Topic 1 What is nuclear radiation?

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You may be surprised to learn that radiation is all around you. Nuclear radiation is usually only present in very small amounts, but other kinds of radiation are present in large quantities. Light, heat and radio waves are all forms of electromagnetic radiation. These are only three of the types of electromagnetic radiation shown in the spectrum in Figure 1.1.

The different types of electromagnetic radiation are often explained as packets of light (photons) of different energy (ie the type of radiation depends on the amount of energy it has). All electromagnetic radiation moves at the speed of light.

Figure 1.1 The electromagnetic spectrum.

Electromagnetic radiation can be very useful. It can also be dangerous, although it is often easily controlled – an electric heater, a light, an X-ray machine or a mobile phone can readily be switched on or off. A fire can usually be put out, and a chemical reaction can be slowed or stopped, but how can nuclear radiation be controlled?

In 1896 French scientist Henri Becquerel discovered a new kind of invisible radiation that seemed to be emitted from a uranium-rich rock. This radiation could not be stopped, increased or decreased. This was nuclear radiation and it was something completely new to science. Marie Curie, working in Paris, coined the term 'radioactivity' to describe this new property, and discovered three new radioactive elements. Further studies by New Zealander Ernest Rutherford showed that this new radiation was actually three different types. He called these alpha (α), beta (β) and gamma (γ) radiation. Each of these types of radiation has its own characteristic properties. Different radioactive substances emit different combinations of these radiation types.

Figure 1.2 The properties of alpha, beta and gamma radiation.

Early scientists used a variety of methods to discover and investigate these new kinds of radiation. A common way of detecting radiation is to use a Geiger counter. A common measure of radiation is counts per minute (cpm). If you had a Geiger counter with you now, it would register a very tiny amount of nuclear radiation, probably between 1 and 100 cpm. This is background radiation. It includes nuclear radiation from the sun and stars and from elements naturally present in the Earth and in materials around you (eg bricks). In every breath, you take in radioactive radon gas, and there is radioactive potassium present within your own body from the food you eat. However, all this background radiation is less than the amount you would receive over the same time period from a medical procedure such as X-ray or the location of a tumour using radioactive tracers. It is many times less than the amount of radiation produced by uranium in a nuclear reactor or weapon.

Atoms and radioactivity

To understand what nuclear radiation is, you first need to understand the structure of the atom. Indeed, it was the discovery and use of nuclear radiation that led scientists of the early 20th century to investigate the internal structure of the atom.

Earlier theories (in the late 19th century) had pictured an atom as being like a plum pudding – the negatively charged electrons (plums) were thought to be embedded in a positively charged 'pudding'. This was shown to be incorrect by a famous experiment designed by Rutherford in 1911. Rutherford used radioactivity to show that all the positive charge in an atom is concentrated into a tiny and very dense nucleus at the centre of the atom. The electrons orbit this nucleus at a distance, a little like the planets around the sun. If an atom was as large as a cricket stadium, its nucleus at the centre would be only the size of a pea, but it would weigh about 200,000 tonnes! The rest of the atom (the whole cricket ground) would be empty containing only the very much tinier orbiting electrons. This means that you and everything around you is mostly empty space, since matter is made of atoms and atoms are mostly empty space.

Figure 1.3 An atom consists of a positively charged nucleus surrounded by one or more negatively charged electrons.

Figure 1.4 An atom that loses one or more of its orbiting electrons becomes a positively charged ion. An atom that captures one or more extra electrons becomes a negatively charged ion.

Alpha, beta and gamma radiation are known as ionising radiation. This is because they can make atoms become ions when they pass through a substance. However, radioactivity cannot be understood by examining an atom's orbiting electrons. The part of the atom that produces nuclear radiation is the nucleus. Details of what alpha, beta and gamma radiation are made of are described in Topic 2.

When the nucleus was first discovered, it was correctly thought to contain positively charged protons, equal in number to the number of negatively charged orbiting electrons. The positive and negative charges cancel one another out, making the atom neutral overall. The number of protons in the nucleus determines an atom's identity as one element or another, and is referred to as its atomic number (Z). In the 1930s it was discovered that the nucleus also contains uncharged particles called neutrons. Neutrons and protons have about the same size and mass. As they are each about 2000 times the mass of an electron, they make up most of the mass of an atom. Consequently, the sum of neutrons and protons in the nucleus gives the mass number of an atom (A). These two numbers summarise key information about an atom or element, and are written as shown below.

Figure 1.5 The nucleus of nearly all atoms contains protons and neutrons. The hydrogen atom (as shown) is the only atom that does not contain neutrons. The number of protons is the atomic number (Z), the number of protons plus neutrons is the mass number (A).

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