nEXO photo-detector development

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The aim of this proposal is to support the development of photo-detectors for the nEXO experiments by 1) enabling the development of a state of the art Canadian technology and 2) allowing the Canadian group to continue addressing critical issues pertaining to the use of silicon photo-multipliers (SiPMs) within the nEXO detector. Item 1) is a unique opportunity for NSERC to promote the 3 dimensional integration technology (3D-SiPM) that is driven by the université de Sherbrooke (QC) and is expected to revolutionize high speed photon detection. Item 2) is critical to maintain the Canadian leadership within the nEXO photo-detector group, and to ensure that the nEXO experiment presents a full-proof case to the US Department of Energy (DOE) for the neutrino-less double beta decay down select expected to happen in 2017. The Canadian group is playing a major role within the nEXO collaboration with D.Sinclair being chair of the board, D.Sinclair and F.Retière being member of the executive council, and K.Graham and F.Retière convening the simulation and photo-detector groups respectively. The developments of photo-detectors and of the high voltage system are the two highest priority R&D items to be addressed before the down select. The nEXO photo-detector group has shown that 1% or better energy resolution can be achieved at 2456 MeV using SiPMs manufactured by the Fondazione Bruno Kesler (FBK). Indeed measurements at Stanford and TRIUMF show that the FBK SiPMs meet all specifications: photon detection efficiency larger than 15%, dark noise rate less than 50Hz/mm^2, and correlated avalanche probability less than 20%. The U.Alabama group has shown that the SiPM radioactive content is consistent with the limits defined by the simulation group. The high voltage group has also made progress in identifying sources of breakdown under the leadership of R.Gornea using a setup built at the University of Bern (Switzerland) that has now been moved to Carleton U. The photo-detector development effort within the nEXO collaboration is shifting from identifying suitable photo-detectors to optimizing performances and addressing integration issues: high voltage compatibility, operation in liquid Xenon, readout, interconnection and mechanical assembly. The 3D-SiPM technology originally developed by the U.Sherbrooke group for Positron Emission Tomography promises to solve all the integration issues, essentially by having photons entering the front face of the SiPM and bits coming out of the back side of a 8x12cm^2 silicon interposer. This solution is a state of the art Canadian solution requiring significant R&D but promising outstanding rewards. In this grant we request funding for supporting the R&D on 3D-SiPM, upgrading the TRIUMF test setup and investigating operation of SiPMs within a large electric field which is one of the major issues of the current nEXO design.