high power dissipation and additional electronic noise, due to the high bias currents required in the input stage of the preamplifier. The results of the study, performed on a handly 2nd order SiPM model, are confirmed by both circuit simulations on a more accurate 4th order model and by lab experiments on commercially available devices.