As discussed in the Experiment Overview, the sodium-iodide crystals that are at the very core of SABRE South are suspended in a volume of liquid scintillator (LAB- linear alkyl benzene), which helps us to distinguish false signals from hoped-for real ones.
The crystals and their light-detecting photo-multiplier tubes (PMTs) are encased within copper cyclinders that hang in the centre of the vessel. These crystal modules protect the very pure NaI crystals and their PMTs, as well as shielding the PMTs from any light flashes which may occur in the surrounding LAB which are due to background (i.e., unwanted noise) events. The modules are sealed and are purged with pure nitrogen gas to eliminate moisture from the interior of the modules, as any moisture would degrade the crystals over time. There is a PMT at each end of a crystal.
The vessel itself is made of low-radioactivity stainless steel, and is a cylinder 2.6-metres in diameter and 2.6-metres high, with curved ends. It will contain some 11,600 litres of LAB, and it stands vertically. At the top, there are access ports for the various electrical and gas lines, and a larger central port to be used for inserting and removing (only when absolutely necessary) the crystal modules. Including its support legs and the access ports at its top, the overall vessel stands around 3.3-metres tall.
On the interior wall of the vessel will be mounted 18 PMTs which will detect flashes of light within the LAB volume; such flashes are due to unwanted background events. This system of LAB plus PMTs constitutes the veto system for the detector, reducing the effect of undersirable background events on the data.
An engineering cutaway of the detector in its shield.
The crystal modules (copper-coloured ) hang vertically near the centre of the veto vessel. Lining the interior of the vessel are PMTs (brown globes with yellow bases) angled towards the centre of the vessel.
Surrounding the vessel on all sides is a shield composed of sandwiched layers of steel and polyethylene. On top of the shield is a detector for counting muons which enter SUPL from overhead.
To access the ports on the top of the vessel, the muon detector and parts of the top shield must be removed.
When in use, the entire vessel (including below it) will be surrounded by a shield made of sheets of steel and polyethylene. Even though the experiment is located some 1,025 metres underground to reduce the effects of the naturally-occurring radation that is found at the Earth’s surface, there are still some particles to be found at that depth.
Some of these make it directly through the overlying rock layers, and some are generated within the rocks by interactions with such incoming particles. Thus, a shield around the vessel will be used to reduce even further the amount of background radiation that could enter the vessel from its surrounding environment.
Whilst access to the interior of the vessel is needed when putting all of the components of the system together, in use the vessel will be enclosed within its shield except for the rare occasions when access is required for maintenance. Calibration of the system will be undertaken by souces which are either built into the interior of the vessel permanently, or are remotely lowered into sealed access tubes as required.