Novel Techniques for Neutrinoless Double Beta Decay Search - bridging grant

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Experiments like SNO and Super-K have observed neutrino oscillations and convincingly demonstrated that these elusive particles have finite masses. But neutrino oscillation experiments cannot determine the absolute mass scale nor probe if the neutrino is a Majorana particle, in contrast to charged fermions known to be Dirac particles. The observation of neutrinoless double beta decay may bring insight into the neutrino mass generation. The measurement of the effective neutrino mass, related to the half-life of the decay, will help determine the neutrino mass hierarchy. The next generation neutrinoless double beta decay experiments will require a very large active mass and ultra low background. The proposed nEXO, a 5-tonnes liquid xenon time projection chamber (TPC), is designed to have a sensitivity to the half-life of Xe-136 decay of the order of 10^28 years. All next-generation detectors face tremendous challenges for reducing the background induced by the residual natural radioactivity. Current generation detectors, like EXO-200 which has provided limits on the Xe-136 neutrinoless double beta decay half-life of the order 10^25 years, are built with carefully screened materials using special design and construction techniques. To reach the nEXO physics goal, further background reduction is mandatory. Standard techniques have reached a limit and therefore a novel one must be developed. The double beta decay of Xe-136 produces a Ba-136 ion that is the only element for which there is experimentally demonstrated single-ion detection and identification capability aka tagging. Therefore it is envisaged that Ba ion tagging can lead to the total elimination of the background due to residual radioactive impurities or induced by cosmic rays. However, applying efficiently the Ba ion tagging technique to a massive cryogenic TPC is a challenge. We are presenting an essential R&D program aimed at developing, optimizing and characterizing a probe displacement device for ion collection from a liquid xenon TPC using radioactive ions. This project is done in coordination with the nEXO collaboration and will provide a definitive answer regarding the feasibility of applying the Ba ion tagging technique to neutrinoless double beta decay search. The work will focus on three main aspects: accurate ion tracking in liquid xenon including studying the ion charge state during its drift in the TPC, efficiency evaluation of electrostatic ion grabbing, extraction and subsequent displacement outside of the TPC and HV issues related to the high electric fields surrounding the TPC cathode and the ion collection probe. This proposal, in addition to EXO-200 data taking and analysis, will provide an interesting research program for graduate students at both Master's and PhD levels. Only support for one year is asked since afterwards the applicant will join the main SAP - Project application for EXO Canada for following years.