With the current interest in airport screening, a review of the current technical capability seems appropriate and useful. Here is a short rundown on the types, operation, advantages & disadvantages of some explosives detection equipment.
Explosive detection technology orients on known characteristics common to most explosives; these are:
Three basic approaches are being taken to detect the presence of explosives by use of various technologies. These three approaches are: nuclear based devices, x-ray based devices, and vapor detection devices.
Nuclear based devices are intended primarily for high-volume, luggage-screening applications and can be automated. Included are thermal neutron analysis (TNA), fast neutron analysis (FNA), pulsed fast neutron analysis (PFNA), nuclear resonance absorption (NRA), nitrogen-13 production-positron emission tomography (PET), and pulsed neutron backscatter (PNB). In this approach, the luggage or other item being screened is moved past a nuclear source which releases gamma rays, ionized radiation, neutrons, etc. A detector on the opposite side receives the radiation after it penetrates and then interprets the 'pattern' of penetration, absorption, or emission it receives. It then matches that pattern or image against known profiles of explosives to determine the content of the luggage or other item. If the profile matches that of a known explosive, the machine alerts the operator. In some cases, it also gives a location in the screened item. The exact method and reliability of the 'image' or 'profile' match varies with each type of nuclear source.
X-Ray machines can be used in either high-volume, luggage screening with some automation possible, and in personnel operated, low-medium volume passenger-carried luggage screening. They include the usual scanners with which everyone is familiar, but also include dual- or multi-energy scanners, backscatter x-rays, and computerized tomography (CT) scanners. Standard X-rays go through the luggage to produce an image which is then interpreted electronically or by a human operator. A dual- or multi-energy scanner simply produces two or more images using different x-ray frequencies. This increases the validity of the interpretation either human or electronic. A backscatter system produces the usual image, but also produces a second image based on 'reflected' x-rays. This increases the reliability of interpretation.
Vapor detection systems are intended primarily for point screening of passenger-carried luggage and cargo-entry points. They may be used as the second check when another system alerts to a possible problem. They are found in five technological types: gas chromatography with chemiluminesence (GC/C), gas chromatography with electron capture detector (GC/ECD), ion mobility spectrometry (IMS), two stage mass spectrometry (MS), and fluoroimmunoassay (antigen-antibody reactions).
Gas chromatography devices operate by sampling the molecules in the usual fashion; the chemiluminesence type then analyzes the color present while the electron capture type ionizes the molecules and compares the rate of ionization. Both compare the results to known explosive profiles for color spectrum and ionization. IMS devices pull air through a filter and then ionizes it. After ionization, the rate of arrival at measurement point produces an electric current which is compared against a known explosive profile. Two stage MS devices first ionize an air stream, then sort it by weight. The second stage measures the rate of heavier ions by creating an electric current which is measured against a known explosive compound profile. Fluoroimmunoassay devices depend upon drawing vapors into solutions which have antibodies for specific explosive compounds. The antibody reaction can then be measured and identify specific explosives.
Nuclear based devices are large, expensive and have significant operation, training and maintenance problems. They have trouble differentiating plastics from explosives.
X-Ray devices of all types have good resolution, vary in cost, can see weapons as well as explosives, and may miss small quantities of explosives. They are very dependent upon operator image-interpretation skill. CT devices can identify small objects, but usually cost more and tend to be larger and slower in operation.
Vapor detection devices tend to be slow in usage, may require frequent cleaning or calibration, may require daily operating supplies, and have difficulty differentiating between plastics and explosives.
As can be seen from this short review, bomb detection is not simple in high-volume situations such as passenger or cargo screening.