NMR-Based Liquid Explosives Detector

Abstract

Liquid explosives pose a threat to security on airplanes and other public places, since they can easily be concealed as benign liquids. A detector, able to quickly identify liquids, would increase the possibility to detect such threats and speed up security checks. As a step towards a long-term goal to develop a liquid explosive detector, we have constructed an experimental setup based on a low-cost 1.1 T permanent magnet with huge static magnetic field gradient of 4.8 T/m, which allows us to measure proton relaxation times T ₁ and T ₂ and the self-diffusion coefficient D in liquid samples in a thin slice excited by radio-frequency pulses. We have developed a simple model in order to explain diffusion-enhanced non-exponential magnetization recovery in inversion recovery T ₁ experiment in this setup. Measuring a wide variety of liquid samples, we have demonstrated that it is possible to discriminate between the liquids based solely on these parameters. We discuss further improvements to the detection method, among those the choice of magnetic field, based on the fast field-cycling measurements.