| 1 | A | B | C | D | E | F |
| --- | --- | --- | --- | --- | --- | --- |
| 2 | Radioisotope thermoelectric generator - Criteria for selection of isotopes |  |  |  |  |  |
| 3 | Material | Shielding | Power density (W/g) | Power density (W/g) | Half-life (years) | Half-life (years) |
| 4 | 238Pu | Low | 0.54 | - | 87.7 | - |
| 5 | 90Sr | High | 0.46 | - | 28.8 | - |
| 6 | 210Po | Low | 140 | - | 0.378 | - |
| 7 | 241Am | Medium | 0.114 | - | 432 | - |

Question: What is 90Sr's half life?
Answer: 90Sr's half life of 28.8 years is shorter than that of ²³⁸Pu and it also has a lower a power density of 0.46 watts per gram.