To understand what we mean by radioactivity, consider a nucleus with Z protons and N (equals A-Z) neutrons.  It turns out that this nucleus can only be stable for certain combinations of Z and N.  By “stable”, we mean that the nucleus remains in the same form (with the same Z and N) for an infinite period of time.  

For example, consider an atom with 27 protons and 32 neutrons (Z=27, A=59).  These are the atoms of which stable cobalt (a metal) is composed, and is written in the form cobalt-59.  The nucleus is stable, and does not change its composition over the course of time.  However, consider now a nucleus with 27 protons and 33 neutrons (Z=27, A=60, called cobalt-60), that is, with an extra neutron compared with the stable cobalt-59.  This nucleus is not stable, and can only exist in that form for a limited amount of time.  After a certain amount of time has passed, the nucleus changes its form to a configuration that is stable, and cobalt-60 is said to be radioactive.  This involves a change in the numbers of protons and neutrons that are present; in this case, one of the neutrons becomes a proton and an electron is emitted from the nucleus – but more of that in the next section.  Atoms with nuclei that have the same number of protons but differing number of neutrons are called isotopes and the transformation from one nucleus to another nucleus is called radioactive decay.

Predicting the exact amount of time that elapses before any particular unstable nucleus decays is not possible. However, if there are a large number of such nuclei present, one can define an average decay time, called the radioactive half life.  This is defined as the time required for half of the nuclei present to decay.  For the case of cobalt-60, the radioactive half life is about 6 years.  That is, if we start out with 1000000 cobalt-60 atoms, after about 6 years we can expect to be left with approximately 500000 cobalt-60 atoms. 

The existence and decay of atoms with unstable nuclei is the basis for the existence of radiation and radioactivity.  In the example of cobalt-60, the radioactive transformation of the nucleus to a more stable form is accompanied by the emission of an electron, as one of the neutrons in the nucleus transforms itself into a proton.  In the case of cobalt-60, this transformation is also accompanied by the emission of a photon of electromagnetic radiation (called a gamma ray).  It is the ultimate fate of these gamma rays and the emitted electron that is of concern when considering the radioactive problems posed by cobalt-60 (cobalt-60 sources are often used in medical applications of radioactivity). 

In the next section we will discuss in more detail the type of radioactive decay described above for cobalt-60.  In addition, we will look at the other types of radioactive decay that can occur, and why it is that radiation produced in this way can be dangerous to us. 

Summary 

To summarise this section, we have found out that: 

1.                  All matter is composed of atoms;

2.                  Atoms consist of a central nucleus, around which rotate a number of electrons;

3.                  The nucleus consists of protons and neutrons;

4.                  Only certain combinations of protons and neutrons are stable

5.                  Radioactive decay can be specified in terms of the radioactive half life

Radioactivity 

In the previous section, we discovered that radiation and radioactivity arise as a result of the inherent instability of some types of atomic nucleus.  Whereas some nuclei are inherently stable, others change their form after a certain period of time.  The resulting nucleus after this transformation may be stable, or it may not, and a further transformation may take place.  In this section, the various types of transformation are discussed, along with the effects that such radioactive transformations have on humans and other living creatures. 

Alpha Decay 

Alpha decay is a type of radioactive decay in which the nucleus emits an alpha particle – a particle that consists of two protons and two neutrons bound together.  In fact, the alpha particle is the nucleus of the stable helium atom.  The alpha particle is emitted with an energy that is characteristic of the nucleus undergoing the decay.  An example of an isotope that undergoes is plutonium-239.  The alpha decay of plutonium-239 can be written as follows: 

            Plutonium-239  ->  uranium-235  +  alpha particle 

The radioactive half life of this decay is about 24000 years – plutonium-239 is therefore a very long-lived isotope.  With a few exceptions, alpha decay is only observed in nuclei with more than 82 protons. 

Beta Decay 

Beta decay is another type of decay in which either a neutron is converted to a proton and the nucleus emits an electron, or a proton is converted into a neutron and the nucleus emits a positron (a positively charged electron).  In some cases, a given nucleus can decay in either of these two ways.  Some examples of beta decay are: 

            Iodine-131  ->  Xenon-131 + beta particle (electron)

            Sodium-22  ->  Neon-22 + beta particle (positron) 

In contrast to alpha decay, the energy of the emitted beta particle can assume any value from zero up to a maximum value, the mean energy of emission being about a third of the maximum value.  In order to explain this, the existence of another type of particle, called a neutrino, was proposed.  It is now known that every beta decay is also accompanied by the emission of a neutrino – though because neutrinos are thought to have no mass or charge, they are extremely difficult to detect. 

Orbital Electron Capture 

Orbital electron capture is a type of decay in which the nucleus captures one of the electrons orbiting the nucleus.  The electron interacts with one of the protons, producing a neutron and ejecting a neutrino from the nucleus.  Sodium-22 can decay by orbital electron capture, as well as by positron emission. 

Gamma Decay 

Before we discuss gamma decay, it is necessary to understand a little more about the arrangement of the protons and neutrons in the nucleus.  Protons and neutrons are very small particles, and as such are subject to rules of behaviour governed by quantum mechanics.  Quantum mechanics is a branch of science that determines the behaviour of systems of very small particles, for example electrons, protons and neutrons.  The results of quantum mechanics sometimes provide a direct conflict with the results of our everyday experiences.  One such result is the following: 

            Any particle or system of particles can only exist in certain well-defined energy states. 

This is an odd result – it tells us that our nucleus can only exhibit certain values for the total energy of the system.  It turns out that the same result holds for everyday objects, such as cars, tennis balls, etc, but closer examination shows that the allowed energies for such objects are so close together in value that it is not possible for us to distinguish between them, and that to all intents and purposes, the energy of a tennis ball can exhibit a continuum of possible values.

Let’s get back to our nucleus.  Gamma rays are pulses of electromagnetic radiation (called photons) that are emitted when the nucleus moves from one of its allowed energy states down to a lower energy state.  The energy of the gamma ray is equal to the energy difference between the higher and lower energy states of the nucleus, and the emission is required to conserve energy.  Radio waves, microwaves and visible light are more familiar examples of electromagnetic radiation. 

A common situation in which a gamma ray is emitted is following beta decay (and occasionally, alpha decay).  Once the decay has occurred, the resultant nucleus is left in one of its higher energy states.  It moves back down to its lowest energy state by emitting one or more gamma rays.  Not all beta emitters emit a gamma ray as well, and so two types of beta decay are often considered in radiation protection: 

            Pure beta emitters (e.g. hydrogen-3), emit beta particle only;

            Beta-gamma emitters (cobalt-60), emit beta particle and gamma ray(s).

Internal Conversion 

Internal conversion is an alternative to gamma decay. Instead of shedding its excess energy by emitting a gamma ray, an excited nucleus can dispose of its excess energy by interacting with one of the inner electrons orbiting the nucleus, which in turn absorbs the excess energy and is ejected from the atom.  An example where this occurs is with the atom of radioactive caesium, caesium-137.  This decay by beta decay to an excited state of barium-137, which in 89% of decays loses its excess energy by emitting a gamma ray, and in the remaining 11% of cases by internal conversion.  Caesium-137 is a particularly important radionuclide in the context of radiological accidents, as it is one of the principal by-products of the operation of nuclear reactors.  The most important radionuclide released during the Chernobyl accident was caesium-137. 

When internal conversion occurs, other orbiting electrons can ‘fall’ into the orbit vacated by the electron that was ejected.  When this happens, the electron loses energy, and an X-ray is emitted.

On to Effects of Radiation