The need for improved security extends beyond cargo screening to a whole new set of security needs at airports. For years, carry-on bags have been screened, but checked luggage has been loaded onto airplanes without screening. As of the end of 2002, however, the U.S. Federalization Security Act is requiring that all checked bags be screened by devices that can detect explosives.
      Screening luggage for explosives poses unique challenges. Current screening systems for carry-on bags are not sensitive enough to do the job. These systems use stationary low energy X-ray tubes and line-by-line scanning to show two-dimensional shapes. They detect a knife within a pile of clothes, for example.
      Detecting explosives, however, is harder; it requires the ability to distinguish materials of different densities as well as their shapes. And since an immense number of bags pass through the nation’s airports in a single day, they must work quickly.
      To tackle the job, the security industry has turned to CT scanning, which uses higher energy metal/ceramic X-ray tubes that spin around the luggage at two revolutions per second. They can detect and differentiate between the densities of scanned materials. They operate within explosive detection systems (EDS) that generate three-dimensional color-coded images highlighting suspicious items. EDS tubes represent some of the latest advances in X-ray tube technology.

How X Rays Are Made
X rays are a form of high energy light with very short wavelengths that make it possible for them to pass through solid objects. They are created by accelerating electrons to a very high speed and driving them into a metal target. The resulting subatomic collisions release energy in the form of X rays (1%) and heat (99%).
      In an X-ray tube, electrical energy is applied to a filament, heating it up to white-hot temperatures so that it ‘boils off’ electrons. To accelerate these electrons, the tube is equipped with a cathode (a negative electrode) and an anode (a positive electrode). The application of a high voltage across the positive and negative electrodes creates a differential that causes the electrons to speed towards the anode at a very high velocity. This assembly is housed within a vacuum, which eliminates resistance so that the electrons can attain higher speeds by accelerating more rapidly.
      The cathode incorporates a focusing cup to concentrate the electron stream and its kinetic energy onto a small focal spot, or target, within the anode. This target is usually made of tungsten or some other metal that can withstand very high temperatures.
      The collision of electrons with the tungsten unleashes X rays that are channeled out of the tube through a small window or aperture. The velocity achieved by the electrons before they strike the anode is directly proportional to the amount and penetration power of the resulting X rays.
      EDS series tubes operate at very high electrical voltages — between two and four times the voltage used in systems for screening carry-on luggage. This results in the higher contrast resolution needed for differentiating between materials and detecting explosives.
      Varian’s new line of EDS X-ray tubes are engineered to meet the specifications for CT based explosive detection systems that are now being installed at more than 400 U.S. airports.