Physics (A-level)/Nuclear physics


Key WordsEdit

Some of the key words and definitions you will need to know for A2 nuclear physics are:

  • Nucleon - A component of a nucleus
  • Proton - A positively charged nucleon
  • Neutron - A nucleon with no charge
  • Nuclide - A combination of nucleons to form a nucleus
  • Atomic number - The number of protons in a nucleus
  • Nucleon number - The number of nucleons(Protons+Neutrons) in a nucleus
  • Isotope - A different nuclide of a specified element


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A - nucleon number
Z - atomic number
N - neutron number
m - unstable nuclide likely to experience gamma decay
α - Helium nucleus (He2+)
β-+ - electron/positron
γ - gamma wave

Discovery of the nucleusEdit

The nucleus of an atom was first discovered in the early 19th century by Ernest Rutherford. Before this discovery, atoms were thought of as lumps of positive charge dotted with smaller electrons on it's surface. In order to find out more, Rutherford decided to fire alpha particles at atoms in a vacuum to observe the effects. Despite not knowing much about the atom, they did know of alpha particles and their high energies.
For this, he had his colleagues, Geiger and Marsden use an alpha radiation source to fire alpha particles(He2+) at a very thin gold leaf with a width of as few gold atoms as were possible at the time. This gold leaf was suspended and surrounded by a screen made of zinc sulfide. The point of this was that the screen would give a tiny flash of light when it was hit by an alpha particle. The surprising result of this was that they detecting alpha particles hitting the screen even behind the alpha source but only a very small amount. From this, Rutherford realised that a small percentage of the alpha particles were being back-scatted by the nucleus whereas the rest of them were passing straight through the gold leaf. Deductions from that went on to the discovery that the nucleus of the atom was incredibly small compared to the size of the atom itself. An atom is approximately 100,000 times the size of it's nucleus



Nuclides with a low Z value are generally stable when the number of protons is equal to the number of neutrons. Bigger nuclides require a greater proportion of neutrons to gain stability. Radioactive decay occurs when an unstable nuclide loses potential energy(Ep) to stabilize. The biggest possible stable nuclide contains 83 protons and 126 neutrons which is Bismuth-209(209Bi). For any nuclide bigger than that, the strong nuclear force isn't capable of holding the nucleus together so it will generally experience α emission
The four types of radioactive emission are: α(alpha), β-(Beta minus), β+(Beta plus), and γ(gamma)

β- EmissionEdit

β- emission is the emission of an electron from a nucleus. This type of emission converts a neutron to a proton, increasing the atomic number by 1 but keeping the Nucleon number the same. This explains the slight difference in mass between a proton and a neutron with a neutron being very slightly more massive. It also releases an electron antineutrino.
np + e
+ ν

This form of emission occurs when a nuclide has too many neutrons to remain stable. In this case the nuclide is said to be neutron rich. The electron antineutrino is emitted to balance the lepton numbers throughout the equation. The electron has a lepton number of +1 and the antineutrino has a lepton number of -1.

β+ EmissionEdit

β+ emission is very similar to β- emission, except it is the emission of an antielectron (positron). This converts a proton into a neutron, decreasing the atomic number by 1 but keeping the nucleon number the same. This decay also releases an electron neutrino.
pn + e+
+ ν

This form of emission occurs when a nuclide has too few neutrons to remain stable. In this case the nuclide is said to be neutron light. As with β- emission, the electron neutrino is emitted to balance lepton numbers. The positron has a lepton number of -1 and the neutrino has a lepton number of +1.

α EmissionEdit

α emission is the emission of a nuclide consisting of two protons and two neutrons. This is called an α particle. This is equivalent to a Helium nucleus. The α particle has a nucleon number of 4 and an atomic number of 2. α decay often occurs with nuclides that have a very large atomic mass and are neutron light. This is because when an α particle is emitted, the proportion of neutrons increases, making the nuclide tend to a more stable nuclide.
235U → 231Th + 4α
In the emission above, Uranium-235 which has 92 protons in it's nucleus, emits an α particle to form Thorium-231 which has 90 protons in it's nucleus. It should be noted that both the nucleon number and atomic number on both sides of the equation are balanced.

γ EmissionEdit

γ decay usually occurs after an α or β emission. It takes the daughter nuclide out of it's "excited state". A daugter nuclide from α or β decay will be in a higher energy state called an "excited state".


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