Alpha, beta and gamma radiation, nuclear equations, half-life, fission and fusion.
Radioactivity is the spontaneous emission of radiation from unstable nuclei. Alpha: a helium nucleus (⁴₂He), charge +2, strongly ionising, least penetrating. Beta: a fast electron (⁰₋₁e), charge −1. Gamma: electromagnetic radiation, no charge or mass, least ionising, most penetrating. In a magnetic field α and β bend in opposite directions; γ is not deflected.
Which radiation is deflected by neither electric nor magnetic fields?
Gamma, since it is uncharged.
Which is the most ionising and which is the most penetrating?
Most ionising: alpha. Most penetrating: gamma.
In every nuclear equation the mass number A (top) and the atomic number Z (bottom) must balance. Alpha decay: A falls by 4, Z by 2. Beta decay: A unchanged, Z rises by 1.
Alpha decay of uranium-238.
²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He. Check: A 234 + 4 = 238; Z 90 + 2 = 92.
Alpha decay of radium-226.
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He. Check: 222 + 4 = 226; 86 + 2 = 88.
Beta decay of carbon-14.
¹⁴₆C → ¹⁴₇N + ⁰₋₁e. Check: A 14 = 14; Z 7 + (−1) = 6.
How many α and β particles are emitted when ²³⁸₉₂U finally becomes ²⁰⁶₈₂Pb?
α: (238 − 206)/4 = 8. These lower Z by 16, to 76. To reach 82, β: 82 − 76 = 6. So 8 α and 6 β particles.
Half-life is the time in which half of the radioactive nuclei decay. Fission splits a heavy nucleus (e.g. U-235 hit by a neutron) into two lighter ones, releasing neutrons and energy. Fusion joins light nuclei (e.g. isotopes of hydrogen) into a heavier one, also releasing energy; it needs a very high temperature.
A sample of 80 g has half-life 5 days. How much is left after 15 days?
15/5 = 3 half-lives. 80 → 40 → 20 → 10 g left (= 1/8 of the original).
Fission of uranium-235.
²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy. Check: A 235 + 1 = 236 = 141 + 92 + 3; Z 92 = 56 + 36.
A fusion reaction.
²₁H + ³₁H → ⁴₂He + ¹₀n + energy. Check: A 2 + 3 = 5 = 4 + 1; Z 1 + 1 = 2.
1. What is radioactivity?
2. Name the particle emitted in alpha decay. Give its charge and mass number.
3. What change in A and Z occurs in alpha decay? In beta decay?
4. Complete: ²¹⁴₈₄Po → ? + ⁴₂He
5. Define half-life.
6. A source has half-life 2 h. What fraction remains after 8 h?
7. Distinguish nuclear fission from fusion in one line each.
8. Give one use of radioactive isotopes in medicine and one in industry.
9. Why must radioactive material be kept in lead containers?
10. Where does the energy released in fission come from?