Evaluation of Photomultiplier Tube Candidates for the CTA and Upgrade of the MAGIC LIDAR System
School
Technische Universität München
Abstract:
The Cherenkov Telescope Array (CTA) is paned as the next generation Imaging Atmospheric Cherenkov Telescope (IACT) for very high energy gamma ray astrophysics. CTA will cover an energy range from a few tens of GeV to above 100 TeV. The goal of this project is to achieve a ten times better sensitivity than current major IACTs. An important component of IACTs is the camera system which currently uses Photomultiplier Tubes (PMTs) as standard light sensors. A dedicated development program with the companies Hamamatsu Photonis K.K. and ET Enterprises for new PMTs for the CTA project was started six years ago. Finally, Hamamatsu finished their development during this thesis and ET Enterprises is close to a final version. After several iterations, PMTs with exceptional good parameters became available. During this Master’s thesis important parameters like quantum efficiency of the photocathode, gain, pulse width, timing and afterpulse probability were measured. For this purpose, a low-noise setup sensitive for single photoelectrons (1ph.e) was optimized and the analysis software for the raw data was improved. The second part of this Master’s thesis is dedicated to upgrade the micro-power Light Detection and Ranging (LIDAR) system of the MAGIC (Major Atmospheric Gamma Ray Imaging Cherenkov) telescopes on La Palma. Since 2011 a single wavelength LIDAR system is operated alongside the observations of the MAGIC telescopes. It is used for real-time monitoring of the atmospheric transmission and for detecting cloud layers in the field of view of MAGIC. Light absorption and scattering losses in the atmosphere play an important role for IACTs and have a large influence on the data quality. The LIDAR observations are used to estimate the data quality of the MAGIC data and offer the possibility to correct energy spectra of data affected by adverse atmospheric conditions. The MAGIC telescopes LIDAR system uses a pulsed Nd:YAG laser with 532 nm wavelength and a pulse energy of 5 μJ as transmitter. The receiver is mounted to a 60 cm spherical single mirror telescope. To compensate for the low light intensities a sensitive detector with the capability of single photon detection as well as charge integration is needed. For this purpose, a hybrid photo diode is used in a custom designed detector module. After five years of data taking the MAGIC LIDAR system has been upgraded with a new detector unit including a separate high voltage supply and new readout electronics during this thesis. Additionally, a stronger laser was tested for range extension. At the end of this thesis, the new hardware components were installed at the MAGIC site on La Palma and first test measurements have been started.