Identification of concealed materials, including explosives, by terahertz reflection spectroscopy

Abstract

Abstract. We report on a method for extracting the characteristic features of covered materials, including hexogen, in the range from 0.5 to 1.8 THz. This time-domain spectroscopy-based technique takes into account only part of the signal reflected from a covered sample and analyzes it by using the Fourier transform. The obtained power spectrum has distinctive peaks that correspond to peaks measured in the transmission configuration and can be used for further identification of the materials. © IntroductionTerahertz (THz) radiation in the range of 0.3 to 3.0 THz has a strong potential for use in identification of covered explosive materials because of two key factors. Some commonly used explosive materials, such as hexogen (RDX), octogen (HMX), and penthryte (PETN) possess distinctive transmission 1,2 and reflection 3,4 spectral features (called "fingerprints") in the THz regime. Also, most commonly used nonpolar dielectric covering materials, such as cotton, polyester, paper, and plastic materials, show low absorption of THz waves. Since the attenuation properties of most explosive materials are relatively high, 2 the transmission configuration seems to be more applicable to samples that are a few millimeters thick, e.g., for use in mail scanners. In contrast, the reflection configuration is safer for inspection applications and is more suitable for performing detection on people and inside envelopes or bags. However, the reflection configuration is more complicated owing to the small reflectance from pure explosive materials, 4 the strong attenuation properties of water vapor, the scattering effects related to the morphology of the sample 5,6 as well as to the attenuation properties of the covering layers.

Effective criteria developing for the identification of substance using the reflected THz signal

“…In [11] was reported on a method of extracting the characteristic features of some materials, including RDX, by so called a p-Spectrum method. The proposed technique takes into account only the part of the main reflected pulse, called a p-signal, and analyzes it by using the Fourier transform.…”

“…Nowadays the THz Time-Domain Spectroscopy is a widely used technology for the problems of defense and security [1][2][3][4][5][6][7][8][9][10][11][12]. THz-TDS technology is based on the analysis of the absorption or reflection spectra of THz radiation, transmitted through or reflected from the medium.…”

Properties of two-component nematic liquid crystal mixtures in the range of 0.3–3.0 THz

“…[13,14] The setup contains a mode-locked titanium:sapphire laser with a centre wavelength of 800 nm and a pulse duration of sub-100 fs at a repetition rate of 80 MHz and has a maximum average output power of 300 mW. The optical pulse from the laser is divided into two beams.…”

Advances in explosives analysis—part II: photon and neutron methods

“…), surface roughness of the samples, and water vapor or humidity present in the air [30,37]. Many researchers have addressed these challenges through developing methods to separate spectral fingerprints of explosives from barrier materials [30,31,[38][39][40], and signal-processing algorithms to compensate for water-vapor absorption [32].…”