CoMET is the name given by Linnaeus University to a project to optimize wide-field Very-High-Energy (VHE) gamma-ray observations from the ground.
CoMET is conceived to study cosmic VHE gamma-rays above the energy of few hundreds Gigaelectronvolt (GeV), which are the messengers of the most powerful phenomena in the Universe. The detection of VHE gamma-rays bears witness to the presence of relativistic processes in the vicinity of black holes, in Pulsars, in Supernova Remnants and in Active Galactic Nuclei.
The goal of CoMET is to reach an optimal sensitivity on extragalactic sources. Such a sensitivity would allow catching VHE AGN flares and thus to study the black hole/jet emission in more detail and enrich the catalogues of Gamma Ray Bursts detected at VHE.
Improving the sensitivity of the WCDA technique
For ALTO, there are a number of points through which the current design of WCDAs can be improved as: the altitude of the observatory, the use of a layer of scintillator below the water tank, the construction of smaller tanks, and the use of more precise electronics and time-stamping.
Altitude of the observatory
By increasing the altitude of the observatory location one can approach the point at which the number of particles in the gamma-ray induced shower is maximum, and this gives the possibility to decrease the energy threshold of the gamma-ray events. In particular, we plan to increase the altitude up to 5.2 km which allows a decrease of 40% in the energy threshold with respect to HAWC.
Water Cherenkov tanks
The construction of smaller tanks with respect to HAWC will allow to have a finer-grained view of the shower particles on the ground which helps in the reconstruction of the arrival direction of the incoming event and in the background rejection. The current design of the ALTO water tank is hexagonal, to be close-packed.
Background rejection
In our current ALTO design, we have added of layer of scintillator below the water tank. This is very important in order to be able to “tag” the passage of muons, which are the almost unambiguous signature of the nature of the particle cascade, as background proton-initiated showers are muon-rich. The implementation of this new “muon-tagging” allows to reach an increased signal over background discrimination by analysis, and thus allows an increase of the sensitivity of the detection technique. One of the solution for the liquid scintillator under consideration is a non-toxic oil with some non-toxic dopants which is relatively cheap, while being effective and environmentally friendly.
Improvements in front-end electronics and timing
ASIC Analogue Memories allow to store an electronic signal for a short time in a circular buffer (Switched-Capacitor Array, SCA), only reading it out if a trigger condition has passed. For systems where the read-out is not continuous, but happens in “windows” at a low rate, this allows the digitization to be done at a slower rate, avoiding the need for expensive, fast, and power-hungry flash Analogue to Digital Converters (ADCs).
For ALTO, we propose to take advantage of the improvements in integrated electronics which have been made in the last 10-15 years, in particular for the aspects of the Analogue memories and the timing distribution.
For the prototype we are using the WaveCatcher electronics.