229Th

Thorium-229 is one of my favourite isotopes. With the lowest-energy first excited state of any known nucleus, it's the isotope of choice for interacting directly with things such as lasers, atoms and molecules, whose characteristic energies are much smaller than normal nuclear transition energies. One of the most promising practical uses of ²²⁹Th is in making a new time standard which is more accurate than existing atomic clocks. My friend and fellow blogger Rob Jackson has just published a paper on the chemical side of implanting ²²⁹Th in material for possible use in such a clock standard. He's blogged about it here.