Atmospheric Dependence of Fractoluminescence and Phononic Emissions in Inorganic Scintillator Fractures

Abstract

Background signals in particle detectors can originate from several categories of sources, including radioactivity found in the detector or its housing and cosmic radiations. High energy physics relies heavily on the use of particle detectors that are capable of identifying the desired particles while ruling out backgrounds. Fractures in the detectors create a background that makes it difficult to anticipate the timing of their occurrence. Given that typical rare-event search experiments last months or longer, it would be advantageous to find a way to identify when fractures occur and how to discriminate against them. The Double Cleavage Drilled Compression (DCDC) geometry allows creating fracture-inducing stresses in the material in a reliable manner. Given the brittle nature of Bismuth Germanate (Bi3 Ge4 O12 or BGO) single-crystal scintillators and their negligible afterglow, this thesis investigates backgrounds due to fractures in BGO crystals in the DCDC geometry. A series of experiments were conducted where both photonic and phononic emissions were recorded along with the crack propagation to study the fractoluminescence and acoustic signals generated, and how they would interfere with particle detection. Fractoluminescence in global fracture was confirmed in BGO over many conditions. Acoustic emissions due to fractures were correlated to the crack growth, and significant acoustic events were measured during subcritical fracture. The subcritical fracture was measured using image analysis of the crack, and the crack growth model was identified using evolutionary algorithms. The thesis also discusses the design of micro-fabricated chips to test the effects of cryogenic temperatures on the rate of crack growth, and to study the possibility of quantum effects dominating the subcritical fracture mechanism.