Contamination vs irradiation

How Half-Life Affects Hazard Risk

The half-life of an isotope determines how long it remains a hazard. Isotopes with a short half-life have a very high initial activity, emitting many particles per second. This makes them dangerous in the short term, but they quickly decay to safe levels.

Isotopes with a long half-life present a persistent contamination risk. They have lower initial activity but continue to emit radiation for years or even centuries, making them difficult to manage.

Which isotope is a greater contamination risk over a 5-year period: Isotope X (half-life 2 hours) or Isotope Y (half-life 30 years)?

Step 1: Consider the short-term decay rate.

• Isotope X will undergo many half-lives in just a few days, dropping to a completely safe activity level very quickly.

Step 2: Consider the long-term decay rate.

• Isotope Y will barely decay over 5 years.

Step 3: State the conclusion.

• Isotope Y is the greater contamination risk because it remains radioactive for the entire 5-year period, requiring long-term containment.

Activity Calculation Example

A contaminated surface has an initial activity of $1200\text{ Bq}$ from an isotope with a half-life of $3\text{ days}$. What is the activity after $9\text{ days}$?

Step 1: Calculate the number of half-lives.

• $9\text{ days} \div 3\text{ days per half-life} = 3\text{ half-lives}$

Step 2: Halve the initial activity three times.

• $1200 \rightarrow 600 \rightarrow 300 \rightarrow 150\text{ Bq}$

Step 3: State the final answer.

• The final activity is $150\text{ Bq}$.

Safety and Prevention

We use different methods to protect against these two risks. To prevent external irradiation, we rely on distance and shielding. Standing further away or using lead-lined barriers physically blocks the radiation path.

To prevent internal contamination, we rely on containment. Workers use airtight suits, face masks, and tongs to stop radioactive atoms from physically transferring to their skin or being inhaled into their lungs.

Real-World Applications in Medicine and Industry

We apply these principles logically in real-world scenarios. When sterilising food, the items are exposed to a powerful gamma source inside a lead-shielded facility. The gamma rays kill bacteria, but because the food never touches the source, it is not contaminated. Therefore, the food is not a source of radiation and is perfectly safe to eat.

Conversely, medical tracers deliberately use temporary internal contamination. A patient is injected with a gamma-emitting isotope with a very short half-life. Gamma is chosen because its high penetrating power allows it to pass safely out of the body to a detector, while its low ionising power minimises cellular damage.