SNO+ is a kilo-tonne scale liquid scintillator detector that studies neutrinos. The experiment is located approximately 2km underground in VALE's Creighton mine near Sudbury, Ontario, Canada. The heart of the SNO+ detector is a 12m diameter acrylic sphere filled with approximately 800 tonnes of liquid scintillator which floats in a water bath. This volume is monitored by about 10,000 photomultiplier tubes (PMTs), which are very sensitive light detectors. The acrylic sphere, PMTs and PMT support structure has been re-used from the SNO experiment. Therefore, SNO+ looks almost exactly the same as SNO except that, in addition to the ropes that currently hold the acrylic vessel up, we have added ropes to hold the vessel down.
Liquid scintillator is an organic liquid that gives off light when charged particles pass through it. SNO+ detects neutrinos when they interact with electrons and nuclei in the detector to produce charged particles which, in turn, create light as they pass through the scintillator. The flash of light is then detected by the PMT array. This process is very similar to the way in which SNO detected neutrinos except that, in the SNO experiment, the light was produced through the Cherenkov process rather than by scintillation. It is this similarity in detection schemes that allows the SNO detector to be so efficiently converted for use as a liquid scintillator detector. The scintillator in the SNO+ experiment is primarily composed of linear alkyl benzene (LAB), which is a new scintillator for this type of experiment. LAB was chosen because it has good light output, is quite transparent, and is a "nice" chemical to work with (it has properties much like those of mineral oil). It also seems to be compatible with acrylic (which is obviously important for SNO+). LAB is used commercially to manufacture of dish soap, among other things, which means that it is available in the large quantities needed for SNO+ at a relatively low price. As an added bonus, there is a plant in Quebec that produces very good LAB, meaning that SNO+ can have a "local supplier" of high quality scintillator. (Read more)