Search

The LUX-ZEPLIN experiment needs acoustic sensors to monitor the liquid xenon contained in its time projection chamber (TPC) for irregularities that could spoil a stable liquid xenon environment that is required for a dark matter search. I have developed and implemented algorithms that distinguish the occurrence of bubbling and other sounds, and I have carried out tests using a PVDF sensor and a readout system developed at Oxford; demonstrating that the hardware and analysis meet the requirements for their purpose in LUX-ZEPLIN. Acoustic sensors can be used as a veto to strengthen the fidelity of detector data. The primary event data in LUX-ZEPLIN will come from arrays of photomultiplier tubes, which detect light emitted from recoil events in the TPC. Waveform data from TPC events will be collected through a bespoke data acquisition system. A simulation of the detector response is needed in order to ensure that reliable event analysis will be possible when the experiment starts taking data. I have constructed an analytical model of the full signal chain from the photomultiplier tubes to the digitisers, including all analogue front-end electronics and interconnecting cabling, and I created a Monte Carlo simulation software package in which I have embedded this model. I have used the model and the software to carry out simulations to evaluate the design of the LUXZEPLIN electronics readout system. The model and the simulation I have created play a central role in performing event analysis on simulated data in preparation for real data taking with the LUX-ZEPLIN experiment.

LUX-ZEPLIN (LZ) is a next-generation liquid xenon dark matter detection experiment, searching for direct interactions with WIMPs. As it is a low-background experiment using high-purity xenon, the sensors installed to monitor the experiment must be custom-made from a short list of acceptable materials. This includes the position sensors that will monitor the relative position of the top photomultiplier tube (PMT) array with respect to the cryostat vessel that surrounds it. LZ will operate at around -100°C, so must undergo a significant cooling period, during which the vessel that supports the PMT array is expected to shrink. It is crucial to monitor this. The capacitive position sensors couple a change in relative position to a change in capacitance. However, the size of the LZ detector necessitates the use of cables around 12m long to read out this signal. These cables will have a capacitance themselves of around 1 nanofarad, which can vary as the cable environment changes, while the sensor capacitances are around a thousand times smaller, so small variations in the cable capacitance could be misinterpreted as changes in position. To combat this, we created a novel feedback circuit for the sensor readout electronics, designed to suppress the effect of the cable capacitance. To monitor precisely the position of the PMT array with respect to the cryostat vessel, we will use six position sensors mounted as a Gough-Stewart Platform Mechanism (GSPM) between the PMT truss and the cryostat vessel. The use of six sensors in this configuration will enable the reconstruction of all six degrees of freedom of the position of the PMT array. I have developed an algorithm that returns these six values given input from the position sensors. The six capacitive sensors connected as a GSPM, with the novel readout electronics, form the system of position sensors that will be used in the LUX-ZEPLIN experiment.

Galactical and cosmological evidence show that a quarter of the energy budget of our universe is made of collisionless, non-relativistic, and non-baryonic dark matter. Its potential coupling to standard model particles, however, has not yet been understood. One of the leading candidates - Weakly Interacting Massive Particles (WIMP) - allows the production of a dark matter relic density as observed today and couples to standard model particles at or below the weak scale. LUX-ZEPLIN (LZ) is a future tonne-scale two-phase xenon TPC aiming to detect WIMP recoils with xenon nuclei. The experiment will begin WIMP search data-taking in 2020 at the Sanford Underground Research Facility (SURF) in Lead, South Dakota and has a projected sensitivity of 3 × 10-48 cm2 or better in probing a 40 GeV/c2 WIMP. The main observables of particle interactions in LZ are the primary scintillation (S1) and secondary scintillation (S2). However, optimising and achieving a stable S2 signal in such a tonne-scale TPC is non-trivial. Effects from the structural design of the S2 production region (top-corner structure), TPC tilt, and the xenon circulation system requires precise monitoring of the liquid surface. Such monitoring is achieved by the capacitive liquid level sensors developed within this thesis. The sensors are strategically placed to ensure that nonuniformity of the S2 signal due to the effects can be understood and corrected. In this thesis, the development of a monitoring system designed to optimise the quality of the S2 signal, based on the capacitive level sensors is discussed. A design of the electronics scheme based on a differential measurement allows femtofarad precision measurement of sensor's capacitance at picofarad level, even in the presence of cable capacitance at nanofarad level. A systematic study of the response of such a sensor to LXe and the application of the precision level sensors to two-phase TPC was carried out. Findings of intrinsic influences from LXe artefacts and LXe dielectric constant variation with its saturated temperature are identified; the result on the application of the sensors contributes to the designs of LZ circulation and the top-corner region. The final LZ level sensors show an artefact-free liquid level measurement and a 12 μm precision in measuring liquid nitrogen level (projection for LXe: ∼ 9 μm) over a 20 mm measurement range.

