# 4.5 Nuclear fission and fusion  (Page 4/4)

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## Age of atoms ( ${10}^{13}$ S - ${10}^{15}$ S)

As the universe expanded further, it cooled down until the electrons were able to bind to the hydrogen and helium nuclei to form hydrogen and helium atoms. Earlier, during the Age of Ions, both the hydrogen and helium ions were positively charged which meant that they repelled each other (electrostatically). During the Age of Atoms, the hydrogen and helium along with the electrons, were in the form of atoms which are electrically neutral and so they no longer repelled each other and instead pulled together under gravity to form clouds of gas, which evetually formed stars.

## Age of stars and galaxies (the universe today)

Inside the core of stars, the densities and temperatures are high enough for fusion reactions to occur. Most of the heavier nuclei that exist today were formed inside stars from thermonuclear reactions! (It's interesting to think that the atoms that we are made of were actually manufactured inside stars!). Since stars are mostly composed of hydrogen, the first stage of thermonuclear reactions inside stars involves hydrogen and is called hydrogen burning . The process has three steps and results in four hydrogen atoms being formed into a helium atom with (among other things) two photons (light!) being released.

The next stage is helium burning which results in the formation of carbon. All these reactions release a large amount of energy and heat the star which causes heavier and heavier nuclei to fuse into nuclei with higher and higher atomic numbers. The process stops with the formation of ${}^{56}\mathrm{Fe}$ , which is the most strongly bound nucleus. To make heavier nuclei, even higher energies are needed than is possible inside normal stars. These nuclei are most likely formed when huge amounts of energy are released, for example when stars explode (an exploding star is called a supernova ). This is also how all the nuclei formed inside stars get "recycled" in the universe to become part of new stars and planets.

## Summary

• Nuclear physics is the branch of physics that deals with the nucleus of an atom.
• There are two forces between the particles of the nucleus. The strong nuclear force is an attractive force between the neutrons and the electromagnetic force is the repulsive force between like-charged protons.
• In atoms with large nuclei, the electromagnetic force becomes greater than the strong nuclear force and particles or energy may be released from the nucleus.
• Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting particles or electromagnetic waves.
• The particles and energy released are called radiation and the atom is said to be radioactive .
• Radioactivity was first discovered by Henri Becquerel, Marie Curie and her husband Pierre.
• There are three types of radiation from radioactive decay: alpha ( $\alpha$ ) , beta ( $\beta$ ) and gamma ( $\gamma$ ) radiation.
• During alpha decay , an alpha particle is released. An alpha particle consists of two protons and two neutrons bound together. Alpha radiation has low penetration power.
• During beta decay , a beta particle is released. During beta decay, a neutron is converted to a proton, an electron and a neutrino. A beta particle is the electron that is released. Beta radiation has greater penetration power than alpha radiation.
• During gamma decay , electromagnetic energy is released as gamma rays. Gamma radiation has the highest penetration power of the three radiation types.
• There are many sources of radiation . Some sources are natural and others are man-made.
• The half-life of an element is the time it takes for half the atoms of a radioisotope to decay into other atoms.
• Radiation can be very damaging. Some of the negative impacts of radiation exposure include damage to cells, genetic abnormalities and cancer.
• However, radiation can also have many positive uses . These include use in the medical field (e.g. chemical tracers), biochemistry and genetics, use in food preservation, the environment and in archaeology.
• Nuclear fission is the splitting of an atomic nucleus into smaller fission products. Nuclear fission produces large amounts of energy, which can be used to produce nuclear power, and to make nuclear weapons.
• Nuclear fusion is the joining together of the nuclei of two atoms to form a heavier nucleus. In stars, fusion reactions involve the joining of hydrogen atoms to form helium atoms.
• Nucleosynthesis is the process of forming nuclei. This was very important in helping to form the universe as we know it.

## Summary exercise

1. Explain each of the following terms:
1. electromagnetic force
2. For each of the following questions, choose the one correct answer :
1. The part of the atom that undergoes radioactive decay is the...
1. neutrons
2. nucleus
3. electrons
4. entire atom
2. The radioisotope Po-212 undergoes alpha decay. Which of the following statements is true ?
1. The number of protons in the element remains unchanged.
2. The number of nucleons after decay is 212.
3. The number of protons in the element after decay is 82.
4. The end product after decay is Po-208.
3. 20 g of sodium-24 undergoes radoactive decay. Calculate the percentage of the original sample that remains after 60 hours.
4. Nuclear physics can be controversial. Many people argue that studying the nucleus has led to devastation and huge loss of life. Others would argue that the benefits of nuclear physics far outweigh the negative things that have come from it.
1. Outline some of the ways in which nuclear physics has been used in negative ways.
2. Outline some of the benefits that have come from nuclear physics.

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