Three types of security scanners have currently been developed
for airport security use. These are X-ray units using
backscattered X-rays, X-ray units using transmission X-rays and
units (see table in Background). Each of these is described in
more detail below. The information on the operating parameters
and safety systems of the scanners has been obtained from a
number of sources including the equipment suppliers and the UK
Health Protection Agency reports written under contract to
manufacturers, suppliers and potential users.
Backscatter radiation is
the radiation that is reflected (scattered) from a material back
towards the X-ray radiation source.
X-ray security units using backscatter
radiation operate by
exposing the subject to low energy X-radiation. This low energy
radiation passes through clothing but is readily scattered by
dense objects. Some of the radiation is scattered back into a
series of radiation detectors, and creates an image of the
subject’s body, showing any items concealed under the
Backscatter X-ray systems use a narrow, pencil shaped beam
that scans the subject at high speed in a horizontal and
vertical direction. Large detectors are installed on the same
side of the subject as the X-ray source. The person stands in
front of the enclosure and is scanned by the X-ray beam, which
has a typical cross-sectional area of approx. 25 mm2.
Usually the person is scanned twice, once from the front and
then from the back. Sometimes lateral scans are also performed.
Typical systems use an X-ray set operating at fixed peak voltage
(kVp) and current (mA) settings. These are typically 50 kV and 5
mA. The total filtration to reduce the low energy component in
the X-ray beam, which is ineffective in the detection mechanism,
is in the range of 1 mm to 7 mm aluminium equivalent. The
duration of a single scan can be up to 8 seconds.
Transmission radiation is
the radiation that passes directly through the person being
examined. This radiation can be measured by a detection system
placed on the side of the person opposite to the X-ray
Transmission X-ray security units use significantly higher
X-ray energies than backscatter units to create a radiographic
image of the subject. The image is similar to those used for
medical purposes and shows the skeletal structure of the
subject, on which can be seen any contraband items, with
sufficient X-ray absorption contrast, which the subject has
swallowed as well as any weapons hidden on the body beneath the
Transmission X-ray systems generally use a vertical fan-shaped
beam of X-rays and a linear array of detectors. The person
stands between the X-ray tube and the detector array and is
scanned by an X-ray beam having a typical width of approx. 2 mm.
The limiting quantity for the spatial resolution is the size of
the detector elements. Typical systems use fixed settings: X-ray
peak voltage in the range 140-220 kVp and current in the range
0.1 to 4 mA. Filtration is deliberately incorporated in the
X-ray beam to reduce the quantity of low energy X-rays that do
not have sufficient energy to contribute to the imaging process,
but do add to the radiation
dose received by the person.
The total filtration in the X-ray beam is generally in the
range of 4 mm to 8 mm aluminium equivalent. This value includes
the inherent filtration that is a consequence of the X-ray tube
construction as well as the added filtration.
Some units have the capability to operate in either a “low
dose” mode (160 kVp, 0.1 mA) or “medium dose” mode (160 kVp, 0.3
mA). The mode used depends on the dimensions of the subject and
the nature of the items being searched for. The duration of the
exposure is in the range 5 to 15 seconds, depending on the model
Figure 2 shows a modern transmission scanner unit with the
side panel removed to reveal the X-ray set. The X-ray beam
originates from an X-ray tube mounted on one side of a conveyor
unit that the person undergoing examination stands on. The
conveyor system moves the person past the X-ray tube. X-rays are
initiated at the start of the scan sequence. Sensors terminate
the exposure once a person has passed through the unit. If no
person is present or the sensor fails, X-rays are terminated
after a maximum of 12 seconds (the time the belt takes to move
from one end to the other).
X-ray systems that use both backscatter
transmitted radiation in a single scan procedure are also
3.3.3 Non-ionising backscatter
A range of scanners using non-ionising
radiation are currently
being developed and assessed for security screening purposes.
There are two types of this technology. Active scanners emit
radio waves to produce an image. Passive scanners detect natural
radiation emanating from the person.
The main scanners in the active scanner category are
millimetre wave scanners, which emit radio frequencies within
the 24–30 GHz frequency range. The radio waves are transmitted
from two antennae simultaneously as they rotate around the body.
The wave energy reflected back from the body, or other objects
on the body, is used to construct a three-dimensional image,
which is displayed on a remote monitor for analysis. During a
scan, the individual is exposed to an
for a time not exceeding 2 s. The published surface power
densities measured during a scan are low and vary between 60
μW/m2 and 640 μW/m2.
The established health effects associated with non-ionising
limited to thermal effects, although uncertainty remains
concerning long-term effects of extremely low frequency (ELF)
and radio frequency (RF) fields. Millimetre wave body scanners
operate at outputs well below those required to produce tissue
Passive systems detect the very low levels of non-ionising
radiation that are
naturally emitted from the human body or objects concealed on
the body. These systems produce no radiation, either ionising or
non-ionising and hence present no radiation hazard.
Non-ionising security scanners are not considered further in
3.4 Safety systems
X-ray security units are designed and supplied with
comprehensive and modern safety systems. The type of systems
that are installed on a particular model will depend on whether
the scanner is a backscatter unit or a transmission unit, but
will include most of the following:
- Password control. The X-ray set can only be operated from
the control console and the controls are password protected.
- Warning lights. The units have clear warning lights that
indicate the condition of the X-ray set. These lights normally
consist of a green light that is illuminated when the power is
switched on but no X-rays are being generated, and a red light
that is illuminated when X-rays are being generated.
- Emergency stop buttons. Buttons positioned close to the
operator’s position can be pressed to immediately terminate the
generation of X-rays.
- Access panel interlocks. Panels that can be removed to
provide access to the X-ray set are interlocked to ensure that
X-ray generation is terminated and cannot be initiated when a
panel is removed.
- Operational interlocks. These will terminate the
generation of X-rays in the event of a range of fault modes,
including operational software malfunction, failure of a warning
light or failure of the conveyor mechanism in the case of
- Local shielding. Lead shielding is incorporated into the
scanners to ensure that radiation dose rates at accessible
locations outside the scanning area are very low.
These examples are not exhaustive and additional safety
systems may be fitted, depending on the type of scanner.
Consideration of the required safety systems is an optimisation
issue, and will be part of the dialogue between the supplier and
The American National Standard ‘Radiation Safety for Personnel
Security Screening Systems Using X-ray or Gamma Radiation’,
ANSI/HPS N43.17-2009, specifies the operational interlocks that
must be fitted to each type of scanner, and also requires that
the generation of X-rays is automatically terminated in the
event of any malfunction or fault mode. This standard is not
formally endorsed in Europe, but as similar equipment is likely
to be used as in the US, compliance with the standard is assumed
(at least until a European standard is introduced). Compliance
is, however, the responsibility of the manufacturers, unless
required by the airport or travel safety authority upon
purchase. Insufficient data are available to estimate the
probability of any malfunction occurring, but the required
interlock systems will ensure that, in the event of a
malfunction, radiation doses to the person being scanned, the
operators and any other persons in the vicinity will remain