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Four channels data acquisition system for silicon photomultipliers

  Silicon Photomultiplier (SiPM) is an array of photodiodes (cells) connected together in parallel and operated in Geiger mode [1]. Each element of this array consists of diode and a quenching resistor to limit current flowing through the junction. When SiPM is biased beyond electrical breakdown (Geiger mode), typical gain is between 105 and 106 [2]. Each photon can be a source of an avalanche which spreads out in the whole volume of a single cell. This means that in each avalanche approximately the same charge is generated. The total output is a sum of current from all microcells hence it is proportional to the number of avalanches, which in turn is proportional to the light intensity [7]. The main advantages of SiPM with respect to the standard photomultiplier are the following: compact size, smaller power consumption and lower operating voltage - less than 100 V [5]. Dark current, caused by thermally generated avalanches, is the main disadvantage. These pulses appear even if there is no photon detection. The process of thermally generated avalanches is stochastic and can be eliminated by simultaneous measurement of a signal in two or more SiPMs using a coincidence mode. Acquisition system Measurement system is shown in Fig. 1. The system converts pulses of blue light coming from LED to an electric form using SiPM. Applied front-end ASIC (Fig. 2) consists of four channels readout. Each channel can process the data from a single SiPM individually. The signal from SiPM, proportional to the input light, is propagating through ASIC, analog-digital converters and FPGA device respectively. Then the data are saved on PC’s hard drive. Front-end ASIC The designed integrated circuit consists of two crucial elements: preamplifier and peak detector and hold (PDH) [3]. The purpose of preamplifier is to amplify the signal coming from SiPM which afterwards is shaped in Pole[...]

Silicon photomultiplier as fluorescence light detector

  Fluorescence measurements are often used in biochemical and biotechnological analysis where a fluorescent dyes are used as markers. Dye-labelled number of molecules can be estimated by measurement of fluorescent light intensity. Fluorescein and resorufin are dyes commonly used in biology. Both pigments have different wavelength of light for absorption and emission. Their efficiency of absorption and emission in solution with pH factor equal 8 is shown in Fig. 1. Detection of fluorescent light is very complicated, especially for a low number of fluorescence substance molecules. For this purpose many companies usually use vacuum photomultiplier tubes (PMTs). Constant innovations of these sensors have contributed to their adaptation in many fields of science because of their speed and sensitivity for single photons. Their main drawbacks are: sensitivity to the influence of magnetic field and high voltage supply. Silicon photomultipliers (SiPM) which were invented in 1998 didn’t have these defects [2, 3]. They have very small dimensions and low breakdown voltage which provide opportunities for reducing the size of light probes. Silicon photomultiplier consists of an array of avalanche photodiodes operated in Geiger mode. Each photodiode is connected to quenching resistor which is responsible for returning to steady state from avalanche breakdown. Parallel connection all circuits photodiode-resistor causes a current flows through a photomultiplier which is proportional to number of photons. Internal photodiodes circuits and structure of a single pixel of SiPM are presented in Fig. 2. Silicon photomultiplier is not without drawbacks. From the perspective of designers, the most important ones are: thermally generated dark current, cross-talk and after-pulsing. System described in the paper doesn’t eliminate these drawbacks during the acquisition data process. It was taken into account only in a data analysis process. In [...]

Front-End electronics for Silicon Photomultiplier detectors implemented in CMOS VLSI integrated circuit

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Silicon Photomultiplier (SiPM) detectors are of great interest mostly because they can operate with light levels of few photons at room temperature and have fast response with typical rise time of 2-5ns. The paper presents an integrated circuit of front-end electronics designed in CMOS technology, dedicated for Silicon Photomultiplier (SiPM) detectors. The circuits was produced in the AMS 0,35m technology and preliminary test results show its high performance. Streszczenie. Krzemowe fotodetektory cieszą sie dużym zainteresowaniem e względu na możliwość rejestracji światła w temperaturze pokojowej na poziomie pojedynczych fotonów. W artykule przestawiono układ scalony elektroniki odczytowej do krzemowych fotopowielaczy zrealizowany w technologii CMO (AMS 0,35 m) oraz wstępne wyniki testów potwierdzające jego funkcjonalność. (Układ scalony elektroniki odczytowej do krzemowych fotopowielaczy zrealizowany w technologii CMOS) Keywords: SiPM, CMOS, photodetector, photon counting Słowa kluczowe: Krzemowy fotopowielacz, CMOS, fotodetektor, zliczanie fotonów Introduction Silicon photomultipliers (SiPM) are high performance photodetectors capable of dealing with extremely low photon fluxes. They are strong competitors of widely used Photomultiplier Tubes (PMT) which require high voltage (kV range) for operation and are quite expensive. State-of-theart SiPM’s work with bias voltages at the order of 25-100V, have much smaller dimensions than a PMT’s and its unit cost can be very low (for large quantities the cost can be ~10 euro) [1, 2]. Moreover the SiPM can be operated in strong magnetic field. There are several important applications where silicon photomultipliers appear to be extremely attractive: Positron Emission Tomography, Real-time dosimetry (i.e. Mammography) and High Energy Physics experiments (i.e. calorimetry). There is a lot of research ongoing to exploit the potential of SiPM sensors related to those[...]

Method of temperature fluctuations compensation in the silicon photomultiplier measurement system

  Independently of the direction the p-n junction is polarized in, the currents in it are strongly dependent on temperature. In the state of the avalanche breakdown, the increase of the temperature contributes to the more vivid vibrations of the particles of the crystal lattice. The vibrating atom occupies more space and the probability of the collision with an accelerated carrier increases. The collisions occur earlier so the free path is shorter. It means that the carriers are accelerated on shorter path and have smaller kinetic energies. Insufficient energy results in a reduction of the probability of knocking out carriers pairs. Avalanche multiplication becomes weaker and the avalanche current decreases. Silicon Photomultiplier operates in Geiger mode which mean that it is polarized beyond the breakdown voltage. Variation of breakdown voltage especially influence the parameters of SiPM. This voltage depends on temperature. When temperature increase value of the breakdown voltage increase also [1]. This variation leads to fluctuation of the current of SiPM because polarization voltage is constant during measurement. Distance between polarization voltage and the breakdown voltage is changed. To set operating parameters of SiPM on the stable level there is need to control temperature. This article introduce new method for compensating temperature fluctuations. This method is based on controlling distance between breakdown voltage and polarization voltage to set current of the p-n junction on the stable level. If the temperature of the Silicon Photomultiplier is higher than 0K, inside the detector, due to vibrations of the lattice, pairs of the electron-hole carriers are created. It is called the thermal generation of the carriers. The probability of detecting the photon (detecting the absorption of the photon resulting in the generation of avalanche current) is directly proportional to the value of bias voltage of the detector.[...]

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