Magneto-optical trap and mass-separator system for the ultra-sensitive detection of 135 Cs and 137 Cs

Abstract

We report the first magneto-optical trapping of radioactive 135Cs and 137Cs and a promising means for detecting these isotopes at ultra-sensitive levels by coupling a magneto-optical trap (MOT) to a mass separator. A sample containing both isotopes was placed in the source of a mass separator, ionized, mass-separated, and implanted in a Zr foil within the trapping cell. After implantation, atoms were released from the foil by inductive heating and then captured in a MOT that used large-diameter beams and a dry-film-coated cell to achieve high trapping efficiency. Trapped-atom numbers in the case of either isotope ranged from 104 to 107, as determined from the MOT fluorescence signal. Over this trapped-atom range, the MOT fluorescence signal was found to increase linearly with the number of atoms implanted in the foil and without isotopic bias to within 4%. In principle, this method can then provide a measurement of the 137Cs/135Cs ratio accurate to within 4% through the direct ratio of MOT fluorescence signals. The fluorescence signal from stable 133Cs, when implanted and released from the foil, was suppressed relative to MOT signals by more than seven orders of magnitude when the system was tuned to trap 135Cs or 137Cs. When combined with the isotopic selectivity of ≥105 for the mass separator, the overall suppression of 133Cs is expected to exceed 1012. At present our system delivers atoms from sample to MOT with an efficiency of 0.5%, has a trapped-atom detection limit of 4000 atoms, and achieves a sample-detection sensitivity of one million atoms.