Design of single phase liquid argon detectors for dark matter searches

Abstract

Within our current understanding of the makeup of the universe, dark matter makes up 25% of the total energy and over 80% of the matter in the universe. Little is known about the makeup of dark matter, but its existence has been indirectly measured using the rotation curves of galaxies, clusters of galaxies, and the Cosmic Microwave Background. To gain a greater understanding of this component of the universe, direct detection of dark matter is a major objective in particle astrophysics. One popular candidate for dark matter is the weakly interacting massive particle, or WIMP. The allowed rate of interaction between a WIMP and normal matter is extremely low, requiring new detection technologies with greater sensitivity to be explored. Though several experiments have already been conducted, no direct detection experiment has unambiguously identified a dark matter signal. This work explores the use of noble liquids, in a single liquid phase design, to detect single scatters of dark matter particles. The goal of current experiments is to investigate matter-dark-matter interaction cross-sections down to 10 -45 cm2 . With that in mind, the MiniCLEAN detector has been designed with a 500 kg liquid argon detector volume and will be viewed by a spherical 4π configuration of 92 photo-multiplier tubes. In order to determine the ability for single phase noble liquid to detect nuclear recoils from dark matter, several R&D experiments have been performed. These experiments undertook the measurement of how dark-matter-like nuclear recoils and background-like electronic recoils behave in liquid argon. In addition to reviewing the measurements of pulse shape discrimination and other noble liquid properties, my measurement of the scintillation efficiency is described. The scintillation efficiency characterizes the differing energy responses for nuclear and electron recoils. This was the first measurement of the scintillation efficiency in liquid argon for nuclear recoils over a wide energy range. Additionally, this work covers the design and testing of the front-end electronics and data acquisition software I developed for the MiniCLEAN experiment. This system has been designed to record and process thousands of physics events per second and has been tested using novel simulators, that I developed, that approximately represent the expected PMT signals of the MiniCLEAN detector.