The Search for the Permanent Electric Dipole Moment of Radium-225
We are part of the Radium EDM Collaboration along with Argonne National Lab and the University of Kentucky. Our approach is to probe cold Ra-225 atoms that are tightly confined in a laser trap. Based on theoretical studies of laser traps for diamagnetic and paramagnetic atoms, a sensitive EDM search is feasible. Because cold atoms are so slow, they are less sensitive to “vxE” and “geometric phase” systematic effects. Because the atoms are tightly confined spatially, the requirements for magnetic shielding are far less stringent. We currently get Ra-225 from the National Isotope Development Center at Oak Ridge National Lab. Because of the low vapor pressure, an high temperature oven is used to produce a hot atomic beam of Ra. This beam is then collimated and slowed for loading into a magneto-optical trap. The atoms are transferred into a transport optical dipole trap, which delivers the atoms into a standing wave optical dipole trap. This trap, which is located between two high voltage electrodes, is where the atoms are spin-polarized using a brief pulse of circularly polarized light. As the atoms precess around the vertical magnetic field, their absorption probability is modulated at the Larmor precession frequency. By sending carefully time laser pulses through the confined atoms, we can build a spin precession curve and measure the spin precession frequency. An EDM would couple to the vertical electric field causing a very small frequency shift to the precession frequency. In the Fall of 2014, we demonstrated the first “proof-of-principle” measurement of the atomic EDM of Ra-225. This measurement also represents the first EDM measurement in a laser trap as well as the first EDM measurement of any “pear”-shaped nucleus.
We are now planning several upgrades to the experiment to improve the measurement sensitivity by several orders of magnitude. At MSU, we plan on building a Yb-171 laser trap that will be used to prototype several aspects of the experiment and may eventually be used to measure systematic effects as a “co-magnetometer.” In addition, we are also planning on harvesting Ra-225 and possibly other isotopes of interest from the water beam dump of FRIB, when it comes online around 2020.