How does SABRE South work?

SABRE South is a direct-detection experiment for dark matter (DM) particles. This means that the intention is that dark matter particles will interact directly (i.e., by collision) with some part of the hardware- in this case, atoms within a crystal made of sodium iodide that has been doped with thallium.

Under certain assumptions relating to the physical properties of putative dark matter particles, such an interaction is expected to result in the release of a brief burst of light within the crystal. Such a light pulse can be detected using very sensitive and very fast light detectors called photomultiplier tubes; these PMTs then give an electronic pulse which can be measured. Thus, a DM particle hitting some part of the crystal lattice should result in a measurable pulse from the PMT that physically is coupled closely to the crystal (actually, one at each end of the cylindrical crystal; the crystal and its PMTs are housed in a sealed, light-tight enclosure).

A further expectation based on our ideas about the distribution of DM within our galaxy, is that the Earth, as it orbits the sun every year and the centre of our galaxy every 230 million years or so, sometimes is moving with the DM “wind” that results from our motion around the galactic centre, and sometimes is moving against that wind.

For the portion of the year when we’re moving against that DM wind, we expect the Earth to face more DM particles in a given amount of time (per day, for example). When we’re moving with that wind, we expect the Earth to face fewer particles in that same amount of time (much the same reason as why cycling against a head-wind is harder than riding with a tail-wind).

We would thus anticipate the DM count rate from our direct-detection experiment to vary with a one-year cycle, with the peak of that count rate to occur on a specific day of the year. (The two sites of SABRE- North and South- are designed to pry out any possible local seasonal environmental effects which may have an annual variation. The DM rate should vary identically at each site, but environmental factors will not.)

That is the core of SABRE.

However, that’s not the entire story… Materials used in the construction of the equipment- copper, steel, glass, the crystals themselves, for example- and the laboratory surroundings- also produce particles that can and will interact with the detectors to give us a false signal.

To reduce the effect of this background as much as possible, the crystal detector modules will be suspended within a liquid that produces a flash of light when such a background particle passes through that liquid. This liquid scintillator veto is held in a cylindrical tank around which are mounted a number of PMTs looking into the veto volume; if we see a flash in the liquid scintillator and in the crystals at the same time, we suspect that this is due to a background event. If we see a flash only in the liquid scintillator, we would have a background event again.

However, if we see a flash only within the crystal, we can be more confident that that is a true DM interaction event.

So we will run the detectors for some time, looking for an annual modulation in the count rate coming from the crystals (accounting for vetoed events, of course), and especially looking for a modulation that is consistent across both the North and South installations.

Associated with the detector hardware is a raft of sensors that will continually monitor the laboratory environment and other factors such as power supply voltages, in order both to check the correct operation of the experiment, and to look for possible correlations between the monitored quantities and the count rate given by the detectors.

A possible layout of the South vessel, used for simulations to optimise the layout of the veto PMTs. The containing vessel is in blue, the veto PMTs in yellow/pink, and the crystal detector modules are in the centre.