Procedures for the operator

The actual operation of an x-ray baggage scanner is straightforward. Vendors will provide recommended procedures for operating their specific equipment, and each school will probably tailor this for its own environment. However, as with the radiologist who examines medical x-rays, the challenging part of operating x-ray equipment for weapon detection is knowing what to look for. The untrained or disinterested operator can negate any possible benefit that could be gained in a weapon detection program.

The TV monitor that displays the black-and-white x-ray images of baggage it is scanning can usually be used in the positive or negative, i.e., solid objects can be displayed as light or dark objects. There are two types of color systems on the market. There are colorized single-energy (one radiation source) systems in which the color is arbitrarily assigned based on the level of energy transmitted. The second type is a dual-energy (two radiation sources) system that assigns color based on the effective Z-number of the material. The first type is inexpensive but adds no useful information to the display. The second type adds useful information but would normally be cost-prohibitive for most schools (exhibit 3.13).

Some general guidelines for the operator of an x-ray detector are:

• The different models of x-ray detectors utilize various techniques and angles for transmitting the radiation and receiving it on its sensors. Your vendor will inform you as to the best orientation for items being scanned by your equipment. For example, for an x-ray detector that uses a fan-shaped beam emanating from the top of the equipment's interior in a downward direction, the vendor will give instructions similar to:
  Do not put a bag down on a conveyer belt such that the images captured will be of the narrowest perspective of the bag. Lay the bag down on its widest side to allow the x rays to penetrate the least amount of material. Be careful that no part of the bag is outside the zone of detection, which is generally defined by the width of the conveyor belt that is used.
  
• What you are looking for is a solid dark object (if display is set this way) that could be a weapon, part of a weapon, or hiding a weapon. A best case scenario (for the operator) is a revolver that is lying on its side so that its shape is obvious. The same is true for a knife of substantial size if it is lying on its flat side. What becomes difficult, and where most operator training and judgment come into play, is when a weapon is in a different orientation so that it is viewed from the top, bottom, or back of the weapon. A revolver will generally still have a revolver shape that reveals its cartridge. An automatic weapon viewed from the top, however, will produce an image that is an innocuous rectangle 4 inches or more in length. (Keep in mind that there are some weapons available today such that the length is less than 3 inches.) An automatic or semiautomatic weapon viewed from the back is an even smaller rectangle. And, unfortunately, a knife can be very difficult to detect if it is made of any material other than metal.
  
• Clutter occurs where several dark items are grouped together in an x-ray image, such that the actual size and shape of each item cannot be reasonably determined. More often than not, clutter is the cause of manual searches in weapon detection programs.
  
• Surprisingly, band instruments can usually be put through an x-ray machine successfully; the normal thinness of the metal of most instruments will allow the x-ray detector to see within and behind the instrument for any hidden weapon. The school should screen all of the different types of instruments beforehand to determine if any of the instruments (or their cases) will be a problem for the x-ray detector.
  
• When in doubt about an object in a bag, investigate!

   

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  Research Report:   The Appropriate and Effective Use of Security Technologies in U.S. Schools