2016 JINST TH 002
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.