Two views of the atom trap used in the experiment. On the left, the signal indicates that the trap is empty, while the picture on the right shows a “hot spot” indicating that an atom of argon-39 is contained within the trap. Image: Argonne National Laboratory |
Radioactive
dating is used to determine everything from the age of dinosaur fossils to
Native American arrowheads. A new technique recently developed at the U.S.
Department of Energy’s Argonne National Laboratory may give researchers another
tool for radioactive dating that could be of particular use in studying the
history of climate change.
Argonne physicist Peter Mueller and his colleagues
created a special “atom trap” to isolate and study argon-39, an
exceptionally rare isotope of a noble gas that occurs naturally in small
quantities in the atmosphere.
According
to Mueller, most atmospheric argon is argon-40, which means it contains 18
protons and 22 neutrons. However, occasionally an incoming high-energy proton
from outer space—also known as a cosmic ray—collides with the nucleus of an
argon-40 atom to kick out one neutron and create an unstable, radioactive
isotope.
This
process happens so infrequently that radioactive argon-39 is present in the
atmosphere in only extremely minute quantities: less than one argon-39 atom per
one quadrillion atoms of argon-40 (one quadrillion equals
1,000,000,000,000,000).
“Even
deciding to study this exotic isotope raised a number of different
challenges,” Mueller said. “When we have so little of our target atom
to work with naturally, we have to make sure that we’re counting as many as
possible of them out of the samples we have while not making any mistakes in
sorting them out.”
The
advantage of using argon-39 for radioactive dating lies in its unique half-life,
which is approximately 270 years. “Some well-established radio dating
isotopes, like carbon-14, have half-lives of several millennia, while others,
like tritium or krypton-85, have half-lives on the range of only years to
decades,” Mueller said. “Argon-39, even though it is exceptionally
rare, is really a kind of Goldilocks isotope for some uses. Its half-life is
not too short, not too long—it’s just right.”
According
to Mueller, scientists studying the dynamics of groundwater or ocean currents on
the timescale of 100 to 1,000 years might find argon-39 dating to be a
particularly useful tool in their analysis. “The good news for people who
want to study things like groundwater flows or ocean mixing,” he said,
“is that we have a pretty good understanding of how argon behaves in these
environments. Argon-39 is almost an ideal clock for these purposes.”
To isolate
argon-39 atoms from argon gas, Mueller and his colleagues built a device called
an atom trap. The atom trap uses six laser beams to form a “cage”
that catches only argon-39 while letting through all atoms of argon-40 or any
other isotopes present in the sample. Once trapped, an argon-39 atom appears as
a bright dot on a sensitive video camera.
“Argon-39
is so rare that we have to measure and detect the atoms one at a time,”
said Robert Janssens, director of Argonne’s
Physics Division. “It is an involved and painstaking process.”
A lot of the experimental research involved looking at an empty trap,
Mueller explained: “Every few hours, we’d detect the signatures from just
a handful of individual atoms. Because we’re looking for something so rare, we
need to be both patient and precise in order to find it.”