  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.

Instrumentation and Detectors