In this work, three strategies for shortening the output pulse of a silicon photomultiplier (SiPM) are reported. The first strategy is passive filtering, where band-pass filtering removes the lowest frequency components in the signal, getting a noticeable reduction in pulse width (a compression ratio of 10∶1 was obtained). In the second place, a reflectometric scheme is proposed where the amplified signal coming from the SiPM is injected into a signal splitter with one of its stubs connected to a short-circuited stub. In the last strategy, the reflectometric part is replaced by an analog subtractor circuit. In this approach, a signal splitter with stubs of different lengths is used. All solutions provide good compression ratios, up to 10∶1. Best pulses obtained are single narrow peaks, with width below 10 ns, preserving the photonic modulation and with good pseudo-Gaussian shape, single polarity and low ringing. The potential of pulse shortening for improving the capability of the detector to resolve single photons is demonstrated by mean of single photon counting patterns. The detection error probability is reduced in one order of magnitude when shortening is used for conditioning the output photosignal.
Photomultipliers have a wide range of applications, from nuclear medicine to nuclear physics. In particular, they are commonly used in high energy physics and astrophysics. The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope is an example of their applications. This work presents a test system for the characterization of the transient response of the photomultipliers used as fast light detectors in the telescope. The bias of the photomultiplier is described, taking into account its effects in the response to fast light pulses. Two methods are used to generate these pulses: 1. a specifically designed pulse generator, and 2. a plastic scintillator. For the former case, we also calculate the anode charge we expect in this setup to cross-check it with the actual measurements.
In this work we have built an electro-optical system for the transmission of low frequency analogue signals through optical fibre. The main goal was to achieve minimum pulse distortion with maximum dynamic range. The system has been used in the framework of the MAGIC telescope experiment for the transmission of the analogue output from a photo-multiplier dedicated to optical observation of astrophysical objects, in particular pulsars. The received signal polarizes an infrared LED (λ=850 nm), which converts the pulse into an optical analogue pulse. The electro-optical pulse is transmitted by means of a multi-mode optic fibre and finally amplified and filtered by the optical receiver. The whole system has been tested using a pulse generator resembling the type of pulsed signal we expect from pulsars, that is with period of about tens of milli-seconds and few milli-seconds wide. The system was calibrated in order to: a) obtain a fixed relation between the received pulse and the final data and b) enhance the dynamic range and low distortion. In what follows, we show the behaviour of the optical transmitter under different pulse shapes, amplitude and frequencies up to several hundred Hz. The electro-optical system has been mounted on the MAGIC telescope and tested successfully with the observation of the pulsed optical signal from the Crab pulsar.
A simple and cost-effective integrated synthesizer of fast light pulses has been designed, analyzed and tested for the characterization of the time response of photo-multipliers (PMT). This synthesizer consists of an integrated pulse generator based on Schmidt Trigger Inverters, a broadband matching network and a high speed LED. It enables the generation of pulses as short as less than 10 ns with variable pulse width, amplitude and repetition frequency. In order to accurately know the shape of the pulses applied to the PMT under test, a circuital model of the LED has been developed and verified at frequencies up to 2 GHz. This model accounts for the nonlinear behavior of the LED capacitance as well as the package parasitics. The influence of the mismatch at the different frequency components of the synthesized pulse has been investigated. The pulse transmitter has been used to test the time response of MAGIC telescope pixels.
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