Towards CYGNUS-UK: Developments in Negative Ion Charge Amplification and Readout Technologies for Directional Dark Matter Detection

Dark Matter (DM) is known to constitute the majority of mass in the Universe; however, its true particle nature remains a mystery. Particles in the Standard Model (SM) are not considered viable candidates for DM and thus alternatives must be pursued. The prevalence of one well motivated particle candidate called a Weakly Interacting Massive Particle (WIMP) could explain the observed influence of DM. Two-phase Time Projection Chambers (TPCs), using liquid and gaseous xenon, currently lead the direct search for rare WIMP-nucleon scattering events. The sensitivity of these experiments has improved by several orders of magnitude since their inception and are now faced with a new irreducible background of neutrinos, predominantly produced by processes in the Sun. However, the most viable method for irrefutably identifying a WIMP-induced signal, particularly in light of this neutrino background, is the directional measurement of recoiling nuclei; an approach which is not feasible with conventional two-phase experiments.

There are several detector technologies capable of directional detection; however, low pressure gaseous TPCs are the most developed and widely used. Previous directional searches like the Directional Recoil Identification From Tracks (DRIFT) experiment have established the groundwork for the next generation of directional searches, like that of the CYGNUS and CYGNO collaborations, by pioneering the fundamental principles of operation. These collaborations are largely in a phase of small scale research and development aiming to identify optimal technologies for a global network of nuclear recoil observatories. Major avenues for research include: the investigation of helium gas mixtures capable of more cost-effective atmospheric operation, which ideally possess Negative Ion Drift (NID) characteristics for high-fidelity event reconstruction; innovative gain stage devices, which are capable of producing large charge amplification in NID gases comparable to conventional gases; high-granularity charge readout technologies, which can improve the event reconstruction resolution compared to previous technologies used by DRIFT; and new scalable charge readout and data acquisition systems, which can handle the increased number of readout channels associated with high-granularity technologies.

In this thesis, significant developments towards these major research goals are presented. Charge amplification on the order of 10⁴ was successfully demonstrated in CF₄:He mixtures, which prioritised directional potential, at low and atmospheric pressures with a single Thick Gaseous Electron Multiplier (ThGEM) for the first time. Significant work with a novel multi-stage Multi-Mesh ThGEM (MMThGEM) demonstrated large charge amplification on the order of 10⁴ in an NID gas, low pressure SF₆, for the first time. The MMThGEM also showed promise for SF₆:He mixtures, demonstrating large 10⁴ gas gains up to 100 Torr. This device was then coupled to a high-granularity micromegas detector and demonstrated high gain of order 10⁵ in low pressure SF₆; constituting a gas gain in an NID gas which is an improvement of 2 orders of magnitude. This detector was also used to demonstrate 2-dimensional nuclear recoil detection in a large 1 m³ volume of low pressure SF₆, with 32 channels, for the first time. Subsequent efforts focussed on scaling up the readout electronics and demonstration of full area instrumentation of the micromegas readout plane. This featured a comparison with alternative micromegas designs, and the first novel operation of the scalable readout system in low pressure pure CF₄. Ultimately, the results presented in this thesis successfully demonstrate order of magnitude improvements in the gas pressure of mixtures with directional potential, charge amplification in NID gases, and the granularity of charge measurements compared to the technologies utilised by the DRIFT experiments.