  The NEXT experiment aims to observe the neutrinoless double beta decay of
$^{136}$Xe in a high pressure gas TPC using electroluminescence (EL) to amplify
the signal from ionization. Understanding the response of the detector is
imperative in achieving a consistent and well understood energy measurement.
The abundance of xenon k-shell x-ray emission during data taking has been
identified as a multitool for the characterisation of the fundamental
parameters of the gas as well as the equalisation of the response of the
detector. The NEXT-DEMO prototype is a ~1.5 kg volume TPC filled with natural
xenon. It employs an array of 19 PMTs as an energy plane and of 256 SiPMs as a
tracking plane with the TPC light tube and SiPM surfaces being coated with
tetraphenyl butadiene (TPB) which acts as a wavelength shifter for the VUV
scintillation light produced by xenon. This paper presents the measurement of
the properties of the drift of electrons in the TPC, the effects of the EL
production region, and the extraction of position dependent correction
constants using K$_{\alpha}$ X-ray deposits. These constants were used to
equalise the response of the detector to deposits left by gammas from
$^{22}$Na.
