First Events Seen at Sudbury Neutrino Observatory

Fifteen years after it was initially proposed, the Sudbury Neutrino Observatory (SNO) has begun collecting data on solar and atmospheric neutrinos. Built by a collaboration of scientists from Canada, the US, and the UK, SNO is the second of a new generation of powerful neutrino detectors (see Physics Today, July 1996, page 30 and December 1997, page 56). It consists of an acrylic container holding 1000 metric tons of heavy water, surrounded by an array of about 9600 photo‐multiplier tubes (PMTs) in ordinary water and buried 2 km underground in a nickel mine near Sudbury, Ontario.