In an enormous cavern, 700m (almost 2,300ft) beneath a wooded hill in southern China, an extraordinary scientific instrument is being built. The Jiangmen Underground Neutrino Observatory (JUNO), a 20,000-tonne (almost 19,700ton) sphere of a detector liquid surrounded by 43,000 super-sensitive light detectors, is set to probe the secrets of nature’s most elusive subatomic particle: the neutrino.
“We’ve had to overcome many difficulties and challenges in building JUNO,” says Dr Yifang Wang, the Director of the Chinese experiment, who came up with the concept and design in 2008 after reading a paper by Italian theorist Prof Serguey Petcov. “JUNO is a formidable feat of engineering,” says Prof Jennifer Thomas, a particle physicist at University College, London, who sits on the international advisory panel of the Chinese experiment. “It’s scheduled to start operating in 2025 and we’re extremely excited.”
Neutrinos are the second most common subatomic particles in the Universe after photons of light. They’re abundant in the extreme, but practically refuse to interact with any type of physical matter, hence they’re commonly referred to as ‘ghost particles’. Hold up your thumb. An astonishing 100 billion or so neutrinos are passing through your thumbnail every second. They were created by the nuclear reactions that generate sunlight. And eight and a half minutes ago they were in the heart of the Sun. Neutrinos only interact with the atoms of matter on exceptionally rare occasions. Your thumbnail is not nearly enough to stop them in their tracks. As American novelist Michael Chabon observed: “Eight solid light-years of lead… is the thickness of that metal in which you would need to encase yourself if you wanted to keep from being touched by neutrinos.”