Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
FACT—The first Cherenkov telescope using a G-APD camera for TeV gamma-ray astronomy
Introduction
Since the first ground-based detection of very high energy (VHE) gamma-rays from outer space in 1989 [1], the field of gamma-ray astronomy with Imaging Air Cherenkov Telescopes (IACT) has made a significant progress, resulting in the detection of more than a hundred known gamma-ray sources of both galactic and extragalactic origin. This achievement was mainly driven by technological developments enabling a giant leap in sensitivity as achieved by the most recent instruments, the CANGAROO-III, H.E.S.S., MAGIC, and VERITAS telescopes. Now the field is standing at the crossroads, seeking another significant increase in sensitivity compared to the currently best instruments for the next generation instrumentation, CTA [2].
As the sensitivity of IACTs depends on the overall photon detection efficiency, i.e., on the conversion of Cherenkov photons reflecting from the primary mirror into measurable photoelectrons, it is only natural to seek for better devices for photon detection. For all IACTs built up to now, Photomultiplier tubes (PMT) have been the first choice. Recently, a new semiconductor device with excellent single photon response became available: the so-called Geiger-mode Avalanche Photodiode (G-APD).
Section snippets
Geiger-mode avalanche photodiodes
PMTs have been the workhorse in detecting single or few photons ever since their invention. This is mainly due to a photon detection efficiency (PDE) of 20–30% around 300–450 nm wavelength and their high intrinsic amplification (). But due to their
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limited possibilities to further increase their quantum efficiency (QE),
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sensitivity to even weak magnetic fields,
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needs of stabilized HV power supplies,
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easy damage by high light levels,
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expensive production techniques
The 36-pixel test camera M0
In the first step of the test to study G-APDs as possible replacements in IACTs, a small test camera was built [10]. This camera was made up of 144 G-APDs of the type Hamamatsu MPPC S10362-33-50-C [11]. The signals of groups of four G-APDs were combined in an analog sum to make one pixel. Thus, the camera consisted of 36 pixels being arranged in a lattice. The trigger decision was derived from a majority coincidence from the innermost 16 pixels. Upon a trigger, all signals from the 36
FACT
The First G-APD Cherenkov Telescope (FACT) will be based on a former HEGRA telescope, still situated at the Roque de los Muchachos Observatory on the Canary Island of La Palma at about 2200 m a.s.l. The telescope will receive a complete technological upgrade, including refurbished mirrors, a new drive system, and a new data acquisition system (as previously reported [17], [18], [19]) and will be equipped with a new camera, based entirely on G-APDs.
Outlook
The construction of the camera as well as the assembly of the new telescope components will be carried out during this winter, so that commissioning will start in early 2011. A successful test of the novel G-APD camera will be a first step to consider the new photosensors for the next generation of IACTs, including CTA. FACT itself might be the first telescope to be included in a world-wide network of Cherenkov telescopes [33], [34] for monitoring bright blazars in the northern hemisphere [35].
Acknowledgments
Testing novel photo-sensors for advanced Cherenkov cameras is partially funded through the German BMBF Grants 05A08WW1 and 05A08PEA which are gratefully acknowledged.
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Cited by (0)
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Also at Max-Planck-Institut für Physik, D-80805 München, Germany.
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Also at Technische Universität München, D-85748 Garching, Germany.