Towards CYGNUS: A Large Scale, Low Threshold TPC for the Direct Detection of Dark Matter

The evidence supporting the existence of a new type of matter, know as dark matter, is both considerable and wide ranging. From this evidence, it is predicted that ∼84% of the matter in the universe is dark. Further to this, multiple theories, that attempt to extend our understanding beyond the incomplete standard model of particle physics, naturally produce dark matter candidate particles. One popular hypothesis states that dark matter is both weakly interacting and massive, leading to the term WIMP (Weakly Interacting Massive Particle). The three methods used to find such a particle are indirect, direct and collider searches. The focus of this work is the development of a large scale direct detection experiment, capable of reconstructing the direction of a recoiling target nucleus. The detector is envisioned as a Negative Ion Time Projection Chamber (NI-TPC) with a 20 Torr SF6 target volume. The ultimate goal is to use the discriminatory power of directionality to explore the dark matter parameter space that is subject to a neutrino background. The detector would be either a single 1000 m3 structure, named CYGNUS-1000, or multiple smaller structures that sum to the same target volume. A first step in this process would be the construction of a 10 m3 prototype, named CYGNUS-10. The achievable low energy (1-10 keVee) gamma background rejection of these detectors was studied using simulation and found to be 10−4 keV−1 yr−1 above a 6 keVee threshold. This result was used, along with a neutron limit of < 1 yr−1, to prove the feasibility of a CYGNUS TPC from a background perspective, using a dedicated GEANT4 Monte Carlo simulation. A method to enhance the sensitivity of a future detector, using machine learning (ML), was demonstrated on existing data from DRIFT-IId (Directional Recoil Identification From Tracks). The ML algorithm produced an improved WIMP cross section reach of 32% compared to the standard analysis. The results of this study were used to produce WIMP search reach predictions for a CYGNUS-1000 and CYGNUS-10 TPC, which showed that a 1000 m3 target volume, of 20 Torr SF6, could eventually observe a neutrino background with directional sensitivity. A readout constructed from an MWPC (Multi-Wire Proportional Chamber) and ThGEM (Thick Gaseous Electron Multiplier) amplification stage was found to produce the least background for a CYGNUS TPC. This result led to the construction of an MWPC-ThGEM hybrid prototype using a 10 cm diameter ThGEM and a 30 cm long wire array with sub-mm pitch (the lowest achieved for an MWPC). The chosen array size reflected the scale up requirements of a future detector with large readout areas. For the prototype, 2D track reconstruction in SF6 gas was demonstrated using alpha tracks and initial gain measurements were performed in CF4, resulting in gains O(103). A large scale ThGEM device, with an area of 40×40 cm2, was tested separately from the readout and was shown to produce ∼2 orders of magnitude lower gain than it’s smaller counterpart, suggesting that the performance of a ThGEM is reduced when scaling the device to larger areas. A CYGNUS TPC prototype, called CYGNO, is planned for construction at LNGS (Laboratori Nazionali del Gran Sasso). This prototype uses camera based readouts and an atmospheric gas mixture of He and CF4. The gamma background from the camera was studied here and a TPC design aimed at reducing this was considered. It was found that introducing shielding into the TPC geometry had little effect on the resulting background rate, due to the need for transparent windows, suggesting that other background mitigation methods are required for this readout.