What is the half-life of Cesium?

• A. 30 years
• B. 3 years
• C. 300 years
• D. 30 days

Answer: (A)

Half-life is the time it takes for a radioactive substance’s activity to drop to half of its initial value. For the cesium source commonly used in industrial gauges, the isotope involved is cesium-137, whose half-life is about 30 years. That means after roughly 30 years, the activity is halved; after 60 years, it’s down to a quarter; after 90 years, an eighth, and so on. The other options—30 days, 3 years, and 300 years—don’t match the actual decay rate of Cs-137, so 30 years is the best-fit value. If you were thinking in terms of the stable natural cesium isotope, that one doesn’t have a half-life, but in this context the cesium-137 source is what's relevant.