STEEL (Sheffield TEst stand Experiment with Liquid argon)

Background

LAr R&D work has continued towards characterising pixel readout with a liquid argon test stand in Sheffield and a comparison of its performance with multi-wire readout in APAs, with applications for both gaseous and liquid argon detectors. The first LAr test detector and its purification/circulation systems were commissioned in 2021. The two-week run was with continuous argon circulation and purification, and nitrogen cooling. A few detector operation issues were identified during this first commissioning run and then corrected and validated in a new three-week-long run in 2023. Using data from these two detector commissioning runs, we are now optimising the online event monitoring system, developing the argon purity measurement and the full event reconstruction algorithms.

Operation Principles

A schematic diagram of the STEEL system is shown in the image below. STEEL consists of a 20 l steel dewar filled with argon, an internal photomultiplier tube (PMT), four external cosmic-ray taggers (CRTs), and a cylindrical field cage with 10 cm drift distance, allowing for the detection of ionisation charges and cosmic-ray event tagging.

LAr evaporates into GAr, flowing through a purification system that filters out electronegative impurities (such as water and oxygen molecules), before passing through a condenser system, where circulating 78 K liquid nitrogen cools the argon back to liquid at 87 K, falling back to the bottom of the dewar.

Key components of the internal detector system shown in the photograph below include,

• Adapter board plane with two LArASIC chips - Amplifies and shapes raw charge information into processed signals,
• Anode pixel plane - PCB-based charge readout plane,
• TPC Field Cage - Field strength varies from 0 - 600 V/cm, directed downwards, drifting ionisation electrons upwards,
• Cathode mesh - Wire mesh with a negative voltage transparent to scintillation light,
• Grounding mesh - Protects the PMT from E-field interference,
• TPB-coated plastic sheet - Shifts 128 nm argon scintillation light to the visible range so the PMT can read out photons,
• Cold PMT - Amplifies the photon energy to a measurable readout signal, and operates optimally at 1700 V.

The image below shows inside the STEEL dewar. 

Pixel imaging is motivated by wire planes exhibiting ambiguities in event reconstruction, as well as detector pile-up. STEEL uses a pixelated anode Printed Circuit Board (PCB) designed and constructed at the University of Bern shown in the image below.

The pixel anode plane consists of 1008 pixels, 2.54 mm apart, divided into a 6x6 grid, each surrounded by a region of interest (ROI). Each ROI is voltage-biased to induce a signal and concentrate charges to be collected by the individual 900 μm wide pixels. The pixel DAQ has a total of 64 readout channels: 28 ROIs and 36 pixels, where multiplexing allows for each pixel in the same relative 6x6 grid position to share one readout channel.

STEEL Physics

STEEL collects cosmic-ray muon data, reading out an event when a muon simultaneously triggers the side/top CRTs, cold PMT and pixels. In the summer of 2023, STEEL collected 22 continuous days of cosmic-ray muon data, maintaining consistently low impurity levels for the entire run. The events were run through a noise filter, and hit-finding algorithms identified signal peaks in the pixel readout channels. 2D and 3D event displays visualise hits on the pixel plane, with an additional time axis for the 3D event display, shown below. Signal waveforms from cosmic-muon events tagged by the CRTs and cold PMT in coincidence are plotted for noise filtering and time-of-flight studies. 

R&D for Future LAr and GAr Experiments

Current and future work with the LAr test stand is focussing on developing a novel low-noise hybrid readout technology based on pixels combined with a Thick Gaseous Electron Multiplier (ThGEM) for the DUNE phase-II near detector with gaseous argon, NDGAr. This high gain detector is expected to enhance DUNE phase-II NDGAr, and hence, the whole near detector sensitivity to low-energy hadron shower topologies which is crucial for DUNE’s ultimate precision CP violation measurements.

The STEEL test stand was designed and constructed to be small-scale and versatile, capable of easily plugging in novel LAr and GAr detector components to test them for future generations of experiments. The STEEL group will continue to collaborate closely with working groups that are developing new components, for instance, readout technologies. A suite of validation tests is under development, aiming to be independent of the plugged-in hardware components, to facilitate systematic assessments of their capabilities in the LAr and GAr detector environment.

The current STEEL group consists of Neil Spooner, Rhiannon Jones, Anthony Ezeribe, Ala Zglam, Harry Scott and Hannah Burn.