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Spectrometers — versatile tools for weapons detection

n. The science that deals with … spectrum analysis.

n. An array of entities…ordered in accord with the magnitude of a common physical property.

There are many varieties of spectra, the most familiar is the spectrum of light, as produced by a rainbow. Spectrometers are the tools used to measure spectra to provide important insights into the properties of matter.

Pacific Northwest National Laboratory is using four kinds of spectroscopies to develop sensors that can be used to detect weapons of mass destruction: mass spectroscopy, Raman spectroscopy, neutron spectro-scopy and optical spectroscopy. In mass spectrometry, the ordered array of entities is the masses or molecular weights of proteins. Molecular weights are important in identifying the protein. In optical spectrometry, which includes the ultraviolet and infrared part of the spectrum, absorption or emission of light provides information about the structure of chemical species or composition of a mixture of chemicals. In Raman spectrometry, a subset of optical spectroscopy, the change in the light's wavelength caused by its interaction with matter provides detailed information about the mole-cular structure of that matter that cannot be obtained from emission or absorption. In neutron spectrometry, the kinetic energy distribution of neutrons provides important information about their source or interaction history.

Building a file on potential pathogens

Forget fingerprinting bioterrorists. Fingerprint their weapons.

Researchers at Pacific Northwest National Laboratory are using mass spectrometry to build an automated library of information that can be used as a rapid means of identifying and analyzing bacteria, a potential weapon of mass destruction.

MALDI-MS, Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry, identifies bacteria by measuring the mass of a cross section of their cellular proteins. Using a series of multiple laser shots, MALDI-MS simultaneously volatizes, separates and detects the proteins in a matter of microseconds. MALDI-MS relies on special statistical algorithms developed at the Laboratory to recognize the unique features of bacteria. The statistics-based software constructs and compares MALDI fingerprints automatically, in addition to providing a confidence level. "The automated process prevents bias in interpreting the MALDI spectra," said Karen Wahl, senior research scientist on the project. "So a non-technician may operate the device and get the same results as an expert."

Pacific Northwest scientists are working with DARPA to develop a field portable MALDI-MS system that would be less expensive than lab-based models.

Fingerprinting chemicals

While MALDI-MS fingerprints bacteria, Raman spectrometry fingerprints chemicals. When configured a particular way, a Raman spectrometer can detect dangerous chemicals in translucent containers, including glass as dark as a beer bottle and plastic as opaque as a bleach bottle.

Pacific Northwest scientists are proving that Raman spectrometers work for materials found in the real world, whether they be in dark containers or on the ground, as well as developing a library of chemical fingerprints to match with Raman spectra.

In field tests, the spectrometer correctly identified both liquid and solid samples in a variety of containers and solid samples in the environment with close to 100 percent accuracy. Because the database includes spectra from many of the most hazardous chemicals, lack of a spectral match eliminates many dangerous chemicals from suspicion. "In many cases, knowing what a compound isn't may be more important than making a reasonably correct identification," said Bob Wright, a senior scientist involved with the spectrometer.

Laboratory scientists are currently developing improved algorithms to help identify chemicals in mixtures. "You're more likely to find chemicals mixed with other things in the real world," Wright said. "Users might need to identify chemicals that are mixed with a carrier material or even disguised with other materials."

Detecting neutrons helps identify plutonium

Fast, portable and accurate, the neutron spectrometer can detect plutonium in sealed containers with the same ease it detects fat in hamburger.

Pacific Northwest's neutron spectrometer identifies radioactive substances based on the energy of the neutrons. In line with growing interest in global nonproliferation, this device may allow dismantled weapons to be inspected with-out opening sealed containers, thus preventing the spread of sensitive information.

The neutron spectrometer uses the Laboratory's patented glass fiber technology to create a sensor that detects both neutrons and gamma rays. "This sensor is unique because it is the only inspection tool based primarily on neutron measurement that is portable and fast," said Mary Bliss, chief scientist for the project.

The detector consists of six layers of glass fibers sandwiched between sheets of plastic. Fast moving neutrons pass through the plastic, which slows them down and allows them to interact with the Laboratory's scintillating optical fibers to produce light. This light travels to the end of the fiber, where photomultiplier tubes convert it to an electrical signal. The layer where the neutrons are detected provides information about the neutrons' energy, which is analyzed to identify their source. "With this kind of information, we can identify radioactive material or the 'flavor' of weapon that emitted the neutron in five minutes or less," Bliss said.

Experts prefer using neutron detection over gamma ray detection to confirm plutonium, an important ingredient in nuclear weapons, because neutron detection provides more detailed information about the plutonium, and its accuracy is less affected by surrounding materials. This device also is one of the few solid-state detectors in a world of pressurized gas detectors. "Solid-state detectors are more rugged and transportable for field operations than their counterparts. High-efficiency gas tubes contain too much pressure to be flown on commercial airliners," Bliss said.

Beyond detecting weapons, the neutron spectrometer also could be used commercially to measure fat in hamburger and determine the steam or water content in pipes. The spectrometer has a patent pending and Bliss expects the device to be commercialized sometime in 2003 for use by arms control experts.

Using the infrared spectrum to detect chemicals

Scientists in Pacific Northwest's remote sensing and electro-optics group are developing sensors that use infrared light to find chemical traces of explosives, weapons, narcotics and other contraband associated with terrorist activities.

The way a chemical of interest radiates or absorbs infrared light as a function of the light's wavelength (color), is called the infrared spectrum. The chemical's infrared spectrum forms a unique optical "fingerprint" that can be used to identify contraband, illicit production processes, or warn occupants of public buildings or military units of chemical attack with high reliability. Researchers are particularly interested in mid-wavelength infrared (MWIR) light because optical fingerprints are more likely to be unique in that region of the spectrum.

Pacific Northwest and Bell Laboratories also are jointly developing a new type of MWIR laser, called a "quantum cascade laser." DARPA is funding this project because the laser is essential to the practicality of the new sensing techniques and has other military applications.

In related work, Pacific Northwest scientists are measuring the spectra of about 550 vapor-phase chemicals in collaboration with the researchers developing the sensors. These measurements are needed because the infrared spectra of most chemicals have never been measured with the resolution or accuracy needed to identify chemicals with high confidence.

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