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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

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

Michael D. Di Rosa, S. G. Crane, J. J. Kitten, Wayne A. Taylor, David J. Vieira, Xinxin Zhao

Published Mar 29, 2002• 1 citation
Publisher

Abstract

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.

DOI
10.1117/12.463836

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Apr 7, 1995•0 citations
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.
We report the first magneto-optical trapping of mdioactive {sup 135}Cs and {sup 137}Cs and a promising means for detecting these isotopes to ullrasensilive lcvels by a system coupling the magneto-optical trap (MOT) to a mass separator. The mass separator efficiently delivers a 20 kV ion beam of either isotope into a quartzcube MOT cell having in one corner a small-diameter Zr foil, on which the ion beam is focused and into which the ions are implantetl. Inductive heating of thc foil releases {approx}45% of the implanted atoms into a MOT that uses large diameter beams and a dry-film coating to capture 3% of the released vapor. MOT fluorescence signals were found to increasc linearly with the number of foil-implanted atoms over a range of 10{sup 4} to 10{sup 7} in trapped-atom number. The measured slope of MOT signal versus number implanted was equal in the case each isotope to within 4%, signifying our ability to measure {sup 137}Cs/{sup 135}Cs ratios to within 4%. The isotopic selectivities of the mass separator and MOT combine to suppress interfering signal from {sup 133}Cs by a factor of greater than 5 x 10{sup 12} in the case of detecting {sup 135}Cs or {sup 137}Cs. Our present sample detection sensitivity is one million atoms.
The use of an optical trap for the quantitative detection of small numbers of radioactive atoms is featured. Presently under construction is a system consisting of a high-efficiency magneto-optical trap (MOT) coupled to a mass separator. Mass-separated samples will be implanted into a thin platinum foil, subsequently released from the foil by heating, and resonantly trapped in the MOT. The number of atoms in the trap will be quantitatively deduced from the amplitude of the fluorescence signal. Measurements for different isotopes are planned by appropriately shifting the trapping and repumping laser frequencies of the optical trap. Our initial investigations will concentrate on the determination of trace amounts of 135Cs and 137Cs.
The authors describe the coupling of a magneto-optical trap to a mass separator for the ultra-sensitive detection of selected radioactive species. As a proof of principle test, they have demonstrated the trapping of {approximately} 6 million {sup 82} Rb (t{sub 1/2} = 75 s) atoms using an ion implantation and heated foil release method for 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 (0.3%). 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.
As part of an effort to measure parity non-conserving atomic transition rates in a series of Cs radioisotopes, the authors are working on the efficient coupling of a magneto-optical trap (MOT) to a mass separator. They have established a test stand for optical trapping which is used for the off-line development of a high-efficiency MOT. Their trap features a dry-film coated cell with large (5 cm) windows to permit the use of large diameter, high power laser beams to increase the efficiency of the trapping process. Studies of the effect of detuning, magnetic field gradient, laser beam intensity and different beam sizes on the number of trapped atoms are presented.
We report the successful trapping of $6\ifmmode\times\else\texttimes\fi{}{10}^{6}$ 82Rb{}^{82}\mathrm{Rb}82Rb (t1/2=75s){(t}_{1/2}=75\mathrm{s})(t1/2​=75s) atoms with a trap lifetime of $\ensuremath{\sim}30\mathrm{s}$ in a magneto-optical trap that is coupled to an off-line mass separator. Efficient sample introduction is achieved by implanting 82Rb{}^{82}\mathrm{Rb}82Rb ions into a small yttrium catcher foil located inside the trapping cell. Upon heating, the radioactive atoms are released from the foil and trapped without significant gas loading. This advancement makes a variety of high-precision electroweak interaction experiments possible, including the measurement of \ensuremath{\beta}-decay correlations associated with spin-polarized 82Rb{}^{82}\mathrm{Rb}82Rb nuclei.
We have studied the user of vapor-cell magneto-optical traps for the collection and investigation of trace elements of radioactive isotopes. The fundamental processes governing efficient collection of neutral isotopes in a trap have been identified and integrated into a model that accurately describes the collection process. The model has been verified experimentally; a 6% capture efficiency has been demonstrated for stable Cs isotopes and we predict a simple change in trapping cell geometry will allow a 50% capture efficiency. This paper discussed the issues involved in efficient capture, such as the trapping laser properties and wall-coatings. It describes a new cell design for efficient capture and discusses current work towards trapping small numbers of 221Fr (T1/2 equals 4.8 minutes) atoms.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.