Half-Life (CIE A Level Physics)

• Half-life is defined as:

The time taken for the initial number of nuclei to reduce by half

• This means when a time equal to the half-life has passed, the activity of the sample will also half
• This is because the activity is proportional to the number of undecayed nuclei, A ∝ N

Determining half-life from a graph

When a time equal to the half-life passes, the activity falls by half, when two half-lives pass, the activity falls by another half (which is a quarter of the initial value)

• The half-life of a radioactive sample can be calculated using the equation:

• Where:
  • t_½ = half-life of the sample (s)
  • λ = decay constant of the sample (s^-1)

• This equation shows that:
  • the half-life and the decay constant of a sample are inversely proportional
  • the shorter the half-life, the larger the decay constant and the faster the rate of decay

Derivation of the half-life equation

• To find an expression for half-life, start with the equation for exponential decay:

N = N₀e^–λt

• Where:
  • N = number of nuclei remaining in a sample
  • N₀ = the initial number of undecayed nuclei (when t = 0)
  • λ = decay constant (s^-1)
  • t = time interval (s)

• When time t is equal to the half-life t_½, the activity N of the sample will be half of its original value, so N = ½ N₀

• First, divide both sides by N₀:

• Then, take the natural log of both sides:

• Finally, apply the properties of logarithms:

• Therefore, half-life t_½ can be calculated using the equation: