# Chapter 18- Ionising Radiation and Risk

A summary of chapter 18 from OCR A2 Advancing Physics

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• Created by: R_Hall
• Created on: 18-04-14 12:05

• Radioactive decay is a random process which can be modelled by exponential decay
• Radioactive emissions are known as ionising radiation as when a radioactive particle hits an atom, it can knock off electrons to create an ion
• A gamma source will emit radiation in all directions, which will spread out as you get further away from the source. The amount of radiation per unit area (intensity) will decrease as you move away from the source
• When gamma radiation travels through an absorbing material, its intensity decreases exponentially
• Absorbed dose (in grays, Gy) = the amount of energy absorbed per kilogram of tissue
• Effective dose (in seiverts, Sv) = absorbed dose x radiation quality factor. Allows comparison of damage to body tissues that have been exposed to different types of radiation
• Alpha has the largest quality factor. They are strongly +ve, so can easily ionise atoms as they can pull electrons. The ionisation transfers some energy from the alpha particle to the atom; the particle quickly ionises many atoms and loses all its energy- causes lots of damage.
• Beta-minus has lowers mass and charge, but a higher speed. It can still knock e- off atoms, but can ionise less atoms before losing all its energy- causes less damage to tissues
• Risk = probability x consequence
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## Nuclear Decay

• The strong nuclear force holds the nucleus together, and is balanced against the electromagnetic force pushing protons apart. A nucleus will become unstable if it has too many neutrons, too few neutrons, too many nucleons or too much energy
• Alpha emission only happens in heavy atoms, the nuclei of which are too massive to be stable. When an alpha particle is emitted, the proton number decreases by 2 and the nucleon number decreases by 4
• Beta-minus decay is the emission of an e- and an antineutrino. Happens in neutron-rich isotopes. A proton gets changed into a neutron, and the proton number decreases by 1 and the nucleon number stays the same
• Gamma radiation is emitted from excited atoms with excess energy. A gamma ray is emitted, but there is no change to the nuclear constituents
• In every nuclear reaction energy, momentum, proton number/ charge and nucleon number must be conserved
• Mass doesn't have to be conserved as Einstein's equation says that mass and energy are equivalent
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## Binding Energy

• If the mass of the nucleus is less than the mass of its constituent parts, the missing mass is a mass defect.
• According to E = mc^2, the 'lost' mass is converted into energy and is released. Amount of energy released = mass defect. Energy released when nucleus formed = energy needed to pull nucleus apart
• Binding energy (in MeV) - the energy needed to separate all of the nucleons in a nucleus. Equivalent to the mass defect
• Can compare the binding energies of different nuclei by looking at binding energy per nucleon
• A graph of binding energy per nucleon against nucleon number shows a curve- the nuclear valley.
• The most stable nuclei occur around the minimum point on the graph- at Fe. T
• The more negative the binding energy per nucleon, the more energy needed to remove nucleons from the nucleus
• Nuclear fusion- the combination of small nuclei. Increases the size of binding energy per nucleon dramatically, means a lot of energy is released during fusion
• Nuclear fission- where large nuclei and split in two. Nucleon numbers of the 2 new nuclei smaller than that of the original nucleus, increasing the binding energy per nucleon. Energy also released through fission (not as much as fusion)
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## Nuclear Fission and Fusion 1

• Large nuclei are unstable so are prone to nuclear fission. The process is spontaneous or can be induced. The larger the nucleus, the more likely it is to spontaneously fission. Spontaneous fission limits the number of nucleons a nucleus can contain- limits the number of possible elements
• The energy released through fission can be harnessed in a nuclear reactor, but must be carefully controlled.
• Rods of 235U rich uranium are used as fuel that undergoes fission. They release neutrons which induce other nuclei to undergo fission- chain reaction.
• Neutrons must be slowed down to cause chain reactions, as it allows them to be captured by uranium nuclei. They are thermal neutrons.
• Fuel rods are placed in a moderator (eg water) to slow down (allow further fission to keep steady rate of reaction) or absorb neutrons (decrease the chance of meltdown if the reactor overheats)
• Must choose a critical mass of fuel to allow the chain reaction to continue at a steady rate. Nuclear reactors used supercritical masses and control the rate using control rods. They limit the number of neutrons in the reactor by absorbing them, and can be inserted by varying amount to control reaction rate
• Coolant is sent round the reactor to remove heat produced. The heat can be used to make steam for powering electricity-generating turbines
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## Nuclear Fission and Fusion 2

• The waste products of fission are neutron rich (larger proportion of neutrons than stable nuclei of similar atomic number) so are unstable and radioactive.
• Can be used for practical application (eg tracer in medical diagnosis) but may be highly radioactive so handling and disposal requires great care.
• It is placed in cooling ponds to allow the temperature to fall to a safe level, and is stored underground in sealed containers until activity falls to safe level
• Two light nuclei fuse to create a larger nucleus in fusion. Nuclei only fuse if they have enough energy to overcome the electrostatic repulsion and get close enough for the strong attraction to bind them
• Energy emitted by stars and sun results from fusion. Can occur as the temperature in the core is so high. The temperature is so high that atoms don't exist, have plasma instead which is positively charged nuclei and free electrons
• The large amounts of energy released (as the new heavier nuclei have a larger binding energy per nucleon) help maintain the temperature for further fusion reactions
• Experimental fusion reactors try to recreate these conditions to generate electricity (without the waste like from fission). Currently, the electricity generated is less than the amount needed to raise the temperature high enough
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