Cover Story
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.
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 the
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SCIENCE REPORTER, April 2010
process, 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.
Cover Story
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.
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
Airport
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
metal detectors rely on pulse induction (Photo courtesy L-3 Communications)
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
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
SCIENCE REPORTER, April 2010
9
Cover Story
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.
A baggage scanner
Sniffer dogs to detect explosive at the airport
10
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 nonmetallic 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
Cover Story
A passenger at a backscanner
X-ray portal (left).
Adapted from
Scientific American
Image as displayed at the control
room (below)
Rotating
collimator
X-ray
Watch
Detectors
X-ray source
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.
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
Backscatter
X-ray
Handgun
Signal to
operator
station
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 Xrays, 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
Explosives
Ceramic knife
The radiation
penetrates the clothing
and reflects off the
person’s body and any
concealed objects.
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 bombdetonating wires due to their reduced
backscattering ability as compared to
the rest of the body surface.
SCIENCE REPORTER, April 2010
11
Cover Story
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.
Adapted from Scientific American
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”
Signal to
operator
station
Enterance ramp
12
SCIENCE REPORTER, April 2010
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 Xrays and the resulting health effects. Xrays 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
Transmitter/receiver
antenna
A person standing in a
millimeter wave portal for
scan (right).
Wave front
The image is displayed at
the control room (below)
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 360degree 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.
Cover Story
Millimeter waves are nonionizing 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
Human trafficking in a cargo container
Report for Luggage Check
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 highenergy 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
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.
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.
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 Xray 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
SCIENCE REPORTER, April 2010
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