# 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) 