The EXO search for Neutrino-less Double Beta Decay

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This application seeks support for the EXO program searching for neutrino less double beta decay in xenon. The proposed work is comprised of three main, tightly coupled, activities: work on the existing 200 kg detector, EXO-200; work to establish the first detector to explore the inverted hierarchy, nEXO; and work to find a roadmap to reach the normal hierarchy.******We are participating in the operation and exploitation of the EXO-200 detector. This pioneering detector was the first to reach sensitivity at the hundred kilogram scale and with this detector we have made the first observation of the two neutrino decay of 136Xe. We established a limit for the neutrino less decay mode which at the time was a world record and it was the first measurement to seriously challenge the claimed observation of this decay process by the Heidelberg-Moscow collaboration. It has been decided to operate for another three years to get an improvement of about a factor of 3 in the sensitivity to double beta decay. This will again make the detector comparable in sensitivity to the best in the world. This work also provides physics topics for our students and research associates.******The second part of the work is the development of the design for the proposed 5 tonne detector nEXO, that the collaboration plan to install at SNOLAB. If approved this should be the world's most sensitive detector for this critical physics study and would provide coverage of the entire inverted hierarchy region of neutrino masses. While the detector has its basis in the EXO-200 design, there are a number of area where improvements can be made in the design or where the design of the 200 kg detector does not scale nicely to the tonne scale. The Canadian team has been very active in this process. We have undertaken major responsibilities for the light sensitive system which will employ SiMs. Another area of responsibility is the design of calibration delivery systems. These activities form a major part of our activities for the coming period. We have also started to explore if less expensive techniques can be found to enrich the xenon. Specifically, we will carry out studies to see if the vapour pressure differences of the xenon isotopes are large enough to permit separation by distillation.******Finally the group is looking ahead to develop concepts for detectors that can reach the normal hierarchy. One of the techniques we have been exploring is to tag each event with the presence of otherwise of a daughter barium ion. We have developed concepts for this and have been successfully testing each of the steps in the process but now we must combine the steps to study a complete process. It will be critical to understand the overall efficiency for the process and to measure any inherent backgrounds to the measurement.