Laboratory: Argonne National Laboratory

Laboratory Organizational Unit: Chemistry Division

Project Title: Heavy Element Chemistry and Separation

Science

Principle Investigators: G. K. Liu and J. V. Beitz

Our spectroscopy and photophysics studies are providing the fundamental
basis for understanding and improving rapid, ultrasensitive laser
techniques for determination of fission product lanthanide elements as
well as most actinide elements. In general, the photon emission rate for
a given excited f-electron state of a lanthanide or actinide ion is
several thousand [per second]{.underline}. In practice, laser-induced
fluorescence methods have not achieved sustained photon emission rates
of several thousand per second per heavy element ion in condensed
phases. Our past studies have identified several factors that contribute
to this deficiency. Hyperfine splitting and spin transfer can result in
spectral hole burning that prevents sustained pumping of a given excited
state. In addition to multiphonon relaxation, facile ion-ion energy
transfer among electronically excited ions nonradiatively can deplete
excited state populations, and excitation-induced structural
rearrangement of the ions or ligands surrounding a heavy metal ion can
markedly reduce its ability to absorb laser light of a given wavelength.
Our FY2001 studies on photoinduced structure variation and ion-ion
energy transfer in liquid and solid phase have addressed several of
these issues. Our projected FY2002 and FY2003 work on optical detection
of heavy element hyperfine interactions and the photophysics of uranyl
fluoride and actinide oxide aggregates will extend our understanding of
the factors that presently limit laser-induced fluorescence detection of
heavy element ions and may provide opportunities for mitigating or
eliminating them.

 

Theft, diversion, transport, and assembly of nuclear weapons, of bombs
that contain radioactive elements or of radiological devices designed to
contaminate waterways, subway tunnels, or building air inlets will
release traces of the constituent radionuclides into the biosphere.
Analysis of currency, blood, hair, fibers, plants, agricultural produce
or air filters on vehicles at ultratrace sensitivity (a few analyte
atoms) for characteristic elements has the potential to identify a
developing nuclear or radiological threat. Laser-based methods that
detect f-state photon emission have the potential for detecting and
quantifying even long-lived actinides such as uranium, neptunium, or
plutonium in a few seconds at the level of a few atoms in a condensed
phase sample that has undergone preconditioning. Conventional nuclear
counting methods are inherently limited by the decay rate of the
radionuclides being counted. Such methods have good sensitivities only
for short-lived isotopes and are expensive due to low throughput. For
example, the nuclear decay probability per U-235 atom is approximately
one in 10^16^ seconds, that per Pu-239 atom is one in 10^12^ seconds,
and that per Np-237 atom is one in 10^14^ seconds. Even for relatively a
short-lived isotope in spent reactor fuel such as Cm-242 (162.8 day half
life), the potentially achievable sustained f-state photon emission rate
is more than 2 billion times larger than its nuclear decay rate. The
promise of ultrasensitive laser-induced fluorescence methods based on
f-electron states is rapid, ultratrace, relatively low cost,
nondestructive analysis for most lanthanide and actinide elements.