Topic 2 Radioactivity and the atom

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All atoms of a particular element have the same number of protons in their nucleus – they have the same atomic number (Z). For example, every atom of carbon has 6 protons (and every atom with 6 protons is a carbon atom). However, the number of neutrons in the nucleus may vary. Atoms of the same element with different numbers of neutrons are called 'isotopes' and have different mass numbers (A). The nuclei for the five smallest isotopes and their chemical symbols are shown in Figure 2.1.

Figure 2.1 The nuclei of the five smallest isotopes: ¹₁H, ²₁H, ³₁H, ³₂He, ⁴₂He.

Nuclear decay

Additional neutrons do not change an atom's chemical properties. For some elements, however, extra neutrons make the nucleus unstable and it eventually undergoes spontaneous radioactive decay. This decay produces nuclear radiation, and so these unstable variants are known as radioisotopes.

When a radioisotope decays, its atomic number usually changes, and if so, so it becomes a different element. The dream of the ancient alchemists to change lead into gold is therefore not as fanciful as it might seem, although with their technology is was very optimistic indeed! Even today it would not be possible in practice.

Part of the radiation that Henri Becquerel discovered being emitted from his uranium rock in 1896 was the result of the radioactive decay of the radioisotope uranium-238 into thorium-234. This daughter element is also radioactive and, in turn, decays into other radioactive elements, as part of an ongoing decay series. Eventually, stable isotopes are produced and the decay series stops. For example, when uranium-238 decays, 14 different radioactive elements are produced before the series finally ends with stable lead-208.

Half-life

Some radioisotopes are more stable than others, and last for different amounts of time before they decay. This will affect their use and disposal. On average, it takes 4.5 billion years for an atom of uranium-238 to decay through all 14 isotopes into lead. However, more than 99.99% of this time is spent waiting for the first decay to occur. The other steps in the series are very much faster; some taking millions of years, and most taking just days or minutes. For polonium-214, the average is just 160 microseconds.

We refer to the average amount of time taken by a sample of a particular radioisotope to decay as its half-life. More correctly, the half-life is the time taken for half the atoms in a sample to decay. For example, the half-life of carbon-14 (C-14) as it decays into nitrogen-14 (which is not radioactive) is about 5,700 years. If you started with 20 million C-14 atoms, there would be 10 million C-14 atoms remaining after 5,700 years. Wait the same time again and there will be 5 million.

Figure 2.2 Two half-lives of decay of radioactive carbon-14. The darker balls indicate C-14 atoms that have decayed.

So now we know a fair bit about nuclear radiation, except ... what it is!

Alpha radiation

Alpha radiation is made up of streams of the tiny particles emitted from decaying nuclei. Ernest Rutherford collected these alpha particles and found he had collected helium! Alpha particles are helium-4 nuclei, that is they have two protons with two neutrons. During alpha decay, these are emitted from the nucleus at around 1/20th the speed of light (ie about 15,000 km/s). Because they are highly charged and relatively large (by subatomic standards), they slow down rapidly as they plough through the air. As they travel through the air or another substance, they knock electrons from surrounding atoms and molecules, which then become charged particles (ions) and so alpha radiation is called ionising radiation. As they slow down, the alpha particles grab the electrons knocked from surrounding particles to become neutral helium atoms.

Figure 2.3 Uranium-238 undergoes alpha decay to become thorium, and releases an alpha particle in the process. The alpha decay is followed by an emission of gamma radiation.

Beta radiation

Early experiments revealed that beta radiation also has an ionising effect, though this is not as strong as alpha radiation. Beta radiation is made up of negatively charged particles: beta particles. Beta particles were shown to be electrons ejected from the nucleus. How this happens involves some advanced physics, but, briefly, a neutron in the nucleus splits to produce a proton (which stays put) and an electron. This electron has too much energy to stay in the nucleus and is emitted as beta radiation. Beta particles are given off with a range of speeds, some as high as 90% of the speed of light.

Gamma radiation

Gamma radiation is very different from the alpha and beta radiation discovered by Rutherford. It is classified as ionising radiation but because it is uncharged, it does not usually cause ionisation directly. It interacts with ions already present and causes changes in the materials it passes through. Gamma radiation is not particle radiation - it is electromagnetic radiation - like light or X-rays. If you look at Figure 1.1, you can see that gamma radiation is at the extreme high energy end of the spectrum, higher even than X-rays. There are gamma rays from space (perhaps from Betelgeuse or Alpha Centauri) that are passing through your body right now.

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