SCIENCE REPORTER, April 2010
O
N 25 December 2009, a Nigerian passenger on board a Detroit-bound flight from Amsterdam was arrested on charges of attempting to blow up the plane. Sewn to his underwear was a pouch containing an explosive powder known as pentaerythriol tetranitrate. During the flight, he tried to ignite the explosive powder by adding a liquid chemical he had carried in a syringe. In theprocess, it made a popping sound and his trouser caught fire. Alert passengers immediately overpowered him and prevented what could have been a disaster for the 278 passengers.
Following the incident, the question that everyone asked was how the Nigerian managed to smuggle on board the explosive powder. Though he had passed through the routine security check at the Amsterdam airport, it had failed to detect the
explosives carried on his body.
This was not the first time that terrorists attempted to smuggle explosives on board. In 2001, a British passenger on board an American Airline plane was caught in the act of igniting an explosive hidden in his shoes. On 10 August 2006, authorities at the Heathrow Airport in Great Britain foiled an attempt by some passengers to smuggle liquid explosives in shampoo bottles in their cabin baggage.
M. S. S. MURTHY
Airports at Risk!
Securing airports against troublemakers is a tough job. Because of limitations of current screening methods, security personnel around the world, including those in India, are recommending the widespread use of what are called “full body scanners” that would see through the clothing and detect concealed metallic as well as non-metallic objects without touching the passenger.
Airports
at Risk!
SCIENCE REPORTER, April 2010
Lines of Defence
It is a big question whether foolproof security is at all possible at airports. However, airports round the world try to put together a layered security system.
The first line of defence is made up by fences, barriers and walls. To keep an eye on intruders there are regular security patrols, security checkpoints and all access gates are monitored by either a guard station or surveillance cameras. Further, to guard against the risk of someone driving a truck or car containing a bomb up to the airport terminal entrance and blowing up the airport itself, airports have large concrete barriers, designed to block vehicles up to the size of large moving
trucks. Passenger parking areas have also been moved far away from the terminal.
Currently, most of the airports around the world adopt two methods to screen passengers for concealed weapons and drugs. First, each
Caption: ...
Metal detectors can only detect metallic items like a gun concealed under one’s clothing and not the non-metallic items.
The second method is a pat-down search, in which the security personnel physically touch the passenger to detect any concealed objects.
It too has limitations.
Metal detectors can only detect metallic items like a gun concealed under one’s clothing and not the non-metallic items.
The second method is a pat-down search, in which the security personnel physically touch the passenger to detect any concealed objects.
It too has limitations.
passenger passes through a portal housing a metal detector. Almost all airport metal detectors are based on pulse induction (PI). Typical PI systems use a coil of wire on one side of the arch as the transmitter and receiver.
This technology sends powerful, short Airport metal detectors rely on pulse induction (Photo courtesy L-3 Communications)
SCIENCE REPORTER, April 2010
bursts (pulses) of current through the coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This subsequent current is called the reflected pulse and lasts only about 30 microseconds.
Another pulse is then sent and the process repeats.
If a metal object passes through the metal detector, the pulse creates an opposite magnetic field in the object.
When the pulse’s magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard
surfaces, you probably hear only a very brief echo, or you may not hear one at all. But if you yell into a room with a lot of hard surfaces, the echo lasts longer. In a PI metal detector, the magnetic fields from target objects add their “echo” to the reflected pulse, making it last a fraction longer than it would without them.
Metal detectors can only detect metallic items like a gun concealed under one’s clothing and not the non- metallic items.
The second method is a pat-down search, in which the security personnel physically touch the passenger to detect any concealed objects. It too has limitations. Many passengers consider a pat-down search as too invasive and hence it is less thorough.
In the case of the Nigerian passenger, it is reported that he had sewn the pouch containing the A baggage scanner
Sniffer dogs to detect explosive at the airport
Currently, most of the airports around the world adopt two methods to screen passengers for concealed weapons and drugs. First, each passenger passes through a portal housing a metal detector.
Currently, most of the
airports around the world
adopt two methods to screen
passengers for concealed
weapons and drugs. First,
each passenger passes
through a portal housing a
metal detector.
SCIENCE REPORTER, April 2010 explosive to his undergarment near his
scrotum. Because of these limitations security personnel around the world, including those in India, are recommending the widespread use of what are called “full body scanners”
that would see through the clothing and detect concealed metallic as well as non-metallic objects without touching the passenger.
Currently there are two types of full body scanners, which are in limited use in some major airports in the world. To prevent the kind of incidents that happened on December 25, these systems may become routine even in Indian airports in future.
What are these systems and how do they work? While one is based on the backscatter of X-rays, the other uses millimeter waves.
Backscatter Technology
We are all familiar with medical X-rays and X-ray scanners to inspect passengers’ baggage in airports. When X-rays fall on an object, they pass through the object, are absorbed and then scattered. Which of these processes predominate depends upon the energy of the radiation as well as the nature of the object—particularly with reference to its atomic number. Atomic number is the number of protons in the atoms of elements making up the object.
