Design and implementation of the double read-out system for the calorimeter of the HERD experiment

The High Energy cosmic-Radiation Detection (HERD) facility is a China-led international space mission that will start its operation around 2027 aboard the future China's Space Station. The mission is expected to extend the direct measurements on cosmic rays and gamma rays by at least one order of magnitude in energy respect to the limits of the experiments currently operating in space. This is possible thanks to its novel design, based on a 3D, homogeneous, isotropic and finely-segmented calorimeter, with good energy resolution (< 1% for electromagnetic showers, < 30% for hadronic showers) and a good effective geometric factor (> 3 m2sr for electromagnetic showers, > 2 m2sr for hadronic showers). The main challenge of the hardware design is the implementation of the read-out system for each of the almost 8000 LYSO crystals of the calorimeter, since a large dynamic range of 107 is needed and absolute energy scale calibration is crucial for space instruments. In this contribution, we describe the double read-out scheme chosen for the final configuration, made of wave-length shifting fibers coupled with an intensified scientific CMOS and photodiodes connected to a specifically designed front-end electronics. In particular, we will discuss the advantages of both read-out systems and the benefits offered to the apparatus by this double read-out scheme.