Worlds Largest Neutrino Detector Is Collecting Data In China
To say that neutrinos aren’t the easiest particles to study would be a bit of an understatement. Outside of dark matter, there’s not much in particle physics that is as slippery as the elusive “ghost particles” that are endlessly streaming through you and everything you own. That’s why its exciting news that JUNO is now taking data as the world’s largest detector.
First, in case you’re not a physics geek, let’s go back to basics. Neutrinos are neutral particles (the name was coined by Fermi as “little neutral one”) with very, very little mass and a propensity for slipping in between the more-common particles that make up everyday matter. The fact that neutrinos have mass is kind of weird, in that it’s not part of the Standard Model of Particle Physics. Since the Standard Model gets just about everything else right (except for dark matter) down to quite a few decimal points, well… that’s a very interesting kind of weird, hence the worldwide race to unravel the mysteries of the so-called “ghost particle”. We have an explainer article here for anyone who wants more background.
The JUNO vessel from inside the (then empty) water jacket. Note the outwards-facing PMTs.
With JUNO, China is likely to take the lead in that race. JUNO stands for Jiangmen Underground Neutrino Observatory, and if you fancy a trip to southern China you can find it 700 metres under Guangdong. With 20,000 tonnes of liquid scintillator (a chemical that lights up when excited by a subatomic particle) and 43,200 photomultiplier tubes (PMTs) to catch every photon the scintillator gives off, it is the largest of its type in the world.
The liquid scintillator — linear alkyl benzene, for the chemists — is housed within an acrylic sphere surrounded by PMTs, suspended within an extra sixty thousand tonnes of ultra-pure water for radiation shielding. The arrangement is similar to the Sudbury Neutrino Observatory, but much larger. More PMTs point outwards to monitor this water jacket to serve as coincidence detectors for things like muons. With all of those PMTs, we can only hope everyone has learned from Super-K, and they don’t all blow up this time.
Assuming no catastrophic failure, JUNO will have great sensitivity in particular to antineutrinos, and will be used not just for astroparticle physics but as part of a beam experiment to study neutrino oscillations from neutrinos emitted by nearby nuclear reactors. (Virtually all nuclear reactions, from fusion to fission to beta decay, involve neutrino emission.) Neutrino oscillation refers to the strange ability neutrinos have to oscillate between their three different ‘flavours’ something related to their anomalous mass.
In this schematic diagram of a neutrino detection, PMTs around the detector are coloured according to the photons detected. The neutrino’s path has been recreated as a green line.
While JUNO is the biggest in the world, it won’t be forever. If everything goes according to plan, Japan will take the crown back when HyperKamiokande comes online inside its 258,000 tonne water vessel in 2028. Of course the great thing about scientific competition is that it doesn’t matter who is on top: with openly published results, we all win.
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