Shaped Femtosecond Laser Pulse Spectroscopy for Nuclear Forensics
PI: I. Jovanovic, Penn State University
August 2012 Report
The first scanning electron microscopy (SEM) images of LIBS ablation spots
on copper and boron carbide have been collected (Figure 1). The instrument used is
a Nova NanoSEM, and will eventually be used to conduct experiments that
demonstrate the limited destructiveness of LIBS on uranium. The SEM technique
will be combined with profilometry, which accurately characterizes the volume of
the laser ablation craters.
A manuscript is being prepared for submission to Spectrochimica Acta Part B.
The paper is based on studies done this past summer to determine boron isotopic
ratios based on signals from molecular transitions. The paper will be tentatively
entitled “Measurement of Boron Isotopic Ratios with Femtosecond, Non-gated
LAMIS”.
Figure 1. Left: scanning electron microscope image (384x magnification) of craters
formed on copper metal from multiple laser pulses. Right: ablation craters on boron
carbide (409x magnification).
A code has been written to extract the 2-D integrated refractive index
distribution of radially symmetric laser produced plasma using shadowgraphy.
Shadowgraphy is a technique in which a laser pulse is sent through the plasma and
imaged on a CCD. In Figure 2 a simulation is shown of the intensity distribution that
would be observed if a Gaussian beam were allowed to pass through spherically
shaped plasma with a constant refractive index (left). In Figure 2 (right) the
intensity observed after the laser pulse passes through the plasma is compared to
the intensity before it passes through the plasma. This information can be used to
determine the 2D refractive index distribution (Figure 2, bottom). At this stage, the
code is being used for simulation only in order to determine both its accuracy and
its reliability. Once that is complete, the code will be used to analyze data that has
already been collected from plasmas generated on the surface of a graphite sample.
Figure 2. Left: simulated intensity distribution observed when a Gaussian beam is
allowed to pass through a spherically symmetric plasma distribution with constant
refractive index. Right: comparison of the intensity distribution before and after the
beam passes through the plasma. Bottom: reconstructed 2D refractive index
distribution.