4.5 Nuclear fission and fusion  (Page 3/4)

You will remember that nuclei naturally repel one another because of the electrostatic force between their positively charged protons. So, in order to bring two nuclei together, a lot of energy must be supplied if fusion is to take place. If two nuclei can be brought close enough together however, the electrostatic force is overwhelmed by the more powerful strong nuclear force which only operates over short distances. If this happens, nuclear fusion can take place. Inside the cores of stars, the temperature is high enough for hydrogen fusion to take place but scientists have so far been unsuccessful in making the process work in the laboratory. One of the huge advantages of nuclear fusion, if it could be made to work in the laboratory, is that it is a relatively environmentally friendly source of energy. The helium that is produced is not radioactive or poisonous and does not carry the dangers of nuclear fission.

Nucleosynthesis

An astronomer named Edwin Hubble discovered in the 1920's that the universe is expanding. He measured that far-away galaxies are moving away from the earth at great speed, and the further away they are, the faster they are moving.

What are galaxies?

Galaxies are huge clusters of stars and matter in the universe. The earth is part of the Milky Way galaxy which is shaped like a very large spiral. Astronomers can measure the light coming from distant galaxies using telescopes. Edwin Hubble was also able to measure the velocities of galaxies.

These observations led people to see that the universe is expanding. It also led to the 'Big Bang' hypothesis. The 'Big Bang' hypothesis is an idea about how the universe may have started. According to this theory, the universe started off at the beginning of time as a point which then exploded and expanded into the universe we live in today. This happened between 10 and 14 billion years ago.

Just after the Big Bang, when the universe was only ${10}^{-43}$ s old, it was very hot and was made up of quarks and leptons (an example of a lepton is the electron). As the universe expanded, ( $\sim {10}^{-2}$ s) and cooled, the quarks started binding together to form protons and neutrons (together called nucleons ).

Age of nucleosynthesis (225 s - ${10}^3$ S)

About 225 s after the Big Bang, the protons and neutrons started binding together to form simple nuclei . The process of forming nuclei is called nucleosynthesis . When a proton and a neutron bind together, they form the deuteron . The deuteron is like a hydrogen nucleus (which is just a proton) with a neutron added to it so it can be written as ${}^2\mathrm{H}$ . Using protons and neutrons as building blocks, more nuclei can be formed as shown below. For example, the Helium-4 nucleus (also called an alpha particle ) can be formed in the following ways:

center then:

or

center then:

Some ${}^7\mathrm{Li}$ nuclei could also have been formed by the fusion of ${}^4\mathrm{He}$ and ${}^3\mathrm{H}$ .

Age of ions ( ${10}^3$ S - ${10}^{13}$ S)

However, at this time the universe was still very hot and the electrons still had too much energy to become bound to the alpha particles to form helium atoms . Also, the nuclei with mass numbers greater than 4 (i.e. greater than ${}^4\mathrm{He}$ ) are very short-lived and would have decayed almost immediately after being formed. Therefore, the universe moved through a stage called the Age of Ions when it consisted of free positively charged ${\mathrm{H}}^{+}$ ions and ${}^4\mathrm{He}$ ions, and negatively charged electrons not yet bound into atoms.