Cs
Alkali metal
Cesium
Z 55A 133mass 132.91
Decay curve
Cesium-137
half-life
30.17 years
half-lives elapsed1 half-life
0123456
left
50.0%
real time
30.2 years
Sample of 240 nuclei
99 leftEach dot is one nucleus. Decay is random for each one — but the pattern that emerges is the exponential curve on the left.
The numbers
| Half-lives | Time elapsed | Remaining | Of 1000 |
|---|---|---|---|
| 0 | — | 100.0% | 1000 |
| 1 | 30.2 years | 50.0% | 500 |
| 2 | 60.3 years | 25.0% | 250 |
| 3 | 90.5 years | 12.5% | 125 |
| 4 | 121 years | 6.25% | 63 |
| 5 | 151 years | 3.13% | 31 |
| 6 | 181 years | 1.56% | 16 |
N = N₀ × (½)ⁿ — never exactly zero
Real half-lives, for scale
Thorium-23214.05 billion years
Uranium-2384.47 billion years
Potassium-401.25 billion years
Neptunium-2372.14 million years
Plutonium-23924,110 years
Carbon-145,730 years