Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

OCR H556 chapter questions and answers

CH23 Nuclear and Particle Physics — Questions

5 min read 8 parts Lesson 1 of 2

0%read
Lesson summary

CH23 Nuclear and Particle Physics (Questions) — OCR A LEVEL PHYSICS A H556 notes: Describe the structure of the atom and nucleus, the types of radioactive decay and the laws of radioactive decay.Rutherford scattering — Most alpha particles pass through gold foil, a few are deflec…

  • Nuclear notation
  • Nuclear radius
  • Strong nuclear force
  • Electrostatic repulsion
  • Alpha decay
  • Beta-minus decay

Section 1 of 8Atomic structure and the nucleus

Atomic structure and the nucleus
Rutherford scattering
  • Most alpha particles pass through gold foil, a few are deflected through large angles, showing a small, dense, positive nucleus.
Nuclear notation
  • The nuclide is written with its proton number Z and nucleon number A, as A over Z X.
Isotopes
  • Nuclei with the same number of protons but different numbers of neutrons.
Nuclear radius
  • R = r0 A1/3 with r0 about 1.2 fm, so nuclear density is about the same for all nuclei.
Strong nuclear force
  • It attracts nucleons at short range and becomes repulsive at very short distances below about 0.5 fm.
    • (up to about 3 fm)
Electrostatic repulsion
  • The nucleus is stable when the strong force outweighs the repulsion between protons.

Section 2 of 8Radioactive emissions

RadiationNatureIonising powerPenetration
AlphaHelium nucleus (2 protons, 2 neutrons)StrongStopped by paper or a few cm of air
Beta minusFast electronMediumStopped by a few mm of aluminium
GammaHigh-energy photonWeakReduced by thick lead or concrete
Alpha decay
  • The nucleus loses 4 nucleons and 2 protons: A decreases by 4 and Z by 2.
Beta-minus decay
  • A neutron changes into a proton, releasing an electron and an antineutrino; Z increases by 1.
Beta-plus decay
  • A proton changes into a neutron, releasing a positron and a neutrino; Z decreases by 1.
Conservation
  • Nucleon number, charge and energy are conserved in each decay.
Antineutrino evidence
  • Beta particles show a range of energies, so a third particle shares the energy.

Section 3 of 8Decay laws

Random and spontaneous
  • Which nucleus decays next and when cannot be predicted.
Activity
  • A = change in N / change in t, measured in becquerel.
    • (decays per second)
Decay constant
  • A = lambda N, where lambda is the probability of decay per unit time.
Exponential decay
  • N = N0 e-lambda t and A = A0 e-lambda t.
Half-life
  • The time for the number of undecayed nuclei or the activity to halve: t1/2 = ln 2 / lambda = 0.693 / lambda.
Example
  • A source with half-life 6.0 hours has activity reduced to 1/8 after 18 hours.
Background radiation
  • Radiation from rocks, cosmic rays and medical sources must be subtracted from count rates.

Section 4 of 8Using radioactivity

Radiocarbon dating
  • The fraction of carbon-14 remaining in organic matter gives its age, using a half-life of 5730 years.
Thickness gauging
  • Beta particles passing through a sheet are detected, and a falling count signals an increase in thickness.
Smoke detectors
  • Alpha particles ionise air in the detector, and smoke reduces the current.
Safety
  • Use tongs, keep time short, increase distance, and use shielding.
Detection
  • A Geiger-Muller tube counts ionising radiation.

Section 5 of 8Detection and experiments

Detection and experiments
Absorption experiment
  • Place different absorbers between a source and a detector and record the count rate.
Inverse square law for gamma
  • The count rate falls with the square of the distance from a point source.
Cloud chamber
  • Alpha particles leave thick, straight tracks, and beta particles leave thin, wandering tracks.
Half-life experiment
  • Plot ln(count rate) against time; the gradient is -lambda.
Uranium series
  • Radioactive decay series pass through many isotopes until a stable lead isotope is reached.
Hazards
  • Ionising radiation damages DNA and can cause cancer.
Dose
  • Exposure is reduced by limiting time, using shielding and keeping a distance.

Section 6 of 8Worked examples

  1. A sample has an initial activity of 640 Bq and a half-life of 12 minutes. Calculate the activity after 48 minutes. [2]2 marks

  2. Write the nuclear equation for the alpha decay of uranium-238 (Z = 92). [2]2 marks

  3. A radioactive isotope has a decay constant of 0.0231 s-1. Calculate its half-life. [2]2 marks

Section 7 of 8Key terms

TermMeaning
IsotopeAtoms of the same element with different numbers of neutrons.
Half-lifeThe time for the activity to halve.
Decay constantThe probability of decay per unit time.
ActivityThe number of decays per second.
BecquerelOne decay per second.
NucleonA proton or neutron.
Background radiationIonising radiation always present in the environment.
Strong nuclear forceThe attractive short-range force between nucleons.

Section 8 of 8Exam technique

1
Writing that radioactive decay is affected by temperature or chemical state.
2
Forgetting to subtract background radiation.
3
Mixing up the proton number and nucleon number in equations.
4
Saying alpha particles are the most penetrating.
\n