New fluorescence technique to search for neutrino masses by identification of 0vββ decay 136Ba+ ion daughters in liquid xenon

Abstract

This work presents the initial research on the development of a new fluorescence technique for laser tagging of single 136Ba+ daughters from 0vββ decay of 136Xe. 0vββ decay is the only probe which is able to reach the absolute neutrino mass scale with meV sensitivity. The current limit of mass sensitivity is about 0.33-1.5 eV, and the next generation Ovββ experiments with meV sensitivity require new techniques for background rejection in order to gain the full benefit of large fiducial mass and long running time. By detecting the decay daughter Ba+ in liquid xenon at the observed position of the decay, we expect to suppress essentially all of the radioactive backgrounds. As a first step in realization this technique, the measurement of the mobility of Ba+ in liquid xenon is finished and the study of the optical spectra of Ba+ in liquid xenon is in progress. In this work, measurements of the mobility of alkaline earth ions, Mg+, Ca+, Sr+, and Ba+, in liquid xenon are presented for the first time. The mobility of Tl+ is also measured in order to make a comparison to the reported value by others with a different measurement system. Akins's cluster model of positive ions in non-polar liquids, based on the electrostriction effect, gives general agreement with the magnitude of the mobility values. This might indicate that the positive ions form a snowball structure in liquid xenon. The excitation and fluorescence spectra of Ba+ in liquid xenon are also observed for the first time. The evidence that Ba+ does fluoresce in liquid xenon is a positive sign for achievement of the long-term goal. The spectra show much narrower absorption width and smaller emission shift compared to alkali atoms in solid noble gas matrix. Agreement is better with the spectra of Ca+ in solid argon. More detailed and careful experimental data will be required for a full understanding of the spectra properties of Ba+ in liquid xenon.