Direction-sensitive radiation detectors for rare event searches

Abstract

Future rare event searches, such as the search for WIMP dark matter, will be sensitive to unshieldable backgrounds that could be separated from desired signals using direction-sensitive detectors. Recoil-imaging techniques with real time event-by-event reconstruction such as gaseous time projection chambers provide directional information by recording the resulting ionisation track of a recoiling particle and represent the most mature direction-sensitive detector technology in the energy range relevant for a dark matter search. The gas mixture selected for a gas-based detector is a highly optimised composition tailored to the specific requirements of the experiment. The scattering interactions between drifting electrons and gas molecules however, cause signal diffusion in gas-based detectors. As future rare event searches require large detector volumes in order to increase observable event rates, the longer drift distances result in poor spatial resolution and therefore direction sensitivity. Some current detectors utilise electronegative gases as an additive to gas mixtures, in order to drift negative ions instead of electrons to minimise signal diffusion. Despite the improved spatial resolution permitted by negative ion drift, detectors using electronegative gases such as SF6, experience decreased gain due to the increased energy required to detach electrons from this negative ion species when compared with electrons. This makes direction reconstruction from the observed signal difficult, and applies limits to amplification stages and readout systems required for future large-scale detection systems operating with electronegative gases. In this thesis, prototype direction-sensitive detectors are presented that use novel readout techniques based on combined charge-optical and image-intensified camera technologies. Opportunities for improving gas-based detectors and their performance utilising electronegative phenomena are discussed, including a method of drifting negative ions in a gas-based detector without requiring electronegative additives which is proposed and measured for the first time. The new method makes use of the dissociative electron attachment process to produce negative ions from fragments of the parent gas species and is demonstrated in pure CF4, a common conventional detector gas. The demonstrated process is dependent on the reduced electric field of the experimental setup and resulting ions promisingly provide gain characteristics similar to electrons. Areas of future work investigating electronegative gas processes in detectors are additionally outlined. Show more