Why emit alphas, not other nucleon groups?

[catkin]

[SOLVED] Why emit alphas, not other nucleon groups?

Homework Statement

(Advanced Physics; Adams and Allday; Spread 8.18, Question
Section 8.18, question 3)
Why do you think helium-4 nuclei (alpha particles) are often emitted from unstable heavy nuclei whereas bundles of neutrons or protons alone are not?

Homework Equations

?

The Attempt at a Solution

I have absolutely no idea but would love to know. I've often wondered and have been looking out for an answer with no success.

The ejection of an alpha particle creates a different nucleus that sits approximately in the same place on the #neutrons vs. #protons "stability" graph. Not quite -- because the stability line curves, hence the ocassional beta emission to re-align.

That explanation says it's energetically attractive to emit equal numbers of protons and neutrons but not why they come out in fours -- twos would do equally well or perhaps singly followed very soon afterwards by the "balancing" nucelon of the pair (because of leaving the nucleus in an excited, unstable state).

 

[einstein_a_go_go]

Is it perhaps to do with the biding energies of the other possibilities?

Look at http://en.wikipedia.org/wiki/Binding_energy graph.

There must then be other reasons for C-12 not being given off!

 

[andrevdh]

For the decay to take place the mass of the daughter nucleus plus the mass of the emitted nucleons need to be smaller than that of the parent nucleus. This is always possible for the tightly bound alpha particles, but not generally so for the emission of single or pairs of nucleons. For some nuclei such decay are possible - energy will be liberated - but in such a case the lifetime of the parent is extremely short and it is not normally observed.

 

[catkin]

Thank you both

Equipped with that information I found a nice explanation at http://www2.slac.stanford.edu/vvc/theory/nuclearstability.html

SOLVED!