William M. Fairbank, Jr
B.A., Pomona College (1968)
Ph.D. Stanford University (1974)
Fellow, American Physical Society
Search for Majorana Neutrinos in EXO-200 and nEXO
Neutrinoless Double Beta Decay
Neutrinoless Double Beta Decay (0nbb) experiments are testing the fundamental properties of neutrinos and are looking for physics beyond the Standard Model. If and when 0nbb decay is observed, it will demonstrate that neutrinos are Majorana particles, that is, neutrinos are the same as antineutrinos and that the law of Conservation of Lepton Number is violated. A measured lifetime for 0nbb decay could determine the neutrino mass hierarchy and add the information needed to extract the individual neutrino masses, when combined with neutrino oscillation data. To date, 0nbb decay has not yet been discovered. The EXO-200 experiment , that our group is working on, and the KamLAND-Zen and GERDA experiments have set very stringent limits on the half life of 0nbb decay, more than 1025 years. The EXO-200 0nbb detector uses 200 kg of isotopically enriched liquid Xe-136 and has run for more than two and a half years. The next generation EXO experiment, called nEXO, is expected to use about 5 tons of enriched liquid Xe-136 and to run for ten years.
Single Barium Daughter Atom Detection
A key technology that we are developing for use in the second five years of nEXO operation is “Barium Tagging” . When a Xe-136 atom decays by 0nbb decay, it produces two energetic electrons with 2.458 MeV total energy and a Ba-136 daughter atom or ion that is left at the decay site. The techniques used in EXO-200, that measure the electron energy and event multiplicity, are very powerful at distinguishing potential 0nbb events from similar-appearing background events. But despite efforts to reduce background to a miniscule level by radioactivity testing of every screw, wire, etc., a small number of indistinguishable 0nbb or background events remain each year. nEXO will have greater discrimination power due to self-shielding in a larger detector and other improvements, but a few background events will likely limit the ultimate sensitivity for 0nbb decay that can be reached.
If there were a way to detect the existence of a Ba-136 daughter atom or ion or not at the decay site of a potential 0nbb decay event, complete distinction between 0nbb decays and background events, that don’t produce a Ba-136 daughter, could be obtained. This is called Barium Tagging. If we and our nEXO colleagues can successfully implement efficient Ba tagging, the nEXO sensitivity could exceed 1028 years half life, an improvement of three orders of magnitude over current levels. Today’s experiments probe the “degenerate” mass hierarchy possibility for neutrino mass. A nEXO experiment with Ba tagging could completely probe the “inverted” mass hierarchy possibility and begin to probe the “normal” mass hierarchy.
The method of Ba tagging we are developing involves inserting a cryogenic probe to the position of the candidate 0nbb decay, freezing the Ba-136 daughter atom or ion in solid xenon on the end of the probe, and then using single atom/ion laser fluorescence detection to produce an image of any Ba-136 atoms or ions in the solid xenon. A characteristic Ba-136 daughter atom/ion fluorescence peak indicates a true 0nbb decay. The absence of such a peak is indicative of a background event .
Single dopant atom placement for next-generation nanoscale devices and a quantum computer chip [4,5]
Today’s nanoscale electronic devices, i.e., transistors and diodes, have become so small that the junction region may contain only a modest number of dopant atoms. In next generation devices, the junction may contain only a few dopant atoms. At that level, the exact number of dopant atoms in the junction and their exact positions may become critical for proper device performance. Thus the ability to place single dopant atoms in semiconductors to ~1 nm precision could be a very important technology to develop for the future. Researchers have already shown, in larger devices, a completely different quantum behavior of a device when there are 6 dopant atoms vs. when there are 12 dopant atoms in the junction . In addition, researchers have shown that an ordered placement of dopant atoms produces more repeatable device properties than the usual random dopant placement .
We are also working towards an exciting proposal for a quantum computer chip that requires the placement of single phosphorous ions in silicon 10 nm below the surface and 20 nm apart to this same ~1 nm precision .
The method we are developing to accomplish precision placement of dopant atoms or ions in semiconductors includes the following steps [4,9]:
(1) laser cooling and trapping of one and only one dopant atom of the desired species in a magneto-optical trap
(2) laser resonant ionization of this single atom at threshold to produce a cold single ion with a very small “phase space”
(3) transporting this single dopant ion with a precision ion column to the desired position in the semiconductor and depositing it with ~1 nm precision at modest energy and minimal “straggling” just below the semiconductor surface
(4) adding extra layers of semiconductor to produce the desired depth or device structure
References J. B. Albert et al., “Search for Majorana neutrinos with the first two years of EXO-200 data”, Nature 510, 229 (2014).  M. Moe, “Detection of neutrinoless double beta decay”, Phys. Rev. C 44, R931 (1991).  B. Mong et al., “Spectroscopy of Ba and Ba+ deposits in solid xenon for barium tagging in nEXO”, Phys. Rev. A 91, 022505 (2015).  W. M. Fairbank, Jr., S. A. Lee, W. P. Czajkowski and J. S. Kluck, “A Laser-cooled Single-atom-on-demand Source for Si Quantum Computing”, Quantum Communication, Measurement and Computing (QCMC), AIP Conf. Proc. 1363, 173 (2011).  S. A. Lee and W. M. Fairbank, Jr., “Measurement of the Hyperfine Structure and Isotope Shifts of the 3s23p2 3P2 -> 3s3p3 3Do3 Transition in Silicon”, Phys. Rev. A 82. 042515 (2010).  E. Prati, M. Hori, F. Guagliardo, G. Ferrari and T. Shinada, “Anderson-Mott transition in arrays of a few dopant atoms in a silicon transistor”, Nature Nanotechnology, 7, 443 (2012).  M. Hori, T. Shinada, Y. Ono, A. Komatsubara, K. Kumagai, T. Tanii, T. Endoh and I. Ohdomari, “Impact of a few dopant positions controlled by deterministic single-ion doping on the transconductance of field-effect transistors”, Applied Physics Letters 99, 062103 (2011).  B. E. Kane, Nature 393, 133-137 (1998).  S. B. Hill and J. J. McClelland, Appl. Phys. Lett. 82, 3128 (2003).