SNO+

The international Sudbuty Neutrino Observatory Plus (SNO+) collaboration’s main goal is to solve a key mystery in neutrino science: whether a neutrino is its own antiparticle.

Based at SNOLAB, SNO+ is one of five TRIUMF-SNOLAB collaborations involving some of the world’s most advanced experiments in neutrino science (nEXO, HALO), and the search for dark matter (DEAP-3600, SuperCDMS). SNOLAB is the deepest underground laboratory in North America, based two-kilometers underground near Sudbury, Ontario, in order to shield experiments from cosmic ray background.

Since Wolfgang Pauli’s prediction of the existence of the neutrino in 1930, the experimental study of these fundamental particles has remained elusive. This reflects the neutrino’s nature. After photons, neutrinos are the most abundant particles in the cosmos, but they are enormously difficult to detect because they very rarely interact with other matter. Trillions of neutrinos pass through your body every second.

The 2015 Nobel Prize in physics was co-awarded to SNOLAB’s predecessor, the Sudbury Neutrino Observatory (SNO) collaboration, for the co-discovery that neutrinos oscillate between different forms on their journey from Sun to Earth.

Yet, much about neutrinos remains unknown, and physicists think that understanding neutrino nature is a window into beyond-Standard Model physics. The Standard Model assumes that, like photons, neutrinos are massless, but the discovery of neutrino oscillation, and other experimental detections implies that neutrinos have a distinct mass.

The SNO+ experiment is tackling the neutrino challenge using an indirect way of determining neutrino’s deeper nature: searching for the putative and extremely rare neutrinoless double beta decay. The SNO+ experiment is repurposing the original SNO detector, using the radioactive isotope tellurium-130 (¹³⁰Te) as a source of double beta decays.

The observation of neutrinoless double beta decay would be a major discovery of new physics, confirming that the neutrino is its own anti-particle and providing an indirect absolute measurement of neutrino mass.

SNO+ is an international collaboration involving researchers from more than two dozen institutions in six countries, including Canadian participation from the University of Alberta, Laurentian University, Queen’s University, SNOLAB and TRIUMF.

SNO+ is repurposing the core experimental equipment from SNO, to which TRIUMF also supplied key components. SNO+ involves two key components: a double beta decay source; and a specialized scintillator material and related light detectors.

Neutrino-less double beta decay

SNO+ will use radioactive ¹³⁰Te as a source of double beta decay. In normal double beta decay two neutrons in a nucleus each decay to a proton by each emitting a beta particle (a high-energy electron) and an anti-neutrino. Thus, each nucleus emits two beta particles and two neutrinos. However, some theories predict that very rarely a nucleus will decay through neutrinoless double beta decay.

Since the half-life of ¹³⁰Te beta decay is more than a billion times the age of the universe and the predicted rate of putative neutrinoless double beta decay is also very rare, the SNO+ experiment will use a large amount of tellurium, approximately 2.3 tons, in search of neutrinoless double beta decay.

The detection of such a decay would provide an important indirect window on neutrinos. One of the key questions in neutrino physics is whether neutrinos are Marjorana or Dirac particles. For Dirac particles, including protons and electrons, there are distinct anti-particles, in these cases antiprotons and positrons. However, Marjorana particles, including photons, are their own antiparticle.

If neutrinoless double beta decay is observed, it would prove that neutrinos are Majorana particles, which would help explain their low mass, and would provide a measurement of it—the rate of neutrinoless double beta decay is theoretically related to the square of the neutrino mass.

Scintillator and Detectors

The heart of SNO+ is a huge, suspended 12-meter diameter acrylic sphere filled with a mix of approximately 800-tonnes of liquid scintillator into which is dissolved the heavy metal ¹³⁰Te. The sphere is monitored by about 10,000 photomultiplier tubes (PMT), very sensitive light detectors. The acrylic sphere, detectors and detector support structure is being repurposed from the SNO experiment.

SNO+ will detect neutrinos through their interaction with electrons and nuclei in the detector to produce charged particles which, in turn, create light as they pass through the scintillator, an organic liquid that emits light when charged particles pass through it. This light is detected by the PMT array, a process very similar to the way SNO detected neutrinos.

TRIUMF’s Design Office and Machine Shop managed the design and construction of two key SNO components: the universal interface and rope equalizers for the detector. TRIUMF also provided testing for all of SNO’s electronics, designed and built at the University of Pennsylvania. For SNO+, upgrades for both the universal interface and the apparatus used to move calibration sources around in the detector were carried out at TRIUMF.