G. Llosa

Electrical Characterization of Silicon Photo-Multiplier Detectors for Optimal Front-End Design

Silicon Photo-Multiplier (SiPM) detectors represent an attractive solution for the detection of low energy photons in several fields of both high energy physics and medical imaging. We present here an accurate electrical model for this kind of detectors, which can be conveniently used to perform reliable simulations at circuit level. A suitable extraction procedure for the parameters involved in the model is also described, based on both static and dynamic measurements. The proposed model allows to reproduce accurately the waveform of the signal generated by the SiPM when coupled to the front-end electronics, as shown by excellent fittings obtained between simulations and measurements taken on real devices. This is particularly useful in order to choose the most suitable front-end architecture for SiPM detectors, since the performance of the whole detection system, especially in terms of dynamic range and timing resolution, can be correctly predicted as a function of the detector parameters and of the main characteristics of the coupled electronics.

Recent developments on silicon photomultipliers produced at FBK-irst

In this contribution, new developments on the silicon photomultipliers (SiPMs) fabricated at FBK-irst (Trento, Italy) are reported. With respect to the first series of devices produced in 2005/2006, there have been major improvements on both the the layout and the technology. Concerning the first aspect we fabricated SiPMs with increased fill factor and with different geometries (square/circular devices, arrays and matrices of SiPMs) to meet the requirements of different applications. Concerning the technology, we identified a process technique able to reduce significantly the dark count rate. In this paper we will describe the main electro-optical characteristics of these devices.

Energy, Timing and Position Resolution Studies With 16-Pixel Silicon Photomultiplier Matrices for Small Animal PET

A high resolution small animal PET scanner that employs Silicon Photomultiplier (SiPM) matrices as photodetectors is under development at the University of Pisa and INFN Pisa. The first SiPM matrices fabricated by the Center for Scientific and Technological Research, FBK-irst (Trento, Italy), are being evaluated for this purpose. The devices are composed of 16 (4 times4) pixel elements of 1 mmtimes1 mm in a common substrate. The first tests have been carried out employing the ASIC MAROC2 for the readout. Energy and timing resolution, and position determination tests have been performed coupling both pixellated and continuous LYSO scintillator crystals to the matrix, and the results have been compared with the ones obtained for single SiPMs. The first tests on position determination with continuous crystals and SiPM matrices have been performed. An intrinsic spatial resolution of 0.61 mm FWHM has been obtained.

Novel Silicon Photomultipliers for PET Applications

Silicon photomultipliers (SiPMs) with quantum efficiency maximized for a wavelength between 420 and 470 nm have been developed at ITC-irst Trento (Italy), and are being tested for their application in the construction of a ultra high resolution small animal PET tomograph. The devices have an area of 1 mm times 1 mm and 625 microcells. The breakdown voltage is around 30 V, and the gain of the order of 106. The intrinsic timing resolution is 70 ps rms at the single photoelectron level. The first tests as readout for scintillators show an energy resolution of 21% FWHM with Na-22 employing LSO crystals. The first matrices of SiPMs have been produced and are being tested.

Energy and Timing Resolution Studies With Silicon Photomultipliers (SiPMs) and 4-Pixel SiPM Matrices for PET

A high performance detector head with matrices of silicon photomultipliers (SiPMs) as photodetectors is under development at the University of Pisa and INFN Pisa. The silicon photomultipliers fabricated at FBK-irst (Trento, Italy) are being evaluated for this purpose. Single SiPM detectors of 1 mm x 1 mm size and the first SiPM test matrices composed of four (2 x 2) pixel elements in a common substrate have been tested with LYSO crystals. The energy and timing resolution of single SiPMs have been evaluated, obtaining an energy resolution of 20% FWHM at 511 keV, and a coincidence timing resolution of 1.4 ns FWHM. In addition, the first tests performed with the SiPM matrices prove the feasibility of such devices.

Silicon photomultipliers and SiPM matrices as photodetectors in nuclear medicine

A high performance detector head with matrices of silicon photomultipliers (SiPMs) is under development at the University of Pisa. The detector head will be employed in the construction of a high spatial resolution, MR compatible small animal PET scanner. Silicon photomultipliers from FBK- irst (Trento, Italy) are being evaluated for this purpose. SiPM elements of 1 mm x 1 mm size and SiPM matrices composed of four (2 x 2) pixel elements have been tested. An intrinsic timing resolution of 60 ps sigma has been measured. The results with LSO crystals show an energy resolution of 20% FWHM at 511 keV, and a coincidence timing resolution of 600 ps rms. New devices with improved characteristics and active area, as well as SiPM matrices with 16 (4 x 4) SiPM elements have been produced, and will be evaluated.

Preliminary results from a current mode CMOS front-end circuit for silicon photomultiplier detectors

We propose a CMOS front-end circuit suitable for Silicon Photomultiplier detectors (SiPM) based on a current buffer, as input stage, which features small input impedance and large bandwidth, thanks to the application of current feedback techniques. The current mode approach enhances the dynamic range of the front-end and does not suffer from possible voltage limitations due to deep-submicron CMOS implementation. We report the first measurement results obtained by coupling the circuit prototype to a SiPM detector excited by a blue LED light source. The measurements confirm the effectiveness of the proposed front-end approach and demonstrate its capability of managing large current signals with good linearity.

Silicon photomultiplier performance tests in magnetic resonance pulsed fields

We are investigating the performance of silicon photomultipliers (SiPM) in a magnetic resonance imaging (MRI) environment with a view to exploiting the potential advantages in a combined positron emission tomography / magnetic resonance imaging (PET/MR) system. SiPMs are attractive for PET because they are compact, provide high gain at low bias voltage, and fast response times. Importantly for combined PET/MR applications, they offer performance comparable to that of the photomultiplier tube (PMT) and are insensitive to magnetic fields. To study the effect of MR static magnetic, pulsed gradient and radio-frequency fields on SiPM performance, we have acquired data whilst switching magnetic fields with the SiPM positioned in the MR system. The wide dynamic range of SiPMs facilitates their use in both low light level and PET applications; we have acquired single photoelectron and 22Na energy spectra to demonstrate SiPM performance for this application. As expected, no degradation in inherent SiPM performance in an MR environment is observed.

New results on the characterization of ITC-irst Silicon Photomultipliers

In this paper we report briefly on the development of Silicon Photomultipliers at ITC-irst. First, details on the technology and geometry are given. Then, experimental data are shown including static IV characterization, signal characterization, noise properties and photodetection efficiency.

Evaluation of the first Silicon Photomultiplier matrices for a small animal PET scanner

A high resolution small animal PET scanner that employs Silicon Photomultiplier (SiPM) matrices as photodetectors is under development at the University of Pisa. The first SiPM matrices fabricated by the Center for Scientific and Technological Research, FBK-irst, Trento, Italy, are being evaluated for this purpose. The devices are composed of 16 (4×4) pixel elements of 1 mm × 1 mm in a common substrate. The first tests have been carried out employing the ASIC MAROC2 for the readout. Energy and timing resolution, and position determination tests have been carried out coupling both pixellated and continuous LYSO scintillator crystals to the matrix, and the results have been compared with the ones obtained for single SiPMs.