  Silicon photomultipliers (SiPMs) are photosensors widely used for imaging in
a variety of high energy and nuclear physics experiments. In noble-gas
detectors for double-beta decay and dark matter experiments, SiPMs are
attractive photosensors for imaging. However they are insensitive to the VUV
scintillation emitted by the noble gases (xenon and argon). This difficulty is
overcome in the NEXT experiment by coating the SiPMs with tetraphenyl butadiene
(TPB) to convert the VUV light into visible light. TPB requires stringent
storage and operational conditions to prevent its degradation by environmental
agents. The development of UV sensitive SiPMs is thus of utmost interest for
experiments using electroluminescence of noble-gas detectors. It is in
particular an important issue for a robust and background free neutrinoless
double-beta experiment with xenon gas aimed by NEXT. The photon detection
efficiency (PDE) of UV-enhanced SiPMs provided by Hamamatsu was determined for
light in the range 250-500 nm. The PDE of standard SiPMs of the same model
(S10362-33-50C), coated and non-coated with TPB, was also determined for
comparison. In the UV range 250-350 nm, the PDE of the standard SiPM is shown
to decrease strongly, down to about 3%. The UV-enhanced SiPM without window is
shown to have the maximum PDE of 44% at 325 nm and 30% at 250 nm. The PDE of
the UV-enhanced SiPM with silicon resin window has a similar trend in the UV
range, although it is about 30% lower. The TPB-coated SiPM has shown to have
about 6 times higher PDE than the non-coated SiPM in the range 250-315 nm. This
is however below the performance of the UV-enhanced prototypes in the same
wavelength range. Imaging in noble-gas detectors using UV-enhanced SiPMs is
discussed.

Instrumentation and Detectors;High Energy Physics - Experiment