Archway Metal Detectors

The basic rule in airports is that all passengers, as well as their cabin baggage, must undergo security screening before being permitted to have access to an aircraft or a sterile area.

Screening of passengers is usually carried out by the use of Metal Detectors, supplemented by manual search. (Passenger and cabin-baggage screening should always be carried out immediately adjacent to each other to prevent the transfer of prohibited items from the person to the baggage and vice versa).

Modern threats include plastic and ceramic weapons, as well as plastic and liquid explosives. These items cannot be detected with metal detectors. So to compensate for these limitations when screening passengers with metal detectors, and to introduce a random element into the security process, there should be the additional manual hand searches of some passengers.

An Archway Metal Detector (AMD), also called a Walk-Through Metal Detector, is a stand-alone structure that resembles a wide door frame. When a questionable item or material is detected by an AMD, it produces an alarm signal (a light or audible alarm, or both). The typical AMD will take up a space on the floor about 90 cm across and 60 cm deep. The typical height of most AMDs is around 2 m. Weight of an AMD can vary from around 25 kg to about 70 kg. AMDs are generally freestanding and are rarely attached to the floor or surrounding structures but their awkward shape prohibits their being easily moved manually by staff. Very light-weight AMDs are available that can be quickly assembled, or disassembled and transported for use in portable applications. Conventional AMDs are difficult to use in portable applications, because they are too big to fit in an automobile and therefore require additional equipment to make them mobile. And power consumption of the electronic system limits the conventional AMDs ability to operate from a battery source for more than a few hours, requiring mains electricity to be accessible or frequent battery recharging. It would be difficult to conduct a metal detection screening programme with only the use of AMDs. Hand-Held Metal Detectors (HHMDs) are usually also required for use on passengers who have triggered the alarm walking through the AMD.

Weapons detection using metal detectors always involves a trade-off between detection of all weapons and detection of innocuous items. Unfortunately, a metal detector alone cannot distinguish between a gun and a large metal belt buckle, so trained operators are needed to make these determinations. Metal detectors work very poorly if operators are not aware of their limitations and are not trained and motivated to operate them correctly. Metal detectors are also not effective for screening baggage and handbags because there are usually a large number of different metallic objects and materials located in or as part of the composition of these carried items that would cause the equipment to alarm. These items need to be screened by X-ray. (An X-ray baggage scanner is not classed as a “metal detector” because it images metallic objects rather than merely detecting their presence). Metal detectors are a convenient screening tool, since a person has only to walk through an AMD at a normal walking speed range. They are limited however, in that they are subject to variations in detection, depending on how they are set up by the operator, and where on the person a metal object is located.

AMDs respond differently as objects are passed through the detector’s archway at different locations. An AMD usually emits its weakest response when an object is passed through at the vertical and horizontal centre of the archway. Sensitivity to an object will also vary according to the object speed through the archway, and even the direction the object is pointing. Therefore, the sensitivity needs to be set to the level where the metal detector alarms at the weakest response to speed, direction, and location. Often, manufacturers scale down the sensitivity near the top of the AMD. This is because the processor on top of the portal emits its own electromagnetic interference, and, depending on how well the detector is engineered and insulated, the interference in that top portion of the field/portal causes the AMD to alarm when no metal is present. So to compensate, the manufacturer lowers the sensitivity in that zone, thus creating a vulnerable spot where some threat objects may be able to get through the portal without detection, depending on the object size and metal type. The same practice can occur with AMDs that are particularly susceptible to the effects of steel reinforcement (rebar) in concrete floors. Again, manufacturers will lower the sensitivity in the bottom portions of the magnetic field in order to eliminate rebar-induced false alarms, thus creating vulnerable spots in the lower portions of the portal’s magnetic field, specifically in the area of the person’s shoes. Very high-quality detectors employ sophisticated multiple detection zones, often overlapping each other, to ensure that weak areas of vulnerability in the portal detection field are eliminated. Each zone, in the case of a true multiple zone AMD can be considered as an individual metal detector that has been housed in a single frame assembly. The sensitivity of each zone can be precisely controlled. This permits the location of a weapon to be precisely pinpointed. In addition to showing the height at which it is carried it also identifies if it is in the centre, left or right of sector of the detector.

