<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> Low <col_header> Shielding </col_header> </cell> </table>
²³⁸Pu has the lowest shielding requirements.