Things That Go Boom in the Night

With the exception of far-less-common biological or radiological threats, explosive devices are the nightmare scenario for people charged with protecting people and places. But advanced techniques can help detect these threats.

Tim Dees・Officer (Ret.)

Read Tim's Posts

June 30, 2010

With the exception of far-less-common biological or radiological threats, explosive devices are the nightmare scenario for people charged with protecting people and places. In most cases, no one knows of the device until detonation, and it may take out first responders as well as private citizens.

This is because devices reside inside commonplace objects like books or backpacks that people see and don't notice. They may trigger via a timer, a switch activated by pressure, proximity or movement, or by remote command. Reliable detection of the devices is difficult because of their small size relative to their lethality. Science is getting better at this, but the more traditional methods also have their place.

Know Your Explosives

The earliest explosive was black powder or gunpowder, made of sulfur, charcoal, and potassium nitrate (also called saltpeter). Anyone reading this article is probably familiar with the characteristics of gunpowder. It's fairly easy to manufacture, but it's a low explosive, capable of propelling a bullet out of a gun barrel, but not destroying the gun itself (unless one overpacks the cartridge, which is more of a function of stupid than physics).

Nitroglycerin came along a bit later. It has a greater energy density than black powder, but its sensitivity to shock makes it difficult to work with, and it is rarely used today for anything except as a vasodilator medication.

Alfred Nobel invented dynamite by combining nitroglycerine with sodium carbonate and diatomaceous earth. The latter ingredient is available at any garden nursery, made of ancient decomposed sea shells. Dynamite was the first commercially viable explosive, and made Nobel wealthy enough to endow an annual prize for advancements in science and in peacemaking.

Dynamite is usually formed into sticks and wrapped in paper. Over time, the nitroglycerine in the dynamite will "weep" or "sweat," forming unstable crystals on the wrappers. Proper storage of dynamite dictates the packages be turned periodically to prevent this.

Some people think trinitrotoluene or TNT and dynamite are the same thing, but they aren't. TNT starts with toluene, a common solvent used in model airplane glue, among other things. The explosive is produced by repeatedly nitrating it with nitric and sulfuric acids. TNT is more stable than dynamite, and can be poured (it melts at 176 degrees Fahrenheit) into molds or combined with other explosives. Maybe that's why it's the most common explosive in conventional military munitions such as artillery and mortar shells.

TNT is the standard for measuring the destructive power of other compounds and weapons. The "Little Boy" atomic bomb dropped on Hiroshima in World War II was equivalent to about 15,000 tons (15 kilotons) of TNT. Modern nuclear weapons are measured in megatons, or million tons of TNT. To provide perspective, a kiloton of solid TNT would be a cube measuring about 10 yards on each side. Interesting bit of trivia: TNT is yellow, and skin that's in repeated contact with it turns the same color.

Terrorist bomb makers prefer plastic explosives. These are compounds containing one or more of the explosives already described, plus pentaerythritol tetranitrate (PETN) or RDX, plus wax or some other plasticizer. The resulting compound is a moldable, castable substance that can take almost any physical shape. PETN is a major ingredient of Semtex, while RDX is used in Torpex and C-4.

Umar Abdulmytallab stashed PETN in his underwear in December 2009 in his unsuccessful attempt to destroy Northwest Airlines Flight 253 between Amsterdam and Detroit. RDX was the primary explosive in the 2006 railroad bombings in Mumbai, India, and in the bombing of the Moscow Metro earlier this year.

Most, if not all, explosives developed since black powder have nitrate compounds as a base component. It's these nitrate molecules and ions that high-tech bomb "sniffers" look for. Plastic explosives tend to be denser than other common materials carried by travelers, and the explosives are sometimes surrounded by wires, batteries, or electronics that complete the bomb. These are detectable with X-rays and other visual scanning methods.

When these methods fail, or when the gadget locates a suspicious lump that might be a bomb, somebody has to take a look with his or her own eyes or a remotely operated device. Of course, you can always blow the thing up wherever it happens to be, but the people who own the venues usually prefer you not do that.[PAGEBREAK]

Computerized Tomography

One method used to identify explosives is a CT scan. Most people are now familiar with the term, even if the only place they've heard it was on a medical TV drama. The "CT" stands for computerized tomography, which is a method for generating a three-dimensional image from multiple 2-D X-ray images captured from multiple angles.

Modern CT scanners use multiple emitters to fire X-rays through an object to a sensor on the opposite side. The sensor records the gross structures it "sees" as well as the amount of energy received, giving an indication of the density of the material it passed through. The computer combines all of these images into one virtual object that appears on a computer display. The virtual object can be rotated on any axis and zoomed in or out to inspect details of interest.

