EXO is a ton-class neutrino-less double beta decay experiment employing 136-Xe and is unique among such experiments as it utilizes a novel technique to tag the atomic species produced in the final state (Ba) of the double beta decay using laser spectroscopy. This technique has the promise of drastically reducing radioactive backgrounds, providing a clean and un-ambiguous measurement with unprecedented sensitivity. In the last few years, EXO R&D has been equally divided between the construction of a 200 kg intermediate scale detector (EXO-200) without Ba tagging and the development of the Ba tagging system. EXO-200 construction is now nearing completion.
The work proposed here concentrates on Ba extraction from the liquid xenon (LXe) and identification. Some new ideas, presented here, have the potential of providing the key for the design of the full scale EXO experiment. Previous R&D has resulted in an ion trap capable of clear single Ba ion detection in the presence of a cooling gas, and the initial experimentation with a number of Ba transfer techniques. The R&D proposed here will revolve around three parallel themes, each to be developed by one of the three collaborating institutions. The conclusion of EXO Ba tagging R&D, along with the successful operation of the EXO-200 detector, will result in the design of a formidable next generation double beta detector to be sited at an underground laboratory in the near future.
The techniques to be developed in this project span a broad range of topics, from nuclear and particle astrophysics, to AMO, surface physics and material science. This research, if successful, is likely to be applicable to other problems in science and technology, where the high efficiency transfer and identification of single atoms would be of interest. Examples include trace analysis for homeland security applications and the detection of rare phenomena.
With the support of this grant we advanced the technology for recognizing and transporting individual atoms (or Barium). The work was motivated by the EXO double-beta decay experiment that is seeking to detect an exceedingly rare nuclear decay, but it could be applied to other field where the need arises of handling single atoms in the presence of very substantial backgrounds. At present we can identify Ba atoms with an efficiency of about 1% and we are planning a future R&D program to develop the tools to reach efficiencies above 50%. This will be important for the EXO experiment that may take shape at a national underground facility. In the course of this work we also developed a sensor capable of detecting single atomic layers of a frozen species in a bath of the liquid and a source of single atoms where the atom emission is tagged by an electrical signal. The source is very flexible and can be adjusted to produce atoms of many chemical species. Most of the work was performed by two graduate students, one of which will receive her PhD soon. For two summers we also hosted in our labs a high school teacher who contributed to the research.
