Shaped Femtosecond Laser Pulse Spectroscopy for Nuclear Forensics. Phyllis Ko ANS Student Conference 2011

Transcription

1 Shaped Femtosecond Laser Pulse Spectroscopy for Nuclear Forensics Phyllis Ko ANS Student Conference 2011

2 Nuclear Forensics Analyzing nuclear material recovered from unused, intercepted devices or post-detonation debris Attributing nuclear material to origin, identifying source, manufacturing process or features of nuclear material 2

3 Outline Name of Institution: Pennsylvania State University Principal Investigator: Igor Jovanovic U.S. DEPARTMENT OF HOMELAND SECURITY, DOMESTIC NUCLEAR DETECTION OFFICE NUCLEAR FORENSICS JUNIOR FACULTY AWARD PROGRAM Narrative leveraged to lay the foundation for a combined molecular dynamics and hydrodynamics model for predicting fs laser plasma and fs-libs signals obtained using various laser parameters. Development of numerical models can significantly increase the utility of fs-libs to nuclear forensics through improved understanding of the scaling of fs-libs signals with pulse shape, pulse duration, laser frequency, repetition rate, environment, etc. Preliminary modeling performed in this work will be validated by Data display experiments. 1. Laser induced breakdown spectroscopy (LIBS) 2. Project overview Laser Detector Focusing Lens Spectrometer Sample stage 3. Pulse shaped fs LIBS for nuclear forensics t Plasma (a) (b) Fig. 1. (a) Comparison of signals produced by fs-libs and ns-libs [3]; (b) fs-libs with adaptive feedback for optimization of emission fluorescence signals. 4. Summary The impact of the proposed work on the science and technology of nuclear forencics is: (a) it will elucidate the important physics of interaction of shaped fs laser pulses with laser-produced plasmas, (b) it will produce a database of optimized laser pulse shapes for detecting materials of relevance for nuclear forensics analysis, (c) it will provide an experimental comparison with another laser method used for nuclear forensics (RIMS), as well as ns-libs, and (d) it will provide ample, sustained educational opportunities to help train the next generation of nuclear forensic scientists and engineers needed by the t government agencies, national laboratories, industry, and academia. 3. Preliminary 3-year research plan PSA! 3.1. Implementation of fs-libs setup with temporal pulse shaping and adaptive feedback The primary approach that will be used experimentally is to employ a temporal pulse shaping technique that can generate pulses with arbitrary shapes and durations between ~30 fs 6 ps, including simple pulse trains. Control of temporal waveform of fs laser pulses, to the best of our knowledge, has not! been explored for nuclear detection applications, but preliminary work indicates its promise for chemical detection, where it has lead to SNR improvements compared to simple unshaped fs-libs [4]. It is hypothesized that the front of the pulse (or the first pulse in the pulse train) will ablate the target material, and the back of the pulse (or another pulse in the pulse train) will resonantly excite a particular frequency band of species in the laser plasma to enhance the signal of interest. Since the emission signals are determined by various lifetimes of excited states, it is possible to manipulate and enhance the signals by adjusting the pulse shape or pulse-to-pulse separation in the pulse train. Shaped fs pulses 3and pulse trains will be synthesized by modulating the pulse frequency content using the acousto-optic programmable dispersive filter (Fig. 2(a)) [5], with the option for additional crude tuning using the compressor grating t separation in the laser. t This advanced fs pulse shaping capability is

4 Sample preparation vs in situ analysis Chemical separations and sample preparation Collection In situ analysis Identification and attribution Sample preparation + High sensitivity - Destructive - Time consuming - Generates toxic waste In situ analysis + Rapid + No sample preparation + Stand-off capabilities - Lower sensitivity 4

5 Laser-induced breakdown spectroscopy Data display Pulse duration - ns, ps, fs Laser Focusing Lens Detector Time Resolution - gated vs non-gated detection Spatially resolved - signal from different plasma regions Ambient conditions - atmospheric, vacuum, gaseous, pressurized Spectrometer Applications - solid, liquid, gas, stand-off Sample stage Plasma 5

6 Project overview Objective: Explore the potential for performance enhancement of LIBS for nuclear forensics by employing femtosecond pulse shaping. Optimize the SNR in LIBS by adaptive feedback and advancing the methods for signal detection. Innovative approach: 1) femtosecond pulse shaping 2) temporally resolved spectral diagnostics 3) adaptive feedback z (b) 6

7 Spectral features of LIBS signals are time-dependent e- e- e- e- e- Continuum emission E E Emission of discrete atomic lines 1 µs 1 µs LIBS signal LIBS signal 1 ns 10 ns 100 ns 1 µs 10 µs 100 µs Time after laser ablation Delay time (gate delay) and integration time (gate window) are important in optimizing SNR. Optimum value of gate delay and gate width is a function of the target composition and plasma parameters. 7

8 Short-pulse LIBS vs long-pulse LIBS Long pulse: ns Short pulse: fs-ps Nd:YAG laser Ti:sapphire laser D.R. Alexander, U. Nebraska-Lincoln 8

9 Pulse shaping for SFS-LIBS Stretcher Short pulse oscillator Short seed pulse Stretched seed pulse Stretched amplified pulse Laser amplifier Pulse shaper Amplified short pulse t t Compressor PSA Pulse shaper Pulse shaping has been successfully applied to coherent control.!! 9

10 We will explore operation at various pressures and attempt to measure isotopic ratios Vacuum operation may improve SNR Isotopic identification is possible LIBS on Si 238 U (93.5%) 235 U (5.3%) 239 Pu (49%) 240 Pu (51%) 10-6 torr 20-3 Pa (air) 10 2 Pa (He) J.S. Cowpe, et al., Vacuum 82, 1341 (2008) C.A. Smith, et al., Spectrosc. Acta Pt. B-Atom. Spectr. 57, 929 (2002). W. Pietsch, et al., Spectrosc. Acta Pt. B-Atom. Spectr. 53, 751 (1998) 10

11 Experimental setup Combination of ultrafast laser pulses, optical pulse shaping and feedback algorithms to control multi photon excitations laser shutter pulse shape control pulse shaper focusing lens collection lens target manipulator vacuum chamber controller spectrometer analysis adaptive feedback light sensor vacuum pump 11

12 Progress to date: modeling ~2% of emission light captured 4 output window focusing lens f=25 cm w0 = 24 µm 30 fs, W/cm 2 >6 µj win = 1 cm 1 cm 1 cm sample input window adjustable spot size normalized LIBS signal B = 2π λ n 2 I(z)dz B-integral sets the maximum spot size on sample Normalized LIBS signal B rad operating range B<1 B-integral ww 0 12

13 Project summary Collaborations established DAQ system under development Modeling Future work: Complete experiment setup Sample analysis Genetic feedback algorithms Impact: Advances in LIBS have a potential to contribute to the development of rapid, sensitive nuclear forensics analysis methods with minimal or no sample preparation. 13