• Blog Stats

    • 76,905 hits
  • Archives

  • Enter your email address to subscribe to this blog and receive notifications of new posts by email.

    Join 157 other followers

  • Copyright notice

    This blog entry and all other text on this blog is copyrighted, you are free to read it, discuss it with friends, co-workers and anyone else who will pay attention.

    If you want to cite this blog article or quote from it in a not for profit website or blog then please feel free to do so as long as you provide a link back to this blog article.

    If as a school teacher or university teacher you wish to use content from my blog for the education of students then you may do so as long as the teaching materials produced from my blogged writings are not distributed for profit to others. Also at University level I ask that you provide a link to my blog to the students.

    If you want to quote from this blog in an academic paper published in an academic journal then please contact me before you submit your paper to enable us to discuss the matter.

    If you wish to reuse my text in a way where you will be making a profit (however small) please contact me before you do so, and we can discuss the licensing of the content.

    If you want to contact me then please do so by e-mailing me at Chalmers University of Technology, I am quite easy to find there as I am the only person with the surname “foreman” working at Chalmers. An alternative method of contacting me is to leave a comment on a blog article. If you do not know which one to comment on then just pick one at random, please include your email in the comment so I can contact you.

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.

Advertisements

Go on, Have your say !

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: