  COSINUS is a dark matter (DM) direct search experiment that uses sodium
iodide (NaI) crystals as cryogenic calorimeters. Thanks to the low nuclear
recoil energy threshold and event-by-event discrimination capability, COSINUS
will address the long-standing DM claim made by the DAMA/LIBRA collaboration.
The experiment is currently under construction at the Laboratori Nazionali del
Gran Sasso, Italy, and employs a large cylindrical water tank as a passive
shield to meet the required background rate. However, muon-induced neutrons can
mimic a DM signal therefore requiring an active veto system, which is achieved
by instrumenting the water tank with an array of photomultiplier tubes (PMTs).
This study optimizes the number, arrangement, and trigger conditions of the
PMTs as well as the size of an optically invisible region. The objective was to
maximize the muon veto efficiency while minimizing the accidental trigger rate
due to the ambient and instrumental background. The final configuration
predicts a veto efficiency of 99.63 $\pm$ 0.16 $\%$ and 44.4 $\pm$ $5.6\%$ in
the tagging of muon events and showers of secondary particles, respectively.
The active veto will reduce the cosmogenic neutron background rate to 0.11
$\pm$ 0.02 cts$\cdot$kg$^{-1}$$\cdot$year$^{-1}$, corresponding to less than
one background event in the region of interest for the whole COSINUS-1$\pi$
exposure of 1000 kg$\cdot$days.
