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Alpha Radiation

An alpha particle is a helium nucleus, made of 2 protons and 2 neutrons. It has a charge of +2e and a nucleon number of 4.
Alpha emission happens in large unstable nuclei, such as uranium-238 and radium-226. The strong nuclear force only acts over about 3 fm, while the electrostatic repulsion between protons acts across the whole nucleus. Ejecting a tightly bound group of four nucleons is an energetically favourable way for such a nucleus to become more stable.When a nucleus emits an alpha particle, its nucleon number A decreases by 4 and its proton number Z decreases by 2, so it becomes a different element: AZX → A-4Z-2Y + 42α.
In every decay equation the total nucleon number and the total proton number must be the same on both sides. Alpha particles from a particular nuclide are emitted with the same, discrete kinetic energy.
Alpha particles are strongly ionising, so they lose energy quickly and have a short range. They travel only a few centimetres in air and are stopped by a sheet of paper.

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Specification: AQA Physics (7407), AS-level, 3.2.1.2 Stable and unstable nuclei. Topic: Particles and Radiation.

Key terms in this lesson

Strong nuclear force
The short-range attractive force between nucleons that overcomes the electrostatic repulsion between protons and holds the nucleus together, becoming repulsive at very short separations.
Antiparticle
A particle with the same rest mass as its corresponding particle but with opposite charge and opposite values of other quantum properties.
Weak nuclear force
The fundamental interaction responsible for beta decay, in which particles interact through the exchange of W bosons.
Muon
A negatively charged lepton, similar to but much heavier than the electron, produced for example by cosmic rays in the atmosphere.
Neutrino
An uncharged, almost massless lepton that interacts only very weakly with matter and is emitted, along with a beta particle, in beta decay.

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