The Story of the Neutrino

"Neutrinos, they are very small.
They have no charge and have no mass
And do not interact at all.
The earth is just a silly ball
To them, through which they simply pass,
Like dustmaids down a drafty hall
Or photons through a piece of glass." -John Updike

It was so much fun talking about neutrinos that I thought I'd take the time to tell you what all the fuss is about.

Let's go back -- way back -- to the late 1920s. Not only did we know that everything on Earth was made out of atoms, we knew that atoms were made out of atomic nuclei, which were positively charged, massive, and tiny, and electrons, which were negatively charged, much less massive, and also tiny.

We even had some idea of how the energy levels worked (above) thanks to Niels Bohr, and how radioactive decay worked (below), thanks to Marie Curie, among others.

There was a problem, however, with some of these radioactive decays. Specifically, the ones that underwent Beta Decay in the image above. What's the problem? It looked like the conservation of energy didn't work for them!

Hang on, you say. I know what the conservation of energy says: that energy can't be created or destroyed, but can be converted from one form into another! Even mass, if Einstein's got it right, is just a form of energy.

That's what E = mc2 tells us, after all!

So what do I mean that the conservation of energy didn't work? Let's give you one of the simplest examples: tritium. Nearly all of the hydrogen that exists in the Universe is the simplest atom you can imagine: one proton for the nucleus, and one electron orbiting it. But I can make a hydrogen atom with a proton and a neutron for the nucleus, called deuterium, or I can make it with a proton and two neutrons, called tritium.

(Apparently, wasserstoff -- "water-substance" -- is hydrogen in German.)

And while plain, no-neutron hydrogen and rare, one-neutron deuterium are stable, tritium, with its two neutrons, will decay after about 12 years!

Big deal, you say. All you have to do is -- if energy and momentum are conserved -- measure the mass of tritium, of Helium-3 and the beta particle (electron), and then you'll know how much Kinetic Energy the electron ought to have. And that should be easy to measure; it should come out with the same energy every time! How much energy? Just a little bit over 18 keV.

And when we measure the energy of the electron, what do we see?

That a teeny, tiny fraction of them have around 18 keV of energy, almost up to the expected amount, but not quite. But the vast majority of these emitted beta particles have much, much lower energies. How do you fix this problem?

Well, Niels Bohr, if you asked him, put forth the idea that maybe energy wasn't really conserved for all decays. Maybe, whenever you have beta decay, you lose just a little bit of energy, and the stuff you start with has more overall energy than the stuff you wind up with.

But if you asked Wolfgang Pauli (left, in the above photo, with Heisenberg and Fermi), he had a bold new idea. Maybe, he postulated, energy and momentum really were conserved, but we just weren't seeing all the particles.

Maybe, in addition to the helium-3 nucleus and the electron, there was a little, uncharged, neutral particle that was carrying away the missing energy and momentum. He named it "neutrino", for "little neutral guy", and this was in 1930.

Nuclear and particle physics developed a lot over the 1930s, 1940s, and early 1950s with the discovery of the neutron, the advent of the atomic bomb, the Hydrogen bomb, and the construction of the first nuclear power plants.

Well, here's the thing. Nuclear power plants work by taking radioactive Uranium rods, sticking them in water, where -- in addition to being enriched to make fissionable materials -- the Uranium radioactively decays, through a great chain of events, eventually culminating in stable lead. Along the way, every time there's a beta decay, an atom produces one of these theoretical neutrinos (well, technically antineutrinos) of moderately high energies.

But here's the deal, and this is interesting. If I can take tritium, and have it radioactively decay to form helium-3, an electron, and an antineutrino, I can make the reverse happen!

What's the reverse? Take one of these high-energy antineutrinos and smack it into helium-3! What do I get out? An atom of tritium -- which is hard to find -- and a positron, which is easy to find!

Why so easy?

Because matter is full of electrons, the anti-particle of positrons. When an electron and a positron collide, they produce two photons of the same energy that the mass of an electron (or positron) has: 511 keV!

Sure, other things make these photons, too. That's why you run the experiment twice: once near the nuclear power plant (which gives you the antineutrinos), and you get something like 500 reactions, and once far away from the nuclear power plant, and you only get something like 200!

And that's how you discover the neutrino! Despite being proposed in 1930, it wasn't found until 1956, by Fred Reines and Clyde Cowan, who used Cadmium-Chloride instead of Helium-3, but the underlying physics is the same.

Not only did Pauli win a Nobel Prize for this, Enrico Fermi, who worked out the reaction rates and cross-sections for neutrinos, also won one, and so did Reines and Cowan, who used Fermi's work to design their experiment!

