Cryogenic Alkali Halide Scintillators for Rare-Event Searches

Abstract

Rare-event searches such as the direct detection of dark matter require very low backgrounds, as well as large volumes of material because of the low rate of signal. Scintillators are an excellent particle detector candidate for such searches because of the flexibility in material choice for the selection of different signals, as well as the possibility of purification to remove backgrounds. Cryogenic scintillation detectors employ further background discrimination by utilizing the signal of heat input when a particle interacts in the material. The ratio of light emitted to detected heat can be used as a selection criteria to remove backgrounds such as gamma radiation. DAMA/LIBRA is a dark matter direct detection experiment that claims the detection of an annual modulation signal using thallium-doped sodium iodide (NaI(Tl)) scintillating crystals that is consistent with a dark matter hypothesis. Many other dark matter direct detection experiments have failed to verify this signal using other materials. One popular idea is to try to verify this signal using the same or similar materials to DAMA/LIBRA, such as alkali halide scintillators to remove any material-dependent assumptions. Several collaborations, such as SABRE and COSINUS, are currently developing experiments using NaI and NaI(Tl) to test DAMA/LIBRA in the near future. This dissertation will present the ability of a small detector with background discrimination, such as a cryogenic scintillation detector, to detect a modulating signal. These simulations indicate that even a relatively small exposure could investigate an annual modulation signal if the background was reduced. Towards the development of such a detector, this dissertation will also present measurements done using an optical cryostat at Queen's University to characterize the light emission characteristics of alkali halide crystals, including cesium iodide (CsI), NaI, and NaI(Tl) at low temperatures. Light yields and scintillation time constants are reported, as well as the ratio of light emission between alpha and gamma excitations as a function of temperature, a measurement available to our experiment because of a high light collection efficiency and novel data processing. It shows that CsI, NaI, and NaI(Tl) have sufficient light yield at low temperatures for use in a cryogenic scintillation detector to further understand the DAMA/LIBRA modulation claim.