BERKELEY, CA -- Exotic searches for physics beyond the Standard Model, as well as tests of known phenomena with unprecedented precision, could be carried out at the Lawrence Berkeley Laboratory's 88-inch Cyclotron using a new technique for trapping rare types of radioactive isotopes. The technique, which was developed by researchers in LBL's Nuclear Science Division, enables these radioactive isotopes to be captured and collected 100 times more efficiently than ever before.
Working with beams of sodium ions produced at the 88-inch Cyclotron, the researchers, led by physicist Stuart Freedman, became the first group ever to capture radioactive isotopes in a laser trap. The key to their success was using laser light in combination with a magnetic field to cool down the atoms and slow their motion so that they could be actively loaded into the trap. A competing approach has been to try to snag radioactive atoms at random in an area of containment.
"Our technique is somewhat like pushing ping-pong balls with a bulldozer," says Freedman. "It has to be done very gently."
In their first experiments, Freedman and his group captured sodium-21, a radioactive isotope with a half-life of 22.5 seconds. Their trapping technique should, however, be applicable to other radioactive isotopes as well.
"We selected sodium-21 because the laser technology and basic techniques for manipulating stable sodium are well established," says Freedman. "Furthermore, the beta decay of sodium-21 can be used to study the weak interaction and I have always been interested in doing high energy physics at low energies."
In their experiments to date, Freedman and his colleagues used a beam of high energy protons (25 MeV) to irradiate a heated atomic oven loaded with file shavings of magnesium. From this reaction emerges a beam of sodium-21 atoms. The sodium-21 beam is focused and sent down the length of a beamline through a decreasing magnetic field. At the same time, a beam of laser light is sent through the line in the opposite direction. The light is tuned so that its frequency is just below the excitation frequency of the sodium-21 atoms. This enables the laser beam's photons to act as a brake on the atoms. The decreasing magnetic field keeps the frequencies of the decelerating atoms and the laser photons in resonance.
"Each collision between a photon and an atom slows the velocity of the atom by about three centimeters per second," says Freedman. Atoms in the beamline start out at a speed of about 1,000 meters per second, which is comparable to the speed of a jet plane.
The slowed atoms are directed into a "magneto-optical" trap, a sphere inside a quadrupole magnetic field through which shoot six circularly polarized laser beams. Atoms are captured at the point in the center of the trap where the six beams meet. 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.
Slowing the motion of the atoms also cools them. By the time the sodium-21 atoms were trapped, their effective temperature had been lowered by several orders of magnitude to a few milli-Kelvin. This not only helps boost trapping efficiency, it also makes possible the accumulation of the high atomic densities necessary for a number of important physics experiments.
"For high densities, the motion of ultracold atoms is affected by the force from re-radiated photons and there is evidence of interesting collective phenomena," says Freedman. "This is leading to a new picture of the behavior of atoms in intense laser fields. It will also help us probe the long-range components of the interatomic force."
Perhaps the most tantalizing of applications to be realized from this new trapping technique, Freedman says, is the possibility of using it to study the weak interaction.
"By comparing the lifetime of sodium-21 with a precise measurement of the beta-decay asymmetry," says Freedman, "we should be able to learn why parity is violated in the weak-interaction, or why only the left-handed component is involved. A decisive experiment could point to the existence of a new family of heavy intermediate vector bosons and a revolutionary discovery."
In addition to their work with sodium-21, Freedman and his group have also begun designing an experiment to produce and trap the first workable quantities of francium, a radioactive element with numerous 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.
LBL is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.