And that's all it takes to discover a new particle! And an experiment like this demonstrates, beyond a shadow of a doubt, that neutrinos exist. Finally, you, too, know how to prove that neutrinos exist!


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you call It wonderful; I call it crass.

P.S. Nice writeup.

By Anonymous Coward (not verified) on 05 Oct 2010 #permalink

This doesn't have much to do with the posting but I thought the lyrics were appropriate for a quote.

There's an old song "Little Neutrino" from the 70's by a band called Klaatu. I love the last line...
Across your open mind, I trace erratic lines, In motion and in time

I fought a battle won, To the surface of the sun, Through fires on and on

It's only you
It can't be me
For I myself refuse to be
I am someone you'll never know
I am the little neutrino

Solus is not far away, It's face is brighter than a day, So don't turn me away


And now I'm passing through, The one who's known as you, And yet you'll never know I do
Just thought I'd share,

I went to school where Clyde Cowan did his research, and the physics chair while I was there had some great stories from those days. IIRC, it turns out that sugar water is decent way to try and detect neutrinos, and hey, there are tons of abandoned, water-filled mines in Virginia! So Clyde got arrested at least once for suspected moonshining because he was running around the backwoods of Virginia with a truck full of sugar. The chair of the department had to vouch for him to some small-town sheriff :)

And he didn't actually receive the Nobel since he died before it was awarded. Boo.

This might be a dumb question, but how does a neutral particle have an anti-particle? Or rephrased, which property is reversed between neutrino and anti-neutrino?

"This might be a dumb question, but how does a neutral particle have an anti-particle? Or rephrased, which property is reversed between neutrino and anti-neutrino?"

Spin direction.

@5: There is also a quantity called "lepton number" which must be conserved. Electrons and neutrinos have a lepton number of 1; positrons and antineutrinos have a lepton number of -1, and baryons have a lepton number of 0. I'm not sure how you prove this theoretically, but it is observationally true.

That's in addition to the spin preference mentioned @6.

By Eric Lund (not verified) on 06 Oct 2010 #permalink

I don't understand what healthphysicist means by "spin direction" (or what Eric Lund means regarding "spin preference") regarding the difference between the neutrino/antineutrino.

My understanding was that they're both s=1/2 particles, so they can both have m_s = +1/2 or -1/2. What's different?

By Anonymous Coward (not verified) on 06 Oct 2010 #permalink

One fact that always amazes me is that in a supernova there are so many neutrinos produced in the core collapse that they area (the?) major component of the pressure that blows the outer layers off. Given how weakly neutrinos interact with ordinary matter, think how many of them you'd need to blow a star apart!

Anonymous Coward/Mu

Depends actually. The question of whether the neutrino is identical to an antineutrino has large implications and is a subject of current research (in the jargon - whether the neutrino is a "Majorana" particle or not).

At the moment we label the two particles as different based on
their helicity. It's a quantity which measures the "handedness" of a travelling particle with respect to it's spin direction. A neutrino is always left-handed, an anti-neutrino is always right handed. This is loosely (very very loosely) analogous to a ball spinning around a direction along which the ball is moving. It can spin clockwise (if looking down the direction of propagation) or anti-clockwise. A neutrino always spins anti-clockwise, and an antineutrino always spins clockwise.

Of course these may just be two states of the same particle.....

By ANeutrinoGuy (not verified) on 07 Oct 2010 #permalink

..Oh and Eric, you can't prove lepton number conservation
theoretically. It's a symmetry that we say must hold based on the
fact that we've never seen it *not* holding. As such it's built
into the theory from the top down, rather than just appearing in
what would be a more satisfying theory. If the neutrino is Majorana
then you could violate lepton number symmetry. Searches continue...

By ANeutrinoGuy (not verified) on 07 Oct 2010 #permalink

Apologies for the necropost, but my SIWOTI syndrome doesn't like this:

Nuclear power plants work by taking radioactive Uranium rods, sticking them in water, where... the Uranium radioactively decays... culminating in stable lead

No. Most nuclear power plants work by arranging uranium rods (previously enriched in U-235) in water in such a way that the neutrons from spontaneous fission provoke more and more fission in other uranium atoms, until the desired power level is reached, at which point the reactor is levelled off (by control rods, for example) at 100% power.

The fission products are mostly very highly radioactive with a lot of beta decay (beause uranium is far richer in neutrons that the fission products can be stable with). So a working nuclear reactor is indeed a neutrino-rich environment, but not because of the uranium decay chain.

I enjoy this discussion of neutrino energy, but am confused.
All these posts (and the article) speak of neutrino energy as
a particle. Doesn't quantum mechanics tell us that these
energies are also experienced as waves? JUst asking.

y neils bohr named the atomic orbitals as K,L,M,N,....