MOONRAY: A Permanent High-Energy Cosmic-Ray Observatory on the Surface of the Moon

The forthcoming decades will see a rapid development of space programs aiming at the implementation of habitats on our satellite. Therefore it makes sense to evaluate the feasibility of a permanent cosmic-ray (CR) observatory on the Moon. Its large sensitive area would allow to carry out a very rich observational program over a time span of a few decades with an unprecedented energy reach. A thorough exploration of the energy region around the CR spectral anomaly located at a few PeV, also known as the "knee", will become possible. In this paper we propose an innovative concept of a modular lunar observatory designed to overcome the limitations of the present generation of cosmic-ray telescopes in Low Earth Orbit. It consists of an array of fully independent modules with limited individual size and weight. This would allow an ample flexibility in the gradual deployment of a progressively larger active volume, while ensuring the collection of meaningful scientific data during the intermediate stages of its implementation. Each independent module consists of three main instruments: a combined Charge and Time-of-Flight detector to identify individual elements from proton to nickel (and beyond), a tracker providing the direction and impact point of the incident particle, and a calorimeter to measure its kinetic energy. The design of each instrument contains innovative solutions that are well within the reach of the present technology.