Noninvasive mail inspection using terahertz radiation

Detecting hazardous materials and illicit drugs inside posted mail is necessary because of security concerns and to deter drug trafficking. In Japan, confidentiality of private mail is guaranteed by law, and only noninvasive inspection methods are permitted. Detection (sniffer) dogs and x-ray imaging have been used, but x-rays cannot identify suspect materials and dogs are only useful if drug vapors leak from a package. Systems using terahertz (THz) radiation have recently been demonstrated as quick and reliable mail-inspection devices.1–4 Like radio waves, THz radiation is not significantly scattered by soft materials such as paper, wood, and plastics, and creates clear images of hidden objects. In addition, many materials exhibit unique THz-absorption spectra—fingerprint spectra— which can be used to identify the contents of suspicious packages. A prototype apparatus has been built to inspect all mail handled in Japanese international post offices (around 100,000 items per day). However, the THz spectrometer takes too long to examine every package. Therefore, to achieve complete inspection, the process has been divided into two stages. The first involves rapid screening using x-rays and THz waves, and the second identifies the suspicious substances selected in the first stage. The initial screening stage uses x-rays to exclude envelopes containing only paper. Images revealing shadows are then scanned and measured at 0.54THz. A diagram of the THz system is shown in Figure 1. According to Mie scattering theory,5 which describes electromagnetic-radiation scattering by spherical particles, THz waves are intensely scattered when the particle size is comparable to the wavelength. Our experiment confirms that powders with particle sizes greater than 100μm result in a significantly stronger scattering signal than empty envelopes. Therefore, the rapid-screening system flags envelopes showing strong THzwave scattering as suspicious mail. Figure 1. (top) THz rapid-screening system. A Schottky diode is characterized by a very low forward-voltage drop. (bottom) THz-scattering signal intensity of sucrose powder of different particle sizes. The Miescattering extinction curves are for nonabsorbing (solid line) and partially absorbing spheres (dashed line). The data point labeled ‘envelope’ illustrates the extinction for a paper-only envelope.