A position-sensing system for the top PMT array in the LUX-ZEPLIN dark matter experiment

• Main Author: Boast, Kathryn
• Other Authors: Kraus, Hans
• Published: University of Oxford 2017
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.757692

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.