Radioactivity — KCSE Physics

Radioactivity involves the emission of particles and energy from unstable nuclei. The three main types of radiation are alpha (α), beta (β), and gamma (γ) radiation. Each has distinct properties:

• Charge:

  • Alpha radiation is positively charged (2+).
  • Beta radiation is negatively charged (1-).
  • Gamma radiation is neutral (0).

• Mass:

  • Alpha particles have a relatively large mass (4 amu).
  • Beta particles have a very small mass (approximately 1/2000 of a proton).
  • Gamma rays have no mass.

• Ionising Power:

  • Alpha radiation has high ionising power and can ionise atoms effectively over short distances.
  • Beta radiation has moderate ionising power.
  • Gamma radiation has low ionising power.

• Penetrating Power:

  • Alpha particles can be stopped by a sheet of paper or skin.
  • Beta particles can penetrate paper but are stopped by a few millimetres of aluminum.
  • Gamma rays have high penetrating power and can pass through most materials, requiring dense substances like lead for shielding.

In radioactivity, the half-life is the time required for half of the radioactive isotope to decay. The formula to determine the remaining quantity of a radioactive isotope is:
N = N₀ × (½)ⁿ
Where:

• N = remaining quantity of the isotope
• N₀ = initial quantity of the isotope
• n = number of half-lives that have passed

To apply this formula, follow these steps:

1. Identify the initial quantity (N₀) of the radioactive isotope.
2. Determine the number of half-lives (n) that have elapsed.
3. Substitute these values into the formula to find the remaining quantity (N).

Example:
If you start with 80 grams of a radioactive isotope and 3 half-lives have passed, calculate the remaining quantity.

• Initial quantity, N₀ = 80 g
• Number of half-lives, n = 3
• Remaining quantity, N = 80 × (½)³ = 80 × (1/8) = 10 g.
  Thus, after 3 half-lives, 10 grams of the isotope remain.

In nuclear physics, it is essential to write balanced nuclear equations to represent radioactive decay processes accurately. Alpha decay involves the emission of an alpha particle (2 protons and 2 neutrons) from a nucleus. The general equation is:

• For alpha decay of Uranium-238:
  [ _{92}^{238}U \rightarrow _{90}^{234}Th + _{2}^{4}He ]
  This shows the transformation of Uranium-238 into Thorium-234 and the release of an alpha particle.

Beta decay occurs when a neutron is converted into a proton, emitting a beta particle (electron). The general equation for beta decay is:

• For Carbon-14:
  [ _{6}^{14}C \rightarrow _{7}^{14}N + _{-1}^{0}e ]
  This indicates Carbon-14 decays into Nitrogen-14, releasing a beta particle.

Nuclear fission is the splitting of a heavy nucleus into smaller nuclei, while nuclear fusion is the combining of light nuclei to form a heavier nucleus. An example of fission is:

• For Uranium-235 fission:
  [ _{92}^{235}U + _{0}^{1}n \rightarrow _{56}^{144}Ba + {36}^{89}Kr + 3{0}^{1}n ]
  This demonstrates the fission of Uranium-235 when it absorbs a neutron.