The EDELWEISS experiment is one of the dark matter direct detection experiments. It aims to detect WIMP interactions using an array of cryogenic germanium detectors. In the previous EDELWEISS-II phase, the cables and connectors used have been identified as a major source of neutron background in the experiment, which means that further effort aimed at better WIMP-nucleon interaction detection sensitivity requires a new, different cold cabling solution connecting the detectors to the front-end electronics. Motivated by this, a new two-section cold cabling system based on semi-flexible laminated copper and stainless steel cables has been developed for the EDELWEISS- III phase at Oxford. Batches of prototypes have been tested first in a cryostat at Oxford as part of a phonon-scintillation detector module, and then at the LSM underground laboratory in several EDELWEISS-III commissioning runs. Following that, a final set of cabling has been produced and installed in the EDELWEISS-III setup, which is currently conducting a science run aiming to improve its sensitivity reach compared to the previous results. This new cold cabling system has shown similar electrical performance as the previous coaxial cabling when comparing different cold cabling configurations in a commissioning run at LSM. Also, its background contribution is within the EDELWEISS-III requirements, according to radioactivity level tests and Monte Carlo simulations. In addition, the assembled connectors have allowed hundreds of signal tracks to be installed within a few days and the low material and space budget has made the cables compatible with the compact cryostat design. Besides reading out detectors for dark matter detection, prototypes of this cabling solution for a wider application range have also been produced at Oxford. The next generation dark matter direct detection experiments aim to achieve detection sensitivity better by a few orders of magnitude. This requires a target mass at tonne-scale, which converts to thousands of cryogenic detectors. Cryogenic phonon-scintillation detectors used in current dark matter searches can provide excellent performance but they usually require individual tuning and attention, making operation in large-scale experiments difficult. It is also technically challenging to stably produce such detectors in large quantity. Therefore, a scalable, robust novel detector concept for cryogenic phonon- scintillation detectors to be used in future rare event search experiments has been developed in this work. This detector module consists of a phonon detector based on a CaMoO4 scintillating crystal as the target with an attached NTD-Ge sensor as the thermometer, and a light detector based on a low-temperature PMT. To provide the high voltage necessary for PMT operation while ensuring the detector module can be cooled down and that the performance of the phonon detector is unaffected, a high voltage supply system based on a Cockcroft-Walton generator (CWG), a transformer and a small AC input has been designed and tested in the cryostat. The laminated cabling system is chosen for reading out the phonon channel and connecting the CWG and the PMT. A test run has demonstrated that, the high voltage can be provided to the PMT without causing a problem to the detector operation, and it is feasible to operate the low-temperature PMT at a temperature as low as 17 mK. Testing with a cobalt-57 gamma source, the phonon detector and the light detector have achieved resolutions of 1.07 keV and 34.2 keV for the 122.06 keV peak respectively. This is close to the performance of detectors used in the current dark matter direct searches, proving this detector concept can be applied to future large-scale dark matter direct detection experiments and other rare event searches. Using the light channel in this detector setup, the scintillation properties of CaMoO4 has been studied. In this work, the experimental data of its scintillation decay time constant has been extended from the previous 7 K to milli-Kelvin temperatures. The data are interpreted using a three-level model, confirming the existence of a metastable emission level in CaMoO4, and giving various parameters of its emission centre. This suggests that the work related to producing a high voltage supply and demonstrating the excellent performance of a low-temperature PMT could also be attractive to scintillator studies at cryogenic temperatures.

Cryogenic detectors are an advanced technology for both dark matter and neutrinoless double beta decay searches, having the key advantage of a range of possible absorber materials that can be used for the detectors. Neutron transmutation doped germanium sensors are highly sensitive thermometers ideal for use at milli kelvin temperatures, with a simple repeatable resistance temperature relation. To discriminate between candidate events and background events simultaneous measurements can be made of the energy deposited in the detector as phonons and the energy emitted by the absorber crystal as scintillation light. Phonon detectors with a calcium tungstate or calcium molybdate crystal as the target and an NTD sensor as a thermometer were made in Oxford, along with a light detector with a light-absorbing silicon layer on a sapphire crystal, also with an NTD thermometer. A system of electronics was designed and tested in Oxford to bias and readout the NTD thermometers, while the setup inside the cryostat was developed to provide a thermally and mechanically stable shielded environment for the detectors. As part of this, prototype semi-rigid kapton cabling for use in the EDELWEISS experiment was installed and tested in the cryostat. Three different NTD germanium sensor types were characterized and calibrated in the cryostat and two of these selected for use on the phonon and light detectors. The detectors were operated at temperatures as low as 9 mK and tested with radioactive sources to produce energy spectra. Baseline resolutions of 1.7 keV and 2.5 keV, respectively, were achieved for the calcium molybdate and calcium tungstate phonon detectors. A working scintillation light detector was demonstrated as part of a phonon-scintillation detector module with a suggested application in double-beta decay searches.