S. G. Crane

Ultrasensitive detection of radioactive cesium isotopes using a magneto-optical trap

We report the first magneto-optical trapping of radioactive 135Cs and 137Cs and a promising means for detecting these isotopes to ultrasensitive levels by using a magneto-optical trap (MOT) coupled 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 MOT 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. MOT fluorescence signals were measured for trapped-atom numbers from 104 to 107 and were found to increase linearly with the number of atoms implanted in the foil. The slope of signal versus number implanted was equal for each isotope to within 4%, signifying our ability to measure 137Cs/135Cs ratios to within 4% for MOT signal levels exceeding that associated with our present detection limit of 4000 trapped atoms. The MOT-based detection scheme was shown capable of suppressing interference from stable 133Cs by more than seven orders of magnitude. Including an isotopic selectivity of 105 of the mass separator, the overall suppression of 133Cs in the case of detecting either 135Cs and 137Cs is expected to exceed 1012. At present, the overall sample detection sensitivity is less than one million atoms.

Developing optical traps for ultrasensitive analysis

We describe the coupling of a magneto-optical trap to amass separator for the ultra-sensitivity detection of selected radioactive species. As a proof of principle test, we have demonstrated the trapping of approximately 6 million

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

_82)Rb atoms using an ion implementation and heated foil release method of introducing the sample into a trapping cell with minimal gas loading. Gamma-ray counting techniques were used to determine the efficiencies of each step in the process. By far the weakest step in the process is the efficiency of the optical trap itself. Further improvements in the quality of the nonstick dryfilm coating on the inside of the trapping cell and the possible use of larger diameter laser beams are indicated. In the presence of a large background of scattered light, this initial work achieved a detection sensitivity of approximately 4,000 trapped atoms. Improved detection schemes using a pulsed trap and gated photon detection method are outlined. Application of this technology to the areas of environmental monitoring and nuclear proliferation are foreseen.