The T-REX Dark Matter experiment is a medium scale radiopure gaseous TPC whose aim is to search for low-mass WIMPs, potentially composing our galactic dark matter halo. TREX-DM consist of two side-by-side common-cathode TPC modules read by two microbulk Micromegas high-granularity readout planes. The radiopurity of the microbulk planes and the rest of detector components combined with the topological information promise very low background levels. The amplification in the gas, together with high signal-to-noise ratio obtained by state-of-the-art self-triggered TPC DAQ electronics allows for very low energy threshold. The combined prospects of low background and very low (sub-keV) threshold is a unique combination with very promising prospects in the search of WIMPs with particularly low masses (e.g. below 10 GeV).
Design of the TREX-DM detector. See this for details.
The Micromegas planes are 25×25 cm2 and are the largest single Micromegas built so far with the microbulk technique. This novel technique allows to build the Micromegas amplifying structures out of double-clad kapton and copper foils, two very radiopure materials.
The sensing pixels of the Micromegas are arranged in X-Y strips with a 1 mm pitch. All signal are extracted out of the same kapton foil of the microbulk, avoiding connections or soldering close to the detector. The readout design is an evolution of previous ones developed for the CAST experiment.
This patterns allows to precisely image the event’s topology in the gas, and use it to discriminate background from potential signal events. TREX-DM has about 1000 readout channels in total.
Some sample events registered with TREX-DM
The detector can hold 20 l of pressurized gas up to 10 bar (corresponding e.g. to 0.30 kg of Ar or 0.16 kg of Ne). The whole body of the TPC is built with the highest radiopurity standards. For example the cylindrical vessel (0.5 m diameter and 0.5 length) is made of 6 cm thick pure copper.
The field cage is made of a teflon structure and the shaping rings are copper strips engraved on a flexible kapton structure. The cathode is a very thin aluminized mylar foil. All detector components have been carefully screened.
The detector is fully surrounded by a shielding made of high purity lead (20 cm) and an innermost layer of high purity copper (5 cm), able to stop all the environmental gamma radiation down to negligible levels. The setup is surrounded by water tanks to shield neutrons. The inside of the shielding is flushed with clean gas (either nitrogen out of LN2 evaporation, or Rn-free air) to prevent the presence of radioactive Rn close to the detector.
TREX-DM installed in the hall A of LSC, with its lead+copper shielding half mounted
The experiment was approved by the LSC at the beginning of 2017. During 2017 into 2018 the experiment was installed at the hall A of LSC. Then it entered the commissioning phase. End of 2018 the experiment has started its first engineering runs at low pressure, and it faces the first physics runs for 2019.