UltraRadioPure Sodium Iodide Detectors for Dark Matter

The quest to understand dark matter is one of the major activities in modern particle physics and astrophysics. It is widely believed that dark matter is composed of a new type of subatomic particle that is not yet well understood. An important discovery in this area is the annual modulation signal measured using sodium iodide scintillation crystals deep in an underground laboratory in Italy. While this result runs contrary to the findings of some other experiments that utilized different methods, there are also some recent results that support the findings. Therefore, there is a clear need for confirmation of the experiments. A new sodium iodide experiment with lower radioactivity background levels would have a higher sensitivity that could confirm or refute the annual modulation result. The goal of this project is to develop sodium iodide detectors with considerably lower background. This is being accomplished by intensive purification of the raw materials, followed by special processes for crystal growth and detector fabrication. In the Phase I project, considerable progress was made in the purification and crystal growth aspects, which demonstrates the feasibility of the approach. Unprecedented low levels of radioactive impurities were achieved in large sodium iodide. Scintillation properties of the highpurity sodium iodide crystals were also found to be suitable for dark matter experiments. The proposed Phase II project will follow the approach demonstrated in Phase I and scale it up for the full size detectors that are needed. Prototype dark matter detectors will be built with the necessary purity and components to provide suitable performance for the annual modulation experiments to be replicated with even better sensitivity. The scientific impact of this result will be significant in furthering the understanding of a fundamental mystery in modern physics and cosmology. The availability of highpurity sodium iodide crystals will enable several largescale dark matter experiments that are under development worldwide.