Manufacturers of AMDs sometimes quote throughput rates of up to 60 people per minute. It is totally unrealistic to assume that people can be organised to pass through an AMD at a rate of one per second! These figures are hypothetically possible but impractical in a real-world environment. One compelling reason is that people object to being too close or touched by others. In an airport environment where threats like small handguns and boxcutter knives are being targeted, a throughput rate of 10 to 15 people per minute is more practical. The frequency of unwanted alarms for any given detector is actually affected by many factors, including: weather, season, time of day, sophistication of people being screened, layout of screening area and the manner in which security personnel supervise the screening operation. Weather and season impact upon the amount of clothing people wear. Bad weather means more clothing, pockets and footwear containing metal. Fewer variations in clothing usually occur when tests are conducted in a domestic departures area rather than an international terminal. The time of day influences the flow rate of passengers, heavy traffic increases the incidence of tailgating (more that one person being in the detection field at a given instant.) Sophisticated travellers that pass through detectors on a regular basis automatically remove items of metal, whereas passengers travelling only very occasionally (e.g. on annual holiday) are less likely to be prepared.

The typical pulsed-field AMDs generate electromagnetic pulses that produce very small electrical currents in conductive metal objects within the portal archway which, in turn, generate their own 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. The receiver portion of an AMD can detect the rapidly decaying magnetic field during the time between the transmitted pulses. Most modern AMDs are “active” in that they generate a magnetic field that actively looks for suspicious materials or objects. Magnetometers, which are passive devices, were much more in use 20 years ago in the detection of weapons. A magnetometer depends on the Earth’s magnetic field – it looks for a distortion caused by the presence of ferromagnetic (attracted to a magnet) material.

Magnetic Imaging Portals are a relatively new technology. Like traditional AMDs, they illuminate their detection space with radio-frequency electromagnetic waves; however, they do so using a number of small antennas arranged ringlike around the portal, pointing inward. Each of these antennas transmits in turn to the antennas on the far side of the array; each antenna acts as a receiver whenever it is not transmitting. A complete scan of the detection space can take place in the time it takes a person to walk through the portal. Using computational techniques adapted from computed axial tomography (CAT) scanning, a crude image of the person (or other object) inside the portal is calculated and displayed. The magnetic imaging portal may for some purposes be classed as a metal detector rather than as an imaging system because it does not produce a detailed image of the metal object detected, but only reveals its location and approximate size.

It is relatively simple to build a detector that detects metal, but it takes considerable expertise to produce an efficient detector that produces consistent results and minimal false and unwanted alarms. Almost without exception manufacturers claim uniform detection, however, in a number of cases the claims are not true. Counter to intuition, the mass of a particular item is not significant in metal detection. The size, shape, electrical conductivity and magnetic properties are the important properties. For example, the foil in a packet of cigarettes may alarm the detector, even though the actual mass of metallic material is small. If a long thin wire is taken through a AMD, and the wire is in any geometry except one in which the two ends (or any two points on the wire) are touching, it will rarely be detected. However, if you shape this same piece of wire into a closed circle, the metal detector will most likely alarm even though the mass of the wire has not changed. Delving even deeper into metal detector sensitivity, consider the orientation of an object. Take the same closed-loop wire described and lay this loop on its side so that it is parallel to the ground. In this configuration, the AMD is less likely to see it. But if the wire loop is upright and parallel to the side panels of the metal detector, the detector will be much more likely to alarm in this orientation. So the position and angle of an object greatly changes an AMD’s response to that object. The reliability of response may change if a firearm is passed through flat, vertical with the handle pointing straight ahead, or the handle pointing to a side of the metal detector. Because handguns and knives are manufactured from a variety of ferrous and non-ferrous metals and alloys a detector must be capable of detecting a broad range of materials in a single operating mode.

A person about to walk through an AMD at an airport checkpoint needs room to place his/her carried items on the X-ray machine and room to put his/her pocket items (coins, keys) in a special pass-through container (to be visually examined), or into a plastic tray to go through the X-ray machine. If a screening area is cramped there is a greater likelihood of tailgating and the detector being jostled. Experienced security personnel can help to control and thus minimise this problem. It is very important that there be neither space nor opportunity for people, including employees, to walk around the AMD. Very definitive boundaries must be established to prevent circumvention of the system and prevent passback of contraband (where prohibited items are handed from outside the screening area to those who have already successfully cleared the scanning process).

