Laser-trapping of rare element gets unexpected assist
ARGONNE, Ill. (May 1, 2007) — Argonne researchers have successfully laser-cooled
and trapped atoms of radium — the first time this rare element has been captured
in a magneto-optical trap — with an assist from an unexpected source.
The group of physicists was attempting to trap the rare, radioactive element
for studies of time-reversal violation, explained Argonne Compton Postdoctoral
Fellow Jeffrey Guest of Argonne's Physics
Division. Finding examples of this
effect has implications for physics beyond the Standard
Model and for explaining
why the Big
Bang yielded an imbalance between matter and antimatter in the
universe.
Starting with less than a millionth of a gram of radium, the scientists vaporized,
laser-cooled and captured the radium atoms in a magneto-optical
trap. "This
is the first time this rare element has been laser-cooled and trapped," Guest
said. "It is the heaviest atom and only the second element with no stable
isotopes — after francium — laser-trapped so far. It was particularly challenging
to trap radium because quantities are scarce, and the atomic structure is not
well studied and understood."
Radium atoms were slowed to a crawl and captured with magnetic fields and
laser beams tuned near the atoms' resonant frequency. Future experiments will
probe the cold radium atoms with lasers as they spin in place in a large electric
field. The atoms will precess — wobble about their axes like tops winding down — as
they spin. The frequency of this precession may reveal a slight offset between
the negative and positive charge within the atom along its spin axis, a signature
of time-reversal violation.
"Because their nuclei are egg-shaped, radium nuclei should be very sensitive
to the time-reversal effects we want to investigate," Guest said. "However,
radium is difficult to work with. Atoms tend to drift out of the trap, and
because of radium's chemistry, it would stick to the walls of the vacuum chamber."
However, researchers were surprised to find the radium atoms were staying
put much longer than expected. "We were surprised to discover that room
temperature blackbody radiation actually played a pivotal and supportive role," Guest
said.
Blackbody
radiation is essentially heat; in this case, infrared radiation
coming from the room-temperature walls of the apparatus. It's often a nuisance
for experiments in physics, causing heating, contributing to background noise
and scrambling quantum phases. However, when the radium atoms fell into metastable
atomic states— in which the atoms could no longer “see” the trapping lasers — during
the laser-cooling, the blackbody radiation added enough energy to the atoms
to "recycle" them back to a configuration in which they could “see” the
lasers again. This allowed the lasers to do their work and hold the atoms in
place.
"This mechanism may be helpful in trapping other atoms with complex structure," Guest
said.
The current effort in the laboratory is focused on adding a dedicated measurement
apparatus to the experiment to begin the search for evidence of time-reversal
asymmetry. Experiments with radium nuclei will begin in earnest.
A report on this achievement was recently published — and marked as a “suggestion” by
the editors — in Physical
Review Letters (PRL 98, 093001 (2007)).
It was also featured in the American Institute of Physics Physics
News Update Feb.
20.
Physics Division researchers on this project include Guest, Nick Scielzo (now
at Lawrence Livermore National
Laboratory), Jin Wang, Zheng-Tian Lu, Roy Holt,
Irshad Ahmad and Dave Potterveld, with Kevin Bailey and Thomas O'Connor providing
engineering support. John Greene of Argonne's Physics Division and Del Bowers
of Argonne's Chemical Engineering
Division prepared the radium samples. Health
Physics support was supplied by Marian Williams of Argonne's ESH/QA Oversight
Division.
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For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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