Pruning: a tool to optimize the layout of large scale arrays for ultra-high-energy air-shower detection

The deployment of several large scale arrays is envisioned to study astroparticles ultra -high energies. In order to circumvent the heavy computational costs of exploring and optimizing their layouts, we have developed a pruning method. It consists in i) running a set of microscopic simulations and interpolate them over a dense, regularly spaced array of detection units, and ii) pruning the unnecessary units out of the layout, in order to obtain the shower footprint on a newly shaped layout. This method offers flexibility to test various layout parameters, instrumental constraints, physical inputs, with a drastic reduction in the required CPU time. The method can be universally applied to optimize arrays of any size, and using any detection techniques. For demonstration, we apply the pruning tool to radio antenna layouts, which allows us to discuss the interplay between the energy and inclination of air -showers on the size of the radio footprint the intensity of the signal on the ground. Some rule -of -thumb conclusions that can be drawn for specific case are: i) a hexagonal geometry is more efficient than a triangular geometry, ii) the detection efficiency of the array is stable to changes in the spacing between radio antennas around 1000 m size, iii) for a given number of antennas, adding a granular infill on top of a coarse hexagonal is more efficient than instrumenting the full array with a less dense spacing.