AMDs don’t just react to metallic items immediately within the archway. The magnetic field is outside of the archway as well as inside, like a magnet. In designing the positioning of an AMD, the composition of surrounding walls, furniture, nearby electromagnetic equipment (such as lifts), nearby metal piping in walls and floors in the walls, and even metal rubbish bins must be taken into account. The optimal effectiveness of an AMD can be easily degraded by a poor location, a casually placed chair with metal, or the nearby use of electromagnetic devices. No AMD is manufactured with the correct adjustments that meet all users’ needs. These adjustments or settings are generally made by the vendor when the detector has been installed in the area where it will ultimately be operational. The only way to properly install and set an AMD is to walk through the archway, with no other metal but the object needing detection, carrying the object in different locations, and in different attitudes while adjusting the sensitivity and/or programme until an alarm is given. It is important that security staff operating a particular type of AMD be made aware of its operating parameters and of any inherent dead spots (low sensitivity regions). This type of testing should periodically be conducted according to the vendor’s precise instructions. Once an AMD has been set up and has been demonstrated to operate accurately in its current position and with its current settings, the operators will not be required to adjust the control settings. The operator of the portal should be aware of the possible sources of interference with the equipment; something as seemingly insignificant as setting a metal rubbish bin alongside the AMD after it has been put into operation can introduce an area of less sensitivity within the scanning area of the equipment. Given equivalent environments, however, different facilities have different requirements for equipment sensitivities. A metal detection programme in a prison will have very different equipment settings than a programme for an airport, for example. The optimal settings for each facility will be a set of tradeoffs that balance false-positive errors against false-negative errors. A false-positive error occurs when an alarm occurs for what might be considered an acceptable item, e.g. a metal button. A false-negative error occurs when NO alarm is triggered by an unacceptable item, such as a penknife.

Most AMDs are additive; they will generate an alarm based on the total response received from all the metal detected on a scannee. An alarm does not necessarily mean just one suspicious item has been detected. Because of this, a scannee who has multiple “borderline” items on his other body may cause the equipment to alarm. The vendor of a particular AMD will provide training and procedures that are geared toward the operation of its equipment. In addition, each airport or secure facility will need to develop specific procedures and policies as to the logistics of its metal detection operations. This will include how to process or direct a person who has caused an alarm. The rest of this section will familiarise a facility with what to expect and to provide some general recommendations:

For an AMD that is located semi-permanently in one position, the operator will usually need only to turn the equipment’s power switch on, wait approximately 10 seconds for the unit to warm up, and do a quick performance test. This process should take less than 5 minutes each time screening begins. For an AMD that must be moved into position each morning and put away afterwards, more extensive procedures will be required. The equipment vendor will be able to give advice as to what additional routines will be necessary.

Do not allow a scannee to proceed through an AMD too quickly. Ideally, drawn footprints can be located at the base of the portal within the scanning zone. The operator should insist that each scannee actually place his/her feet on these footprints before proceeding. This will ensure that the scannee has not gone through the AMD so fast that he/she could have been inadequately scanned. Make certain that no other person is located within a 90cm radius of the AMD while a scan is being performed.

Provide a rescan of any person who causes an alarm, even if he/she is able to identify what appears to have caused the alarm, such as a belt buckle or necklace. Confirm that this person no longer causes an alarm after the offending item is removed from his/her possession. (Most airport screening operations will provide for a second, more sensitive scan to be performed by a person with a HHMD rather than by the original AMD).

Do not allow anyone on the outside of the cleared area the opportunity to hand something to a person who has already been cleared by the AMD on the inside of the cleared area. In many facilities, the misconception exists that someone known by the operator, such as a fellow employee or other security personnel, should be allowed to circumvent the system. It must be clearly established that in order to ensure the integrity of any security screening process, everyone must be subjected to the screening requirements, including passengers, airport employees, contractors, air crew, and security personnel.

The instructions provided to passengers before being processed by AMDs need to be as short and simple as possible. The following example instructions may be suitable:

a) Remove any metal items from your body or pockets and put them in your bag to be passed through the X-ray machine, or place them in the pass-through container.

b) Place hats, jackets, bags and all carried items on the conveyer belt for the X-ray machine.

c) Stay back from the AMD until signalled by the operator to proceed. Walk at a moderate pace through the AMD, one person at a time, being sure to momentarily place your feet on the footprints at the base of the AMD before proceeding.

d) If an audible alarm sounds as you go through the AMD, follow the directions of the security officers for further scanning and a body search.

Some people fear the use of AMDs on themselves because of the possible side effects of being subjected to the magnetic field they produce. This fear is unfounded; metal detectors emit an extremely weak magnetic field, weak enough to be of no concern even to heart patients with pacemaker-type devices. Indeed, the use of an electric hair dryer subjects the user to a much stronger field than would be received by a metal detection device. Another widely-held belief about metal detectors is that they are a straightforward technology, where the equipment does all the work. This is not true at all. The average first-time user may expect a metal detector to be much smarter and more helpful than it can possibly be. A metal detector is only as good as the operator overseeing its use.

*The information about AMDs is provided here as a guidance only to compliment and not to replace procedures and guidelines used at your own locality/facility.