3.3 Technology
Three types of security scanners have currently been developedfor airport security use. These are X-ray units usingbackscattered X-rays, X-ray units using transmission X-rays andnon-ionising radiationunits (see table in Background). Each of these is described inmore detail below. The information on the operating parametersand safety systems of the scanners has been obtained from anumber of sources including the equipment suppliers and the UKHealth Protection Agency reports written under contract tomanufacturers, suppliers and potential users.
3.3.1 Backscatter
Backscatter radiation isthe radiation that is reflected (scattered) from a material backtowards the X-ray radiation source.
X-ray security units using backscatterradiation operate byexposing the subject to low energy X-radiation. This low energyradiation passes through clothing but is readily scattered bydense objects. Some of the radiation is scattered back into aseries of radiation detectors, and creates an image of thesubject’s body, showing any items concealed under theclothing.
Backscatter X-ray systems use a narrow, pencil shaped beamthat scans the subject at high speed in a horizontal andvertical direction. Large detectors are installed on the sameside of the subject as the X-ray source. The person stands infront of the enclosure and is scanned by the X-ray beam, whichhas a typical cross-sectional area of approx. 25 mm2.Usually the person is scanned twice, once from the front andthen 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 5mA. The total filtration to reduce the low energy component inthe X-ray beam, which is ineffective in the detection mechanism,is in the range of 1 mm to 7 mm aluminium equivalent. Theduration of a single scan can be up to 8 seconds.
3.3.2 Transmission
Transmission radiation isthe radiation that passes directly through the person beingexamined. This radiation can be measured by a detection systemplaced on the side of the person opposite to the X-raysource.
Transmission X-ray security units use significantly higherX-ray energies than backscatter units to create a radiographicimage of the subject. The image is similar to those used formedical purposes and shows the skeletal structure of thesubject, on which can be seen any contraband items, withsufficient X-ray absorption contrast, which the subject hasswallowed as well as any weapons hidden on the body beneath theclothing.
Transmission X-ray systems generally use a vertical fan-shapedbeam of X-rays and a linear array of detectors. The personstands between the X-ray tube and the detector array and isscanned by an X-ray beam having a typical width of approx. 2 mm.The limiting quantity for the spatial resolution is the size ofthe detector elements. Typical systems use fixed settings: X-raypeak voltage in the range 140-220 kVp and current in the range0.1 to 4 mA. Filtration is deliberately incorporated in theX-ray beam to reduce the quantity of low energy X-rays that donot have sufficient energy to contribute to the imaging process,but do add to the radiationdose received by the person.
The total filtration in the X-ray beam is generally in therange of 4 mm to 8 mm aluminium equivalent. This value includesthe inherent filtration that is a consequence of the X-ray tubeconstruction as well as the added filtration.
Some units have the capability to operate in either a “lowdose” mode (160 kVp, 0.1 mA) or “medium dose” mode (160 kVp, 0.3mA). The mode used depends on the dimensions of the subject andthe nature of the items being searched for. The duration of theexposure is in the range 5 to 15 seconds, depending on the modelof unit.
Figure 2 shows a modern transmission scanner unit with theside panel removed to reveal the X-ray set. The X-ray beamoriginates from an X-ray tube mounted on one side of a conveyorunit that the person undergoing examination stands on. Theconveyor system moves the person past the X-ray tube. X-rays areinitiated at the start of the scan sequence. Sensors terminatethe exposure once a person has passed through the unit. If noperson is present or the sensor fails, X-rays are terminatedafter a maximum of 12 seconds (the time the belt takes to movefrom one end to the other).
X-ray systems that use both backscatterradiation andtransmitted radiation in a single scan procedure are alsocommercially available.
3.3.3 Non-ionising backscatterradiation
A range of scanners using non-ionisingradiation are currentlybeing developed and assessed for security screening purposes.There are two types of this technology. Active scanners emitradio waves to produce an image. Passive scanners detect naturalradiation emanating from the person.
The main scanners in the active scanner category aremillimetre wave scanners, which emit radio frequencies withinthe 24–30 GHz frequency range. The radio waves are transmittedfrom two antennae simultaneously as they rotate around the body.The wave energy reflected back from the body, or other objectson the body, is used to construct a three-dimensional image,which is displayed on a remote monitor for analysis. During ascan, the individual is exposed to anelectromagnetic fieldfor a time not exceeding 2 s. The published surface powerdensities measured during a scan are low and vary between 60μW/m2 and 640 μW/m2.
The established health effects associated with non-ionisingradiation arelimited to thermal effects, although uncertainty remainsconcerning long-term effects of extremely low frequency (ELF)and radio frequency (RF) fields. Millimetre wave body scannersoperate at outputs well below those required to produce tissueheating.
Passive systems detect the very low levels of non-ionisingradiation that arenaturally emitted from the human body or objects concealed onthe body. These systems produce no radiation, either ionising ornon-ionising and hence present no radiation hazard.
Non-ionising security scanners are not considered further inthis report.
3.4 Safety systems
X-ray security units are designed and supplied withcomprehensive and modern safety systems. The type of systemsthat are installed on a particular model will depend on whetherthe scanner is a backscatter unit or a transmission unit, butwill include most of the following:
- Password control. The X-ray set can only be operated fromthe control console and the controls are password protected.
- Warning lights. The units have clear warning lights thatindicate the condition of the X-ray set. These lights normallyconsist of a green light that is illuminated when the power isswitched on but no X-rays are being generated, and a red lightthat is illuminated when X-rays are being generated.
- Emergency stop buttons. Buttons positioned close to theoperator’s position can be pressed to immediately terminate thegeneration of X-rays.
- Access panel interlocks. Panels that can be removed toprovide access to the X-ray set are interlocked to ensure thatX-ray generation is terminated and cannot be initiated when apanel is removed.
- Operational interlocks. These will terminate thegeneration of X-rays in the event of a range of fault modes,including operational software malfunction, failure of a warninglight or failure of the conveyor mechanism in the case oftransmission scanners.
- Local shielding. Lead shielding is incorporated into thescanners to ensure that radiation dose rates at accessiblelocations outside the scanning area are very low.
These examples are not exhaustive and additional safetysystems may be fitted, depending on the type of scanner.Consideration of the required safety systems is an optimisationissue, and will be part of the dialogue between the supplier andthe regulator.
The American National Standard ‘Radiation Safety for PersonnelSecurity Screening Systems Using X-ray or Gamma Radiation’,ANSI/HPS N43.17-2009, specifies the operational interlocks thatmust be fitted to each type of scanner, and also requires thatthe generation of X-rays is automatically terminated in theevent of any malfunction or fault mode. This standard is notformally endorsed in Europe, but as similar equipment is likelyto be used as in the US, compliance with the standard is assumed(at least until a European standard is introduced). Complianceis, however, the responsibility of the manufacturers, unlessrequired by the airport or travel safety authority uponpurchase. Insufficient data are available to estimate theprobability of any malfunction occurring, but the requiredinterlock systems will ensure that, in the event of amalfunction, radiation doses to the person being scanned, theoperators and any other persons in the vicinity will remainlow.
