Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: DerekT on 10/01/2008 19:29:04

Title: Where do the neutrons come from?
Post by: DerekT on 10/01/2008 19:29:04

Hi Folks,

Been watching a show on bbc4 about the atom and they tell me that helium is 4 times heavier than hydrogen because as well as having 2 protons it also has 2 neutrons. Hydrogen on the other hand only has one proton and no neutrons

They go on to tell me that the sun is constantly making helium by fusing together 2 hydrogen atoms. But that just makes an atom with 2 protons, to be helium it also needs 2 neutrons so where do the neutrons come from?

Many thanks,
Derek

Title: Where do the neutrons come from?
Post by: lyner on 10/01/2008 23:38:48

It's a very low probability thing to happen but:
If you take two protons and bash em together, they may produce a Deuterium nucleus (one neutron + one proton) plus a positive electron.
Hit two of these together and you can get a helium nucleus.
Or Hit a Deuterium nucleus with another proton and you can get a Tritium nucleus (2 neutrons + 1 proton) plus another positive electron. Then, another proton can come along and form a Helium nucleus.

In a fusion reactor as we know it, you need to isolate tritium atoms and mix them with hydrogen atoms. Or you can use Deuterium atoms. Either way, you need to do a lot of physically separating out the atoms you want. It is no good trying to 'do fusion' with only Hydrogen atoms - the probability is so low that you could never get enough energy out with the small quantities available anywhere else but in a star.
So 'fusion reactors' are a bit of a cheat, really - they would not be 'just like a star' because the relative amounts of the ingredient need to be different.

Title: Where do the neutrons come from?
Post by: DerekT on 11/01/2008 13:15:23

So a proton can become a neutron during the fusion process? Thanks a lot, I never knew this.

However it leads to the question how does this happen?

Thanks again,
Derek

Title: Where do the neutrons come from?
Post by: lyner on 11/01/2008 23:21:07

Now you're going down another level.
What we used to describe as fundamental particles have been revealed as consisting of other particles, called Quarks.
There appear to be rules governing how they combine and how proton can become a neutron etc.
Whether this makes us any wiser; well, I don't know.
To find out about things at a deeper and deeper level, we have to be able to smash things up and see what they break down into. This requires higher and higher energies. Hence the accelerator at CERN etc.
Incidentally, a neutron doesn't stay a neutron for very long on its own. The proton seems to have the longest life of all.