Noninvasive inspection of C-4 explosive in mails by terahertz time-domain spectroscopy

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.