Flashcards · 9630 · AS Topic 3

Particles, radiation and radioactivity (AS Level) flashcards

18 cards on AS Topic 3, subtopics 3.3.1 to 3.3.3. Pick a subtopic or study them all.

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Charge and mass of the proton, neutron and electron: Proton: +1.60 × 10⁻¹⁹ C, 1.67 × 10⁻²⁷ kg. Neutron: 0, 1.67 × 10⁻²⁷ kg. Electron: −1.60 × 10⁻¹⁹ C, 9.11 × 10⁻³¹ kg (about 1/1840 of a proton).
Nuclide notation: what are A and Z, and how many neutrons?: A = nucleon number (protons + neutrons); Z = proton number. Neutrons = A − Z. Isotopes have the same Z but different A.
Specific charge: Charge ÷ mass, in C kg⁻¹. Proton: 9.58 × 10⁷ C kg⁻¹. Electron: 1.76 × 10¹¹ C kg⁻¹. For an ion use the net charge.
Rutherford scattering: the three observations and what each shows: Most alpha particles pass straight through: the atom is mostly empty space. A few deflect through large angles: positive charge is concentrated in a tiny nucleus (repulsion). Very few rebound: the nucleus is also massive.
Why was Rutherford scattering done in a vacuum with very thin gold foil?: Vacuum: air would absorb or scatter the alpha particles. Thin foil: each alpha particle is scattered at most once (by one nucleus).
Approximate sizes of an atom and a nucleus: Atomic radius about 10⁻¹⁰ m; nuclear radius about 10⁻¹⁵ m. Nearly all the mass is in the nucleus.

How does a particle differ from its antiparticle?: Same mass and rest energy, opposite charge. Neutral particles (neutron, neutrino) still have distinct antiparticles.
Antiparticles of the electron, proton, neutron and neutrino: Positron, antiproton, antineutron and antineutrino. Rest energies: electron/positron 0.511 MeV; proton/antiproton 938.3 MeV; neutron/antineutron 939.6 MeV.
Annihilation: A particle and its antiparticle meet and are converted into photons. Charge, energy and momentum are conserved, so at least two photons are produced, travelling in opposite directions.
Pair production, and the minimum photon energy: A photon near a nucleus becomes a particle and its antiparticle. Minimum photon energy = 2 × rest energy of one particle: 1.022 MeV for an electron-positron pair.
Energy of each photon when an electron and a positron annihilate at rest: 0.511 MeV each (1.022 MeV in total). 1 MeV = 1.60 × 10⁻¹³ J.

Changes in A and Z for alpha, beta-minus, beta-plus and gamma decay: Alpha: A − 4, Z − 2. Beta-minus: A same, Z + 1 (neutron → proton; electron and antineutrino emitted). Beta-plus: A same, Z − 1 (proton → neutron; positron and neutrino emitted). Gamma: no change.
How did the beta energy spectrum lead to the neutrino hypothesis?: Beta particles from one nuclide have a continuous range of energies up to a maximum, and energy and momentum seemed not to balance. A third, neutral, weakly interacting particle was proposed to carry the missing energy and momentum.
Half-life: The time taken for half the unstable nuclei in a sample to decay. The activity and corrected count rate halve in the same time.
Penetration and ionisation of alpha, beta and gamma: Alpha: most ionising; stopped by paper or a few cm of air. Beta: medium; stopped by a few mm of aluminium. Gamma: least ionising; thick lead greatly reduces it.
Inverse-square law for gamma radiation: I = I₀/r² for a point source with negligible absorption in air. Doubling the distance divides the intensity by 4.
Why is technetium-99m used in medical diagnosis?: It emits only gamma, which leaves the body to be detected, and no alpha or beta. Its half-life of about 6 hours is long enough for a scan but short enough to limit the dose.
How do you correct for background radiation?: Count with no source present for a long time, find the background rate, and subtract it from every reading.