IB DP Physics: HL复习笔记12.2.3 Deviations from Rutherford Scattering

Deviations from Rutherford Scattering

  • In the Rutherford scattering experiment, alpha particles are fired at a thin gold foil
  • Some of the alpha particles are found to come straight back from the gold foil
  • This indicates that there is electrostatic repulsion between the alpha particles and the gold nucleus
  • At the point of closest approach, r, the repulsive force reduces the speed of the alpha particles to zero momentarily
    • At this point, the initial kinetic energy of an alpha particle, Ek, is equal to electric potential energy, Ep
    • Then the alpha particle will be electrostatically repelled


The closest approach method of determining the size of a gold nucleus

  • This experiment assumes that the alpha particles are only interacting through electrostatic repulsion

Deviations from Rutherford Scattering

  • If the energy of the alpha particles exceeds 27.5 - 28 MeV, then they will be close enough to interact with the nucleus via the strong nuclear force
  • The Rutherford formula describes this as it states that as the angle of scattering angle increases, the number of alpha particles scattered at that angle sharply decreases
  • Experimental data bears this out, suggesting that the Rutherford formula is correct
  • A number of assumptions are made in deriving this formula, with the main one being that the only force we need to consider is the electric force
  • Deviations from Rutherford scattering are evidence of the strong nuclear force


The observed back-scattering from alpha particles strongly deviates from the predicted relationship based only on electromagnetic repulsion at 27.5 MeV

Worked Example

Step 2: consider what causes deviation from Rutherford scattering

    • Deviations from Rutherford scattering occur when forces apart from the electric force between the nuclei are at play
    • At much smaller distances the effect of the strong nuclear force will be felt

Step 3: deduce the solution

    • The alpha particles gets closer to the aluminium nucleus, and the strong nuclear force affects them
    • More deviation will be seen with aluminium foil than with gold foil

Exam Tip

  • The greatest deviations from Rutherford scattering occur when the energy of the alpha particles is high and the radius of the target nuclei is small (meaning it has a small nucleon number)