The Nobel Prize-winning Sudbury Neutrino Observatory (SNO) is starting a new chapter in its life. After years of preparation and technical developments, liquid scintillator has started to flow into the SNO+ detector, a next-generation experiment now in development at the same site. Liquid scintillator produces flashes of light in response to the passage of charged particles, which are observed using sensitive photon detectors.
The detector’s acrylic vessel, which measures 12 meters in diameter and is located about 1.2 miles underground in a Canadian nickel mine, previously housed heavy water on loan from the Canadian government. Heavy water uses a form of hydrogen known as deuterium.
The SNO experiment was critical in resolving the so-called “solar neutrino problem,” in which researchers found a deficit in the expected numbers of a certain type of neutrino, produced in the solar core, that successfully reached Earth.
Based on the SNO experiment, scientists concluded that neutrinos could “flavor change,” or change from one type of neutrino to another during their journey. The experiment also provided evidence that neutrinos have a non-zero mass.
The legacy data set from SNO still offers unique insights into neutrino properties and also into backgrounds for future experiments due to SNO’s deep location and unique detection capabilities.
Analysis of this data is being led by Orebi Gann, a staff scientist in Berkeley Lab’s Nuclear Science Division and an assistant professor at UC Berkeley. Orebi Gann leads a joint Berkeley Lab-UC Berkeley team that is contributing to SNO+.
The SNO+ vessel, which will soon be completely filled with the liquid scintillator, will have sensitivity to a much lower range of energies than its predecessor. This broadens its physics reach and could enable the first North America-based detection of antineutrinos produced in the Earth’s crust. These so-called geoneutrinos can shed light on heat production beneath the Earth’s surface.
SNO+ will also measure the spectrum of low-energy solar neutrinos, with sensitivity to new physics effects such as so-called sterile neutrinos, and the experiment could resolve uncertainties in the metal content of the sun by a measurement of neutrinos from one of the sun’s fusion cycles, known as the CNO (carbon-nitrogen-oxygen) cycle.
The primary goal of SNO+ is a search for neutrinoless double beta decay, a rare process possible only if a neutrino is its own antiparticle, which scientists would call “Majorana” in nature. SNO+ will add tellurium to the scintillator-filled detector in the summer of 2019 – an amount equivalent to about 0.5 percent of the total mass of the liquid scintillator. This addition is expected to make SNO+ a world leader in sensitivity among experiments of its type.
The start of the scintillator-filling process heralds a new beginning for this experiment. Orebi Gann’s research team is leading the analysis of data from the preliminary water-filled phase, and is preparing for a fast turnaround on the analysis of early scintillator data.
Gabriel Orebi Gann is a staff scientist in Berkeley Lab’s Nuclear Science Division and an assistant professor at UC Berkeley. Gann leads a joint Berkeley Lab-UC Berkeley team that is contributing to SNO+.