The Cooler Trap
The cooler trap is a large cylindrical Penning trap (the electrode structure is shown on the very left of the picture above) placed in the homogeneous field of a 4.7T superconducting magnet (the cryostat can be seen on the photograph). The ions delivered by the RFQ beam buncher are captured in the trap and mass selectively cooled. The present cooler trap system has been optimized for a high mass selectivity in order to resolve isobars and to deliver clean ion bunches to the precision trap, which is essential for highly accurate mass measurements. As an example, the figure to the right shows a mass scan performed with the cooler trap. Shown is the number of ions extracted from the trap as a function of the applied radio frequency for an A=141 ion beam delivered by ISOLDE. Although only a few nucleons from the valley of stability the isobars 141Eu, 141Sm, and 141Pm are clearly resolved. By choosing the correct frequency either can be selected for pulsed ejection and transfer to the second trap. This capability is essential for direct mass measurements on rare earth isotopes, which are deliverd as isobarically contaminated beam by ISOLDE.