  TREX-DM is conceived to look for low-mass Weakly Interacting Massive
Particles (WIMPs) using a gas Time Projection Chamber equipped with micromegas
readout planes at the Canfranc Underground Laboratory. The detector can hold in
the active volume 20 l of pressurized gas up to 10 bar, corresponding to 0.30
kg of Ar or 0.16 kg of Ne. The micromegas are read with a self-triggered
acquisition, allowing for thresholds below 0.4 keV (electron equivalent). A low
background level in the lowest energy region is another essential requirement.
To assess the expected background, all the relevant sources have been
considered, including the measured fluxes of gamma radiation, muons and
neutrons at the Canfranc Laboratory, together with the activity of most of the
components used in the detector and ancillary systems, obtained in a complete
assay program. The background contributions have been simulated by means of a
dedicated application based on Geant4 and a custom-made code for the detector
response. The background model developed for the detector presently installed
in Canfranc points to levels from 1 to 10 counts keV-1 kg-1 d-1 in the region
of interest, making TREX-DM competitive in the search for low-mass WIMPs. A
roadmap to further decrease it down to 0.1 counts keV-1 kg-1 d-1 is underway.
