D. Guberman

Light-Trap: a SiPM upgrade for VHE astronomy and beyond

Ground-based gamma-ray astronomy in the Very High Energy (VHE, E > 100 GeV) regime has fast become one of the most interesting and productive sub-fields of astrophysics today. Utilizing the Imaging Atmospheric Cherenkov Technique (IACT) to reconstruct the energy and direction of incoming gamma-ray photons from the universe, several source-classes have been revealed by previous and current generations of IACT telescopes (e.g. Whipple, MAGIC, HESS and VERITAS). The next generation pointing IACT experiment, the Cherenkov Telescope Array (CTA), will provide increased sensitivity across a wider energy range and with better angular resolution. With the development of CTA, the future of IACT pointing arrays is being directed towards having more and more telescopes (and hence cameras), and therefore the need to develop low-cost pixels with acceptable light-collection efficiency is clear. One of the primary paths to the above goal is to replace Photomultiplier Tubes (PMTs) with Silicon-PMs (SiPMs) as the pixels in IACT telescope cameras. However SiPMs are not yet mature enough to replace PMTs for several reasons: sensitivity to unwanted longer wavelengths while lacking sensitivity at short wavelengths, small physical area, high cost, optical cross-talk and dark rates. Here we propose a novel method to build relatively low-cost SiPM-based pixels utilising a disk of wavelength-shifting material, which overcomes some of these drawbacks by collecting light over a larger area than standard SiPMs and improving sensitivity to shorter wavelengths while reducing background. We aim to optimise the design of such pixels, integrating them into an actual 7-pixel cluster which will be inserted into a MAGIC camera and tested during real observations. Results of simulations, laboratory measurements and the current status of the cluster design and development will be presented.

Using UV-pass filters for bright Moon observations with MAGIC

MAGIC is a system of two Imaging Atmospheric Cherenkov Telescopes (IACT) that observe Very High Energy (VHE) gamma ray sources. The PMTs in their cameras are designed to operate under moonlight, but they are limited to Moon phases below 93% (300 Moon hours per year), as they can get damaged if the amount of light they receive is too high. As a result, they cannot be used in the three to five nights around full Moon. We have selected commercial inexpensive UV-pass filters rejecting light above a wavelength of 420 nm, where the moonlight intensity is stronger. We mounted them on light-weight frames that can be easily installed on the telescope cameras. Test observations have been performed during the last nine months, from which a moonlight transmission of about 20% and a Cherenkov light transmission of about 45% are estimated. This allows the observation of sources down to an angular distance of 5 degrees to the Moon during Full Moon: essentially in the whole sky and all possible moonlight conditions. Therefore, the duty cycle of MAGIC can be extended by about 30%, including nights when VHE observations with IACTs are currently not feasible. Here we evaluate the preliminary performance, in terms of sensitivity and energy threshold, of the MAGIC telescopes equipped with the UV-pass filters under different moonlight intensities, as inferred from Crab Nebula observations and Monte Carlo simulations.

Light-Trap: A SiPM Upgrade for Very High Energy Astronomy and Beyond

With the development of the Imaging Atmospheric Cherenkov Technique (IACT), Gamma-ray astronomy has become one of the most interesting and productive fields of astrophysics. Current IACT telescope arrays (MAGIC, H.E.S.S, VERITAS) use photomultiplier tubes (PMTs) to detect the optical/near-UV Cherenkov radiation emitted due to the interaction of gamma rays with the atmosphere. For the next generation of IACT experiments, the possibility of replacing the PMTs with Silicon photomultipliers (SiPMs) is being studied. Among the main drawbacks of SiPMs are their limited active area (leading to an increase in the cost and complexity of the camera readout) and their sensitivity to unwanted wavelengths. Here we propose a novel method to build a relatively low-cost pixel consisting of a SiPM attached to a PMMA disc doped with a wavelength shifter. This pixel collects light over a much larger area than a single standard SiPM and improves sensitivity to near-UV light while simultaneously rejecting background. We describe the design of a detector that could also have applications in other fields where detection area and cost are crucial. We present results of simulations and laboratory measurements of a pixel prototype and from field tests performed with a 7-pixel cluster installed in a MAGIC telescope camera.

Second generation prototype silicon photomultiplier focal plane imaging detector module tests in the MAGIC telescopes

To date, Imaging Atmospheric Cherenkov Telescopes (IACTs) have primarily been realized using photomultiplier tubes (PMTs) as light detectors. Recently, such telescopes of small size (∼4m) have been built using silicon photomultiplier (SiPM) sensors in their imaging cameras. This work describes a design approach for SiPM based focal plane experimental instrumentation for operation alongside the PMT based camera of a large IACT. The initial information gathered during design and operation of a first prototype single module installed alongside the PMT camera is summarized. This experience has been used to design two new alternative modules currently being implemented.