Remote detection of radioactive material using a short pulse CO2 laser

Detection of radioactive material at distances greater than the radiated particle range is an important goal with applications in areas such as national defense and disaster response. Here we demonstrate avalanche-breakdown-based remote detection of a 3.6 mCi alpha-particle source at a stand-off distance of 10 m, using 70 ps, long wave infrared (lambda=9.2 micron) CO2 laser pulses. This is ~10X longer than our previous results using a mid-IR laser. The primary detection method is direct backscatter from microplasmas generated in the laser focal volume. The backscatter signal is amplified as it propagates back through the CO2 laser chain, enhancing sensitivity by >100X. We also characterize breakdown plasmas with fluorescence imaging, and present a simple model to estimate backscattered signal as a function of the seed density profile in the laser focal volume. All of this was achieved with a relatively long drive laser focal geometry (f/200) that is readily scalable to >100 m.