(455) HL Nuclear stability and evidence for strong nuclear force [IB Physics HL]

Nuclear Stability and the Strong Nuclear Force

Evidence for the Strong Nuclear Force

• Stability of the Nucleus:

  • The nucleus comprises protons and neutrons.

  • Neutrons do not attract or repel; focus on protons.

  • Protons should repel each other due to electrostatic forces, yet the nucleus remains intact.

• The Strong Nuclear Force:

  • This force is short-range and attractive, keeping the nucleus together.

  • Dominates over electromagnetic repulsion at very short distances within the nucleus.

  • At larger distances, electromagnetic repulsion prevails.

Rutherford Scattering

• Positive particles near a positive nucleus are repelled.

• Deflection Evidence:

  • Alpha particles are deflected, sometimes reflected.

• Breakdown at High Energies:

  • Effects observed break down around 28 Mega electron volts (MeV).

  • Increased energy correlates with smaller distances, suggesting close interaction with the strong nuclear force.

Binding Energy Curve

• Relation between mass number (A) and binding energy per nucleon.

• The curve shows a nearly constant binding energy, suggesting strong nuclear forces are at play.

Stability of Nuclides

• Definition:

  • Stable nuclides do not decay; unstable ones do.

• Stability Line:

  • For elements Z = 15 to 20 (first 20 elements), stability follows the line where neutrons (N) are roughly equal to protons (Z).

• Beyond Z = 20:

  • More protons require more neutrons for stability.

  • Neutrons help overcome repulsive forces between protons via the strong nuclear force.

Key Takeaway

• Understanding nuclear stability involves recognizing how the strong nuclear force balances the repulsive forces among protons, leading to stable configurations of nucleons.