Contraband objects with telltale shapes, like guns, are fairly easy to spot. The latest equipment stores data on densities of objects, both common and deadly, and uses X-ray diffraction to identify and display those in a telltale color. An operator at an airport or other high-threat location can pay closer attention to a container colored in red and determine whether it's plastic explosive or cold cream.

There are several limitations to CT as an explosives detector. The software that flags an explosive may give a false positive alert on something innocuous, like a child's Play-Doh. CT scanners are large-typically, the size of a small truck-and expensive. They're a chokepoint for any venue that has to process a lot of people or items in a short time.

The weakest link is the operator. Even when operators are rotated every 20 minutes or more frequently, fatigue causes them to miss objects that a fresher eye might see immediately. Watching a video display of wireframe images of everyday objects gets boring very quickly, and even the most conscientious employee's mind wanders. When the one-in-a-thousand Bad Thing goes by, it's easy to not see it.

Sniffers

Not all explosives detectors rely on the sense of sight. Some called "sniffers" don't exactly rely on the sense of smell. Labeling an Ion Mobility Spectrometry (IMS) instrument as a "sniffer" might be blasphemy to an analytical chemist, but it's a lot easier to say.

Ions are atoms or molecules containing an unequal number of protons and electrons, so they have a positive or negative charge. In an IMS instrument typically used for explosives detection, a sample is heated to produce a vapor, which is then ionized by a radiation source such as 63Ni or 241Am (there's something similar in the smoke detectors in your house).

The ions move through a drift chamber and an electric field that causes them to separate by size, mass, and geometry. As the ions arrive at a detector at the far end of the drift chamber, those size, mass, and shape characteristics form a signature unique to the compound from whence they came. If the signature matches one the device is programmed to detect, like PETN, it sounds an alarm.

Smiths Detection is one of the largest manufacturers of IMS instruments for explosives detection, and you've probably seen their machines at airport security checkpoints. Typically, an operator uses a glorified coffee filter to swab luggage or some other suspicious item, and places it in a recess for analysis. More often than not, a green light comes on, and you're on your way. The magic described above happens that fast. IMS detectors don't have the operator fatigue problems of CT scanners, but they're still expensive and have limited throughput.

SpectraFluidics has developed a new detection technology that is a combination of Free-Surface microFluidics (FSF) and Surface-Enhanced Raman Spectroscopy (SERS). A key component is a consumable "microfluidic chip" exposed to ambient air around the sample. The fluid, the channels that contain it, and nanoparticles suspended in it are specific for the substances to be detected.

A Raman laser, which works by measuring activity on the surface exposed or excited by the laser, produces a profile that is compared to that of the target substance. The effect is a detector that can be either fixed or handheld and costs a fraction of an IMS instrument. The microfluidic chips are consumed in the analysis process. The company plans to ship its first operational units this year.[PAGEBREAK]

Bunk Technology

Many excellent detection devices exist. But potential purchasers of new technology are wise to beware of something that seems too good to be true. This is especially true of explosives and narcotics detectors. In 1993, the Quadro Corporation of Hadleyville, S.C., sold the Quadro Tracker Positive Molecular Locator to around 1,000 police departments and school districts in the United States at prices ranging from $400 to $8,000.

Users placed a sample of the substance to be detected into a "locator card," which was then inserted into a "card reader" attached to the device, which had a swiveling antenna. By moving the antenna, the operator was led to the target substance. The more advanced, expensive models were said to be able to find almost anything-weapons, narcotics (ingested and otherwise), golf balls, missing children, jewelry-using only a Polaroid photo of the substance or item to be found.

However, when the FBI examined the device, it found the device was essentially hollow, the components were not electrically connected to one another, and the antenna came from a transistor radio. The whole mechanism was essentially a high-priced dowsing rod.

Operating on the "if it worked once..." strategy, several other vendors have produced similar worthless devices for explosive detection, called variously Alpha 6, GT200, Sniffex, and ADE-651. The latter device was sold to the Iraqi government as a bomb detector as recently as last year. After some disappointing and literally shattering experiences, in January 2010 the British government banned the device for export and arrested the company director for fraud.

Inexpensive high-technology explosive detection solutions are still elusive. Unless you have a large budget, your best measures for now may be specially trained dogs, conventional hazardous device procedures, and if all else fails, blow the thing up.

FOR MORE INFORMATION:

EXPLOSIVES DETECTION

Allen-Vanguard Inc.

(contraband detection Equipment, metal detectors, inspection cameras)

Visit Allen-Vanguard Online

American Science & Engineering

(x-ray bomb Detection equipment)

Visit AS&E Online

Criminalistics, Inc.

(bomb handling equipment)

Visit Criminalistics Online

Field Forensics Inc.