Remote detection of radioactive material using mid-IR laser–driven electron avalanche

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Science Advances  22 Mar 2019:
Vol. 5, no. 3, eaav6804
DOI: 10.1126/sciadv.aav6804
  • Fig. 1 Radioactive source–seeded mid-IR laser–driven avalanche breakdown of air.

    (A) Copropagating 50-ps (FWHM) λ = 3.9 μm pump and 70-ps (full width) chirped 1.45-μm probe pulses are generated in an OPCPA system (see Materials and Methods for details). Beamsplitter BS1 splits off >99% of the λ = 1.45 μm probe and sends it to PbSe photodetector PD1 for a probe pulse energy reference. This signal is also proportional to the λ = 3.9 μm pump energy (sample trace shown) because of the optical parametric amplification process. (B) Ionizing radiation (5.3-MeV α-particles) from an 18-mm-diameter Po-210 foil source generates a population of free electrons and O2 ions in the focal region of lens L1, seeding collisional avalanche ionization driven by the λ = 3.9 μm pump. At lower average seed density, a seed electron is less likely to appear in a region of highest laser intensity in the focal volume, and thus, a local breakdown will take longer as shown in the simulation panel. The evolving avalanche breakdown plasma backscatters a portion of the λ = 3.9 μm pump pulse, which is collected by lens L2 onto PbSe photodetector PD2, with a sample trace shown. (C) The chirped λ = 1.45 μm probe is transmitted through the plasma, separated from the 3.9-μm pump by beamsplitter BS2 and collected by lens L3 onto InGaAs spectrometer Spec1. The spectral components of the chirped probe pulse correspond to specific time delays, as shown by the shared wavelength and time axis on the inset figure. The rapidly increasing plasma density cuts off the chirped probe at the breakdown time, taken to be the wavelength interval (and corresponding time) where the ratio Sb(λ)/Sref(λ) is reduced by 20%, where Sref(λ) is the probe reference spectrum and Sb(λ) is the probe spectrum transmitted through the breakdown region.

  • Fig. 2 Breakdown enhancement in the presence of a Po-210 α-particle source.

    Pulses with peak intensities 0.6, 1.0, and 1.3 × 1012 W/cm2 (pulse energies of 2, 3, and 4 mJ) focused at f/20 drove avalanche breakdowns as the radioactive source was moved 1.5 to 7 cm from the focal spot, with 2500 shots taken at each position. The 5.3-MeV α-particle stopping distance in air is ~3.5 cm (18). (A) For decreasing radioactive source–focal spot separation, increasing O2 seed densities cause faster average avalanche breakdowns, until a maximum breakdown time advance is reached. (B) Earlier breakdowns result in increased λ = 3.9 μm pump pulse backscattering by the plasma. Backscatter points are time-integrated signals normalized shot by shot to pulse energy. Each curve is again normalized to its peak signal for comparison between different intensities. The inset of (B) shows backscattering scans for several focal lengths at a fixed peak intensity of 0.7 × 1012 W/cm2, with similar normalization. Error bars indicate the SD of the signal generated from the statistics of 2500 shots at each position and intensity.

  • Fig. 3 On-off response of breakdowns to a modulated external source of radioactivity.

    A series of shots measured the pump backscatter (left) and breakdown time advance (right) as the α irradiation was periodically switched on and off using a mechanical shutter.

  • Fig. 4 Comparison of avalanche breakdowns seeded by the Po-210 source and corona discharge ion generator.

    (A) Normalized backscatter signal versus source distance from breakdown seeded by the Po-210 source (top) and versus negative ion concentration from an ion generator (bottom). All shots are for an intensity of ~2 × 1012 W/cm2, which was high enough to ensure breakdowns at every position/ion concentration. (B) Breakdown time advance versus distance from breakdown seeded by the Po-210 source (top) and versus negative ion concentration from an ion generator (bottom). Error bars show the SD over 2500 laser shots.

  • Fig. 5 Simulation of avalanche breakdown.

    (A) Simulation of the breakdown time advance starting from a single electron driven at a range of peak intensities (Gaussian pulse with τFWHM = 50 ps, terminated at t = ± 35 ps). (B) Simulation of the distribution of mean breakdown time advances for different seed O2 densities driven by a laser pulse with a peak intensity of 1.05 or 1.2 × 1012 W/cm2 focused at f/20, matching experimental conditions. Error bars show the SD of the probability distribution, calculated separately for values above and below the mean. Probability of breakdown times for a given density was calculated using Poisson statistics to determine the probability of an electron occupying volume elements of differing peak intensities throughout the focal volume (see Materials and Methods).

Supplementary Materials

  • Supplementary Materials

    The PDF file includes:

    • Legends for movies S1 and S2

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). ON-OFF response of laser-induced avalanche to external radioactive source.
    • Movie S2 (.mp4 format). ON-OFF response of laser-induced avalanche to radioactivity-induced oxygen ions from an airflow source.

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