Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO) was a neutrino observatory located 2100 m underground in Vale's Creighton Mine in Sudbury, Ontario, Canada. The detector was designed to detect solar neutrinos through their interactions with a large tank of heavy water.

The detector was turned on in May 1999, and was turned off on 28 November 2006. The SNO collaboration was active for several years after that analyzing the data taken.

The director of the experiment, Art McDonald, was co-awarded the Nobel Prize in Physics in 2015 for the experiment's contribution to the discovery of neutrino oscillation.

The underground laboratory has been enlarged into a permanent facility and now operates multiple experiments as SNOLAB. The SNO equipment itself is currently being refurbished for use in the SNO+ experiment.

The first measurements of the number of solar neutrinos reaching the earth were taken in the 1960s, and all experiments prior to SNO observed a third to a half fewer neutrinos than were predicted by the Standard Solar Model. As several experiments confirmed this deficit the effect became known as the solar neutrino problem. Over several decades many ideas were put forward to try to explain the effect, one of which was the hypothesis of neutrino oscillations. All of the solar neutrino detectors prior to SNO had been sensitive primarily or exclusively to electron neutrinos and yielded little to no information on muon neutrinos and tau neutrinos.

...
Wikipedia