Fission and Fusion (Open Book paper 2008)

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Discuss, with the use of examples, the main differences between α- and β-decay and explain how nuclear fission reactions differ from natural radioactive decay.

The differences between alpha and beta decay

Table 1 - the differences between alpha and beta decay

Sources [1], [2], [3]

The differences between nuclear fission and natural radioactive decay

In natural radioactive decay, an unstable isotope of an element decays into a different atom and an emission (alpha/beta particle, or energy in the case of gamma/γ radiation). This is a spontaneous natural process with a random rate.

Nuclear fission is also the splitting of a nucleus into two smaller parts, but each of these is an element in itself. Fission does not happen spontaneously; it requires a trigger. An unstable nucleus bombarded with a neutron will “elongate and divide itself like a liquid drop” [3] as its surface tension is decreased due to its charge [4].

Reaction 1 – a possible fission reaction of uranium

Source [5]

100 words

Explain the role of hydrogen nuclei and helium nuclei in the synthesis of elements in stars. Give a detailed explanation of the nuclear changes that happen when lithium forms in stars.

Chemical elements were originally created in stars such as the Sun, through processes collectively called nucleogenesis [6]. Different elements are produced under the different conditions in each star; in the Sun, the primary “building blocks” are hydrogen nuclei. These hydrogen nuclei fuse to form a helium nucleus, as shown in reaction 1.

4 1H  4He + subatomic particles

Reaction 1 – a fusion reaction of hydrogen-1 to form helium-4 [6]

The energy released by this reaction allows more fusion reactions. Once most of the hydrogen is used up, the helium nuclei start a new series of reactions.

2 4He  8Be

4He + 8Be  12C

12C + 4He  16O

16O + 4He  20Ne

20Ne + 4He  24Mg

Reactions 2-6 – helium-4 fusion reactions [6]

As the helium is used up, the carbon produced in reaction 3 goes on to produce more elements.

2 12C  24Mg

2 12C  23Na + 1H

2 12C  20Ne + 4He

Reactions 7-9 – carbon-12 fusion reactions [6]

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In reactions 8 and 9, enough hydrogen and helium nuclei are produced to allow the synthesis of more elements. Lithium has two routes by which it can be made:

Route 1: 4He + 3H  7Li

Route 2: 4He + 3He  7Be

 7Be + 0e-  7Li

Reactions 10-12 – synthesis of lithium through fusion reactions [6]

In the first route and the first reaction of the second route, the nuclei fuse, simply adding together the protons and the neutrons together to form a larger nucleus. However, the second reaction of the second route is more complicated. The ...

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