A scale for neutrinos

 By Eniday Staff

Her name is KATRIN. She’s from Karlsruhe, in the south-east German land of Baden-Württemberg. You can miss her. In fact, she’s rather large – 70 m long and 10 m tall – and extraordinarily heavy, tipping the scales at 200 tonnes. Her ladylike name is down to the etymology of the name Katrin – Catherine, in English – which derives from the Ancient Greek kataròs, meaning “pure”…

To be fair, you need a massive amount of purity to be able to weigh something that probably has a mass that is between 100 thousand and 10 million times less than an electron – itself not exactly hefty. Yes, KATRIN is a weighing scale, albeit an enormous one with an exceptionally precise mechanism and cutting-edge electronic control system. But still a weighing scale. Its goal? To finally settle the question, “Just how much does a neutrino weigh?” Let’s start at the beginning, though. The neutrino was ‘born’ – if that’s the right term – in Rome’s Via Panisperna, home between the two world wards to the University of Rome’s Royal Institute of Physics. The “Via Panisperna Boys”, as the group of young researchers to whom we owe a fundamental piece of contemporary scientific knowledge were known, included the famous Italian scientists Enrico Fermi and Edoardo Amaldi. It was they who decided to call it a ‘neutrino’, affixing the well-known Italian diminutive “-ino” to the word ‘neutron’, the particle that together with the proton forms the nuclei of every atom in the world. The existence of the neutrino had been postulated by Wolfgang Pauli in 1930, to explain the continuous decay of radioactive atoms That the neutrino could not not exist was shown by Enrico Fermi at the Solway Conference in Brussels, 1933. That it actually existed was then proved in 1956 by the Americans Clyde Cowan and Fred Reines. And finally, that the neutrino had specific physical characteristics was shown by experiments carried out from the 1990s onwards in the Italian national laboratories at Gran Sasso, where this miniscule particle was observed without the annoying fog produced by cosmic rays and other such spoilsports.

The sci-fi Kamioka neutrino observatory built under a mountain in Japan (

One of the characteristics observed at Gran Sasso was surprising. Until that time, it was thought that the neutrino not only did not have an electric charge but did not also have a mass, which would have explained the fact that it can move between common matter – including our bodies – without leaving a trace, as if for a neutrino everything were empty space. Instead, the researchers found that the neutrino did have a mass: infinitesimal, but real nonetheless. Its ability to pass through matter as though it were “Alice in Wonderland” is due to the absence of interaction with strong nuclear force (which holds together the nuclei of atoms of what we call matter) or electromagnetic force (since the neutrino, like its giant cousin the neutron, does not have electrical charge and therefore goes wherever it likes. It does, however, interact with gravitational force (and even falls to Earth, only to pass through it serenely) and weak nuclear force (which causes radioactive atoms to decay).

Katrin’s challenge

So let’s get to the point: how to measure the neutrino’s mass.  To avoid the weight of the neutrino being less than the contraption’s bottom limit, you would need a hyper-accurate machine. Indeed the difficulties soon showed up. The immense and ultraprecise  scale KATRIN (Karlsruhe Trtitium Neutrino experiment) found that the neutrino weighed 1.96×10-33 grams (or 1.96 million billion billion billionths of a gram), which is in fact KATRIN’s lower limit. But we can try again, by reducing that limit. How?

KATRIN, the scale for neutrinos (

Well, Katrin is not a scale like the ones we have in the kitchen or even like the ones in the chemistry lab. KATRIN is a detector of mass and it works, on a rudimentary level, as follows: when a triton atom – a radioactive cousin of hydrogen – decays and disintegrates, it emits an electron and a neutrino, exactly like the sun when it beams down on Gran Sasso. Now, if you know the initial energy of the atom and the energy of the electron, the difference gives you the energy of the neutrino and therefore its mass. Essentially it’s like a market scale in reverse, which tells you when you have a kilogram of produce because the scales are perfectly level, with no difference between the two sides. Here, instead, it is the difference that tells us the weight, but it’s so small that it makes researchers lives very difficult indeed. The team was set to try again at the end of October, but to increase Katrin’s sensitivity by five times will take at least a couple of years of experiments and measurements.
Good luck with that, then!

READ MORE: Bowling with asteroids by Eniday Staff

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Eniday Staff