We know that tons of things make neutrinos, like the sun, the earth, and even bananas!
Unfortunately, stacking a bunch of fruit around a detector isn't going to teach us much.
Instead, many neutrino experiments rely on a neutrino beam.
This is a way to make huge numbers of neutrinos with a
particular energy that we can aim at our detector.
But just how do you make a neutrino beam?
Well, the best way is to give it a heartfelt compliment.
But the second best way is by using a particle accelerator.
We'll explore how to create trillions of neutrinos today on Even Bananas.
As you know, neutrinos are neutral,
which means that the magnets in a particle accelerator can't manipulate them.
So, making a neutrino beam doesn't actually start with neutrinos. Instead, the process uses protons.
To irresponsibly oversimplify things,
you take some really speedy protons, aim them where you want your neutrinos to go,
smash them into a target, and then wait. Let particle decay do all the hard work.
Of course, that's easier said than done.
Getting those protons close to the speed of light
requires some incredible engineering in our accelerator complex.
In fact, this kind of physics research isn't even possible
without a host of people who build these systems and keep them running smoothly.
I've asked one of Fermilab's accelerator operators, Laura Bolt,
to help explain how neutrino beams are made in a bit more detail.
Hi Laura
>Hi Kirsty, thanks for having me.
I work as an accelerator operator for the Accelerator Division,
and a big part of my job is monitoring the accelerator systems from the main control room.
All of our particle beams start with the same thing: a tank of hydrogen gas. Hydrogen is made of
a proton and an electron and these are the protons that we're eventually going to be accelerating.
We take a piece of pure molybdenum,
about the size of a tea bag, and use it as a source of extra electrons.
We run electricity through it and we end up with a gas of hydrogen
ions that each have an extra electron.
Because they're charged, we can manipulate these ions with our magnets.
We can use an extractor to pull these ions in the right direction which starts the beam.
The first stop is the RFQ, which can bunch together and speed up the ions.
It hands them off to the linear accelerator, or linac, which gives them even more energy.
At the end, these extra electrons are stripped off of these ions, making our proton beam.
Then it gets handed off to the next accelerator, the booster.
>The protons can take different paths to experiments around the site,
including to the test beam facility, muon experiments, and neutrino experiments.
Fermilab is unique because we can make neutrino beams at two different energies.
For lower energy neutrinos, we use protons that come straight out of the booster.
This Booster Neutrino Beam makes neutrinos for the three experiments
in Fermilab's Short Baseline Neutrino program and the ANNIE experiment.
For higher energy neutrinos,
operators can move the proton beam into an even bigger accelerator, the Main Injector.
The NuMI beam is what we used for the MINERvA and MINOS experiments and now use for NOvA.
The Main Injector will also be used to make neutrinos for DUNE.
But either way, it's a similar process to go from proton beam to neutrino beam.
>That's right, we use the magnets to steer the protons
in the right direction so they're traveling toward a neutrino detector,
which can be anywhere from several hundred feet to several hundred miles away.
Then we get to do the cliche particle physics thing and smash them.
We have different options for these stationary targets, like
solid graphite or beryllium, and these targets have a tough existence.
They're absorbing a lot of energy from the protons.
When the protons collide with the target, it causes this spray of energetic new particles.
This includes some charged particles called pions and kaons, which we can steer using more magnets.
They travel through things called "neutrino horns" or "magnetic focusing horns".
These are essentially big magnetic funnels that get
the particles condensed back into a narrow beam.
>To make a beam of neutrinos,
we set the magnetic field in the horn so that it focuses positively-charged particles towards
where we want the neutrino beam to go and defocuses negatively-charged particles.
As our charged pions and kaons fly away from the target, they decay, producing neutrinos.
For example, a positively-charged pion will decay to a positively-charged muon and a muon neutrino.
We can also reverse the field in the magnetic horn and make a beam of antineutrinos.
And yes, it does feel very cool to say we can make a beam of antimatter.
To do that, we focus the negative pions
and wait for them to decay into a negative muon and a muon antineutrino neutrino.
Experiments regularly run in both neutrino and antineutrino mode,
and that is one of the ways we can study how matter compares to antimatter.
>The last step to making a neutrino beam is stopping
all of the particles that aren't neutrinos.
We put a material like concrete or steel in the beam path and this absorbs the muons, pions,
or kaons that haven't decayed yet, and any stubborn protons that have made it that far.
After they're produced, the neutrinos are on their own. We,
as operators, can no longer control their flight path.
The neutrinos travel straight through whatever's in front of them,
eventually arriving at the detector.
This whole process happens really quickly.
We're able to send trillions of protons through
the accelerator complex every second and produce trillions of neutrinos.
You might be thinking "what does a neutrino beam consisting of trillions of neutrinos look like?"
and people always ask me "can you stand in the neutrino beam?"
Well, here's a video of me sitting in our underground experimental hall, and you can
see the white circle on the wall indicating the center of the invisible NuMI neutrino beam.
I could sit there for about seven minutes before a neutrino would interact with me,
and even if one did, I would never notice.
>Thanks, Laura.
It's mind-boggling to think that in 1930,
Wolfgang Pauli predicted neutrinos existed, but thought we'd never be able to see them.
And now, we have the technology not just to see them,
but to produce trillions per second on demand.
If only Pauli could see us today, standing in our own neutrino beam not feeling a thing.
For more neutrino content on demand, be sure to like, share, and subscribe.
Fun fact: Without neutrino horns, an experiment would lose about 95% of the neutrinos in its beam.
After the protons hit the target, some of the new particles come out at wide angles.
The horn uses magnetic fields to steer them towards the detector.
To do this, operators pulse a huge electrical current along the horn surfaces,
about 200,000 amps.
For comparison, a toaster uses about 10 amps.
To keep the current flowing evenly and to withstand the harsh environment,
neutrino horns have to be really carefully designed and engineered,
all the way down to the shape of the washers.