In the previous articles, we have found that radioactive decay can lead to the following by-products:

Alpha particles (helium nuclei);
Beta particles (electrons or positrons);
Gamma rays (high energy electromagnetic radiation);

To these we must add a fourth category:

Neutrons.

Neutrons are usually produced when a nuclear transformation is induced (that is, is made to happen, rather than through occurring naturally), for example by taking an atomic nucleus and firing another nuclear particle at it, or when radioactive fission occurs (the breaking up of a large unstable nucleus into two roughly equal nuclei, each around half the size of the original and with the liberation of considerable amounts of energy). The point is that most neutron sources occur in the laboratory (or nuclear reactor) under special conditions, as opposed to the other types of decay, which occur naturally. Nuclear fission is fundamental to the operation of many types of nuclear reactor. During the fission process, large amount of energy are released from the nucleus that undergoes fission, and this energy can be used to provide electrical power by heating water.

To understand why these types of radiation are potentially damaging to living entities, it is necessary to understand how they interact with matter, in particular living tissue and cells. In doing this, bear in mind that just like all other matter, living tissue consists of a collection of atoms and molecules bound together to form the tissue mass.

Alpha and Beta Particles

Alpha and beta particles are often referred to as directly ionizing radiation. This is because when an alpha or beta particle enters living tissue, they interact directly with the outer electrons of the constituent atoms and, if they supply enough energy, they can knock the outer electrons away from the atoms. The end product of such an event is a free electron and a positively charged ion. This process is called ionisation.

Because alpha and beta particles have substantially different masses (an alpha particle weighs about 8000 times as much as a beta particle) and different charges, the rates at which the two types of particle cause ionisation are very different:

Beta particle: produces > 100 ionisation events per cm of travel

Alpha particle: produces > 10000 ionisation events per cm of travel

It can therefore be seen that the alpha particle causes considerably more ionisations (and hence radiological damage), though this is offset to an extent by the fact that alpha particles have a very much smaller range of travel in body tissue than beta particles of the same energy (of the order of micrometres, compared with centimeters for beta particles). As the energy of either particle increases, so the range increases.

A consequence of this is that alpha-emitting radioisotopes rarely pose a radiological hazard outside the human body, as the alpha particles are not able to penetrate human skin. When alpha particles are taken into the body, for example the lung, the situation is reversed on account of the very high rate of ionisation as they slow down in human tissue.

Gamma Rays

In contrast to alpha and beta particles, gamma rays induce ionisation in the atoms of living tissue by indirect means, and are therefore referred to as indirectly ionizing radiation. There are three principal mechanisms by which gamma rays interact with living tissue:

1. Compton scattering;

2. Photoelectric effect;

3. Pair production.

In the Compton effect, the gamma rays are scattered from the outer electrons of the atoms, transferring energy to the electrons and in the process reducing the energy of the gamma ray. If enough energy is supplied during scattering, the outer electron will be removed from the atom, leaving an ion and giving rise to a free electron.

In the photoelectric effect, one of the inner electrons of the atom absorbs the energy of the gamma ray, and is ejected from the atom, again leaving a positively charged ion and a free electron. Following this, it is often the case that one of the outer electrons ‘falls’ down to fill the vacancy. As a consequence, an X-ray is emitted from the atom.

In pair production, the gamma ray interacts with the electric field of the nucleus, and is converted into an electron and a positron. A minimum amount of energy is required for this reaction to occur (equal to the energy associated with the mass of the electron and the positron), and if the gamma ray does not have this minimum amount of energy, the reaction cannot occur. The positron, in travelling through the tissue material, will usually react with another electron and be converted back to two gamma rays.

Neutrons

Because neutrons do not have an electric charge, they do not interact directly with the electrons of the atom. Instead, they are scattered from, and collide with, the atomic nuclei of the constituent atoms. Two processes can occur:

1. The neutrons are scattered from the nuclei, transferring energy to the nuclei and in turn losing energy themselves. The additional energy acquired by the nuclei can be released as gamma rays.

2. The neutrons collide directly with the nuclei, and are absorbed, thus creating a new nucleus. This nucleus may be unstable, and so radioactive decay occurs, creating alpha, beta or gamma rays.

