Liquid explosive detection by microwave resonators

It is well known that the direct observation of the complex dielectric permittivity of liquids at microwave frequency enables fingerprint detection of certain categories of liquids, such as aqueous, organic and corrosive. Over the past seven years, we have been working on ultrafast detection of liquid explosives in sealed containers based on the evanescent field of a patented multimode cavity. Our research has led to the development of a bottle scanner for airport checkpoints, which has achieved the certification for use in European airports and passed a large number of field trials in airports worldwide. In this contribution the technology of evanescent field liquid sensing and its implementation into a user friendly software and hardware interface are explained. Moreover, orthogonal sensor approaches and verification schemes aiming to reduce false alarms will be discussed. Scientific background Microwave absorption of liquids is well understood on term of Debye relaxation [1]. As the most common experimental method the complex dielectric permittivity *(f)=1(f)+i2(f) in the range from about f=50 MHz to 50 GHz can be easily determined using a coaxial probe and a vector network analyzer [2]. In this way, a large variety of different liquids can be identified according to the frequency dependence of their complex dielectric permittivity, which varies quite a lot over the microwave frequency range – as a result of molecular relaxation by the intermolecular bond network and the degree of polarity (see Fig. 1). EMISENS possesses a database of complex permittivity data of most commons liquid explosives and precursors. Fig. 1: Frequency dependence of the complex microwave permittivity of distilled water in comparison to a range of semi-polar liquids.