Higher the atomic number of the
material, greater is the absorption. In lower atomic number materials, it is more probable that the rays either pass through or scatter.
Metallic objects such as guns, bullets and knives are considered as high atomic number materials.
However, materials such as skin, soft tissue, plastics, shampoo, liquid explosives and drugs all constitute low atomic number elements like hydrogen, carbon, oxygen, nitrogen, etc. These materials are generally referred to as organics. Since conventional X-ray screening is based on the ability of the object to absorb X- rays, it is good for detecting objects of high atomic number materials such as guns, knives, etc. but performs poorly with organics.
Organics scatter X-rays better than they absorb them. The most important scattering process is known as the Compton scattering in which the incident photon is scattered by the electrons present in the material.
Different materials scatter X-rays differently depending on their electron density. Since the electron density is higher in low atomic number materials, they scatter X-rays more strongly than high atomic number materials. Hence, an X-ray scatter pattern is more specific to identifying organics such as liquid explosives.
Backscatter X-ray technology utilizes
the photons scattered in the backward direction to produce an image of the object.
In actual practice, the passenger will stand on set spots in front of a closet-sized X-ray imaging unit. A rotating collimator projects a narrow beam of low intensity X-rays to scan the subject at high speed from side to side and top to bottom. The rays pass through the clothing and bounce back from the person’s skin and any thing on it. Specially designed detectors situated on the same side of the subject as the X-ray source grab these backscattered photons. A computer program analyses the intensity of the backscattered photons as a function of the position of the incident beam to construct an image.
Then the person turns around for a second screening from the back. The final display will be the image of the naked body of the passenger along with any object the person may be concealing. Organics such as a bottle of liquid explosive or a pouch of drug concealed under the clothing also show up clearly. Thus, non-metallic objects missed by the metal detector are readily picked up here. At the same time, the system recognizes the presence of any metallic objects like a gun or bomb- detonating wires due to their reduced backscattering ability as compared to the rest of the body surface.
X-ray
Backscatter X-ray
Detectors
X-ray source
Signal to operator station
Rotating collimator
Watch
Handgun
Explosives
Ceramic knife
A passenger at a backscanner X-ray portal (left).
Image as displayed at the control room (below)
Adapted from Scientific American
Critics are of the view that no matter what technology is used, clever terrorists always find ways to get over it. It is a never-ending race. Nevertheless, it has to be run.
The radiation
penetrates the clothing and reflects off the person’s body and any concealed objects.
SCIENCE REPORTER, April 2010 The whole process takes only about 30 seconds and the passenger does not experience any inconvenience.
There is no physical contact and no need to remove cloths, shoes, etc. The image is displayed for inspection only in a room located at a remote place from the scanner. If no contraband is detected, the passenger is permitted to proceed further. In case of doubt, the passenger may be taken aside for further questioning.
During initial trials, the backscatter image produced was so sharp and explicit that people complained that it was too invasive and amounted to disrobing in public. In response to these objections, the manufacturers built in an algorithm to the machine software that could blur the fine details of the subject’s private parts and display only a “chalk-like”
outline of the passenger’s body and anything on it. After inspection and clearing the person, the image is automatically erased. To further maintain privacy, the arrangement is such that the officer who assists the passenger never sees the image and the officer who inspects the image never sees the passenger.
Another area of concern to the traveling public is the exposure to X- rays and the resulting health effects. X- rays are a form of ionizing radiation.
Exposure to ionizing radiation may increase the possibility of cancer and genetic effects. However, national and international organizations, which set guidelines to human exposure to ionizing radaiations, assure that there is no cause for concern.
In fact, ionizing radiations are ubiquitous in our environment to which
we are all exposed throughout life.
Compared with this (which is averaged about one millisievert per year; sievert is a unit of radiation dose), exposure from a backscatter scan is trivial (about 0.00004 to 0.00005 millisievert per scan).
It is further estimated that the radiation dose received by a passenger in one scan is equivalent to the dose received due to cosmic radiation while flying in an airplane for two minutes at an altitude of 30,000 feet. Hence, it is too trivial to cause any concern even for frequent fliers.
Millimeter Wave Scanners
An alternative to X-ray backscatter technology for full body screening is the millimeter wave technology.
Millimeter waves are radio frequency radiation in the range of 24 giga hertz (GHz) to 300 GHz (wavelength in the region of millimeters). They are electromagnetic waves like X-rays but of much lower energy. However, like X-rays they can penetrate through clothing and reflect back from the body. Since the reflected beam carries a signature of the surface, they can be used to image the body contour.
For a scan, the person stands with arms up, at a marked place in the imaging portal. Two vertical banks of transmitter/receivers pivot in tandem, each emitting a wave front that scans the subject for ten seconds. The radiation penetrates the clothing and reflects off the person’s body and any concealed objects. The reflected energy is collected by receivers, amplified and analyzed by a computer to create a 360- degree image of the body, showing both metallic and non-metallic objects, if any, hidden under the clothing. However, unlike backscatter X-ray image, the image has a much poorer resolution and looks like a fuzzy photonegative.
