Nuclear Physics

Radioactivity

Radioactivity is to do with unstable nuclei
The nucleus of a radioactive atom emits either an alpha (α) particle, beta (β) particle and/or gamma (γ) rays
An alpha particle is a slow-moving helium nucleus. It has a positive charge
A beta particle is a fast-moving electron. It has a negative charge
Gamma rays are short-wavelength electromagnetic waves. These have no charge and travel at the speed of light
The activity of a source is the average number of nuclei decaying per second
Activity is measured in Becquerel (Bq)
The activity of a radioactive substance decreases with time
Radioactivity is a random process because it is impossible to predict which nucleus will decay at a particular time
The half-life of a radioisotope is the average time taken for half the active nuclei to decay or disintegrate
The activity of a source is directly proportional to the number of active nuclei
The nucleus of an atom may be represented as , where X is the chemical symbol, A is the nucleon or mass number and Z is the proton or atomic number
The protons and neutrons are also known as the nucleons
In alpha decay, the mass number decreases by 4, the proton number decreases by 2 and a new radioisotope is formed
In beta decay, the mass number remains the same, the proton number increases by 1 and a new radioisotope is formed

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Background radiation is always present and is due to radioactive substances in rocks, soil, air and cosmic rays
Sound background radiation comes from man-made sources
E.g. Nuclear and hospital waste
Radioisotopes are used as tracers in industry and hospitals
Gamma-emitting tracers are used to find leaks or blockages in underground pipes
Smoke detectors contain an alpha emitter
Carbon dating is used to date bone, cloth, wood and paper. It relies on the radioactivity isotope of carbon-14

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Nuclear power stations use uranium as a fuel. Most of the waste is radioactive for thousands of years
Reactions produce the heat to produce steam to turn the turbines, which turn the generators to produce electricity
In a fission reaction, a neutron is captured by a uranium-235 nucleus and splits into two smaller (daughter) nuclei and either two or three neutrons
Energy is released when a uranium nucleus splits
Materials become radioactive when they capture neutrons in a nuclear reactor
In a chain reaction, the neutrons can cause further fission reactions
One destructive use of fission is the atomic bomb. One peaceful use if electricity production

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In the process of the fission of uranium-235:
A uranium-235 absorbs a neutron, making it unstable
It splits into two daughter nuclei, which are also unstable, and two or three spare neutrons
The fission products have a lot of kinetic energy, which is removed by a coolant
The coolant generates steam that turns a turbine
The spare neutrons can cause further fission of other nuclei in a chain reaction
If each spare neutron were allowed to cause another fission, the result would be a chain reaction which would be out of control. To maintain the reaction at a steady rate, on average just one of the neutrons released by each fission is allowed to go on and cause further fission
In the core of a nuclear reactor, control rods absorb the spare neutrons to control the rate of reaction. It is also a safety feature

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Nuclear power has one major disadvantage which is how to dispose of the nuclear waste. There are three main categories:
Low-level waste such as laboratory clothing and packaging materials; these are either buried either underground or at sea
Intermediate-level waste such as the casing used for nuclear fuel and reactor parts that have been replaced; these are kept in stores with thick concrete walls or buried in deep trenches with concrete linings
High-level waste such as spent fuel rods; these present a long-term disposal problem since they remain significantly radioactive for thousand of years; much of this waste is in temporary storage in tank of water until the problem of what to do with it can be solved

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