Actinium - rare & radioactive

The first alphabetical element in the periodic table is actinium. It is a heavy radioactive element, as we discover in episode 1 of Elemental, with Professor Allan Blackman from AUT.CORRECTION:In the audio we mistakenly said that Cherenkov radiation happens when particles travel faster than the speed of light. Listener Jennifer Narang wrote in with this helpful and correct explanation:"Cherenkov radiation is not the result of a particle traveling faster than the speed of light (which is physically impossible). It's due to a particle traveling faster than the PHASE SPEED, which is a totally different animal.Imagine I were singing scales, going up and down and up and down with my voice. How fast I ran through the notes and returned to my original note, that would be my phase speed. That's completely unrelated to how fast those sound waves - whatever their frequency - are traveling from my mouth to your ears.Hope this explanation helps shed light on Cherenkov radiation!"It certainly does help - many thanks.Go to this episode on rnz.co.nz for more details