Cryogenic Scintillators for Rare-Event Searches

Abstract

Rare-event searches, such as the direct detection of dark matter, of neutrinoless double beta decay and of the decay of long-lived radioisotopes, require highly-sensitive radio-pure detectors to measure extremely rare interactions amidst a sea of background events coming from natural radioactivity. The DAMA/LIBRA dark matter experiment has a long-standing claim of detecting event excesses inconsistent with known background and attributes these to the annual modulation signature expected from dark matter interactions on Earth. The DAMA/LIBRA detector array is composed of NaI(Tl) scintillation detectors, which do not possess event-by-event background rejection capabilities. Scintillating calorimeters, cryogenic detectors based on scintillating crystals that produce simultaneous heat and light signals, are a promising detector technology capable of a powerful level of background rejection. Rare-event searches employing scintillating calorimeters are on the look-out for crystals with favourable low-temperature properties. This dissertation will describe the optical cryostat that our group has designed, commissioned, tested and is now operating at Queen's University for the purpose of studying samples of scintillating crystals and measuring their properties as a function of temperature. In particular, it details measurements of the light yields of the alkali halides NaI, CsI and NaI(Tl) under alpha- and gamma-excitation down to 3.4 K. The temperature response of the alpha/gamma quenching factors of these scintillators is also presented, along with surface-dependent systematics. Informed by these low-temperature scintillation results, a scintillating calorimeter based on alkali halides is proposed and its expected sensitivity to dark matter is studied. The technical feasibility of such a detector is also discussed. It is demonstrated that if technical challenges can be surmounted and the phonon performance of the alkali halides can be improved to the level of other materials, a short exposure of 10 kg-days would have a good chance of being able to fully test the DAMA/LIBRA claim under standard astrophysical assumptions.