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E.1.8 (HL)—High-energy scattering deviations

Syllabus
First assessment 2025
Objective
—
Level
HL

Explain High-Energy Deviations

HL only

Start with Rutherford scattering

At moderate energies, alpha-particle scattering can be modelled as electrostatic repulsion from a concentrated positive nucleus. The predicted deflections follow the Rutherford picture.

Read the high-energy deviation

At sufficiently high alpha-particle energies, the particles can approach more closely and the observed scattering departs from the electrostatic prediction. This provides evidence that the nucleus has a finite size and that a short-range strong interaction becomes relevant.

State what the evidence supports

The deviation is evidence about the nuclear scale and the interaction at very small separation. It is not evidence about the size of the alpha particle or the weak force.

Common trap

Do not continue applying pure Coulomb scattering after the experiment has entered the regime where the alpha particle probes the nuclear force.

E.1.8 (HL) Exam Analysis

HL only

Assessment in practice

1–2 marks
How it is assessed

Questions ask what was deduced from the deviation or why the electrostatic model fails at closest approach.

Command terms

Identify / Explain

What earns marks

Name the finite nuclear size or the short-range strong interaction, and relate it to the closer approach made possible by higher energy.

Watch for

Attributing the deviation to the size of the alpha particle or to the weak nuclear force.

Retrieve the HL Atomic Model

HL only

Retrieve the HL extensions

Use R=R0A1/3R=R_0A^{1/3} for nuclear scale, recognise when high-energy scattering exceeds the electrostatic model, and use energy conservation for head-on closest approach.

Retrieve the Bohr model

Hydrogen levels obey En=−13.6/n2 eVE_n=-13.6/n^2\,\mathrm{eV}, and allowed angular momentum mvr=nh/(2π)mvr=nh/(2\pi) produces discrete orbits and energies.