<page_title> Radioisotope thermoelectric generator </page_title> <section_title> Criteria for selection of isotopes </section_title> <table> <cell> 238Pu <col_header> Material </col_header> </cell> <cell> 87.7 <col_header> Half-life (years) </col_header> </cell> <cell> 90Sr <col_header> Material </col_header> </cell> <cell> 0.46 <col_header> Power density (W/g) </col_header> </cell> <cell> 28.8 <col_header> Half-life (years) </col_header> </cell> </table>
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.