SUPL and SABRE
As part of the national CoEPP organisation, the Adelaide node of CoEPP is taking part in the development of a new research facility located in Stawell, Victoria.
Known as the Stawell Underground Physics Laboratory (SUPL), this research facility will be located deep underground within a gold mine- necessary due to the nature of the work to be undertaken there. The deep underground location provides shielding from astrophysical particles that would otherwise provide an overwhelming background signal that would cause great difficulties in detecting possible dark-matter particles.
One of the main aims of SUPL will be to host an experiment to look for elusive dark matter particles. Dark matter is a form of matter that does not directly interact with electromagnetic radiation (such as light), and thus is not visible in the sense that "ordinary" matter is visible; the presence of dark matter is inferred from various lines of evidence, such as its gravitational effect on light, and the non-visible but necessary mass within galaxies and galaxy clusters.
That said, dark matter (whatever it may be...) accounts for around 27% of the total mass-energy of the universe; the ordinary matter we see (stars, galaxies, dust, etc) accounts for only around 5% of the total mass-energy of the universe. Dark matter actually constitutes the majority of the matter of the universe- but we do not yet know exactly what it is...
- Dark matter searches so far
A number of experiments have already been conducted in order to try to provide a direct detection of dark matter particles. One of these (DAMA/LIBRA, located deep underground near the Gran Sasso mountain in Italy) has reported a signal that is consistent with some interpretations of dark matter models.
Figure 1: The count rate from DAMA/LIBRA, plotted against time, shows an annual modulation.
The hypothesised explanation is that, as the Earth moves through our galaxy's dark matter halo due to the Sun's orbit around the galactic centre, the count rate varies annually as the Earth orbits our Sun (sometimes the Earth moves upstream against this dark matter wind; at other times, the Earth moves downstream with this wind).
The complication is that results from other experiments do not definitely support the DAMA/LIBRA result. However, this is not the same as saying that these other results disprove the DAMA/LIBRA claim; some of the inconsistencies reduce or are removed, depending on which model of dark matter one considers.
A significant part of this complication arises from the fact that the various experiments use different detection methods- for example, different detector materials. This means that the various results cannot be directly compared.
What is needed is another experiment based on the same fundamental detector methodology as was used in DAMA/LIBRA. Such an experiment has been proposed and is being developed - Silicon iodide And Background REjection (SABRE). SABRE is an international collaboration, including institutions in Australia, Italy, and the US.
- The basics of SABRE
SABRE will use highly-pure crystals of sodium iodide, with a slight deliberate impurity (or doping) of thallium. (This is the same basic technique as used in DAMA/LIBRA, thus negating the complication of dark-matter-model-dependent assumptions when comparing results.)
When a dark matter particle interacts with the material inside the crystal, a brief burst of light will be released into the optically-transparent crystal- a process known as scintillation. The crystal is held in a light-tight enclosure, and light-sensitive detectors (photo-multiplier tubes, abbreviated to "PMT") attached to the crystal will register the scintillation event.
A very significant problem for such experiments is that some level of radioactive impurities in the crystal material (and, indeed, virtually any materials used in the detector) is unavoidable- they may be reduced, but it is very difficult, if not impossible, to remove them completely. Radioactive decay of these impurities (particularly potassium-40) can lead to false signals that mimic those of dark matter interaction, and thus produce a background that interferes with any putative dark-matter-induced positive signal.
With this in mind, the SABRE crystal modules will be suspended within a volume of liquid scintillator that will detect another by-product of the radioactive decay from such impurities; this is known as a veto. If the system detects simultaneous events within both the crystals and the veto, this suggests that the crystal signal is due to a background event, most likely from potassium-40 decay, and not to a dark matter event. If no veto event is seen, then this raises confidence that the crystal event is due to a dark matter interaction.
Figure 2: Cross-section schematic of a proposed detector design for SABRE.
The NaI crystal modules (brown) are suspended in a cylindrical tank containing the veto scintillator liquid. PMTs at the ends of the veto tank detect scintillation events arising from background events such as potassium-40 decay or environmental background such as incoming muons or neutrons.
The entire detector would also be surrounded by layers of passive shielding in order to reduce local environmental background as much as possible.
The veto layer also will be used to detect background events due to particles (muons, neutrons, etc) that are coming into the overall detector from outside (i.e. environmental background).
Placing the experiment deep underground greatly reduces the incidence of noise from environmental factors. This provides natural shielding due to the overlying rock, but the detector itself will also have layers of shielding material surrounding it in the laboratory area.
- Comparison with DAMA/LIBRA
One of the main points of the design of SABRE is that it is fundamentally similar to that of DAMA/LIBRA, and thus it will be possible more directly to compare the results from the two experiments.
But SABRE will improve upon the capabilities of DAMA/LIBRA by increasing the purity of the crystal materials, by using PMTs with better characteristics, and other enhancements which build upon the experience gained in earlier experiments. All of these are intended to lower the background from false signals, and thus to improve the detectability of any proposed dark-matter signal.
Further, DAMA/LIBRA was a single experiment located in one site only. SABRE will consist of two experiments, with one located in the northern hemisphere, and the other in the southern hemisphere at Stawell.
In order to give a more thorough test of the time-dependent signal as reported by DAMA/LIBRA, these two locations for SABRE will allow seasonal and local environmental aspects to be much more thoroughly examined. Not only will each individual SABRE installation be an improved experiment, the two together will greatly enhance the process of testing the putative direct dark-matter particle detection reported by DAMA/LIBRA.
- SUPL specifics
SUPL is located in the Stawell gold mine, on the eastern outskirts of the town of Stawell in Victoria.
The lab itself will be located at a depth of 1025 metres below ground level. This overburden is equivalent to a thickness of 2900 metres of water ("2900 m.w.e."), which is a common way of expressing the depth of such facilities, and gives an indication of the amount of shielding provided from background-inducing particles.
The SABRE South detector at SUPL will work in the same way as the SABRE North facility at Gran Sasso, but there are some design differences. The vessel for the North experiment is a horizontal cylinder (as shown earlier in Figure 2), whilst the South vessel will be a vertical cylinder, as shown below.
Figure 3: Rendering of SABRE South vessel design (as of February 2018).
The vessel is made from low-radioactivity steel, in order to add as little background as possible.
Access ports for the crystal modules are on the top flange. These ports provide physical access for the modules, as well as electrical and other connections. The veto PMTs are positioned at the short posts around the sloping top section of the cylinder.
(Rendering by Tiziano Baroncelli, UMelb)
Figure 4: Sectional rendering of SABRE South vessel design (as of February 2018).
The crystal modules are seen hanging down from their access ports.The exterior of the modules are made from low-radioactivity copper. Within the modules can be seen the NaI crystals (in white), with a PMT at each end.
The interior of the vessel will be filled with scintillator fluid, acting as a veto.
(Rendering by Tiziano Baroncelli, UMelb)
Since the Stawell mine is a functional mine, blasting will be taking place in other areas of the mine. Protection from resulting vibration is incorporated into the design of the mounting of the vessel, and sensors will record motion of the vessel. Further, data-processing procedures will seek to account for the effect of such vibration on the experimental data.