It is interesting to note that the direct collision described in item 2 above can only occur if the energy of the neutrons are low enough (so-called ‘thermal’ neutrons). If the energy is too high, then elastic scattering is the principal energy loss mechanism.

To summarise, when radiation interacts with matter (in particular living tissue), the main effect is to remove the outer electrons of the constituent atoms by ionisation. The result is that a number of free electrons are created, and a number of positively charged ions are created. Of course, it is possible for free electrons and ions to recombine, to give a neutral atom once again. This is offset by the fact that some of the free electrons may be sufficiently energetic to cause ionisations of their own. In any case, it is this process of ionisation that is responsible for the biological damage that can be caused by radiation.

Effect of Radiation on the Human Body

To understand the nature of the damage caused by radiation, it is necessary to look at the microscopic structure of the human body. The human body (indeed the body or structure of any living animal or plant) is composed of a large number of individual cells. These cells can be split broadly into two categories, namely:

1. somatic cells

2. germ cells.

The germ cells are those that are responsible for reproduction of offspring, and constitute the sperm in males, and the ova in females. All other cells fall under the classification of somatic cells.

The genetic information that characterizes any individual is contained within the chromosomes. Somatic cells contain 46 chromosomes (23 chromosomes, occurring in pairs), and germ cells contain 23 chromosomes (23 chromosomes occurring once), so that when a sperm and an ovum come together, they produce a composite with the full 46 chromosomes. All cells in the body contain exactly the same genetic information; when cells divide, the chromosomes are reproduced exactly, so that the new cells resulting from cellular division contain exactly the same genetic information as in the original cell.

The chromosomes, in turn, are composed of linear sequences of genes. Genes are the basic units of heredity, and mammalian cells contain between 60000 and 70000 genes. The chromosomes are composed principally from deoxyribonucleic acid, which is usually shortened to ‘DNA’. A molecule of DNA contains around 10 million atoms, and it consists of two chains that are entwined around each other (the famous ‘double helix’). The diagram below (courtesy of the Microsoft clipart collection) shows the basic structure of DNA. The two chains are held together by various cross-connections (termed ‘hydrogen bonds’ by chemists) between the two chains. The genetic information held in the DNA molecule is defined by the sequence in which various groups of atoms occur on the molecule. Evidently, this is an extremely complex subject, and is beyond the scope of this article.

Now, let us suppose that a collection of cells in the human body is subject to the types of radiation described above. We know that the main effect of this radiation is to cause ionisation of the atoms in the absorbing medium. Thus, when cells are irradiated, it is likely that ionisation of one or more of the atoms on some of the DNA molecules will occur. This can lead to a number of consequences for the affected molecule. These effects include

1. breakage of the chains of molecules comprising the DNA, and

2. breakage of the links between chains.

In many cases, the cell is able to repair the damage, but not always. When the damage cannot be repaired, the affected cell is left with altered or damaged genetic information, compared with the unaffected cells. All descendants of that cell will contain altered or damaged information as well, because cellular division results in exact replication of the genetic information in the original cell.

The direct attack of radiation on the structure of DNA is not the only means by which radiation can affect cells. The majority of the human body (about 60%) is made up of water, and the ionizing effects of radiation on water can lead to an indirect attack on DNA. The effect of radiation on water (via a series of chemical reactions) is to produce a liquid, similar to water in composition, called hydrogen peroxide. Hydrogen peroxide is, in contrast to water, a chemically active compound, and it is capable of reacting with DNA to damage cells and the genetic information contained therein. Cells can therefore be subject to an indirect attack due to the action of radiation on body water, as well as from the direct effects of ionisation at the site of the DNA.

Therefore, if germ cells (sperm and ova) suffer damage to the genetic information and they are subsequently involved in germination with other germ cells (i.e. affected sperm cells uniting with an ovum), the offspring may well carry cells containing the damaged information. Similarly, somatic cells will divide to increase the number of cells in the body with damaged information. The proliferation of damaged cells that cannot perform their normal function is the root cause of cancer.

On to Classifying Radiation