Remotely Discerning Radioactive Source Type via Laser-Driven Electron Avalanche in Air

Detection of radioactive material from standoff distances outside the attenuation range of its decay products is of great interest in areas such as nuclear security and disaster response. Development of a new technique based on mid-infrared (λ = 3.9 μm) laser driven electron avalanche has recently been demonstrated (R. M. Schwartz et al., Sci. Adv. 5, eaav6804 (2019), D. Woodbury et al., Optica 6, 811-820 (2019)). In the work presented here, we extend this technique by comparing the breakdown plasmas generated near two different radioactive sources, Fe⁵⁵ and Po²¹⁰, emitting 6 keV γ-rays and 5.3 MeV α-particles, respectively. We find that the breakdown sites in air irradiated by Fe⁵⁵ are larger on average and grow faster than those in air irradiated by Po²¹⁰. This is detectable at range through a Doppler shift in the backscattered spectrum. Ionization track simulations support this, showing ionization clusters with locally high seed density that lead to faster avalanche growth in the case of Fe⁵⁵, and a more uniform ionization distribution in the case of Po²¹⁰. Our comparison of two different source types, shows that the avalanche plasma detection method is sensitive to the specific local ionization distribution, possibly allowing for source differentiation at long range.