  NEXT-DEMO is a large-scale prototype of the NEXT-100 detector, an
electroluminescent time projection chamber that will search for the
neutrinoless double beta decay of Xe-136 using 100 to 150 kg of enriched xenon
gas. NEXT-DEMO was built to prove the expected performance of NEXT-100, namely,
energy resolution better than 1% FWHM at 2.5 MeV and event topological
reconstruction. In this paper we describe the prototype and its initial
results. A resolution of 1.75% FWHM at 511 keV (which extrapolates to 0.8% FWHM
at 2.5 MeV) was obtained at 10 bar pressure using a gamma-ray calibration
source. Also, a basic study of the event topology along the longitudinal
coordinate is presented, proving that it is possible to identify the distinct
dE/dx of electron tracks in high-pressure xenon using an electroluminescence
TPC.
