FlashCam is an innovative camera designed for the focal plane instrumentation of Cherenkov telescopes. The concept of the FlashCam trigger and readout system is based on the continuous digitization and digital processing of the photo detector signals on FPGAs, and on a high performance, Ethernet-based front-end readout. Because of the modular design, the electronics that have been developed can serve either photomultiplier tubes or silicon-based photon detectors. In the framework of the CTA (Cherenkov telescope array) project, the FlashCam team has developed a PMT-based camera that is suitable for the medium-sized telescopes. With over 100 Cherenkov telescopes, the CTA observatory will run the most sensitive ground-based telescope systems for TeV gamma-ray astronomy when the two arrays in the Northern and Southern Hemisphere (with three different telescope sizes) will go into operation in the upcoming years.
The Cherenkov Telescope Array (CTA) will be the future observatory for ground-based TeV gamma-ray astronomy. At two sites, one in the northern and one in the southern hemisphere, CTA will feature about one hundred telescopes of different size classes in order to significantly improve the sensitivity and energy range with respect to current Cherenkov facilities. FlashCam is a camera system proposed for the medium-sized telescopes of CTA that implements a fully-digital readout and trigger processing, which allows the reconfiguration of the trigger algorithm and the signal shaping. For the mass production of a substantial number of FlashCam cameras, efficient and reliable testing routines have been developed. In this contribution, the concept and the procedures for large-scale testing of the readout electronics are outlined. Additionally, a fast multi-channel pulse generator specifically designed for the functional testing of FlashCam FADC modules setup is presented.
The FlashCam project is preparing a camera prototype around a fully digital FADC-based readout system, for the medium sized telescopes (MST) of the Cherenkov Telescope Array (CTA). The FlashCam design is the first fully digital readout system for Cherenkov cameras, based on commercial FADCs and FPGAs as key components for digitization and triggering, and a high performance camera server as back end. It provides the option to easily implement different types of trigger algorithms as well as digitization and readout scenarios using identical hardware, by simply changing the firmware on the FPGAs. The readout of the front end modules into the camera server is Ethernet-based using standard Ethernet switches and a custom, raw Ethernet protocol. In the current implementation of the system, data transfer and back end processing rates of 3.8 GB/s and 2.4 GB/s have been achieved, respectively. Together with the dead-time-free front end event buffering on the FPGAs, this permits the cameras to operate at trigger rates of up to several ten kHz. In the horizontal architecture of FlashCam, the photon detector plane (PDP), consisting of photon detectors, preamplifiers, high voltage-, control-, and monitoring systems, is a self-contained unit, mechanically detached from the front end modules. It interfaces to the digital readout system via analogue signal transmission. The horizontal integration of FlashCam is expected not only to be more cost efficient, it also allows PDPs with different types of photon detectors to be adapted to the FlashCam readout system. By now, a 144-pixel mini-camera" setup, fully equipped with photomultipliers, PDP electronics, and digitization/ trigger electronics, has been realized and extensively tested. Preparations for a full-scale, 1764 pixel camera mechanics and a cooling system are ongoing. The paper describes the status of the project.
One of the key aspects of a detector material for space-borne hard X-ray and gamma-ray telescopes is the rate of
prompt and delayed background events generated inside the material by charged and neutral particles striking the detector. These particles are Cosmic Rays, particles trapped in Earth's magnetic field, and secondaries
from Cosmic Ray interacting with the atmosphere and the spacecraft. Here, we present a preliminary study of
Cadmium Zinc Telluride (CZT) and its behaviour in space environments. We have used the simulation package
MGGPOD to estimate the background of the CZT detectors in the proposed Energetic X-ray Imaging Survey
Telescope (EXIST) for possible orbital parameters. The EXIST mission will make use of ~6 m<sup>2</sup> of >0.5 cm
thick CZT detectors to record cosmic X-rays in the energy range from 10 keV to 600 keV. The detectors will be
shielded by a fully or partly active shield. For the specific detector and shielding geometry considered here and
an orbit with a low (7°) inclination, the background rate is dominated by diffuse extragalactic photons below
~200 keV. Prompt and delayed hadronic backgrounds grow increasingly important above this energy, becoming
the main contributors to the total background above ~1 MeV. A fully active shield performs slightly better than
a half active/half passive shield.
We report here on the optimization of 0.5 cm thick pixelated Orbotech CZT detectors with regards to the
best contacting materials and the use of steering grids. We evaluated the performance of different contacting
materials. Our study differs from earlier ones in that we investigated the performance of different anode and
cathode materials separately. We obtain the best performance with Au cathodes. For different anode materials
Ti and In give the best energy resolutions. The detector (2.0×2.0×0.5 cm<sup>3</sup>, 8×8 pixels) shows excellent 59 keV,
122 keV and 662 keV energy resolutions of 1.4 keV, 1.9 keV, and 7.4 keV, respectively. Furthermore, we report
on using steering grids to improve on the performance of the pixelated detectors. Previously, the benefit of
steering grids had been limited by additional electronic noise associated with currents between the negatively
biased steering grids and the anode pixels. We are currently exploring the possibility to isolate the steering grid
from the CZT substrates by a thin layer of Al<sub>2</sub>O<sub>3</sub>. We performed a series of measurements to determine by
how much the isolation layer reduces the grid-pixel currents. Comparing the currents between two Au contacts
before and after isolating one of the two contacts from the CZT with a 700 nm thick layer of Al<sub>2</sub>O<sub>3</sub>, we measure
that the isolation layer reduces the currents by a factor of about 10 at 500 V. We present some results from
a detector before and after deposition of an isolated steering grid. The grid indeed improves on the detectors
energy resolution and detection efficiency. We show that simulations can be used to model the anode to cathode
charge correlation in excellent agreement with the experimental results.
Cadmium Zinc Telluride (CZT) achieves excellent spatial resolution and good energy resolution over the broad energy range from several keV into the MeV energy range. In this paper we present the results of a systematic study of the performance of CZT detectors manufacturered by Orbotech (before IMARAD) depending on surface preparation, contact materials and contact deposition. The standard Orbotech detectors have the dimension of 2.0×2.0×0.5 cm. They have a pixellated In anode with 8×8 pixels and a monolithic In cathode. Using the same CZT substrates several times, we have made a direct comparison of the performance of different contact materials by replacing the cathode and/or the anode contacts with several high- workfunction metals. We present the performance of the detectors and conclude with an overview over our ongoing detector optimization.
Cadmium Zinc Telluride (CZT) detectors are having a major impact on the field of hard X-ray astronomy. Without the need for cryogenic cooling they achieve good spatial and energy resolutions over the broad energy range from 10 keV to ~600 keV. In this paper, we briefly review the historical development of detectors used in X-ray astronomy. Subsequently, we present an evaluation of CZT detectors from the company Imarad. The standard 2x2x0.5 cm detectors, contacted
with 8x8 In pixels and an In cathode, exhibit FWHM energy resolutions
of 7 keV at 59 keV, and 10 keV at 662 keV. A direct measurement of the 662 keV photopeak efficiency gives 67%. We have started a detailed study of the performance of Imarad detectors depending on surface preparation, contact materials, contact deposition, post-deposition detector annealing, and detector passivation techniques.
We present first results from contacting detectors with Cr, Ag, Au, and Pt.