Unit 23 Nuclear Physics

Define the decay constant.

• Probability of decay

• of a nucleus per unit time

Suggest why dust that has been contaminated with Strontium-90 presents a serious health hazard.

• small volume of strontium-90 has high activity

• can be breathed in easily so presents health hazard

e^- + e⁺ → 2 γ

Assuming that the kinetic energy of the positron and the electron is negligible when they interact, suggest why the two photons will move off in opposite directions with equal energies.

• momenta must be equal and opposite after

• since momentum before is zero

• equal momentum after so momenta of photon equal and in opposite directions so photon energies equal

A radioactive element has a half-life of approx. 4 hours.

Suggest why measurement of the mass and activity of a sample of this element is not appropriate for the determination of its half-life.

Sample would decay appreciably whilst measurements are being made.

State what is meant by nuclear fission

• Large nucleus splits

• into two nuclei of approximately equal mass

By reference to the figure, explain how fission is energetically possible.

• binding energy = B_E × A

• binding energy of parent nucleus is less than sum of binding energy of the fragments