Millimeter-Wave Chaotic Cavity Detector for Non Metallic Concealed Weapons MILLET Nicolas, LEHUREAU Jean-Claude, LAMARQUE Thierry Thales Research & Technology, Palaiseau, 91767, France

Abstract — New ways to detect non metallic weapons such as explosives and ceramic handguns in order to secure public places have to be developed. To do so, we use millimeter-waves (frequency comprised between 18 and 40 GHz) at which human body is highly reflective and clothes almost invisible. We developed a multipoint sensor based on a chaotic high-Q cavity filled with millimeterwaves. It relies on the contrast that exists between the refractivity of human body and the refractivity of dielectrics. Index Terms — Cavities, Chaos, Diversity methods, Millimeter wave imaging, Security.

I. INTRODUCTION Some devices already exist to secure public places. Xray scanners and metal detectors have greatly improved security in airports and government buildings. However it is still impossible to detect non-metallic weapons such as explosives or ceramic handguns concealed underneath clothes. Several methods have been investigated to fulfill this goal. Chemical vapor detection, terahertz and millimeter-wave sensors are examples of ways to perform concealed explosive detection [1]. In this article we are using millimeter-waves between 18 GHz and 40 GHz. At those frequencies, human body is highly reflective and most clothes are thin enough and not too dense to be transparent. Most of the studies led in Europe and in the USA aim to make a high-resolution millimeter-wave picture of the body [2-4]. Those systems are usually complex and expensive to implement. Our goal is to develop a low cost imaging device. To do so, we have taken several aspects into account: 1. A good detection does not require high-resolution pictures of screened bodies. Imaging is a local measurement of physical properties. Since properties of dielectrics and human body are totally different, a low-resolution image is sufficient to make a good detection. 2. We can rely on modern computers' processing power and their capacity to treat data in real time to design new types of sensors with simpler hardware. In this paper, we present a device complying with those points. For the moment, our work is mainly directed toward a hand-held device, but the concept could be easily extended to a walk-through portal.

In the millimeter-wave band human body has a high refractive index. On the other hand, a dielectric has a relatively low refractive index and can match the impedance between air and human body decreasing its reflectivity at some frequencies. This device is a multi-point imaging sensor using only one millimeter-wave source and one detector. It is composed of a chaotic high-Q cavity filled with millimeter-waves (frequencies comprised between 18 GHz and 40 GHz). It detects the reflectivity drops caused by the presence of a potential threatening object. First we will introduce the concept of chaotic imaging. Then, we will discuss the results we obtained during preliminaries experiments. II. CHAOTIC IMAGING A. Description of the chaotic cavity detector The basic idea was to use chaotic cavities for imaging like in the time-reversal theory [5-6]. It uses the ability of chaotic cavities to distinguish the position of a source anywhere in space by processing complex data. Random energy distribution (interference pattern) Holes coupling resonant wave to the outside

Source

Detector (Schottky diode) Copper reflective walls

Human body

Fig. 1. Scheme of the chaotic cavity detector. The cavity is filled with millimeter waves. Boundaries are highly reflective to increase the Q-factor of the cavity. Holes are drilled in the lower wall of the cavity to permit a small part of the wave to leak out of the cavity. A detector locally measures the power of the wave resonating inside the cavity.

The device is basically a box with highly reflective walls covered with copper (see Fig. 1.). The cavity is nearly rectangular and is not chaotic by itself. Chaoticity is assured by the introduction of metallic elements in the cavity (brace, mechanical stirrer, …). When fed with millimeter waves, a resonant wave will take place; that will result in a random interference pattern inside of the cavity. Since the cavity is chaotic

where ∆Ptotal is the power variation measured by the schottky diode when dielectrics are present in front of a given set of holes {ik}, and ∆PHole ik is the power variation measured when a dielectric is present in front of the ikth hole only. Measuring ∆Ptotal is not yet sufficient to determine the individual response of each hole. For one given interference pattern inside the cavity, we obtain only one measurement of power characterizing the presence of a dielectric object. We need to obtain more uncorrelated measurements by changing the interference pattern. A totally different interference pattern can be obtained in the cavity by changing sufficiently the resonating conditions. When new resonating conditions are totally different, the measured power is fully decorrelated from the former one. By changing several times the interference pattern, which is called modes stirring [8], we obtain several uncorrelated measurements. By stirring many modes, we obtain a signal array stotal. Considering we sweep N uncorrelated modes, stotal is defined by

and has a high Q-factor, this pattern is very sensitive to changes in the resonating conditions [7] (the frequency of the wave or the geometry of the cavity for example). A schottky diode is used to measure the power of the resonant wave at a given point of the cavity. One of the walls is pierced of several holes. Part of the resonant wave will be transmitted outside of the cavity, and then reflected back by the human body. B. Principle of measurement This non metallic weapon detector relies on the great difference that exists between the permittivity of the human body and the permittivity of dielectrics (see table I). In the millimeter-wave band, human body is highly reflective. A dielectric layer will deeply decrease the reflectivity of the body at some frequencies by matching the impedance of air and organic tissues. TABLE I PERMITTIVITY AND LOSS TANGENT AT 30 GHZ Type of media Muscle Dry skin Wet skin Polymers (explosives)

Relative permittivity

Loss tangent

23 15 18 ≈ 2-4

0.92 1.0 0.93