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Asked by lizzie3006 to Ben, Jony, Katharine, Mark, Peter on 21 Nov 2011.
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Katharine Schofield answered on 21 Nov 2011:
Really good question lizzie3006. To understand this you need to know a bit about the quantum behaviour of the nucleus… I posted about this last week in answer to a different question, so this is a bit of a copy and paste job…:
It’s not just the electrons in an atom that arrange themselves into shells…the protons and neutrons in the nucleus do too. Nuclei are quantum objects, and although the maths is messier than with electrons, the shell model of nuclei has helped nuclear physicists explain why some nuclei are stable and some are radioactive (i.e. they decay, emitting radiation). So if the outermost neutron and proton shells are full, the nucleus sits quite happily and doesn’t decay (a bit like atoms with full electron shells not being very reactive with other atoms). If the outer shells aren’t full, the nucleus prefers to get rid of these hangers-on and so is more likely to decay.
It turns out that in the shell the a nucleus made of 2 protons and 2 neutrons is a very stable configuration. It’s stable because it’s a very tightly bound system (which means the potential energy of the configuration is less than the mechanical energy it would take to pull it apart). When a big nucleus is unstable, it can become more stable by getting rid of (emitting) some of it’s protons and neutrons. Because the 2 proton+2neutron combo is so stable, they cluster together and get emitted.
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Peter Williams answered on 21 Nov 2011:
Yes, Katherine is right. It is the most stable combination.
What is tricky is calculating why this is so. We have a good theory of how quarks and gluons behave at high energies when we bash them together, but using this theory to “predict” these stable states at low energy is hard. There is a research group at Glasgow university (and some others like Edinburgh, Cambridge) who use some of the most powerful computers in the world to do these calculations – and they are slowly succeeding 🙂 -
Mark Basham answered on 21 Nov 2011:
This is also the reason why in fusion on earth in Tokamaks is done with Tritium (1Proton and 2 Neutrons) and Deuterium (1 Proton and 1 Neutron). When they fuse together they make the very stable alpha particle and they also release a high energy Neutron which can then be used to heat water or make more tritium etc. Here’s a pic http://en.wikipedia.org/wiki/File:Deuterium-tritium_fusion.svg
All in all a very useful little particle 🙂
Comments
bosslewisbossbyllal commented on :
is there anything more powerful than the brain muscle ?