Searching for light dark matter with a spherical proportional counter

Knights, Patrick Ryan ORCID: 0000-0003-2981-705X (2021). Searching for light dark matter with a spherical proportional counter. University of Birmingham. Ph.D.

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The vast majority of mass in the universe is comprised of an unknown form of matter – Dark Matter. The NEWS-G collaboration are using a novel gaseous detector, the spherical propor- tional counter, in a search for Dark Matter particles with masses down to sub-GeV. Having set the first exclusion limit on the spin-independent DM-nucleon cross sections for a 0.5 GeV DM particle in 2017, NEWS-G is now turning its focus to it’s new detector, SNOGLOBE. The 140 cm-diameter spherical proportional counter was constructed and commissioned in France and has now been shipped to SNOLAB, Canada, for a direct DM search.
For SNOGLOBE to achieve it’s physics potential, several developments are required, in- cluding the understanding of the detector, the properties of gases, background suppression techniques and the physics potential of future experiments. Developments in the spherical proportional counter read-out technology are presented, which uses high-resistivity electrodes to improve stability and energy resolution. The multi-anode sensor, ACHINOS, enables the operation of larger detectors at higher pressures. A simulation framework for the spherical proportional counter has also been developed, which is an important tool for understanding how the detector operates. Another critical component to understanding the operation of the detector when looking for low-energy nuclear recoils induced by DM interactions is the ionisation quenching factor. Measurements of this in gases are scarce, and so a method to calculate this from measurements of the W-value has been developed and applied to several gases. The suppression of radioactive backgrounds is of paramount importance for future NEWS-G spherical proportional counters, and all rare-event search experiments. A method for producing highly radiopure copper is electroforming, which has been used to apply a layer to SNOGLOBE’s inner surface and suppress experimental backgrounds. The application of this technique to produce future NEWS-G detectors is discussed, along with their physics potential.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Physics and Astronomy
Funders: None/not applicable
Subjects: Q Science > QC Physics


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