Equipment for research and development on EXO at TRIUMF

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The next generation of the Enriched Xenon Observatory (nEXO) is designed to achieve unprecedented sensitivity to the possible neutrinoless double beta decay of 136Xenon. It will probe a large fraction of the parameter space allowed by theory which explicitly requires that neutrinos are Majorana particles, i.e. that neutrinos are their own anti-particles. In particular, nEXO will probe the full parameter space available if the neutrino mass hierarchy is inverted. The detection of neutrinoless double beta decay would show that neutrino are Majorana particles and would allow determining their absolute mass. nEXO will hold 50 times more xenon than EXO-200, its predecessor, which is currently running and has established one of the most competitive limits to date. nEXO design borrows heavily from EXO-200 but several key elements have to be revisited. In particular, the TRIUMF group intends to participate to the redesign of the photo-detector planes. In addition, TRIUMF intends to contribute to the development of a Barium tagging system that would dramatically enhance EXO background reduction capability. The photo-detector plane is being redesigned mainly because of issues with procurement of the large area avalanche photo-diodes (APDs) that were used in EXO-200. The leading technology envisioned to replace the APDs is Pixelated Geiger Mode Avalanche Photodiode (PPD) also called Silicon photomultiplier (SiPM, although, it is a brand name). TRIUMF has extensive expertise in the development of PPDs starting with the design, construction, and operation of the T2K Fine Grained Detector in 2007 to the development of PPD based solutions for Positron Emission Tomography or muon Spin Rotation. One of the main goals of this grant application is the construction of a setup for testing PPDs in liquid Xenon condition, at a temperature of 160K and for 178nm photons. The nEXO collaboration indeed still needs to demonstrate that PPDs fulfill the experiment requirement, especially in term of energy resolution. The proposed setup will allow the TRIUMF group to characterize PPDs from several vendors in order to identify suitable candidates. The energy resolution performance will be assessed using simulations including a precise model of the PPD response. Such a model will be developed to fully reproduce measurements following the example of the software package put together for T2K. The nEXO conceptual design is scheduled to be completed by the end of 2015, and the redesign of the photo-detector planes is one of the critical path items. Hence, it is essential that the TRIUMF group starts tackling this task in 2014. The RF funnel system for ion extraction at Stanford has been tested and a controlled Ba-ion source has been installed in the 10 bar GXe chamber. At this point, it is not clear if 100% transfer efficiency can be reached, however, this is needed in order to extract the Xe decay daughter ions. This points towards the need for clear identification of extracted species, the full understanding of loss-mechanisms and optimizing the individual components. One of the current problems, is that the number of expected ions released into the gas from the source and the number of detected ions differ by a factor of 10 000 too many. This could be due to residual gas ionization in the high pressure chamber. While a vacuum of 2-3 10^-9 mbar has been reached, there would be still sufficient residual contamination left to generate the number of detected ions. Moreover, the recirculated gas is filtered to a purity of ppb-level, but this could also be insufficient. The TRIUMF group will set up a test system for ion identification in a high pressure region, such as an RGA mass filter, where the source of unwanted ions can be identified and and reduced.