G.U. Pignatel

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.

Numerical Simulation of Radiation Damage Effects in p-Type and n-Type FZ Silicon Detectors

In the framework of the CERN-RD50 Collaboration, the adoption of p-type substrates has been proposed as a suitable mean to improve the radiation hardness of silicon detectors up to fluencies of 1times10 16 n/cm2. In this work two numerical simulation models will be presented for p-type and n-type silicon detectors, respectively. A comprehensive analysis of the variation of the effective doping concentration (Neff), the leakage current density and the charge collection efficiency as a function of the fluence has been performed using the Synopsys T-CAD device simulator. The simulated electrical characteristics of irradiated detectors have been compared with experimental measurements extracted from the literature, showing a very good agreement. The predicted behaviour of p-type silicon detectors after irradiation up to 1016 n/cm2 shows better results in terms of charge collection efficiency and full depletion voltage, with respect to n-type material, while comparable behaviour has been observed in terms of leakage current density

Thermal and Electrical Characterization of Silicon Photomultiplier

Detection of low levels of light is one of the key aspects in medical and space applications. Silicon photomultiplier, a novel type of avalanche photodetector which operates in Geiger mode, shows promising results and offer superior design options. The performance characteristics of the SiPM realized in FBK-irst are studied and presented in this paper. The leakage current, dark rate and internal gain are characterized as a function of temperature. The investigation has been carried out in the framework of the DASiPM Collaboration and the INFN/FBK-irst MEMS project.

Thermal and electrical characterization of silicon photomultiplier

Detection of low levels of light is one of the key aspects in medical and space applications. Silicon photo-multipliers, a novel type of avalanche silicon photo-detector with Geiger mode of operation show promising results and offer superior design options. The performance characteristics of the SiPM are studied and presented in this paper. The leakage current, dark rate and internal gain are characterized as a function of temperature. The investigation has been done in the framework of the DASiPM Collaboration in the contest of the INFN/FBK-irst MEMS project.

Innovative carbon nanotube-silicon large area photodetector

We report on a new photodetector fabricated using carbon nanostructures grown on a silicon substrate. This device exhibits low noise, a good conversion efficiency of photons into electrical current and a good signal linearity in a wide range of radiation wavelengths ranging from ultraviolet to infrared at room temperature. The maximum quantum efficiency of 37% at 880 nm has been measured without signal amplification. Such innovative devices can be easily produced on large scales by Chemical Vapour Deposition (CVD) through a relatively inexpensive chemical process, which allows large sensitive areas from a few mm2 up to hundreds of cm2 to be covered.

Evaluation of Silicon Detectors With Integrated JFET for Biomedical Applications

This paper presents initial results from electrical, spectroscopic and ion beam induced charge (IBIC) characterisation of a novel silicon PIN detector, featuring an on-chip n -channel JFET and matched feedback capacitor integrated on its p-side (frontside). This structure reduces electronic noise by minimising stray capacitance and enables highly efficient optical coupling between the detector back-side and scintillator, providing a fill factor of close to 100%. The detector is specifically designed for use in high resolution gamma cameras, where a pixellated scintillator crystal is directly coupled to an array of silicon photodetectors. The on-chip JFET is matched with the photodiode capacitance and forms the input stage of an external charge sensitive preamplifier (CSA). The integrated monolithic feedback capacitor eliminates the need for an external feedback capacitor in the external electronic readout circuit, improving the system performance by eliminating uncontrolled parasitic capacitances. An optimised noise figure of 152 electrons RMS was obtained with a shaping time of 2 mus and a total detector capacitance of 2 pF. The energy resolution obtained at room temperature (2degC) at 27 keV (direct interaction of I-125 gamma rays) was 5.09%, measured at full width at half maximum (FWHM). The effectiveness of the guard ring in minimising the detector leakage current and its influence on the total charge collection volume is clearly demonstrated by the IBIC images.

PIN diode and integrated JFET on high resistivity silicon: a new test structure

A new test structure intended as a pixel for segmented X- and /spl gamma/-ray detectors has been designed fabricated and tested. The structure consists of a PIN diode of 0.8 mm/sup 2/ area and of an n-channel JFET integrated on the same high resistivity (6 k/spl Omega/ cm) silicon chip. The electrical parameters-leakage current, the transconductance and the capacitance have good expected values. Instead, noise in excess is present in the transistor. Operated at room temperature as an X-ray detector with the integrated frontend transistor in the charge sensitive configuration, the new test structure shows an equivalent noise charge of about 60 electrons rms at the optimum shaping time of 3-6 /spl mu/s.

A modular prototype detector for scintimammography imaging

In this paper we report the development of a prototype for a new kind of detector intended for scintimammography imaging. Traditional photon counting information is available; moreover digital words representing the amplitude of each event and suitable image elaboration methods significantly enhance detector capabilities. The prototype detector is composed of a collimator and 4 imaging modules. Each module is composed of an 8times8 sensing array of 2 mmtimes2 mm silicon p-i-n photodiode pixels, coupled to a CsI(Tl) scintillator and custom readout electronics. The sensor is fixed to an Al2O3 printed circuit board. Eight readout mixed analog-digital ICs, wire bonded to single sensor pixels and to the PCB, provide analog signal conditioning, A/D conversion, self-triggered acquisition and bidirectional serial data transfer. Digital signals are routed out so that each module can be attached to a main board with an FPGA for overall detector management. PC-based supervision of the detector is accomplished using a LabVIEW simplified graphical user interface. The purpose of the prototype is to test the approach and the chosen architecture; all its elements have been designed in order to permit the construction of the final detector with a significantly larger sensing area

A comprehensive analysis of irradiated silicon detectors at cryogenic temperatures

The effect of particle irradiation on high-resistivity silicon detectors has been extensively studied with the goal of engineering devices able to survive the very challenging radiation environment at the CERN Large Hadron Collider (LHC). The main aspect under investigation has been the changes observed in detector effective doping concentration (N/sub eff/). We have previously proposed a mechanism to explain the evolution of N/sub eff/ whereby charge is exchanged directly between closely-spaced defect centres in the dense terminal clusters formed by hadron irradiation. This model has been implemented in both a commercial finite-element device simulator (ISE-TCAD) and a purpose-built simulation of inter-defect charge exchange. To control the risk of breakdown due to the high leakage currents foreseen during 10 years of LHC operation, silicon detectors will be operated below room temperature (around -10/spl deg/C). This, and more general current interest in the field of cryogenic operation, has led us to investigate the behaviour of our model over a wide range of temperatures.

Evaluation of HPK and FBK silicon photomultipliers for TOF and PET applications

In time-of-flight (TOF) measurements, or positron emission tomography (PET) experiments where two gamma rays are emitted in coincidence, the time resolution of the photon detector is of primary importance. SIPMs are very promising devices for these applications, since their intrinsic response time is very short, typically less than 1ns. In practice, the actual timing resolution of SIPMs is affected by the area (capacitance) of the device, by the type of electronics used to pre-amplify the signal, by the dark count rate which is viewed as background noise, and other second order effects like cross-talk and after dark pulsing. In this work we report the characteristics of different samples of Hamamatsu Photonics (HPK) and Fondazione Bruno Kessler (FBK) SIPMs, with pixel size ranging from 40 to 100 micron. In particular, we have investigated the time response when stimulated with O(100) ps pulsed laser with a wavelength of 407nm. Finally, SIPM performances are compared with that of fast silicon PIN diodes characterized with the same setup.