IN ADDITION TO LAYING THE GROUNDWORK FOR the next generation of accelerators at LBL (see main story), work is also underway to extend the lifetime of the last of the old generation, the 88-Inch Cyclotron. Commissioned in 1962 and named for the diameter of its main magnet, the 88-Inch Cyclotron is especially well-suited for accelerating heavy ions and has played a major role in nuclear science research. A new development by researchers in LBL's Nuclear Science Division, however, may give this venerable old cyclotron an unexpected future role in high energy physics.
A new technique for trapping rare types of radioactive isotopes 100 times more efficiently than ever before could enable physicists to use the 88-Inch Cyclotron to search for phenomena beyond the Standard Model and also test known phenomena with unprecedented precision. NSD physicist Stuart Freedman led a team that became the first ever to capture radioactive isotopes in a laser trap. The key to this success was the use of laser light in combination with a magnetic field to cool the atoms and slow their motion so that they could be actively loaded into the trap.
"Other researchers have tried to snag radioactive atoms at random in an area of containment," says Freedman. "Our technique is somewhat like pushing ping-pong balls with a bulldozer -- it has to be done very gently."
Freedman and his group worked with beams of sodium-21, a radioactive isotope with a half-life of 22.5 seconds, which they produced at the 88-Inch Cyclotron. A beam of high-energy protons (25 MeV) was used to irradiate a heated atomic oven loaded with file shavings of magnesium. From this reaction emerged a beam of sodium-21 atoms that was focused and sent down the length of a beamline through a decreasing magnetic field. At the same time, a beam of laser light was sent through the line in the opposite direction. The light was tuned so that its frequency was just below the excitation frequency of the sodium-21 atoms. This enabled the laser beam's photons to act as a brake on the atoms. Thanks to the decreasing magnetic field, the frequencies of the decelerating atoms and the laser photons remained in resonance.
The slowed atoms were directed into a "magneto-optical" trap, a sphere inside a quadrupole magnetic field through which shoot six circularly polarized laser beams. Atoms were captured at the point in the center of the trap where the six beams met. With this technique, Freedman and his group were able to capture 20 percent of the atoms that went through the trap. The old record for trapping atoms was less than one percent.
This new laser trapping technique should be applicable to other radioactive isotopes in addition to sodium-21. Already, Freedman and his group have begun designing an experiment to produce and trap the first workable quantities of francium, a radioactive element with many isotopes.
Members of Freedman's research group at LBL include Christopher Bowers, Brian Fujikawa, Zhengtian Lu, Justin Mortara, Song-Quan Shang, and Eric Wasserman. Freedman has also been working with two collaborators from Argonne National Laboratory, Kevin Coulter and Linda Young.
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