3. What are the technologies used in the proposed security scanners?
The SCENIHR opinion states:
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 non-ionising radiation 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 clothing.
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 source.
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 clothing.
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 of unit.
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 radiation and transmitted radiation in a single scan procedure are also commercially available.
3.3.3 Non-ionising backscatter radiation
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 electromagnetic field 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 radiation are 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 heating.
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 this report.
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 transmission scanners.
- 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 regulator.
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 low.