Shaped Femtosecond Laser Pulse Spectroscopy for Nuclear Forensics
PI: I. Jovanovic, Penn State University
July-September 2013 Quarterly Report
During the past several months a significant amount of progress has been made working on
molecular isotopic spectroscopy, pulse shape analysis, and laser produced plasma imaging. From
late April through August the ICCD was successfully used to collect fast-gated images of laserproduced plasmas generated by both a ns and fs laser system; the ICCD has now been added to
the front exit of the spectrometer to allow for high resolution time resolved spectral analysis.
Further, the ICCD has now been proven to be able to be controlled through LabVIEW, which
will allow for the use of the ICCD in the previously designed genetic algorithm pulse shaping,
filamentation, and molecular emission LabVIEW experimental controls.
The month of September was spent training an undergraduate research assistant, Annie Hopkins,
who will be working on this project over the next year. From the time of her arrival, she has
assisted with diagnostic assessments of the ICCD LabVIEW control and the collection of both
image and spectral data, as well as the triggering control and stability. She will be working with
Kyle Hartig, who returned from his year long NNSA Graduate Fellowship in August and is
working on this project again, to implement the genetic algorithm pulse shaping - utilizing the
ICCD - in order to study the effects of complex pulse shape on uranium LIBS emissions. Parallel
to this effort, time was spent on activating the Spectral Phase Interferometry for Direct Electricfield Reconstruction (SPIDER) system that will provide complete and accurate characterization
of the resulting complex pulse shapes. The final calibration and testing of the SPIDER system is
in progress. In addition to this study, there is a renewed effort to look at the molecular emission
of uranium for isotopic analysis, as well as filamentation and wavelength dependence of LIBS
measurements that utilizes the unique capabilities of our lab. We have ordered reference material
XRF samples of uranium in a glass matrix that will provide a significant increase in the signal
stability and uniformity across the sample for use in the above efforts.
The following two sections highlight the experimental work performed during the reporting
period.
Fabry-Perot Etalon LIBS Resolution Enhancement
Collaboration research was initiated with Dr. Jill Scott at Idaho National Laboratory this past
summer. The scope of future work involves design and implementation of modifications to
various LIBS experimental setups, especially for comparison of actinide analysis results.
Experiments will be prepared to evaluate various LIBS systems and/or methods for
reproducibility, detection limits, and accuracy. Experimental results will be verified with
appropriate samples with known concentrations and data analysis of the LIBS spectra.
One approach that will be studied involves coupling a relatively inexpensive, compact FabryPerot (FP) etalon to the LIBS system setup for obtaining higher resolution measurements. A
typical FP etalon consists of two highly reflecting, semi-transparent plates that transmit specific
wavelengths of light corresponding to the resonance frequency of the etalon. Constructive
interference form a complex set of concentric ring patterns which is focused into a detector.
Instrumental design and method of data analysis are important to consider for achieving the
optimal resolution and limits of detection of the system. The influence of the optical components
on the resolution of the system and data interpretation will be investigated. Currently an
algorithm is being developed for modeling of the spectral signal projected on the image sensor.
Figure 1: Left: Fringe pattern depending as a function of angle θ. Right: Typical circular fringe
transmission pattern from Fabry-Perot etalon.
Shadowgraphic Analysis of Copper and Uranium Plasma
Ablated by Femtosecond Laser Pulses
Images were taken of copper and uranium plasmas at different delay times, ranging from 10 ns to
200 ns and at varying pressures. In this study we took shadowgrams of both uranium and copper
plasmas at atmospheric pressures. MATLAB code was written to extract the plasma radius from
each image in a data set – each data set contains 10 images per delay step with the total number
of images ranging from 100 to 200 per set – and compare the results to the Sedov-Taylor
expansion model typically used to model laser-produced plasma expansion on this time scale.
Copper vs. Uranium:
Figure 2 shows an image of representative copper plasma and Figure 3 shows an image
representative of uranium plasma. The two shadowgrams were taken at the same time delay and
pressure. The shadowgrams of the two plasma propagation fronts show a clear difference in
plasma shape, as the uranium plasma is much flatter and wider than the copper plasma. Also, in
the uranium plasma, the shockwave is not present as it is seen in the copper plasma. It is not
unusual to find that plasmas of different elements have different shapes, but it is unclear why the
shock front was not visible in the uranium plasma.
Figure 2: A shadowgram of copper plasma taken at atmospheric pressure
Figure 3: A shadowgram of uranium plasma taken at atmospheric pressure
Time Variation:
Uranium plasma at both a small (3.6 ns) and large (8 ns) time delay following plasma formation
are shown in Figures 4 and 5. There is a noticeable difference in the size and shape of the plasma
propagation front at the two delays. This data will be analyzed to see if current models predict
such rapid changes on this time scale.
Figure 4: A shadowgram of uranium plasma 3.6ns after ablation.
Figure 5: A shadowgram of uranium plasma 8 ns after ablation
Pressure Variation:
Shadowgraph images were taken at atmospheric pressure (Figure 3) and near vacuum (Figure 6)
for the same time delay and experimental parameters; thus, any difference could be attributed to
the difference in pressure. It is clear that as the pressure was increased the plasma expanded
quicker in the direction parallel to the sample surface and slower normal to the surface. An
expected result of the modeling effort is to explain this phenomenon. In order to validate the
models across different pressure and temperature regimes this data can be used to build the initial
basis for the model and higher resolution data can be taken to validate any final model produced.
Figure 6: A shadowgram of uranium plasma taken at near vacuum pressure, about 3 mTorr.
Conclusion:
Shadowgraphic images of copper and uranium plasmas were successfully taken at different delay
times and chamber pressures. The shadowgrams were analyzed using an adapted Poisson solver
that transformed the intensity data into refractive index. In the future the images that were taken
in this study will be used to validate current plasma expansion models at early times, between 3
ns and 10 ns. If the plasma models confirm the experimental observations over small delay times
and varying pressures, it will be of interest to see how these models can assist in manipulating
the plasma properties that would lead to an increase of various figures of merit in LIBS.