At this moment, I think it's about 100 billion neutrinos per second is part are passing through your little finger truly ludicrous number of these particles.

If you actually want to stop one neutrino, you need about four light years of lead to have a good chance of making it stop.

Yes, this is kind of a new Z one s.

An article just appeared in the journal Nature, where they have detected neutrinos from the sun and a rather different kind of neutrino than have been detected previously.

From this, I guess we need to back up a little bit and talk about the neutrinos and where they come from.

Okay, So styles like the sun powered by nuclear fusion in their cause.

What's happening in the middle of the sun is by a series of nuclear reactions.

You're turning hydrogen into helium on hydrogen is one proton and the helium you make consists off two protons and two neutrons.

And so it's not just a matter of sticking protons together because you can't make two protons and two neutrons by just taking protons together.

You have to transmute some of those protons into neutrons, and so some of the nuclear processes has to change protons into neutrons.

That's a thing called weak force interaction, but a byproduct of those weak interactions.

You always end up throwing out a neutrino is part of the process.

The fact that the sun is turning hydrogen into helium means that you have to be making neutrinos in in the process for a good number of years.

Now they're being experiments to look for solar neutrinos any day.

We've been detecting solar neutrinos, but up until now, the only solar neutrinos that have been detected or the very high energy ones that are on.

But it turns out the vast bulk of neutrinos are much lower energy neutrinos.

The main reaction that produces neutrinos in the sun is where you take two protons, smack them together and end up creating a thing called deuterium.

Heavy hydrogen, which is one proton, one neutron so you could see you started with two protons.

You've turned it into one proton on one neutron, so therefore you must have done one of these weak interactions, and therefore there has to be these byproduct of a neutrino and it's sort of the first step of turning hydrogen into helium is to take two hydrogen atoms smacking together create deuterium.

In fact, it's sort of the rate limiting step if you just keep smacking protons into each other.

Turns out it would take you about 10 billion years of smacking them into each other, too, Actually convinced them to gun to go this reaction.

So it's a very, very slow process.

But fortunately is an awful lot of hydrogen in the silent son has a very long lifetime on their four, actually thesis, you know, the fact that this is a long time scale process?

Actually, it's probably what dictates the lifetime of the sun is the fact that this this reaction is so slow.

So you also have to create another particle.

We have to create a positron in the process.

So actually the charges conserved as well, because the actual reaction is proton.

Plus proton goes to proton neutron in deuterium, plus a posy strong, which carries away the extra bit of charge plus a neutrino.

I've always been aware that the sun is sharing us with country knows.

Is it not sharing us with positrons, as well, then No.

Positrons don't last very long because a positron very soon meet up with an electron and then they'll annihilate and turn into a couple of gamma rays.

And that this is what remember this is all going on right in the core of the sun on allies opaque to us, because actually, any positrons or gamma rays or anything else that gets created down there scatters around and takes literally thousands of years to make its way out of the sun.

Whereas neutrinos, because they interact so little with anything, Once you've created the middle, they stream straight out of the sun.

So the neutrinos were detecting now were made very recently in the sun.

Where is the rest of the energy we receive from the sun?

Has taken this long path to get out.

That's the key reaction that's going on in the center of the sign.

It's the thing that's creating most of the energy in the sun, but those are the neutrinos that are being previously undetectable because they're too low energy.

The previous generations of neutrino detectives have not be nobody detective, And now, with his new generation of neutrino detector, there finally reaching a point where they can actually start detecting.

These sort of core neutrino is being produced by the sun.

The ones that they were detecting the high energy ones, Where were they from?

So in positions, I mean, there were lots of reactions that so you have to somehow turn your protons into helium, and there's lots of different channels you can go go down to actually make that reaction happen, some of which involved creating your heavier things that helium, which then split up again and so on.

So that's quite a complicated set of the channels you can go down.

And some of those more obscure channels produce very high energy neutrino.

So those are the ones that were being detected before.

If you actually want to stop one neutrino, you need about four light years of lead to have a good chance of making it stop.

Okay?

They really because they just don't interact them direct so weakly with matter, they just happen to go streaming.

They'll go streaming through the earth.

