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Why does alpha decay occur

Dear Reader,

I bemoan the fact that few text books explain why things happen, many text books are content to tell you what happens when they discuss radioactivity but are not able to or willing to explain how it happens. Now some time ago I explained the driving force behind beta decay, today we are going to start to deal with alpha decay.

Alpha decay is when the nucleus of an atom emits the nucleus of a helium-4 atom, now I am sure that some of the smarter readers will have asked (or be considering) the question of why is it always a helium-4 nucleus. Now I have to tell you that helium-4 has a special high stability.

I know as human beings we like to think of ourselves as more than just the sum total of our parts, while one recent estimate suggests that a human body is only worth $ 4.50 the EPA think that a human life is worth $ 9100000. I do not want to get into a debate about the morals or value of human life but it is clear that if we use the EPA estimate that a human is worth much more than the scrap value of the typical human body.

In the same way a group of neutrons and protons which make up an atomic nucleus has a mass which is often different to the sum of the mass of the free nucleons. This is because when they bind to each other some energy is lost. because energy and mass can be interconverted (E = mc2) this means that the mass is changed slightly. In general the more strongly the particles are bonded to each other the lower the energy of the nucleus and the lower the mass is. Now carbon-12 is used as the zero point for many things, the mole is defined as 12 grams of carbon-12 and also it is used as a zero point these nuclear calculations.

If we look at a graph of the excess energy which is due to the extra mass which is associated with taking protons and neutrons out of a very stable system into a less stable system divided by the number of nucleons in the nucleus against the mass of the nucleus then we get a funny looking graph. It has a general downward trend over the mass range 1 to 30 but there are some masses which are extra stable. For this graph I have used the stable nuclides (stable isotopes) except for two points which we will get onto later.

These are 4, 8, 12, 16, 20 and 24 which are magic numbers. It is important to note that these nuclei have even numbers of protons and neutrons. We will get onto magic numbers again some time in the future. Here is the graph below.

A graph of excess energy (keV) per nucleon against the mass of the nucleus

Now I hope that we should be able to see that the helium-4 nucleus is a very stable small fragment. We will continue soon with alpha decay, but before I go you might find this link useful. It is for some lectures on the nuclear physics of radioactive decay.


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