1. Methods
1.1. Manual search
1.2. Metal detectors
1.3. Package searches (x ray, etc.)
1.4. Explosives detection (bulk and trace)
1.5. Chemical and biological agent detection
2. Manual search
2.1. Advantages:
2.1.1. Low hardware investment, flexibility
2.2. Disadvantages:
2.2.1. Slow throughput, higher labor costs
3. Metal detectors
3.1. Magnetometer
3.1.1. Passive technology detects changes to Earth's magnetic field caused by ferromagnetic materials
3.1.2. Does not detect non magnetic metals
3.1.3. Outdated; not used for many years
3.2. Continuous wave
3.2.1. No longer commercially available
3.2.2. Active technology detects changes to its own magnetic field
3.2.3. Generates a "steady state" magnetic field 100 Hz to 25 kHz
3.2.4. Requires the subject/package to pass between transmitter and receiver coils
3.2.5. Magnitude of response depends on metal's conductivity, magnetic properties, shape and size, and orientation
3.3. Pulsed field
3.3.1. Active technology detects eddy currents in metals produced by pulses
3.3.2. 400 to 500 pulses per second
3.3.3. Magnitude of response depends on metal's conductivity, magnetic properties, shape and size, and orientation
3.3.4. Object orientation has less effect on results
3.4. Handheld
3.4.1. Better for detecting small amounts of metal
3.4.2. Considered more intrusive
3.4.3. Takes more time and manpower
4. Package searches
4.1. Manual or active interrogation
4.2. Active interrogation techniques
4.2.1. Single energy transmission X ray (low penetration, simple applications, not effective for explosives detection)
4.2.2. Higher energy X ray and multiple energy X ray (screening large, dense containers and vehicles)
4.2.3. Computed tomography (CT)
4.2.4. Backscatter X ray
4.3. Low Z materials : Materials with low atomic numbers ("Z numbers"); materials starting with hydrogen and ending with Z number 26, Aluminum
4.3.1. More difficult to image with less sophisticated X ray technology
5. Explosives detection (bulk and trace)
5.1. Bulk
5.1.1. Macroscopic, detonable amounts of explosives
5.1.2. Targets specific threat amounts of explosives
5.1.3. Usually uses ionizing radiation that is not safe for use on people
5.1.4. Measures X ray absorption coefficient, X ray backscatter coefficient, dielectric
5.1.5. constant, gamma or neutron interaction, or microwave or infrared emissions
5.1.6. Can determine calculated mass, density, nitrogen, carbon, oxygen content, and
5.1.7. effective atomic (Z) number
5.1.8. Multiple energy X rays and backscatter X rays more readily identify low Z number materials
5.1.9. High energy X rays are large, fixed mechanisms designed to scan large cargo containers
5.1.9.1. May be combined with backscatter X ray technology for detection of low Z number materials
5.1.9.2. Instead of X rays, these devices may use gamma rays or neutrons for detection (very high penetrating)
5.1.9.2.1. Thermal neutron activation (TNA): Detects the presence of nitrogen through gamma wavelengths
5.1.9.2.2. Pulsed fast neutron absorption (PFNA): Determines carbon and oxygen content
5.1.9.2.3. Determination of nitrogen, carbon and oxygen content lends more accuracy to separating explosives from food items
5.1.9.2.4. Food items/cargo cannot be irradiated at energy levels of more than 10 milli electron volts (MeV) (international law)
5.1.10. Low dose backscatter X ray technology is safe for humans, producing ~10 microrem per dose (NRC limit is 100 millirem per
5.1.11. CT scans spin sensors on a gantry around the package and produce a 3D image that detects small threat masses; may be subject to high NARs
5.1.12. Quadruple resonance (QR) technology uses pulsed low energy radio waves to detect nitrogen rich materials
5.1.12.1. Can detect small threat amounts
5.1.12.2. Can be defeated by shielding contents/materials with thin sheets of metal, but can detect and report the shielding efforts
5.1.13. Raman analysis uses laser interrogation and analysis of the spectrum of scattered light to identify threat materials
5.1.13.1. Cannot see through opaque packaging designed for clear package searches
5.1.14. Stand off detection:
5.1.14.1. Still under research and development experiments with distance detection of explosives with infrared cameras, passive and active millimeter wave imaging systems, and lasers sensing fluorescence or atomic emissions
5.2. Trace
5.2.1. Particles and vapor residues associated with handling explosives
5.2.2. Key performance metrics
5.2.2.1. Limit of detection (smallest detectable amount) (may be as low as <1 nanogram)
5.2.2.2. Selectivity (ability to distinguish one material from another)
5.2.3. Sampling methods
5.2.3.1. Swipe (most efficient)
5.2.3.2. Vapor (puffing) (less invasive)
5.2.4. Challenges
5.2.4.1. Low vapor phase concentrations of several common high explosives (parts per billion and parts per trillion)
5.2.4.2. Packaging of explosives with oil based gel or solvent
5.2.4.3. Absorption of explosive molecules upon most materials at room temperature and decomposition upon moderate heating or exposure to high energy, and thus, loss of significant sample material in collection and transport
5.2.5. Technologies
5.2.5.1. Ion mobility spectrometry (IMS):
5.2.5.1.1. Ni 63, ion drift time, Faraday plate; low NAR, sensitive, easy operation, robust, low maintenance
5.2.5.2. Colorimetry
5.2.5.2.1. Colorimetry : sprays, test paper, ampoule; portability is excellent, but high NAR, strong smell, disposal of chemicals
5.2.5.3. Chemiluminescence:
5.2.5.3.1. Chemiluminescence: photochemical detection, fast gas chromatograph, nitrogen oxide, ozone, nitrogen dioxide, phototube, photoemission, electron capture detectors (ECD's); excellent sensitivity to common high explosives but must expensive, longes t analysis time, more maintenance
5.2.5.4. Mass spectrometry :
5.2.5.4.1. magnetic and electric fields, mass to charge ratio, quadrupole mass spectrometry, quadrupole ion trap time of flight, ion detector, parent ions, daughter ions, many configurations; gold standard, high specificity, low limits of detection but high cost, high maintenance, and expert operators required
5.2.5.5. Fluorescence :
5.2.5.5.1. fluorescent polymers, monomers, capillary tubes, photomultiplier tubes; high
5.2.5.6. Canine olfaction :
5.2.5.6.1. Very mobile and effective, but not generally used for check points (fixed locations), requires skilled handlers and healthy dogs, frequent rest breaks, constant retraining, and recurring cost of handlers
5.2.5.7. Trace explosives detection portals :
5.2.5.7.1. : Uses puffer in conjunction with an ion mobility spectrometer or mass spectrometer, 10 25 seconds to screen a subject, automated detection, high sensitivity, non invasive but large, expensive (~$150k), and high maintenance.
6. Chemical & Biological agent detection
6.1. Chemical
6.1.1. Uses point sensors at perimeters
6.1.2. Goal is early warning
6.1.3. NAR is a serious consideration (due to high response level)
6.1.4. May not be appropriate for checkpoint screening
6.1.5. May use optical sensing methods
6.2. Biological agent detection
6.2.1. Different from chemical detection in two ways
6.2.1.1. Most biological agents are not immediately lethal, impacting necessary response times
6.2.1.2. Usually requires several hours for collection and analysis of air samples