Pa=PROTACTINIUM
Protactinium primarly is generated from the decay of the rare isotope uranium 235 (via the very unstable thorium 231). Therefore it only exists in small amounts, most of it is found in nuclear waste. The half-life of the most stable isotope 231 is 32760 years. Outside of science, no applications for protactinium exist. Nearly all protactinium decays further to actinium.
