EXO: Technical Design for a Tonne-Scale Enriched Xenon DoubleBeta Decay Experiment

Stanford University, Stanford CA

Investigators

Abstract

This proposal will fund the search for neutrino-less double beta decay process in Xenon-136. The discovery of neutrino oscillations can be regarded as the first window provided by experiments on physics beyond the standard model. The search for neutrino less double beta decay represents the next step in this exciting field of physics and provides an opportunity to perform transformational science addressing universal topics such as the origin of mass, the role of Dirac?s equation in describing spin half fermions and connections between particle physics and cosmology. Many experiments tackled the double beta decay over the last thirty years, setting half-life limits for this mode of the order of 10**25 years, corresponding to a Majorana neutrino mass just below 1 eV. A new generation of much larger experiments will push this sensitivity to 10**28 years or Majorana masses below 10 meV, exploring much of the parameter space compatible with the oscillation results. Those searches require tones of specific isotopes that, in order to result in viable measurements, must be produced by isotopic enrichment. They also require unprecedented reductions of radioactive backgrounds and substantial attenuation of the cosmic ray flux, making their location deep underground imperative. This proposal plans to establish the engineering and project management infrastructure to design a full scale double beta decay experiment, the ?Enriched Xenon Observatory? (EXO), using a few tons of the isotope Xenon 136. The isotope choice capitalizes on the relative simplicity of xenon enrichment and purification and on its versatility as a detection medium in a liquid or gas phase Time Projection Chamber. The substantial investment in isotopic enrichment can, in this way, result in more than one detector, making it possible to cross check results and upgrade detectors as new technologies become available. In addition a scheme being developed by the EXO collaboration to tag the atomic final state of the decay has the promise of drastically reducing radioactivity backgrounds. Data soon to be collected with EXO-200 will provide very important input for the design work on a multi-ton detector for DUSEL. The broader impact plan of the EXO collaboration is to develop a cross-disciplinary program involving the techniques ranging from nuclear/particle detector physics to AMO and surface science. Therefore many intermediate results may have spin-offs in a broad range of fields. EXO R&D has trained students in a variety of disciplines and a vibrant outreach program at several EXO institutions brings the excitement of science to grade school students and to high school teachers who spend their summer in some of our labs. As part of this proposal we plan to enhance this last component, by connecting it with the DUSEL outreach program.