An Atomic Source for Short-Lived Radium Isotopes

We are developing an effusive oven that produces a thermal beam of short-lived radium isotopes for optical clocks and tests of fundamental symmetries. The oven is loaded with thorium-228 (1.9 year half-life) which decays to radium-224 (3.6 day half-life). Thorium’s vapor pressure is trillions of times lower than radium and therefore remains in the oven when the radium is heated out. Our approach should generate a useful supply of radium atoms over several thorium half-lives, making it suitable for photoionizing and loading the radium into an ion trap. We have produced atomic beams of radium-224 (nuclear spin zero) from an effusive oven and observed the ²S_1/2 → ²P_1/2 (468 nm) transition with photoionized, trapped radium ions. In a parallel effort we are designing an orthotropic oven which is expected to produce a more collimated beam of radium atoms. We will use the techniques developed with radium-224 to prepare a radium-225 (nuclear spin I=1/2, 15 day half-life) atom source via the nuclear decay of thorium-229 (7900 year half-life).