Measurement of the light flux of stars and the night-sky with silicon photomultipliers

摘要

Regarding the study of cosmic rays at the highest energies (E 10^18 eV) the observation of extensive air showers by means of fluorescence telescopes is a well-established and powerful detection technique which provides a direct measurement of the longitudinal shower development. With the prototype FAMOUS the first fluorescence telescope featuring a camera instrumented with novel semiconductor single photon detectors, referred to as silicon photomultipliers (SiPMs), has been developed and produced. SiPMs are arrays of avalanche photo diodes, referred to as cells, operated beyond their breakdown voltage. The advantages of SiPMs compared to photomultiplier tubes commonly used for the fluorescence detection of air showers include a high photon detection efficiency, a small size, a low operational voltage, and the potential for low cost mass production. The knowledge of the light flux of the night-sky can be used to evaluate the performance of fluorescence telescopes regarding their sensitivity to signals from air showers. In this context, bright stars can be regarded as reference light sources. This work presents an SiPM-based photometer to measure the light flux of stars and the night-sky, measurements performed with this photometer, and corresponding data analyses. The photometer features a single pixel which is similar to the ones used by FAMOUS. It employs a commercial Newton reflector with an aperture diameter of 203 mm and a focal length of 800 mm. Its photosensitive device is an S10362-33-100C SiPM manufactured by Hamamatsu with a sensitive area of 3 x 3 mm^2 and a cell pitch of 100 µm. Both the photometer and FAMOUS are photosensitive in the UF-0 and RF-0 bands which range from 310 to 395 nm and 690 to 745 nm, respectively. The photometer is the predecessor to FAMOUS and adduces the proof-of-principle to determine absolute photon fluxes measured under field conditions. The reconstruction of the measured photon fluxes is performed by means of a novel photon counting method. Key characteristics of the SiPM, which are required by this analysis method, are determined. These characteristics include the breakdown voltage, the gain, the thermal noise rate, the probabilities of optical crosstalk and afterpulsing, the timing constants of afterpulses, the recovery time constant, and the dynamic range. The relative systematic uncertainty of photon detection rates determined with the photon counting method decreases with the operation voltage. For the considered voltages it ranges from 6.9% to 4.1%. The light flux of the bright stars Arcturus and Vega is measured and compared to reference data. The latter are obtained from models of stellar atmospheres with the corresponding spectral irradiances being scaled to absolute flux calibrated measurements. The relative systematic uncertainty of the expected photon detection rate is 14% and 12% for Arcturus and Vega, respectively. The data measured with the SiPM-based photometer agree with the reference data, as the latter yield systematically 19% more detected photons. The light flux of the night-sky into directions without bright stars is measured and determined to (1021 ± 22 (stat.) ± 132 (sys.)) x 10^9 and (87 ± 14 (stat) ± 11 (sys)) x 10^9 photons per square meter per second per steradian in the combined UF-0 and RF-0 bands for observations in Aachen, Germany during nights with a visible full Moon and with the Moon being 40° below the horizon, respectively.