Naturally occurring radioactive substances have high nucleon number. It is possible to make artificial radioactive substances by bombarding lighter nuclides with  - particles, protons or neutrons. The radioactive substances produced in this manner are known as radioisotopes.
A nuclide is any species of atom of which each atom has an identical proton number and also an identical nucleon number. Different nuclides, which have the same proton number (but different nucleon numbers) are called isotopes (isotopic nuclides). The first radioisotope was an unstable isotope of phosphorus. It was produced in 1934 by bombarding aluminium with  - particles, i.e.,

Phosphorus - 30 was produced, together with a neutron. Notice that on each side of the equation the sum of the nucleon number is 31 and the sum of the proton number is 15. Phosphorus - 30 decays by ejecting a positron and has a half-life of about 3 minutes.
The positron has not been mentioned before because it does not occurring natural radioactivity. It has a mass equal to that of the electron, and a positive electron. It is denoted by 

When magnesium is bombarded by neutrons a radioisotope of sodium is formed. The reaction is

The sodium decays with the emission of a  - particle.


The important point is that it is now possible to produce any radioisotope. Most of those produced have short half-life periods. This is very important because the activity and hence the danger from radioactive emissions does not last very long.
Most of the radioisotopes found in nature have relatively long half lives. They also belong to elements which are not handled well by the human body. As a result medical applications generally require the use of radioisotopes which are produced artificially.
We have looked at the subject of radioactivity in earlier chapters of this wikibook and have then progressed to cover the interaction of radiation with matter, radiation detectors and imaging systems. We...

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