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

Optimizing capture efficiency in a magneto-optical trap

Michelle S. Stephens, Carl Wieman, Kristan L. Corwin, Zheng‐Tian Lu, Harvey Gould, T. Dinneen

Published Apr 7, 1995• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.206446

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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.
Laser cooling and trapping of rare, radioactive isotopes has the potential for greatly enhancing experiments in high resolution spectroscopy of exotic species, electron electric-dipole moment searches, β-decay asymmetry, and parity nonconservation. The ability to hold atoms for several seconds at high densities (1011 cm-3) and away from the perturbing effects of the cell walls makes laser cooling and trapping attractive for use in the experiments mentioned above. Because many isotopes of interest for these experiments are difficult to produce, the ability to quickly and efficiently collect these atoms from a dilute vapor is crucial. We have demonstrated a 6% collection efficiency in a special wall-coated cell. We have developed a model that predicts the capture efficiency as a function of trap properties and have developed techniques for measuring the pertinent characteristics.
We present the results of the first study of the fundamental processes governing collection efficiency of neutral atoms in a vapor cell laser trap. The experimental test of our model of the collection process closely agrees with our predictions based on the measured properties of the laser beams and cell. This study has led to the development of special vapor cell wall coatings. We have demonstrated a collection efficiency of 6% for cesium and predict more than 50% could be achieved under optimum conditions. This work develops the concepts for and demonstrates the feasibility of new experiments using short-lived radioactive isotopes.
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.
Radioactive 79Rb^{79}\mathrm{Rb}79Rb atoms have been produced in a nuclear reaction and were captured into a magneto-optic trap in a cell. A fluorescence signal was obtained from 80 79Rb^{79}\mathrm{Rb}79Rb atoms in the trap. The efficiency of the process is estimated as 1\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}5}$ atom in the trap for every atom/sec produced. This efficiency is sufficient to develop further experiments to measure atomic properties of Fr with the eventual goal of observing parity nonconservation effects in neutral atoms of different radioactive isotopes of Fr.
We are attempting to produce an atomic vapor that is so cold that the de Broglie wavelength is larger than the interparticle spacing. This is the condition needed to obtain Bose-Einstein condensation of a noninteracting gas. Our approach is to optically trap and cool atomic cesium and then load it into a purely magnetic trap for further cooling. In the course of this work we have investigated a number of magnetic traps and very low temperature collisions that are relevant to the success of this approach. In other work we are developing optical trapping techniques for the efficient collection of radioactive alkali isotopes. This work is aimed at measuring parity violation in these isotopes and thereby stringently testing the standard model of elementary particle physics. We have studied how to optimize the capture efficiency of atoms in an atomic vapor. We are currently developing cells that will allow the atoms to bounce off the walls until they are captured in the optical trap. The efficiency of this capture scheme will be discussed.
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.