Construction and operation of the Bern "EXO-100" cryogenic facility for R&D in liquid xenon: advances in barium ion tagging
by
Madrone
SLAC
The Enriched Xenon Observatory, EXO, is an ongoing multi-stage project searching for neutrinoless double-beta decay in the isotope Xe-136. As detectors, EXO uses time projection chambers (TPC’s) filled with liquid xenon. In the current stage of the project, EXO-200, the two-neutrino double-beta decay of Xe-136 allowed in the Standard Model has been discovered, and furthermore, a lower limit was set on the half-life of the neutrinoless double-beta decay. The next stage of the project, nEXO, is currently being developed. In its first phase, nEXO aims for a sensitivity sufficiently high to survey the complete parameter range of the inverted-neutrino-mass-hierarchy scheme. To further increase sensitivity in the second phase of nEXO, barium ion tagging, a novel background rejection technique, is being developed. The principle of this technique is to identify in realtime the decay product of Xe-136 double-beta decay, a single barium ion. This technique would reject all backgrounds not related to double-beta decay, and it is thus considered the ultimate background rejection technique.
In this talk I will present the construction and operation of the Bern "EXO-100" cryogenic facility and a study of ion properties in liquid xenon. EXO-100 is a very versatile facility and was operated with liquid argon, liquid xenon, and liquid tetrafluoromethane (CF4). To study barium ion tagging, a TPC was constructed and successfully tested in liquid argon. Moreover, high voltage breakdowns in liquid xenon were studied with miniEXO, a miniaturised mock-up of the EXO-200 TPC. During these tests, EXO-100 was equipped with cryo-cameras, which were specifically developed for operation in EXO-100. Furthermore, the ion properties relevant for barium ion tagging were studied for polonium. To study these properties, EXO-200 data were analysed and the delayed coincidence between Rn-222 and Po-218, which is part of the naturally occurring U-238-series, was sought. Clean fully reconstructed events were studied.