A high energy cosmic-ray and gamma-ray observatory at the Moon South Pole: the MoonRay concept

The MoonRay project is pursuing a concept study of a permanent cosmic-ray and gamma-ray observatory on the Moon, in view of the current plans for the implementation of habitats on our satellite. The idea is to build a modular telescope that will be able not only to overcome the limitations, in available power and weight, of the present generation of cosmic-ray instruments in Low Earth Orbit, but also to operate as a powerful high energy gamma-ray observatory from a vantage point at the South Pole of the Moon. An array of fully independent modules (towers), with limited individual size and mass, can provide a geometric factor more than one order of magnitude larger than instruments in flight, or planned to be operational within the decade. The modular telescope is designed to be deployed progressively, along a series of lunar missions, while collecting meaningful scientific data during the intermediate stages of its implementation. Power will be made available by the facilities that will maintain the lunar habitats. With a geometric factor close to 15 m 2 sr and about 8 times larger sensitive area than FERMI-LAT Atwood et al. (2009) , MoonRay will be able to carry out a very rich observational program over a time span of a few decades with the exploration of the CR ”knee” to about 10 PeV and the observation of the Southern Sky with gamma rays, from GeV to multi-TeV. Each tower is equipped with three main instruments. Leveraging on an innovative two-layered array of pixelated Low Gain Avalanche Diode (LGAD) sensors, with sub-ns time resolution, a combined Charge and Time-of-Flight detector (CD-ToF) identifies individual cosmic elements, while providing an unprecedented rejection power against back-scattered radiation from the calorimeter. It is followed by a tracker (equipped with absorbers to provide photon conversion), and by a thick calorimeter of 3cm side cubic crystals (55 radiation lengths, 3 proton interaction lengths at normal incidence) with an energy resolution of 30-40 % (2 % ) for protons (electrons) and a proton/electron rejection in excess of 10 5 . The design of each instrument contains innovative solutions that are well within the reach of the present technology. A test campaign of prototype arrays with 3mm x 3mm LGAD pixels was recently carried out at CERN with a fragmented Pb beam. Results will be reported showing a preliminary time resolution close to 100 ps.