We have designed and implemented a new trigger concept (hereafter called sumtrigger) for Cherenkov telescopes, which allowed us to lower the trigger threshold of the MAGIC telescope by a factor of ...two to 25 GeV. For the new trigger, we subdivided the camera in 24 overlapping patches, each patch consisting of 18 pixels. In each patch the clipped analog signals of all pixels are summed and the trigger decision is subsequently derived from the summed signal. The clipping of the individual analog signals before summing prevented accidental triggers from large afterpulses of the photomultipliers (PMT). Special emphasis were put in the design of the analog part of the electronics to preserve the fast characteristics of the PMT signals. The 2.6 ns FWHM of the analog signals results in an effective coincidence window of about 3 ns, thus strongly suppressing accidental triggers caused by fluctuations of the night sky background (NSB). In the sumtrigger all pixels of one patch contribute to the trigger decision. For 25 GeV gamma-ray showers, a patch size of 18 PMT maximizes the signal to noise ratio of the air shower signal over the fluctuations of the NSB. In combination with the clipping of the signals before forming the sum we achieved a factor of two lower trigger threshold of the sumtrigger compared to the standard MAGIC trigger. The sumtrigger was successfully used to detect pulsed gamma-ray emission from the Crab Pulsar with the MAGIC telescope.
We characterized the spectral response of CdTe sensors with different pixel sizes – namely 75, 150 and 300 μm – bonded to the latest generation IBEX single photon counting ASIC developed at DECTRIS, ...to detect monochromatic X-ray energy in the range 10–60 keV. We present a comparison of pulse height spectra recorded for several energies, showing the dependence on the pixel size of the non-trivial atomic fluorescence and charge sharing effects that affect the detector response. The extracted energy resolution, in terms of full width at half maximum or FWHM, ranges from 1.5 to 4 keV according to the pixel size and chip configuration. We devoted a careful analysis to the Quantum Efficiency and to the Spectral Efficiency — a newly-introduced measure that quantifies the impact of fluorescence and escape phenomena on the spectrum integrity in high-Z material based detectors. We then investigated the influence of the photon flux on the aforementioned quantities up to 180⋅106 cts/s/mm2 and 50⋅106 cts/s/mm2 for the 150 μm and 300 μm pixel case, respectively. Finally, we complemented the experimental data with analytical and with Monte Carlo simulations – taking into account the stochastic nature of atomic fluorescence – with an excellent agreement.
A G-APD based Camera for Imaging Atmospheric Cherenkov Telescopes Anderhub, H.; Backes, M.; Biland, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
02/2011, Volume:
628, Issue:
1
Journal Article
Peer reviewed
Imaging Atmospheric Cherenkov Telescopes (IACT) for Gamma-ray astronomy are presently using photomultiplier tubes as photo sensors. Geiger-mode avalanche photodiodes (G-APD) promise an improvement in ...sensitivity and, important for this application, ease of construction, operation and ruggedness. G-APDs have proven many of their features in the laboratory, but a qualified assessment of their performance in an IACT camera is best undertaken with a prototype. This paper describes the design and construction of a full-scale camera based on G-APDs realized within the FACT project (First G-APD Cherenkov Telescope).
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