While the longest lived known isotopes of technetium have half lives in the millions of years, the longest lived known promethium-isotope only has a half life of 17.7 years, which requires some source constantly replenishing it for it to be still present in measurable quantities.
Technetium isotopes with long lifespans (millions of years) and promethium-isotope's short half-life (17.7 years) necessitate continuous replenishment for measurable quantities.