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Alpha decay and quantum tunneling

You can't know too much about where nucleons are

Radioactive decay isn’t affected by what goes on outside the nucleus but by what happens inside it.

We tend to model our protons and neutrons as static balls but it’s better to think of them as whizzing blurs.  Quantum theory says that the position of any given nucleon is not defined exactly at any given instant.  You can only give it a probability of being in a particular place at a particular time.

Try Why Do Astronauts Float by Julian Hamm

An alpha particle is a very stable arrangement

Let’s consider a nucleus that decays by alpha emission.  Every so often two protons and two neutrons come together in an ‘alpha cluster’.  Protons and neutrons tend to pair up, so an alpha cluster is very stable.

When an alpha cluster forms randomly in a big nucleus the energy released almost allows the alpha to break free.  But it’s never quite enough.

An alpha cluster can just find itself outside the grip of the strong force

Quantum theory assigns a probability to the alpha cluster’s position and there is a probability that it will find itself outside the nucleus.  If the alpha is found outside the pull of the strong force then it is emitted, as the electrostatic repulsion takes over.

Think of the nucleons at the bottom of a deep ‘energy well’.  When an alpha cluster forms it gets a boost to above ‘ground level’ but still not high enough to escape.

Quantum theory allows for the possibility that it will just appear on the other side of the energy barrier.  It’s as if the alpha just ‘tunneled’ through the energy barrier so this is called ‘quantum tunneling’.

Just like electrons, nucleons only have certain 'orbits' or energy levels where they're stable.  The alpha can never be found in the middle of the energy barrier because there are no available energy levels.

back to Lesson 12: Stability, Fusion and Fission