  The photon detection system of the DUNE experiment is based on the X-ARAPUCA
light trap. The basic elements of the X-ARAPUCA are the dichroic filters coated
with wavelength shifter (para-Therphenyl), a waveshifting plate and an array of
SiPMs which detects the trapped photons. A small scale prototype of the
X-ARAPUCA has been installed in liquid argon in a dedicated facility at
INFN-Napoli and exposed to alpha particles from a source. In order to test the
stability of the overall device response the X-ARAPUCA was kept for 10 days in
liquid argon continuously purified. The performed tests allowed for a
preliminary estimation of the X-ARAPUCA absolute photon detection efficiency.
