UCL tests liquid argon purity monitor
Laura Manenti (UCL), 05/12/2017
Close-up of the purity monitor currently under testing at UCL (Image: Laura Manenti, UCL)
DUNE, an international experiment which aims to study neutrinos, selected Liquid Argon Time Projection Chambers (LAr TPCs) for use in its Far Detectors (FDs). As a result, the technology must be tested and refined for optimal results.
Following tests of the monitor in vacuum and gas in May 2017, researchers involved in the AIDA-2020 project continued their test schedule in liquid argon.
LAr TPCs are able to reconstruct the tracks of particles by collecting the scintillation light and ionisation charge produced as they traverse the liquid argon. Ideally, 100% of the electrons produced should be drifted to the anode. In reality, due to the presence of impurities in the liquid, part of the ionisation charge will be lost. To keep such loss to a minimum, purification of the liquid argon is essential.
Although a TPC itself can estimate the purity of the LAr by measuring the electron drift time to the anode (usually using muon tracks), this can only be done once the cryostat is fully filled and a certain level of purity is achieved.
Until that point, it is not possible to track the level of purity through time and know if the detector conditions are improving or worsening. Here comes the necessity of instrumenting the TPC with a device to monitor the liquid argon purity since the very beginning: a purity monitor.
Purity monitors have already been successfully employed in the ICARUS detector and in the 35-ton prototype detector at Fermilab. Both the ProtoDUNE Single-Phase (SP) and Dual-Phase (DP) detectors will also employ purity monitors.
At present, researchers at UCL are working on the design, construction and commissioning of the purity monitors of the ProtoDUNE DP detector.
The monitor’s design resembles that used by ICARUS: a Xenon lamp placed outside the cryostat is coupled through an optical fibre to a gold cathode (in the ProtoDUNE DP a 3 mm Silicon plate coated with 10 nm Titanium and 200 nm of Gold), where VUV photons extract electrons by photoelectric effect.
An electric field drifts the electrons from the cathode to an anode (a simple stainless steel plate) across a drift region, composed of two grids, placed 18 mm above the cathode and 10 mm below the anode. The volume between the two grids is 160 mm long and contains a series of field shaping stainless steel rings, which are connected by a chain of resistors, guaranteeing a uniform drift. By comparing the level of charge released from the cathode and that collected at the anode, researchers can extrapolate the purity of the liquid argon between the two electrodes.
Although previous studies have proven that the design of the purity monitor works, the team at UCL hope to make it simpler and cheaper for the DUNE Far Detector. Next, the possibility of using a VUV LED source and different cathode materials will be investigated whilst working on the final design for the protoDUNE DP prior to its installation in March 2018.
Laura Manenti is an Associate Researcher, currently leading the purity monitor experimental project at UCL.