Subject: [NEXTEL-1] Terahertz rays track down drugs

From: Rick Parrish <[email-masked]>
Date: Tue, 28 Oct 2003 12:31:29 -0600

Terahertz rays track down drugs
28 October 2003

Drug smuggling may become much harder in the future thanks to a 
terahertz imaging system being developed in Japan.

Kodo Kawase and his team from RIKEN (The Institute of Physical and 
Chemical Research) have demonstrated a system that detects the presence 
of illicit drugs that are concealed within an envelope. (Optics Express 
11 2549)

Drug detection

Tests to date have shown that the imaging system can successfully detect 
and identify a range of substances including ecstasy (MDMA) and 
methamphetamine. The researchers are now working with companies to 
develop a mail screening system that could suit use in post offices and 
airports.

The scheme works by spectral fingerprinting -- illuminating a target 
envelope with tunable terahertz radiation and analyzing the absorption 
spectra of the resulting image. The results are cross-referenced with a 
database of spectra to check for the chemicals of interest.

“The ability to check the contents of a suspect envelope without 
violating the correspondence rights has been long sought after,” said 
Kawase. “Since ink is generally transparent to terahertz waves the 
privacy of the correspondence is not violated while the identification 
of concealed drugs is possible.”

At the heart of the Japanese system is a compact and tunable optical 
parametric oscillator (OPO) that emits terahertz waves. The OPO is made 
from a nonlinear crystal (MgO:LiNbO3) that is pumped by a Q-switched 
Nd:YAG laser. It emits terahertz radiation that is tunable between 1 and 
2.5 THz.

Terahertz waves of several different frequencies are scanned over the 
envelope and the transmitted radiation is picked up by a pyroelectric 
bolometer and analyzed by a computer.

In tests, the system correctly identified the contents of three small 
(10x10mm) polyethylene bags that were placed inside an airmail envelope 
and contained 20 mg of ecstasy, methamphetamine and aspirin. The 
detection limit of the system is estimated at around 3 mg/cm2.

Perhaps the biggest drawback of the current system is its measurement 
time of 10 minutes. Kawase now plans to speed this up.

“With the present technology, one minute per image is achievable, 
however we will continue our efforts to obtain higher speeds and we can 
see several ways to try this,” he told Optics.org. “We are now working 
on the development of a parallel (CCD) imaging system that can be used 
much like a regular camera and can output images at a much higher rate 
than the present scanning system.”

RIKEN is not the only research institute to have investigated the use of 
terahertz spectroscopy for analysing suspect powders in envelopes. Last 
year, a group lead by Xi Cheng Zhang at Rensselaer Polytechnic, US, 
demonstrated a system that could detect bacterial spores.

Author
Oliver Graydon

http://optics.org/articles/news/9/10/22/1

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