DEPARTMENT
OF PHYSICS & ASTRONOMY
Atomic Physics
Positron Scattering, Positronium
Formation/Destruction, and 3-gamma/2-gamma Annihilation Ratio
Spectroscopy
Walter E. Kauppila, Talbert S. Stein
(emeritus), and Eugene Surdutovich
Positrons, being the antiparticles of electrons, are rather unique
projectile particles for investigating interactions with regular matter
due to the their ultimate destruction by direct annihilation into two
511 keV gamma rays or via forming positronium (Ps, an unstable
hydrogen-like atom composed of a positron and an electron), which in
turn annihilates into two 511 keV gamma-rays or three gamma-rays adding
to 1022 keV. Astrophysicists have observed positron annihilation
gamma-rays coming from solar flares, the direction towards the center
of our Galaxy and other gamma ray bursting sources in our Universe,
while medicine has developed PET (positron emission tomography)
scanners for investigating the heart, brain, and cancerous
tumors. Other uses of positrons are to study atoms, molecules,
surfaces, and solids in fundamental research. Ps is an intriguing
projectile because it is a neutral particle and has a finite lifetime
ending in annihilation.
Our research group uses low energy (up to a few 100 eV) positron beams
obtained from a sodium-22 radioactive source to investigate positron
scattering from a variety of atoms and simple molecules in three
different experimental systems. Our group is focused on measuring
cross sections for total scattering using a beam transmission
technique, differential elastic scattering using a crossed-beam
approach, and Ps formation by observing the annihilation gamma rays
from its decay. These measurements are providing evidence that
coupling effects between the various scattering channels (e.g. elastic,
Ps formation, and atomic excitation) are playing an important role in
positron-atom (molecule) scattering. We are finding evidence that
capture of electrons from inner subshells of atoms and from inner
orbitals of molecules play a role in Ps formation. Most recently
we have developed a new type of spectroscopy where we measure the ratio
of 3-gamma to 2-gamma annihilation from Ps decay, which enables us to
not only make detailed studies of Ps formation, but also the
destruction of Ps as it interacts with a surface.
This research is supported by the National Science Foundation.
Recent Publications:
"Measurements of positronium formation cross sections in positron-Mg
collisions", E. Surdutovich, M. Harte, W. Kauppila, C. Kwan, T. Stein,
Phys. Rev. A 68, 022709 (2003).
"Investigations of positronium formation and destruction using
3-gamma/2-gamma annihilation ratio measurements", W. Kauppila, E.
Miller, H. Mohamed, K. Pipinos, T. Stein, E. Surdutovich, accepted for
publication in Phys. Rev. Lett. (2004).
Laser-Induced Breakdown Spectroscopy (LIBS)
Steven Rehse
LIBS is an extremely powerful and flexible elemental analysis technique that utilizes
the energy in a short, intense laser pulse to vaporize or “ablate” a small volume of
sample material. The ablated target material (whether solid, liquid, or gas) absorbs
enough energy to ionize the constituent atoms, creating a small cloud of plasma (free
ions and electrons) that expands rapidly. As the cloud expands and cools, a significant
fraction of the ions recombine to form excited atoms, which eventually decay via
spontaneous emission to the atomic ground state (the state of lowest energy in the atom).
The photons given off during spontaneous emission can be collected and spectrally
analyzed which provides a “spectral fingerprint” of all the constituent elements of the
target and the plasma. Since each element has a unique spectral fingerprint, relative
and absolute elemental concentrations within the target material can be determined.
Theoretically, this measurement can be accomplished with one laser pulse (about 10 ns),
resulting in a very rapid analysis of the target (total measurement time less than one
second). Because the target material is interrogated by a laser beam and the plasma is
analyzed by “all–optical” techniques, the analysis can be performed remotely or on
hazardous materials.
Our research group uses an infra-red pulsed laser (1064 nm, 700 mJ/pulse maximum) to create
the ablation plumes and an Echelle spectrometer with an intensified charged-coupled
device (ICCD) to analyze the spectrum of the plasma. We are particularly interested in
analyzing liquid targets and solid particulates in liquids (sometimes referred to as
colloidal suspensions) which have previously been largely ignored due to the difficulty
in generating long-lived plasmas with high signal-to-noise atomic emission lines in
liquids. The ability to perform rapid analyses on these types of systems has applications
in both industrial and environmental settings. We are also interested in performing
automated two-dimensional scanning/stepping elemental analyses of flat substrates to
create “elemental composition” maps. This 2D scanning analysis has applications in
metallurgy and in the field of photovoltaics.
Laboratory astrophysics
Steven Rehse
Laboratory astrophysics is the study of the properties of atoms or ions of interest to
astronomers. If created in a suitable vacuum apparatus, the pulsed-laser-generated plasma
plumes described above may also be used as a source of ions for use is such laboratory
astrophysics experiments. Such an apparatus is sometimes referred to as a laser ion
source or torch.
Our group is particularly interested in measuring the lifetime of a highly excited state
of Ga II (singly-ionized gallium) which is present in high concentration in certain HgMn
stars. An experimental determination of the lifetime of this state (which is observed
spectroscopically by observational astronomers) can be used to reduce the uncertainty on
the oscillator strengths of atomic transitions originating from that state. These oscillator
strengths are essential for determining absolute abundances of the atomic or ionic species
from stellar spectra. Current measurements of the Ga II abundance in the HgMn class of stars
differ by as much as an order of magnitude based on which emission line is observed.
Jogindra M. Wadehra
The emphasis of our work is on studying the scattering of positrons
(antiparticles of electrons) and electrons from various atoms and
molecules. The systems under current investigation include rare gas
atoms, alkali atoms and light molecules. We have also calculated and
compared the cross sections for the ionization of inner shells of
various atoms by impact of both positrons and electrons.
Other research interests include investigations of production of
negative ions by the process of dissociative electron attachment to
simple molecules. The rates of negative ion production by this process
are strongly enhanced if the attaching molecule is initially
rovibrationally excited. We have also calculated the cross sections for
the vibrational excitation and dissociation of simple molecules by
electron impact. Atomic and molecular collision processes play
important and significant roles in astronomy and astrophysics.