L. Middendorf

Dynamic range measurement and calibration of SiPMs

Photosensors have played and will continue to play an important role in high-energy and Astroparticle cutting-edge experiments. As of today, the most common photon detection device in use is the photomultiplier tube (PMT). However, we are witnessing rapid progress in the field and new devices now show very competitive features when compared to PMTs. Among those state-of-the-art photo detectors, silicon photomultipliers (SiPMs) are a relatively new kind of semiconductor whose potential is presently studied by many laboratories. Their characteristics make them a very attractive candidate for future Astroparticle physics experiments recording fluorescence and Cherenkov light, both in the atmosphere and on the ground. Such applications may require the measurement of the light flux on the sensor for the purpose of energy reconstruction. This is a complex task due to the limited dynamic range of SiPMs and the presence of thermal and correlated noise. In this work we study the response of three SiPM types in terms of delivered charge when exposed to light pulses in a broad range of intensities: from single photon to saturation. The influence of the pulse time duration and the SiPM over-voltage on the response are also quantified. Based on the observed behaviour, a method is presented to reconstruct the real number of photons impinging on the SiPM surface directly from the measured SiPM charge. A special emphasis is placed on the description of the methodology and experimental design used to perform the measurements.

The optics and detector-simulation of the air fluorescence telescope FAMOUS for the detection of cosmic rays

A sophisticated method for the observation of ultra-high-energy cosmic rays (UHECRs) is the fluorescence detection technique of extensive air showers (EAS). FAMOUS will be a small fluorescence telescope, instrumented with silicon photomultipliers (SiPMs) as highly-sensitive light detectors. In comparison to photomultiplier tubes, SiPMs promise to have a higher photon-detection-efficiency. An increase in sensitivity allows to detect more distant and lower energy showers which will contribute to an enrichment of the current understanding of the development of EAS and the chemical composition of UHECRs.

Data acquisition for an SiPM based muon detector

Tomeasure cosmic rays at the highest energies, ground based detectors sparsely covering an area of several thousand square kilometers to detect cosmic ray induced air showers are used. One of those experiments is the Pierre Auger Observatory, which is currently being upgraded for a better determination of the muonic component in the showers to allow a better identification of the type of primary particle. For this upgrade multiple different detector concepts and designs were proposed. One of those proposals is the Aachen Muon Detector (AMD), which is a tiled scintillator detector with silicon photomultipliers (SiPMs) as light sensors. To digitize the SiPM signals the EASIROC chip is used, which allows to read out 32 SiPMs and adjusting of their supply voltage with a low power consumption. The complete readout electronics, including the power supply unit as well as the firmware and software needed for operation, are described in this thesis. Furthermore, characterization measurements of the electronics and first measurements with the detector system are presented.

An integrated general purpose SiPM based optical module with a high dynamic range

Silicon photomultipliers (SiPMs) are semiconductor-based light-sensors offering a high gain, a mechanically and optically robust design and high photon detection efficiency. Due to these characteristics, they started to replace conventional photomultiplier tubes in many applications in recent years. This paper presents an optical module based on SiPMs designed for the application in scintillators as well as lab measurements. The module hosts the SiPM bias voltage supply and three pre-amplifiers with different gain levels to exploit the full dynamic range of the SiPMs. Two SiPMs, read-out in parallel, are equipped with light guides to increase the sensitive area. The light guides are optimized for the read-out of wavelength shifting fibers as used in many plastic scintillator detectors. The optical and electrical performance of the module is characterized in detail in laboratory measurements. Prototypes have been installed and tested in a modified version of the Scintillator Surface Detector developed for AugerPrime, the upgrade of the Pierre Auger Observatory. The SiPM module is operated in the Argentinian Pampas and first data proves its usability in such harsh environments.

SiPMs – A revolution for high dynamic range applications

In a wide range of applications, semi-conductor photo sensors (SiPMs) are increasingly replacing classical photo multiplier tubes (PMT). They have the advantage of an easier handling due to their significantly lower bias voltage and a long life time without aging. Usually, detectors need an adapted design for the application of SiPMs due to their smaller size compared to PMTs. While the linear dynamic range of a PMT is inherently limited and usually depends strongly on the individual PMTs, SiPMs promise a dynamic range which only depends on the SiPM type applied and not on the individual sensor. SiPMs are compiled from individual Avalanche Photo Diodes operated in Geiger-mode (G-APD). Every of these diodes is only capable of the detection of a single photon at a time. Thus, the number of G-APDs inherently limits the dynamic range of a SiPM. Strictly speaking, a SiPM is non-linear starting from the first detected photon. If this non-linearity is taken into account, the dynamic range for todays sensors can reach 10^6 for coincident photons. A complication arises for extended pulses from the fact that typical re-charge times of individual cells are in the order of several nano-second. With a 3.8 sqm scintillator detector developed for the upgrade of the Pierre-Auger Observatory, it has been shown that SiPMs can nowadays act as an ideal replacement even in applications which require a high dynamic range. This has been successfully proven by operating two identical detectors on top of each other, one read out with SiPMs and one by a PMT. It is demonstrated that even at very strong illumination the SiPM response is still understood. Furthermore, laboratory measurements confirm that individual sensors are, within the systematic errors, exhibiting identical response. Given the precision of the devices and their advantages in operation, including the possibility of characterizing their response during measurement without any additional calibration device, the application of SiPMs will be a revolution for high dynamic-range applications, significantly reducing systematic uncertainties due to improved stability.

The application of SiPMs in the fluorescence telescope FAMOUS and the Aachen Muon Detector

After huge advancements in SiPM technology made in the last years, they are perfect sensors for light detection in astroparticle physics experiments. They are very robust devices and have an equal or higher photon detection efficiency than conventional photomultiplier tubes (PMTs). In addition, SiPMs can be precisely calibrated exploiting their single photon resolution. We study their performance in various applications. FAMOUS (First Auger Multi-pixel photon counter camera for the Observation of Ultra-high-energy air Showers) is a fluorescence telescope with a 61-pixel camera made of SiPMs. It is a small sized telescope using a Fresnel lens as the focusing element. The Aachen Muon Detector (AMD) is a scintillator detector designed to improve current experiments through a precise determination of the muon content in air-showers. The light produced in scintillating tiles is collected by wavelength-shifting fibers. Through clear fibers the light is guided on one SiPM per tile.

FAMOUS: a prototype silicon-photomultiplier telescope for the fluorescence detection of ultra-high-energy cosmic rays

A sophisticated technique to study ultra-high-energy cosmic rays is to measure the extensive air showers they cause in the atmosphere. Upon impact on the atmosphere, the cosmic rays generate a cascade of secondary particles, forming the air shower. The shower particles excite the atmospheric nitrogen molecules, which emit fluorescence light in the near ultraviolet regime when de-exciting. Observation of the fluorescence light with suitable optical telescopes allows a reconstruction of the energy and arrival direction of the initial particle. Due to their high photon detection efficiency, silicon photomultipliers (SiPMs) promise to improve current photomultipliertube- based fluorescence telescopes. We present the design and a full detector simulation of an SiPM-based fluorescence telescope prototype, together with the expected telescope performance, and our first construction steps. The simulation includes the air showers, the propagation of the fluorescence light through the atmosphere and its detection by our refracting telescope. We have also developed a phenomenological SiPM model based on measurements in our laboratories, simulating the electrical response. This model contains the photon detection efficiency, its dependence on the incidence angle of light and the effects of thermal and correlated noise. We have made a full performance analysis for the detection of air showers including the environmental background light. Moreover, we will present the RandD in compact modular electronics using photon counting techniques for the telescope readout.