Caesium

Caesium was an element with the symbol Cs (TV: Shada) and atomic number 55. (TV: Flatline)

Secondary electrode accelerators in ion jet rockets beat the problem of neutral caesium ions. (TV: The Seeds of Death)

Inside a miniscope, the Third Doctor oberved an early pulse mechanism based on the principle of caesium decay. He considered it "vintage stuff". (TV: Carnival of Monsters)