The goal of the NITEC project was the construction of a Negative Ion Time Projection Chamber (NITPC) with triple Gas Electron Multiplier amplification and pixel readout (GEMPix) for directional Dark Matter (DM) searches. DM is 5 times as prevalent as normal matter in the Universe, but its identity remains unknown. Its mere existence implies that our inventory of the basic building blocks of nature is incomplete: deciphering its nature is one of the most compelling tasks for fundamental physics and astronomy. Direct DM detection experiments aim at observing very low energy (10-100 keV) nuclear recoil caused by a DM particle scattering in the matter and imply detectors with very challenging requirements on the allowed backgrounds. While today leading experiments have reached excellent rejection of electromagnetic component, neutrons and neutrinos (producing a detector response nearly identical to DM) will become dangerous sources of backgrounds for the next-generation detectors. Extremely powerful tools in event discrimination are the topological signature and the directionality of the event. Although inherently challenging, directional gaseous TPCs potentially provide the best observables for a DM search experiment, including the above, thanks to the possibility of measuring the charge (and dE/dx) released along the track. Above all, directionality offers a new and powerful observable. Due to the Earth's motion with respect to the Galaxy and the expected DM distribution, an apparent DM wind coming from the Cygnus constellation is expected to be observable, with a change in direction of about 90 degrees for every 12 sidereal hours. Here lies the strength and timeliness of directional DM searches: no background can mimic a directional correlation with an astrophysical source. For these reasons,we believe that the contribution of directional DM detectors has become crucial at the present moment and developed the NITEC project in this context. In a conventional TPC, image transport is by free electrons. A peculiar modification of this approach involves the addition of a highly electronegative dopant, making it a Negative Ion TPC: NITPC. In this configuration, the primary electrons liberated by the track during gas ionization are captured at very short distances by the electronegative molecules. The resulting anions drift to the anode where they produce a normal electron avalanche. Since anions act as image carriers instead of electrons, their higher mass reduces longitudinal and transversal diffusion to the thermal limit. NITEC innovation resides in the combination for the first time of the negative ion drift with a pixelated charge readout, the GEMPix. The GEMpix is a triple thin GEMs for charge amplification coupled to a Timepix chip for readout (55 x 55 um2 pixels). Timepix can be operated in counting mode, Time Of Arrival or Time Over Threshold mode, allowing to measure time or charge deposited along the track with high precision. A distinctive feature of the Timepix, not available in other chips used in this field, is the adjustable sampling frequency down to 50 kHz, of paramount importance when working with negative ions, whose velocity is 1000-10000 times slower than electrons. With NITEC we proved for the first time negative ion operation with triple thin GEMs, validating GEMs high amplification capability also in this environment. Our work contribute to expanding the knowledge on the negative ion drift, in particular with the innovative capture agent SF6 with the measurement of drift velocities and mobilities at various pressures and in various mixtures. Above all, the most significant NITEC achievement is the demonstration of negative ion operation with a He:CF4:SF6 mixture at nearly atmospheric pressure (610 Torr), opening extremely interesting opportunity for next generation directional Dark Matter detectors.
