Numerical characterization of the ARAPUCA: a new approach for LAr scintillation light detection

Abstract

The ARAPUCA concept has been proposed as a simple and neat solution for increasing the effective collection area of SiPMs through the shifting and trapping of scintillation light in noble liquids, thus with great potential for improving timing and calorimetry resolution in neutrino and dark matter search experiments using time projection chambers. It is expected to achieve a single photon detection efficiency larger than 1\%. The initial design consists of a box made of highly reflective internal surface material and with an acceptance window for photons composed of two shifters and a dichroic filter. The first shifter converts liquid argon scintillation VUV light to a photon of wavelength smaller than the dichroic cutoff, so the surface is highly transparent to it. When passing through the dichroic filter, it reaches the second shifter which allows the photon to be shifted to the visible region and be detected by the SiPM nested inside it. When it enters the box, the photon will likely reflect a few times, including on the dichroic filter surface, before being detected. We present a full numerical description of the device using a Monte Carlo framework, including characterization of the acceptance window, models of reflection of different materials, and sensor quantum efficiency, that can now be used to further improve the detection efficiency by comparing different geometries, positions of SiPM and materials. Estimates of simulated efficiencies, number of reflections and acquisition time are presented and compared to analytical calculations. Those are very promising results, giving a total efficiency for the detection of scintillation light in liquid argon of 1.7$\rm \pm$0.3\%. Comparison of the estimated total efficiency with a preliminary result from an experimental test with an ARAPUCA prototype made in Brazil is also presented.