Further, the software blurs the facial features to protect privacy.
Airports are not the only place where backscatter X-ray technology finds its use. It has been in use for some time by customs authorities in UK for screening cargo containers.
Transmitter/receiver antenna Wave front
Signal to operator station
Enterance ramp
Adapted from Scientific American
A person standing in a millimeter wave portal for scan (right).
The image is displayed at the control room (below)
SCIENCE REPORTER, April 2010 Millimeter waves are non-
ionizing radiations similar to mobile phone radiation. Further, the amount of radiation received in a scan is estimated to be 10,000 times less than that during a cell phone transmission.
Hence, exposure to electromagnetic radiation during a scan is not a concern.
Full body scanners are quite expensive, each costing more than a million US dollars. In spite of the cost, security personnel think it is a worthwhile investment since “the system can detect objects being carried under the clothing that should not be there”. Full body scanners can detect only what is on the body, not what is hidden in body cavities. For example smuggling drugs stashed in body cavities may become more common.
Other Methods
Another version of the millimeter wave scanner is known as the passive millimeter wave technology. It does not irradiate the passenger by an external source, but utilizes the radiation naturally emitted by the body. All objects, including human body, whose temperature is above absolute zero, emit electromagnetic radiations of which millimeter waves are a part. The scanner collects these waves (77 GHz) by a dielectric lens and processes the signal to form an image of the object. Radiation emitted by the human body at body temperature is more intense than that emitted by objects carried on the body. The passive scanners can detect concealed objects by distinguishing between them even when hidden under clothing.
Scientists at the Department of Homeland Security, USA have gone one step further. They are developing a new type of gadget that can read a person’s mind to detect hostile intents, if any. Named “Malintent”, it uses a number of non-invasive sensors to
monitor non-verbal cues like elevated blood pressure, rapid heart beats, facial expressions, eye gaze, diameter of the pupil, and so on to evaluate if the person is planning to commit a crime.
However, it is not a polygraph test.
Subjects are not hooked on to instruments. The sensors do all the work without any physical contact. The system is in advanced stage of testing.
Another non-invasive technique is known as the “Puffer machine”, which blows clean air on passengers to dislodge the residues of any recently handled chemicals settled on the body and clothing. Air samplers collect these residues and analyze the contents for the presence of explosives, narcotics, etc. Since mass spectrometry is used for detection, the system can identify compounds on a molecular level and so is very sensitive. All this is done in a few seconds to allow security personnel to take further action.
Airports are not the only place where backscatter X-ray technology finds its use. It has been in use for some time by customs authorities in UK for screening cargo containers. “Z Backscatter Van”, an equipment manufactured by the American Science and Engineering performs a rapid drive-by screening of the cargo vehicles at the border crossing to check for drugs, explosives and even illicit human trafficking.
While newer technologies are making inroads into body scanning, baggage scanning cannot remain far behind. While conventional X-ray scanning can reveal only metallic objects in the baggage, the Heathrow incident highlighted the need for screening cabin baggage for liquid explosives and other low atomic number materials too. To improve detection in such cases a new technology known as dual-energy X- ray system is employed. By using a technology known as digital subtraction method, it can image separately the high and low atomic number materials and colour-code them. The low atomic number materials are generally shown in orange colour so that the operator can identify them easily.
Have all these multilayered security arrangements made air travel much safer today? Critics are of the view that no matter what technology is used, clever terrorists always find ways to get over it. It is a never-ending race. Nevertheless, it has to be run.
Dr M.S.S. Murthy retired as Head, Radiological Physics Division, Bhabha Atomic Research Centre (BARC), Mumbai. Address: B-104, Terrace Garden Apartments, 2nd Main Road, BSK IIIrd Stage, Bangalore- 560805
While passengers are stepping through the scans, their luggage items are going through the X-ray system. A conveyor belt carries each item past an X-ray machine. After the X-rays pass through the item, they are picked up by a detector. The detector passes the X-rays on to a filter, which blocks out the lower-energy X-rays. The remaining high-
energy X-rays hit a second detector. A computer circuit compares the signals from the two detectors to display the objects on a monitor.
Since different materials absorb X-rays at different levels,
the image on the monitor lets the machine operator see distinct items inside the bag.
Items are typically coloured on the display monitor based on the range of energy that passes through the object.
Most X-ray systems use shades of orange to represent the organic items. This is because most explosives are organic. Machine
Human trafficking in a cargo container
Report for Luggage Check
operators are trained not only to look for guns and knives but also to look for suspicious items that could even be a component of an improvised explosive device.
Electronic items such as laptop computers have so many different items packed into a relatively small area that it can be difficult to determine if a bomb is hidden within the device. That is the reason why you are asked to turn your laptop or mobile on.
Many airports also have a chemical sniffer, which is essentially an automated chemistry lab in a box. If there is a reason to suspect the electronic device, the security attendant quickly swipes a cloth over the device and places the cloth on the sniffer.
The sniffer analyses the cloth for any trace residue of the types of chemicals used to make bombs. If there is any residue, the sniffer warns of a potential bomb.
