Silicon Photomultiplier Research and Development Studies for the Large Size Telescope of the Cherenkov Telescope Array

The Cherenkov Telescope Array (CTA) is the the next generation facility of imaging atmospheric Cherenkov telescopes; two sites will cover both hemispheres. CTA will reach unprecedented sensitivity, energy and angular resolution in very-high-energy gamma-ray astronomy. Each CTA array will include four Large Size Telescopes (LSTs), designed to cover the low-energy range of the CTA sensitivity ($\sim$20 GeV to 200 GeV). In the baseline LST design, the focal-plane camera will be instrumented with 265 photodetector clusters; each will include seven photomultiplier tubes (PMTs), with an entrance window of 1.5 inches in diameter. The PMT design is based on mature and reliable technology. Recently, silicon photomultipliers (SiPMs) are emerging as a competitor. Currently, SiPMs have advantages (e.g. lower operating voltage and tolerance to high illumination levels) and disadvantages (e.g. higher capacitance and cross talk rates), but this technology is still young and rapidly evolving. SiPM technology has a strong potential to become superior to the PMT one in terms of photon detection efficiency and price per square mm of detector area. While the advantage of SiPMs has been proven for high-density, small size cameras, it is yet to be demonstrated for large area cameras such as the one of the LST. We are working to develop a SiPM-based module for the LST camera, in view of a possible camera upgrade. We will describe the solutions we are exploring in order to balance a competitive performance with a minimal impact on the overall LST camera design.Comment: 8 pages, 5 figures. In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions at arXiv:1508.0589

Delčna sestava kozmičnih žarkov ekstremnih energij na observatoriju Pierre Auger

Cosmic rays with energies above 10^18 eV, usually referred to as ultra-high energy cosmic rays (UHECR), have been a mystery from the moment they have been discovered. Although we have now more information on their extragalactic origin, their direct sources still remain hidden due to deviations caused by galactic magnetic fields. Another mystery, apart from their production sites, is their nature. Their mass composition, still uncertain at these energies, would give us a better understanding on their production, acceleration, propagation and capacity to produce extensive air showers in the Earth\u27s atmosphere. Mass composition studies of UHECR try to determine their nature from the difference in development of their extensive air showers. In this work, observational parameters from the hybrid detection system of the Pierre Auger Observatory are used in a multivariate analysis to obtain the mass composition of UHECR. The multivariate analysis (MVA) approach combines a number of mass composition sensitive variables and tries to improve the separation between different UHECR particle masses. Simulated distributions of different primary particles are fitted to measured observable distributions in order to determine individual elemental fractions of the composition. When including observables from the surface detector, we find a discrepancy in the estimated mass composition between a mixed simulation sample and the Pierre Auger data. Our analysis results from the Pierre Auger data are to a great degree independent on hadronic interaction models. Although they differ at higher primary masses, the different models are more consistent, when combining fractions of oxygen and iron. Compared to previously published results, the systematic uncertainty from hadronic interaction models is roughly four times smaller. Our analysis reports a predominantly heavy composition of UHECR, with more than a 50% fraction of oxygen and iron at low energies. The composition is then becoming heavier with increasing energy, with a fraction of oxygen and iron above 80% at the highest energies.Kozmični žarki z energijami nad 10^18 eV, ki jih poimenujemo tudi kozmični žarki ekstremnih energij (UHECR), dosegajo energije trenutno nedosegljive trkalnikom delcev. Pri njihovem prehodu skozi Zemljino atmosfero tvorijo obširne atmosferske plazove sekundarnih delcev, ki jih detektiramo z obširnimi polji vodnih detektorjev Čerenkove svetlobe in detektorji fluorescenčne svetlobe. Zaradi posredne detekcije preko plazov sekundarnih delcev in uklanjanja kozmičnih delcev v galaktičnih magnetnih poljih, pa sta delčna sestava in izvori UHECR še odprti vprašanji. Določitev obeh bi nam omogočala boljši vpogled v njihov nastanek, pospeševanje, propagacijo in zmožnost tvorjenja plazov v Zemljini atmosferi. Raziskave delčne oziroma masne sestave UHECR temeljijo na razliki, ki jih ti povzročijo pri razvoju plazov sekundarnih delcev. V tem delu združimo izmerjene podatke obeh detekcijskih sistemov observatorija Pierre Auger v skupno analizo po večih spremenljivkah za določitev masne sestave UHECR. Tako imenovana multivariabilna analiza (MVA) združi več masno odvisnih spremenljivk in pripomore k izboljšani masni separaciji. Pri tem primerjamo porazdelitve simuliranih dogodkov z izmerjenimi porazdelitvami in s tem ocenimo delež posameznih delcev. Pri vključitvi spremenljivk detektorjev Čerenkove svetlobe prihaja do neskladja med podatki observatorija Pierre Auger in simulacijami. Masna sestava UHECR je pri ekstremnih energijah nezanesljiva zaradi odvisnosti od modelov hadronskih interakcij. Naši rezultati kažejo to modelsko odvisnost le pri težjih primarnih delcih, ki pa se močno zmanjša po združitvi deležev kisika in železa, ter je približno štirikrat manjša kot pri ostalih objavljenih rezultatih. Prav tako naši rezultati kažejo na predvsem težjo sestavo UHECR z več kot 50% deležem kisika in železa pri nizkih energijah, ter več kot 80% deležem kisika in železa pri najvišjih energijah

Mass composition of cosmic rays with energies from [1017.2]eV to [1020]eV using surface and fluorescence detectors of the Pierre Auger Observatory

Ultra-high-energy cosmic rays (UHECRs) are highly energetic particles with []EeV energies, exceeding the capabilities of man-made colliders. They hold information on extreme astrophysical processes that create them and the medium they traverse on their way towards Earth. However, their mass composition at such energies is still unclear, because data interpretation depends on our choice of high energy hadronic interaction models. With its hybrid detection method, the Pierre Auger Observatory has the possibility to detect extensive air showers with an array of surface water-Cherenkov stations (SD) and fluorescence telescopes (FD). We present recent mass composition results from the Pierre Auger Collaboration using observational parameters from SD and FD measurements. Using the full dataset of the Pierre Auger Observatory, implications on composition can be made for energies above [1017.2]eV