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 \ensuremath{\alpha}-particle source at a standoff distance of 10 m, using 70 ps, long-wave infrared (\ensuremath{\lambda} = 9.2 \textmu{}m) ${\mathrm{CO}}_{2}$ laser pulses. This is \ensuremath{\sim}10 times 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 ${\mathrm{CO}}_{2}$ laser chain, enhancing sensitivity by >100 times. We also characterize breakdown plasmas with fluorescence imaging, and present a simple model to estimate backscattered signals as a function of the seed density profile in the laser focal volume. All of this is achieved with a relatively long-drive laser focal geometry (f/200) that is readily scalable to >100 m.