How Things Work: Bomb detectors

Considering the fact that terrorist attacks are becoming frequent in the world today, bomb detection becomes an area of extreme importance and scientific research.
One technique, which may look obsolete to technophiles of the 21st century, is using specially trained dogs. Although it can be very effective, using dogs is not a viable option as the dogs get bored and tired easily. Also, since training the dogs is very expensive, this approach is not very practical.

A large amount of technological progress has been made in the field of explosives detection. With the high risks involved with explosions in the sky, air travel is the biggest test bed for upcoming technological advancements in this field.

Early bomb detection systems were based on conventional X-ray machines, which let the operator see the insides of a bag by showing different objects in different colors based on their densities. This basic method, however, was not very reliable and had to be augmented with advanced digital signal processing algorithms to help the operator make more reliable decisions.

The newer generation of X-ray machines takes a page from the medical imaging industry’s book. These machines create 3-D images by scanning the bag and capturing 24 helical slices across 360 degrees. In other words, the scanner travels both across the scanned item while being revolved around it. With pictures from all these angles, it is possible to create a 3-D model of the item, which is a lot more useful than a regular 2-D image.

One of the rather advanced 3-D X-Ray machines, manufactured by Analogic Systems, scans the bag 720 times over the course of the six seconds that the bag is in the scanner. With its powerful motors that rotate the scanner at 90 rounds per minute, and computers that have 15 gigaflops of digital signal processing power, this system is capable of scanning over 500 bags per hour.
Relying solely on methods adapted from medical imaging technologies nevertheless has its downsides. For example, even with the vast amount of data being accumulated, these systems are not foolproof; something as small as a jar of peanut butter might result in a false positive.
Inspecting a live person for bombs poses even bigger challenges since a lot of the invasive technologies cannot be used on living humans.

Currently, the most common method is to walk people through metal detectors and pat them down if a metal object is detected.

However, as demonstrated by a terrorist attack that involved using liquid explosives and was thwarted by security forces in Britain in August 2006, metal detectors are not very useful against non-metal explosives.

In millimeter-wave technology, or quadruple resonance (QR), travelers are irradiated with high-frequency and low-intensity radio waves that excite the nuclei of materials to higher energy levels.
The amount of energy released as the nuclei of the human body and the denser materials settle and return back to their original states can be used to determine the presence of explosives. Such systems have been installed in a few airports in Europe, but currently none are installed in the United States.

Another interesting and relatively new method actually uses what are called “electronic noses.” In this technique, the machine first blows air over a person who might have chemical particles on him or her. Then, this air is sucked in by the machine and charged with electricity which ionizes the particles in the air.

A scanner then measures the time for the ionized particles to travel across a distance and uses that time to determine the nature of the particles. Several of these “sniffers” are installed in United States airports, including Pittsburgh International Airport.
This technique and many others have serious limitations, especially when used to detect threats like suicide bombers.

Suicide bombers pose a serious challenge to security forces.
Since suicide bombing can result in a large number of casualties, especially in a crowded area, it is crucial to detect such bombs very fast.

In a 2005 study on suicide bombing conducted by researchers at Yale University and Naval Postgraduate School in Monterey, Calif., it was estimated via mathematical models that it would take 150 QR scanners to provide a 10-second warning in a 500-meter-square area.
Hence, using currently available methods, detection of suicide bombers is commercially infeasible and virtually impossible.

Only further research in this field can provide a firm solution to this problem.

Checking cars for explosives poses unique and difficult challenges given the complexity and size of the cars, and the materials that go into making an automobile. One new technology basically shoots the car with neutrons. This causes the items in the car to emit gamma rays, which can be translated into chemical signatures of the materials in the car and helps the operator to determine the presence of explosives.

This method however has its drawbacks; the time to scan can range from seconds to minutes and it works reliably only with a substantial amount of explosive.
The usage of this method, consequently, is being considered at customs checkpoints and similar locations where waiting times of a few minutes are acceptable.

All these detection techniques have their own drawbacks and it is clear that only a combination of all these methods, backed by sound intelligence gathering, can alleviate the danger of explosive devices.