Ph.d. degree
University of Bern, Switzerland, 2015

S�bastien Claude Delaquis

Supervisor: Razvan Gornea

Construction and operation of the EXO-100 cryogenic facility for R&D in liquid xenon: advances in barium ion tagging

Keywords:

• Double-beta decay detectors
• Time projection chambers
• Cryogenic detectors
• Noble liquid detectors (scintillation, ionization, double-phase)

Abstract:

Neutrinolessdouble-betadecayisahypotheticalprocesswhich, ifobserved, impliesthatneutrinos are Majorana particles. Moreover, it is a lepton-number-violating process and thus forbidden in the Standard Model of particle physics. Measuring its half-life would allow the determination of the effective Majorana neutrino mass and shed light on the neutrino mass hierarchy. For decades, experiments have been carried out to search for this process, so far without success. The Enriched Xenon Observatory, EXO, is an ongoing multi-stage project searching for neutrinoless doublebeta decay in the isotope 136Xe. 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 136Xe allowed in the Standard Model has been discovered; its half-life was found to be Tββ(2ν) 1/2 = 2.165(16) � 10^21 yr. Furthermore, EXO-200 set a lower limit on the half-life of the neutrinoless double-beta decay Tββ(0ν) 1/2 > 1.1�10^25 yr at 90% C.L. 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 real-time the decay product of 136Xe 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.

The framework of this dissertation achieved both advances in barium ion tagging and R&D for nEXO.This paper presents the construction and operation of the 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. The results show a qualitative agreement with finite element simulations and help to understand high voltage problems in EXO-200. 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 222Rn and 218Po, which is part of the naturally occurring 238U-series, was sought. Clean fully reconstructed events were studied. The ion fraction was measured to be 52(10) %, and no indication was observed of ion neutralisation during drift. This suggests that the ion life-time is large compared to the drift time. The ion drift speed was measured to be 1.4(4)mms-1 (for a drift field of 380(5)kVcm-1). This confirms results from an analysis of partially reconstructed events, with more statistics, which revealed a behaviour of the ions never observed before. It was shown that the ions have two drift speeds, between which they change with a transition time τ. It was found that τ depends on the purity of the xenon. The present analysis is consistent with these findings.