Radioisotope thermoelectric generator
Criteria for selection of isotopes

Material
Shielding
Power density (W/g)
Power density (W/g)
Half-life (years)
Half-life (years)
238Pu
Low
0.54
-
87.7
-
90Sr
High
0.46
-
28.8
-
210Po
Low
140
-
0.378
-
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.