MainzTPC: design and comissioning of a dual-phase liquid xenon time-projection-chamber for studies of the scintillation pulse shape
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Abstract
In 1933 Fritz Zwicky first claimed the existence of Dark Matter in the universe. Since that time, astronomy,
particle and astro-particle physics have made some effort to understand the effect. Yet Dark Matter remains
an unsolved puzzle. Especially in the last three decades direct detection experiments have been built and
conducted, restricting the parameter space for Dark Matter particles, but none of the experiments could
detect Dark Matter interactions with baryonic matter yet. Some of the leading experiments in the last
decade have been based on liquid xenon as detector material and upcoming experiments based on the same
principle are still most promising. To improve background discrimination and analysis techniques, a refined
knowledge on the properties of xenon as detector material and the microscopic processes in liquid xenon
that lead to scintillation and ionization are necessary.
The MainzTPC, a detector dedicated to study the scintillation and ionization process of liquid xenon
systematically has been built in the course of this thesis. Like the Dark Matter experiments (XENON100,
XENON1T, LUX) the MainzTPC is built as a dual-phase time projection chamber. Hence, it provides
charge readout, which yields 3D position resolution and the possibility to study the influence of the strength
of an applied drift field on the scintillation process.
Pulse shape discrimination (PSD) could be used as a background discrimination technique comple-
mentary to the established method using charge-over-light ratio. For the liquid xenon scintillation pulse
shape it was shown in earlier measurements that two excimer states, singlet and triplet, contribute to the
scintillation. But their short decay time constants of 2 ns and 27 ns poses a challenge to the measurement
of the pulse shape. Although pulse shape discrimination might be possible without measuring the signal
with high time resolution, a detailed study of the ratio of singlet to triplet excimers and the influence of
electron-ion recombination (and its suppression by the drift field) on the pulse shape requires fast photo
detectors and electronics with high bandwidth.
This thesis describes the MainzTPC and the measurement setup, including electronics and photo-sensors.
The abilities for pulse shape measurements with the MainzTPC are investigated and some issues that require
improvement for the future are pointed out. Finally, the thesis shows various measurements of nuclear
and electronic recoils taken at the neutron source nELBE at the Helmholtz-Center Dresden-Rossendorf.
First analysis approaches to model the scintillation signal pulse shape are described. With a signal shape
averaged over many interactions, the results clearly show a difference in the shape of electronic and nuclear
recoils without drift field, and hence qualitatively confirm older results from [KHR78] and [HTF + 83]. The
attempt to determine the pulse shape on an event-by-event basis by fitting individual scintillation signals
with a model function is hampered by low photon statistics. The reason for this difficulty is the shape of the
individual signal, which does not have one single rise followed by a decay as the averaged signal shape, but
shows fluctuations of the signal especially in the decaying part. A Monte Carlo simulation, in which the
signal shape of individual events was simulated based on the knowledge of the average scintillation shape
from the previous measurements and the single photo-electron response of our photo-multiplier tubes showed
that the fluctuations seen on individual measured signals originate indeed from the statistical fluctuations in
photon generation times. Prospects for future improvements are discussed.