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Basics - Poster 2
Neutron Production , Fission & Activation
There are many ways in which neutrons can be produced . Some occur naturally , such as through the fission of heavy elements like uranium- 235 or through the bombardment of the Earth ' s atmosphere by cosmic rays in a process similar to that shown below . These neutron production processes can also be harnessed in devices to provide sources of neutrons for measurement , research and energy production .
Neutron Production
The most common neutron radiation source is a mixture of americium-241 and stable beryllium-9 . As well as a low energy gamma , Am-241 emits alpha particles . A very small proportion of these are captured by a beryllium atom to produce a stable carbon atom and a neutron .
9 4
Be + He
4 2
C+ 1 n+γ
6 0
12
There are about 30 neutrons produced per million alphas produced . Typical sources used in industry have Am-241 activities of the order of a few gigabecquerel ( GBq ).
Nuclear fission refers to either a nuclear reaction or a radioactive decay process , in which the nucleus of an atom splits into smaller parts ( lighter nuclei ), often producing free neutrons and photons ( in the form of gamma rays ) and releasing a very large amount of energy . The two nuclei produced are most often of comparable but slightly different sizes .
Uranium-235 is an example of the heavy isotopes that can decay
Nuclear Fission by fission . It can also decay by other routes .
However the fission process can be stimulated by the capture of a neutron . In some conditions the additional neutrons produced can go on to produce further fissions in what is known as a " chain reaction ". This is the basis of a nuclear reactor , which is designed to control the rate of the chain reaction and to have mechanisms to extract the heat and produce electric power .
Neutron Activation
Radionuclides can be produced by bombarding stable isotopes with neutrons . In some cases an atom absorbs a neutron , increasing the mass number by one and producing an atom that is radioactive - this is neutron activation .
The schematic on the left shows this process for two radionuclides commonly used in industrial radiography . Both decay by the emission of beta particles and gamma rays . The beta particles will be absorbed by the metal source container , but the gammas will easily penetrate it and can be used for radiography .
Images provided by courtesy of Public Health England