It is Iridium or Ir-31 is the closest answer I could find for you. The actual Iridium used is Ir-192. Another problem was that Uranium/U is not the one used its Radium so that can not be it. And Europium is used for helping with curing brain tumors but only in trace amounts
It starts at the bottom and then it goes up into the air as evaporation and that’s when the air gets cooler so gas is cooler and the liquid would be hotter
