Tuesday, 5 February 2013

Neutrons - Part 2

A neutron as 1 up and 2 down quarks
A while back I did a post on neutrons. I have always had a bit of a soft spot for the neutron because while it may seem a little dull, having no charge and all. It is actually an amazing particle.

A neutron is one of the three basic particles we learn about at school. The others being the proton and the electron. We are taught that all atoms are made up of these 3 particles. Great, no problem there.

The neutron, if it finds itself outside of an atom, (say it has been kicked out during the radioactive decay of something like Uranium 235) actually gives up the ghost and changes into a proton, electron and an anti neutrino, represented by this ...
n0 → p+ + e + ν
e
What I find amazing though is the following. If we assume that the Big Bang was correct then we are fairly confident that a besides a load of electro magnetic energy we got some very basic particles, protons, neutrons etc.

Your basic hydrogen atom as a single proton at its centre. The water you drink is made up of molecules consisting of 2 hydrogen atoms and one oxygen atom, good old H20. These protons were probably produced 13 billion years ago at the start of everything. Mind blowing. There is more though. At the beginning some single protons joined with single neutrons to produce a hydrogen atom that we call deuterium and give it the letter D. Hydrogen is the only element that has more than one symbol.

D can form water molecules, D20. In fact 1 in about 6500 water molecules is made of D20. A glass of water contains billions and billions and billions of molecules of the stuff. It is thought that most deuterium was also created soon after the big bang! This stuff has been around for ages which brings me back to neutrons.

What happens if we manage to pop another neutron next to the proton and first neutron? well we end up with Tritium, this does decay with an average half life of about 12 years. So

protons are stable
neutrons are not stable
deuterium is stable
tritium is not stable

when a neutron decays it does so as shown above. Although some think that this is not actually true, what really happens is this
n0 → p+ + W
W→  e + ν
The W particle, is a really, really short lived particle which decays rapidly into the electron and the anti neutrino. By short I mean about 10-25 of a second. This is way to small for the brain to even contemplate. It got me thinking though, inside Deuterium, does the neutron decide to behave itself? Does it think, I've got a proton buddy don't think I'll bother with any more of this radioactive suicide? Or does it pull a little bit of body snatching I wonder.

Imagine if the neutron does actually decay as I've described above, but after the first step

         n0 → p+ + W

The W particle thinks "No chance mate" and instead of decaying into the electron actually pounces on the other unsuspecting proton like so

         p+ + W→ n0 

So we still end up with a single proton and a neutron, only now they have swapped places. Then, a little time later, they do the same dance back. How cool would that be these two little particles doing this dance over and over for billions of years.

Now, imagine the centre of a very large star that is just in the process of undergoing a catastrophic collapse. Most of its fuel has gone and gravity is starting to crush the atoms, closer and closer and closer together. Space-time is getting all bent out of shape and then atoms themselves start to collapse. Electrons actually get pushed into the nucleus, react with protons and produce neutrons. This goes on and in no time at all we have a star that is now only about 20 km across and consists entirely of neutrons.

Lots of people struggle with the last part, after all, electrons have the opposite charge to a proton, since opposites attracted they should jump on each other. But here is one of those really strange things that are "explained" using quantum mechanics. so why don't they just jump on each other, I'll save that one for another post. Suffice to say that to get an electron into the nucleus of an atom you have to put on some tremendous pressure, like the type you get inside a collapsing star.

So now we have something really amazing, a neutron star. Which is pretty much just a super enormous nucleus consisting of loads of neutrons. A nucleus with a radius of about 10 km. The gravitational pull of this star at its surface is about 100 Billion times stronger that on earth, a number far to big for the brain to understand. The gravitational field is so strong that it bends light so much that if you are looking directly at the star you can see some light from the far side! That would be like looking at the moon and being able to see some of the far side, even though it is still on the far side!

Neutron stars spin many times a second, again I find this incredible, these massive objects some spinning hundreds of times a second. The angular momentum is just huge.

I'll end this post pretty much were I started, the decay of a neutron into a proton and an electron. In about 1 in 250,000 of these decays the electron does not have the energy to completely escape the proton and the result is a hydrogen atom.

What I wonder is this... if neutrons can decay into hydrogen atoms, then was there any need for protons to be produced in the big bang? couldn't it just have been a shed load of neutrons, that decayed very very rapidly a short time after.

With a half life of just shy of 15 minutes, even if every particle produced by the big bang was a neutron then within 15 minutes half of them would be protons. Within an hour we would have less that 7% of the free neutrons still in existence. By the end of the second hour this would be less than half of 1%. The big bang theory doesn't think this is what happens, in that theory there are protons as soon as particles begin to form and most of the neutrons join up with the protons to form Deuterium and Helium nuclei, not sure I am convinced.

So there we have... the humble neutron.









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