There, go streaming through you through your little finger Through several light years of lead, you need an awful lot to stop them But of course, because there's so many Ofem, the fact that the probability of stopping in the individual one is very low really doesn't matter because you know sooner if you got 100 billion off the positive your little finger every second.

If you think about building a nice big detector, then there'll be trillions and trillions of neutrinos going through second, even though the chances of any one of them interacting is very small, a few will.

And so this particular detective is ah, uses on organic simulator simulator is just something which gives out light on what happens is a neutrino goes through very occasionally.

One of these neutrinos will bump into an electron as it passes through.

Give the electron a little bit of a kick.

So suddenly the electrons got some energy on the and then the nature of the center Laters is that the electron in dumps that energy into the simulator makes a little flash of light, and you have a whole array of photo multiplier tubes of very sensitive light detectors all around it that just look for these little flashes of light, and so they essentially detect the neutrinos by these very occasionally interactions leading to these interaction in the center, later, which produces a tiny flash of light.

What does it take for a neutrino electron collision?

To be sufficient for this to happen?

Does I have to hit it smack in the face or at a certain angle?

Or an electron?

That said, in a certain state itself for it, really, it's just the very rare in direction.

I mean, they're just the There's no particular threshold for this.

It's just that the vast majority of the time the neutrino in the electron take no notice of each other at all.

And it's just once in a while you get close and you know, if you like, you can think about them is a close enough collision that actually it will bump into it and dump a decent amount of energy into the electrons.

So it is just proximity is just smacking it in the face.

That's really the way that people that you can think about them in two ways you can think about it as a probability, or you can think about the neutrino sort of having a cross section, and if you happen to get within that cross section.

It's a hit and you get an interaction.

But it's not, is probably not right.

To think of the neutrino is a little hardball.

In that sense, I can see how previously the high energy ones were easier to find if there were more likely to shove an electron.

What did they change that made it possible to find these lower engine?

So there's a couple of things firstly, the particular kind of scintillating they're using is capable of.

Actually, it's well scintillating even with these lower energy interactions.

And secondly, they've gone to enormous lengths to push the background down because, as well as these neutrinos creating these little flashes of light, other things can create little flashes of light in your detector as well.

For example, if you got cosmic rays hitting the detector so other particles from space nothing to do doing neutrinos, they can cause installations in the detectives that they stick holding a mile underground.

Any radioactive material in the detector itself, in the body of the detector in the simulator or whatever it is, then the radioactive decays of that will also create little flashes of light.

So they've gone to enormous lengths.

For example, there's a radioactive isotope of carbon that decays produces will also produce these dumps.

Energy into the simulator produces flashes of light.

The way they have dealt with that is they built.

The center later is made of organic molecules that have lots of carbon in them, so that's a bad news.

So they used extremely old oil deposits to make the simulator.

You get the carbon from very, very old oil deposits, where almost all the carbon 14 is already decayed.

Even with that being the case, it's still one of the dominant sources of background in this thing is brighter than the neutrinos are.

So it's actually quite a subtle measurement you have to make in order to dig out from all these possible sources of confusion, where those neutrinos from the sun from this basic reaction coming from that some level, we've got these two measures as to the the luminosity of the sun.

One is how bright the sun appears to be in the sky and remember, it's a sight takes tens of thousands of years for that light to actually get to the surface, and the other is the neutrino flux is telling us how much energy is being produced right now.

So at least on that, we can compare the two to see whether the Sun's brightness has changed on that 1,000,000 year timescale by comparing what's going on now in the center of the sun from what was going on many tens of thousands of years ago.

So in that sense, it's a sort of it is a least a check that the sun sort of behaving itself on that time Scott, which we've never been able to measure before.

I guess it would have been a bit of a surprise if we found anything weird that way.

And it does turn out that actually the sun seems to behaving more.

That's the way we expected it.

So neutrinos are like real time.

Sun and light is the sun thousands of years ago.

Exactly so.

If ever the neutrino flux stops from the sun than we know that in, you know, 30,000 years time, we're gonna be in deep trouble.

It's so difficult to detect these, so the way that they tend to be detected is deep underground.

So the original experiment, the 1st 1 for example that was there to detect neutrinos from the sun was actually a big tank or cleaning fluid carbon tetrachloride.