This award addresses two world-class experiments in areas identified as high priority questions in fundamental physics, one in search of neutrinoless double beta decay and one in search of dark matter. Firstly, this award provides support for the group's involvement in the SNO+ Collaboration. SNO+ is the conversion of the Sudbury Neutrino Observatory detector, a heavy water Cherenkov detector located 6800 feet underground in a working nickel mine near Sudbury, Ontario, Canada, into a new experiment by replacing the heavy water in the acrylic vessel with linear alkylbenzene, a liquid scintillator. This will yield enhanced sensitivity at lower energies enabling the SNO+ experiment to address several interesting scientific questions. By adding the double beta decay emitter 150-Nd to the liquid scintillator, SNO+ has the potential to lead the field of neutrinoless double beta decay experiments. Secondly, this award provides funds for the group to build a trace gas analyzer based on cavity ring-down spectroscopy technology.
The broader impact of the program includes providing opportunities for undergraduate and graduate students from Idaho and South Dakota, including under-represented Native American students, to participate in world-class international experiments in the exciting setting of underground particle physics. Working with BHSU's Center for the Advancement of Math and Science Education and the Education and Outreach arm of the Sanford Lab, education and outreach activities are planned which will involve undergraduate students as well as area high school students and their teachers.
Large-scale liquid noble gas scintillation detectors are the basis for many current and proposed particle astrophysics experiments including dark matter searches, neutrinoless double beta decay experiments, and long baseline neutrino experiments. One such project is the DARKSIDE experiment. DARKSIDE is a program of progressively larger-volume liquid argon-based two-phase (liquid and gas) time projection chambers which will search for dark matter in the form of WIMPs (Weakly Interacting Massive Particles). When an ionizing particle interacts with a detector filled with liquid argon, free electrons form an ionization signal and excited molecular argon is formed which then radiatively de-excites through scintillation light at 128 nm. Singlet and triplet states of the excited molecular argon are formed, each witha different lifetime. These ionization signals and scintillation photons can be independently detected and spatially resolved in the detector to identify a WIMP event and distinguish it from background events. If impurities are present, a non-radiative de-excitation of Ar can occur, reducing the number of excited argon molecules and effectively quenching the scintillation light. In the DARKSIDE-10 prototype detector, a measurable increase in the light yield was observed when purifying the argon to sub-ppb levels. Impurities not only reduce the overall light yield, they effectively reduce the lifetime of the long-lived (triplet) scintillation light without affecting the short-lived (singlet) component, thus changing the pulse shape of the scintillation signal that is used to reject background events. To test the impurity levels in these liquid noble gases, we have developed at Black Hills State University (BHSU) a Cavity Ring-Down Spectroscopy (CRDS) system. CRDS is a method of spectroscopy that utilizes a greatly increased pathlength by using extremely highly reflective mirrors, which allow the light to pass many times through the sample. The resulting instrument measures trace amounts of CH4, H2O and other impurities in noble gases such as argon and xenon that are of interest in dark matter detection. The sensitivity is projected to be at least an order of magnitude better than currently available commercial products. With our current noise level in the ring-down time, we can measure approximately 1.0 ppb water vapor in nitrogen and are in the process of improving this noise level iteratively by more careful mode-matching and better temperature and pressure stabilization of the cavity. Additionally, simply by lengthening our cell to 2 m (under discussion with TigerOptics) and assuming our current noise level (before any mode-matching changes, etc.), our sensitivity will be approximately 0.1 ppb.
