J. Schumacher

Dedicated power supply system for silicon photomultipliers

Silicon photomultipliers (SiPMs) have replaced traditional photomultiplier tubes bit by bit in high-energy physics experiments in the last years. This includes the scientific fields where the demand for highly efficient and stable photo sensors outweigh the need for large active areas. Silicon photomultipliers offer a high photon detection efficiency, low supply voltage and stable operation even under harsh environments, for example bright moon-light conditions. The temperature dependence, however, presents a challenge to the power supply system which has to compensate for this effect along with biasing the SiPMs with a stable voltage with mV precision at up to 100V (10−5 accuracy). Here, we present an intelligent power supply system for silicon photomultipliers. Up to 64 SiPM channels can be driven with one module, where more than 1mA of power can be drained per channel. The operating-voltage can be changed in 1mV steps to allow for temperature variations of the power supply system itself, which is well below 1mVK−1. A built-in micro-controller applies the voltage correction for temperature changes on the SiPM automatically using up to 64 analogue temperature sensors. The data, like the mean current per channel, temperature and applied voltage is communicated via Ethernet, while the user is able to set the bias-voltage to his needs. Measurements concerning the performance of the power supply system are being shown.

Future use of silicon photomultipliers for the fluorescence detection of ultra-high-energy cosmic rays

A sophisticated technique to measure extensive air showers initiated by ultra-high-energy cosmic rays is by means of fluorescence telescopes. Secondary particles of the air shower excite nitrogen molecules of the atmosphere, which emit fluorescence light when they de-excite. Due to their high photon detection efficiency (PDE) silicon photomultipliers (SiPMs) promise to increase the sensitivity of todays fluorescence telescopes which use photomultiplier tubes - for example the fluorescence detector of the Pierre Auger Observatory. On the other hand drawbacks like a small sensitive area, a strong temperature dependency and a high noise rate have to be managed. We present plans for a prototype fluorescence telescope using SiPMs and a special light collecting optical system of Winston cones to increase the sensitive area. In this context we made measurements of the relative PDE of SiPMs depending on the incident angle of light. The results agree with calculations based on the Fresnel equations. Furthermore, measurements of the brightness of the night sky are presented since this photon flux is the main background to the fluorescence signals of the extensive air showers. To compensate the temperature dependency of the SiPM, frontend electronics make use of temperature sensors and microcontrollers to directly adjust the bias-voltage according to the thermal conditions. To reduce the noise rate we study the coincidence of several SiPMs signals triggered by cosmic ray events. By summing up these signals the SiPMs will constitute a single pixel of the fluorescence telescope.

Small size air-Cherenkov telescopes for ground detection arrays - a possible future extension?

The combined use of air-Cherenkov telescopes with ground based particle detectors is investigated. While a hybrid detection technique using fluorescence telescopes and ground arrays has been successfully applied by the Pierre Auger Observatory, this study focuses on the combination of air-Cherenkov telescopes and ground arrays for detection of TeV gamma-rays. After the successful measurement of air showers with the 7-pixel prototype telescope, an upgraded version (FAMOUS) of a combined fluorescence and air-Cherenkov telescope is now operational. It employs a 55 cm aperture Fresnel-lens in a sealed tube and a 61-pixel camera with semiconductor photo sensors (SiPM). For joint measurements, it will be synchronized with the High Altitude water-Cherenkov gamma-ray observatory HAWC (Sierra Negra, Mexico) to evaluate its prospects as an enhancement of ground based detectors. A main focus is the improvement of the energy resolution by accessing complementary shower informations with both technologies. The direct measurement of the air shower particles by the HAWC observatory also allows the characterization of the air-Cherenkov telescope, such as collection area, without any Monte Carlo simulation. Preliminary results from this study will be presented and the future potential of air-Cherenkov telescopes as an extension for ground based arrays will be discussed.

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.

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.