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NITEC: a Negative Ion Time Expansion Chamber for directional Dark Matter search

Periodic Reporting for period 1 - NITEC (NITEC: a Negative Ion Time Expansion Chamber for directional Dark Matter search)

Reporting period: 2015-05-04 to 2017-05-03

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.
We developed a 5 cm drift distance field cage design for low pressure operation, to be coupled to the triple GEMs + Timepix assembly. Field cage rings were manufactured with the 3D printer and silver 1 mm diameter wires were enclosed to ensure uniformity of the drift field. We designed and had an external company build a 50 cm diameter cylindrical vacuum vessel to operate NITEC below atmospheric pressure. Mylar and quartz windows were installed in the vessel, to allow operations with an external particle beam or lasers. In parallel, a proper gas system was setup in the lab, in order to be able to operate with various gas mixtures at different pressures. We started characterised NITEC with conventional electron-carrier gases, as Ar:CO2 and Ar:CO2:CF4 at atmospheric pressure, to test its functioning in well-known conditions. Surprisingly, thanks to the Timepix pixels granularity, single ionisation clusters are nicely visible already in this data, even without the diffusion suppression provided by the negative ion drift. Algorithms for cluster counting are under optimization, but a preliminary version already possess about a 50% efficiency. We then installed NITEC inside the vacuum vessel for low pressure operation and performed measurements with an electrons beam facility (the BTF), 55Fe x-ray sources and cosmic rays. With these, we measured drift velocities and mobilities of pure SF6 (at 75, 100 and 150 Torr), He:CF4:SF6 at 60:40:100 Torr and 360:240:10 Torr and Ar:CO2:SF6 at 192:85:93 Torr and the effective gas gain in pure SF6 at 150, 175, 200, 225, 250 and 370 Torr. We are still optimising reconstruction algorithms to perform a single track data analysis and identify single ionisation clusters and minority carriers.The results of the NITEC project have been presented in six workshops and allowed the project proponents to quickly establish a reputation in the DM search community. The Experienced Researcher has today a leadership position in the development of the next generation directional DM detector at the ton scale (CYGNUS-TPC and UNDER) and had two additional project on negative ion drift studies approved and funded (CERN RD51 and INFN CSN5 projects).
The NITEC project proved for the first time negative ion drift operation with triple thin 50 um GEMs. The most significant result from NITEC is the demonstration of the feasibility of negative ion operation with a He:CF4:SF6 mixture at nearly atmospheric pressure (610 Torr). A NITPC approach with an He:SF6 gas mixture (possibly at atmospheric pressure) with installation in multiple underground sites has been recently proposed by the newly-formed collaboration CYGNUS-TPC for the next-generation directional DM experiment at the ton-scale. The gas choice is dictated by the exploitation of negative ion drift for reduced diffusion and minority carriers for fiducialization, while at the same time extending the experiment sensitivity for spin-independent cross section to low WIMP mass thanks to the He. Our achievement is the first demonstration for a realistic possibility of a development of a negative ion TPC with SF6 at 1 bar, possibly also with optical readout thanks to the CF4 and SF6 scintillation properties. Starting from the NITEC experience and the new partnerships established during and thanks to the MSCA, we are now in fact developing a proposal for a 1 m3 NITPC detector with He:SF6 or He:SF6:CF4 mixture and triple thin GEMs amplification and optical (CMOS + PMTs) readout to be installed at Laboratori Nazionali del Gran Sasso (LNGS) by 2019.
NITEC setup in the vacuum vessel at the electron beam facility BTF
NITEC setup in the vacuum vessel with radioactive sources in the lab
NITEC prototype