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previous articles, we have found that radioactive decay can lead to the
To these we must add a fourth category:
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.
Interactions of Radiation with Matter
Alpha and Beta Particles
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
This process is called ionisation.
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:
produces > 100 ionisation events per cm of travel
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 centimetres for beta particles). As the energy of either particle increases, so the range increases.
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
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:
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.
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.
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.
Effects of Neutrons
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
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.
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
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:
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.
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
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.
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.
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.
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
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.