  NEXT-100 experiment aims at searching the neutrinoless double-beta decay of
the Xe-136 isotope using a TPC filled with a 100 kg of high-pressure gaseous
xenon, with 90% isotopic enrichment. The experiment will take place at the
Laboratorio Subterr\'aneo de Canfranc (LSC), Spain. NEXT-100 uses
electroluminescence (EL) technology for energy measurement with a resolution
better than 1% FWHM. The gaseous xenon in the TPC additionally allows the
tracks of the two beta particles to be recorded, which are expected to have a
length of up to 30 cm at 10 bar pressure. The ability to record the topological
signature of the neutrinoless double-beta events provides a powerful background
rejection factor for the double-beta experiment.
  In this paper, we present a novel 3D imaging concept using SiPMs coated with
tetraphenyl butadiene (TPB) for the EL read out and its first implementation in
NEXT-DEMO, a large-scale prototype of the NEXT-100 experiment. The design and
the first characterization measurements of the NEXT-DEMO SiPM tracking system
are presented. The SiPM response uniformity over the tracking plane drawn from
its gain map is shown to be better than 4%. An automated active control system
for the stabilization of the SiPMs gain was developed, based on the voltage
supply compensation of the gain drifts. The gain is shown to be stabilized
within 0.2% relative variation around its nominal value, provided by Hamamatsu,
in a temperature range of 10 degree C. The noise level from the electronics and
the SiPM dark noise is shown to lay typically below the level of 10
photoelectrons (pe) in the ADC. Hence, a detection threshold at 10 pe is set
for the acquisition of the tracking signals. The ADC full dynamic range (4096
channels) is shown to be adequate for signal levels of up to 200
pe/microsecond, which enables recording most of the tracking signals.
