Noninvasive Inspection of C-4 Explosive in Mails by Terahertz Time-Domain Spectroscopy

TLDR

The explosive C‑4, used in almost all mail‑bomb terrorist activities, exhibits six THz bands between 5 and 90 cm⁻¹ (150 GHz–2.7 THz). The study measured the refractive index, extinction coefficient, and absorption coefficient of C‑4 by THz time‑domain spectroscopy to evaluate its suitability for detecting C‑4 in mail. The authors employed THz time‑domain spectroscopy to obtain optical constants and calculated THz reflectance spectra to assess the feasibility of reflection‑based detection. The 26.9 cm⁻¹ band serves as a strong fingerprint, the spectrum of C‑4 hidden inside an envelope matches that of pure C‑4, and overall the results demonstrate that THz techniques are promising for counterterrorism against C‑4 mail bombs.

Abstract

We have measured the refractive index, extinction coefficient, and absorption coefficient of the explosive C-4 by THz time-domain spectroscopy and examined its applicability to detecting C-4 in mails. The explosive C-4, which is used in almost all mail-bomb terrorist activities, shows six THz bands in the frequency region from 5 to 90 cm-1 (150 GHz–2.7 THz). The 26.9 cm-1 band is very strong and can be used as a fingerprint of C-4. We confirmed that the THz absorption spectrum of C-4 hidden inside an envelope coincides essentially with that of C-4 itself. We also examined the applicability of the THz reflection spectroscopy to C-4 detection by calculating the reflectance spectra. Our results demonstrate that the THz techniques are very promising for counterterrorism against C-4 mail bombs.