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Flashcards · J249 · Topic 6

Radioactivity flashcards

19 cards on Topic 6, subtopics P6.1 to P6.2. Pick a subtopic or study them all.

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P6.1 Radioactive emissions

Charge and relative mass of the proton, neutron and electron
Proton: +1, mass 1. Neutron: 0, mass 1. Electron: −1, negligible mass (about 1/1800).
Atomic number and mass number
Atomic number Z: number of protons (fixes the element). Mass number A: protons + neutrons. Neutrons = A − Z.
Isotopes
Atoms of the same element (same number of protons) with different numbers of neutrons, so the same nuclear charge but different mass.
Alpha, beta, gamma and neutron emission: what each is
Alpha: 2 protons + 2 neutrons (a helium nucleus). Beta: a fast electron from the nucleus, made when a neutron becomes a proton. Gamma: EM radiation. Neutron: a single neutron.
Effect of each emission on mass number A and atomic number Z
Alpha: A − 4, Z − 2. Beta: A unchanged, Z + 1. Neutron: A − 1, Z unchanged. Gamma: no change.
Complete the decay: uranium-238 (Z = 92) emits an alpha particle.
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He (238 = 234 + 4; 92 = 90 + 2)
What happens to an electron when an atom absorbs or emits EM radiation, and what is ionisation?
Absorbs: the electron moves to a higher energy level. Emits: it falls to a lower level. Ionisation: the atom loses an outer electron and becomes a positive ion.
Half-life, and why it can be used although decay is random
The time for the number of undecayed nuclei, or the activity, to halve. Each nucleus decays at an unpredictable moment, but a large sample behaves predictably.
Activity 1600 Bq, half-life 5 minutes. Activity after 20 minutes, and the net decline as a ratio?Higher
4 half-lives, 2⁴ = 16: 1600 ÷ 16 = 100 Bq. Ratio 16 : 1.
Penetrating power and ionising ability of alpha, beta and gamma
Alpha: stopped by paper or a few cm of air; most ionising. Beta: stopped by a few mm of aluminium. Gamma: reduced by thick lead or concrete; least ionising.

P6.2 Uses and hazards

Irradiation vs contamination
Irradiation: exposure to radiation from outside; the object does not become radioactive. Contamination: radioactive atoms on or inside an object or person.
Why is an alpha source more dangerous inside the body than outside it?
Outside, alpha is stopped by skin and a few cm of air. Inside, all its energy goes into nearby living cells, because it is strongly ionising.
Precautions against irradiation and against contamination
Irradiation: shielding, distance, short exposure time, tongs. Contamination: gloves, protective clothing, no eating or drinking, cleaning spills.
Short half-life vs long half-life: activity and hazard
Short: higher activity from the same number of nuclei, but the hazard does not last long. Long: lower activity, but the hazard lasts a very long time.
A spill reads 960 Bq; safe level 120 Bq; half-life 2 hours. How long to wait?
960 ÷ 120 = 8 = 2³, so 3 half-lives: 3 × 2 = 6 hours.
Choice of radioactive tracer for medical imaging
A gamma emitter, so it passes out of the body to the detector, with a half-life of hours. Alpha would not leave the body and would ionise cells inside.
Nuclear fission of uranium-235
The nucleus usually absorbs a neutron first, becomes unstable and splits into two smaller nuclei, releasing two or three neutrons and gamma radiation. The products have kinetic energy.
Chain reaction
Neutrons released by one fission cause further fissions. An average of exactly one new fission per fission gives a steady energy release.
Nuclear fusion, and why it needs extreme conditions
Two light nuclei join to form a heavier nucleus; some mass may be converted into energy carried by radiation. Very high temperature and pressure are needed to overcome the repulsion between positive nuclei.

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