  The NEXT-100 detector will search for the neutrinoless double beta decay of
$^{136}$Xe using an electroluminescent high-pressure xenon gas TPC filled with
100 kg of enriched Xe. An observation of this hypothetical process would
establish a Majorana nature for the neutrino and prove the violation of lepton
number. A scaled-down prototype, NEXT-DEMO, has been built to demonstrate the
feasibility of the technology. NEXT-DEMO includes an energy plane made of PMTs
and a tracking plane made of SiPMs. X-ray energy depositions, produced by the
de-excitation of xenon atoms after their interaction with gamma rays, have been
used to characterize the detector response. With this method, the released
energy by gammas coming from $^{22}$Na source has been corrected, achieving an
energy resolution of 5.691% FWHM and 1.62% FWHM at the 29.7 keV and 511 keV
peaks respectively, which extrapolate to 0.62% FWHM and 0.73% FWHM at Q$_{\beta
\beta}$ value of Xenon.
