Shaped Femtosecond Laser Pulse Spectroscopy for Nuclear Forensics
PI: I. Jovanovic, Penn State University
January-March 2014 Quarterly Report
Radioactive license amendment
In the last three months the approval process of the SNM-95 license amendment by the
Nuclear Regulatory Commission (NRC) has progressed. This amendment would approve
the use of enriched uranium in our research. We plan to inplement additional security
features in our laboratory, according to the NRC request and review committees’
recommendations. A heavy-duty fire-proof safe with a combination and key lock, where
the SNM will be stored, has been installed securely in the laboratory. Additionally, a
mechanical padlock will be installed on the entrance to the laboratory.
Publications and grants
We were notified that Kyle Hartig’s abstract entitled “Effect of Pulse Duration and
Ambient Gas Pressure on Laser-Induced Breakdown Spectroscopy of Uranium” and
Phyllis Ko’s “A Pragmatic Approach to Improving Spectral Resolution for Laser-induced
Breakdown Spectroscopy of Nuclear Materials” were accepted for podium presentations
at the Institute of Nuclear Material’s Management’s annual meeting in July 2014.
Prof. Jovanovic is a member of the 12-university consortium led by the University
Michigan that was awarded the $25M grant from DOE/NNSA to develop novel tools for
nuclear treaty verification. As a part of this 5-year effort, Prof. Jovanovic’s group will
conduct fundamental and applied research of laser-based spectroscopy techniques,
including LIBS, for standoff detection and characterization of nuclear materials and other
materials relevant to proliferation and forensics.
Teaching and student activities
In the past period several undergraduate students involved with the American Nuclear
Society student section and the Institute of Nuclear Materials Management student
section at Penn State have been involved with organizing the 2014 American Nuclear
Society Student Conference at Penn State and making an organized visit to Broohaven
National Laboratory. A special nuclear safeguards panel was organized during the
American Nuclear Society Student Conference.
Michael Moore, an undergraduate student working in I. Jovanovic’s group, has been
awarded a nuclear forensics undergraduate scholarship, which will enable him to spend
the coming summer conducting research at Oak Ridge National Laboratory.
LIBS of glass for nuclear applications
We are currently collaborating with Dr. Cory Trivelpiece of the Material’s research
institute at Penn State to study the advantages and challenges of using LIBS to
characterize glass samples similar to those used in radioactive waste vitrification.
International Simple Glass (ISG) glass samples were studied. The sample composition is
listed below:
Oxide
SiO2
B2O3
Na2O
Weight (%)
56.2
17.3
12.2
Oxide
CaO
Al2O3
ZrO2
Weight (%)
5.0
6.1
3.3
The initial measurements of glass samples were challenging. Fs-LIBS of glass was
difficult beause of insufficient ablation of the sample surface. Emission lines were
successfully detected using ns-LIBS. The results are shown in Figure 1. However, the
data acquired was not consistent over numerous shots. We attribute this to the uneven
surface of the samples, and plan to repeat experiments by minimizing rastering across the
sample and by increasing the number of shots at one position on the sample.
A
B
Figure 1. Ns-LIBS spectra of glass over 20 shots for two spectral regions: 405 nm – 420
nm (A); 635 nm – 650 nm (B).
Collaboration with Los Alamos National Laboratory
In March, Kyle Hartig began his praticum assignment at LANL, where he will work
under Dr. James Barefield on LIBS. First, additional spectra of glass has been acquired
using the LIBS system at LANL and is shown in Figure 2. The results will be compared
to the data collected using the LIBS setup at Penn State.
Figure 2. Ns-LIBS spectra of glass, acquired using the LANL LIBS system, over 10 shots
for the entire spectral region of the spectrometer.
Also, with assistance from LANL, theoretical modeling of plasmas and atomic emissions
is for LIBS is under way. Such models could be used to compare experimental
observation with synthetic data and experimental measurements.
Fabry-Perot etalon for LIBS resolution enhancement
In conventional LIBS, each column of the detector array is automatically integrated to
generate a 1D spectrum of intensity for each wavelength. When a Fabry-Perot (FP) etalon
is placed before the spectrometer entrance and the detector is used for 2D imaging, the
resolution of the vertical axis of the image is determined by the FP etalon, which can be
an improvement from that of the spectrometer grating in the horizontal direction. A
model is being developed for use of FP in our LIBS system, based on the intensity of
interference rings that are described by the equation
I ( l, q ) =
1
1+ (
)
2R 2
1-R
sin ( 2lp d n2 cosq )
(1)
where  is the wavelength of an emission line, and  is the angle of the light entering the
FP etalon relative to the normal of the incidence plane. Simulated fringe profiles for the
uranium 424 nm line under varying isotopic composition and with various line widths are
shown in Figure 3 and Figure 4, respectively. According to the simulation, the resolution
of our system may not be sufficient to resolve the isotopic composition of natural
uranium, depending on the line broadening effects. However, a multivariate calibration
could be constructed using spectra from uranium with varying enrichments.
Figure 3. Simulated Fabry-Perot fringe profiles for the isotope shift of the uranium 424
nm line. Black-natural uranium, blue dashed-10% U-235 and 90% U-238, red dotted50% U-235 and 50% U-238. Fringes shown are of the first order (n=1) to clearly show
differences for varying percent compsitions. The simulated input spectra were delta
functions (no spectral broadening was included) at 424.12 nm and 424.427 nm with
intensity ratios corresponding to isotope ratios of the uranium.
Figure 4. Simulated Fabry-Perot fringe profiles for uranium with the composition: 50%
U-235 and 50% U-238. The input spectral peaks are assumed to have gaussian
distributions. The spectral broadening effects on interference fringes of the etalon are
shown and compared to a delta function input (no broadening - black). As the width of
the input spectrum is increased in 0.005 nm steps, the fringes for U-235 and U-238
become unresolvable. The results indicate the conditions under which spectral
broadening affects have to be taken into account when making isotope measurements
using the FP technique.