Introduction to Radioactive Decay

This decay simulation program is the one used in class. You will be told how to use it in class. Download it at home if you want. It is a self-extracting zip file (about 740 kb in size). Any problems, email me.

Decay simulation has been rebuilt (210401) to correct a small error, please redownload!

Some basic facts

• Radioactive decay is a process that takes place in the nucleus of an atom.
• The nucleus changes in one of several ways (it might break apart for example) and emit radiation. This is called disintegration.
• Once the nucleus has decayed, it will not be able to decay in the same way again.
• There is no way of predicting when an individual nucleus will decay - it is a random process.

Decay is a random process

The most important point is that radioactive decay is a random process. The probability that a particular nucleus will decay (disintegrate) does not change as the nuclei get older. All the nuclei of a particular isotope (uranium-234 for example) have the same probability of decaying. Nuclei of different isotopes have different decay probabilities (oxygen-14 nuclei have a higher decay probability than copper-64 nuclei)

Radioactivity

A substance is radioactive if it contains nuclei which are disintegrating. Radioactive substances emit radiation (can be alpha, beta, gamma or other types not considered in this course). The number of disintegrations per second is called the activity of the sample. The unit of activity is the Becquerel (Bq). Radioactive nuclei are sometimes termed radionuclei for short. Some isotopes are not radioactive - these are called stable isotopes (oxygen-16 is stable).

Law of radioactive decay

Some people find it a little odd that there can be a 'law of radioactive decay' since the decay of individual nuclei is a random process. (This problem really bothered the early researchers of radioactivity (from 1896 on)). Nonetheless, what is random for individual nuclei directly leads to the law of radioactive decay for a huge number of nuclei. It is a mathematical law, which says basically that:

The activity of a sample of radio-nuclei always goes down with time

(Mathematically, the activity decays exponentially with time - you don't need to understand exponentials for standard grade physics.)

Half-life

Half-life is a measurement of time. It is the time taken for the activity of a sample of radioactive material to fall by half. Different isotopes have different half-lives.

There is a huge range in the lengths of half-lives. Finish this section by reading the glossary reference on half-life. Make sure you can do half-life calculations!

Summary

• Radioactive decay takes place in the nucleus of an atom.
• The nucleus changes in one of several ways and emits radiation.
• Once the nucleus has decayed, it will not be able to decay in the same way again.
• There is no way of predicting when an individual nucleus will decay - it is a random process.
• Activity of a radioactive sample is the number of disintegrations taking place per second.
• The unit of activity is the Becquerel.
• It is possible to predict how the activity of a huge number of nuclei will fall away with time.
• Half-life is the time taken for the activity of a sample to fall by half.

Other things you should know

Amongst the other radiation areas you should know about are:

• The structure of the atom.
• The three basic types of radiation.
• Background radiation and how to deal with it in calculations.
• Nuclear medicine - both diagnostic and therapeutic treatments.
• How to detect nuclear radiation.
• The biological effects of radation and dose-equivalent.

Check your class notes!