T. Niggemann

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.

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.

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.

GODDeSS: a Geant4 extension for easy modelling of optical detector components

Scintillator- and fibre-based particle detectors with SiPM readout are an indispensable tool in high-energy particle physics, medical physics and other fields of application. For designing and understanding these detectors, very detailed simulations are necessary, which require an accurate modelling of the optical physics (optics, scintillation, wavelength-shifting effects, ... ), of the optical material properties, and of the geometry. To allow for a reliable usage also by less experienced users, the necessary complexity and flexibility of a suitable simulation framework must not lead to an increasing danger of user mistakes. Additionally, the required effort for creating or modifying a detailed simulation has to be minimised in order to allow for the fast creation of flexible simulation setups. These challenges have been addressed by developing GODDeSS (Geant4 Objects for Detailed Detectors with Scintillators and SiPMs). It is an extension of the particle-physics simulation tool Geant4 and allows for the easy simulation of optical detector components, especially combinations of scintillators, optical fibres, and photodetectors. GODDeSS enables the user to create extensive setups for Geant4 simulations with a few lines of source code. At the same time, GODDeSS helps to avoid typical user mistakes. This paper introduces the basic concepts of the GODDeSS framework, its object classes, and its functionality. Furthermore, test measurements with prototype modules will be presented, which were performed to validate simulation results of the GODDeSS framework.

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.