Prospects for the application of nuclear methods for detecting explosives.
Aviation Week & Space Technology.- 1996 .- 145, No. 15 .- P. 46, 47, 49.
Prospects for the application of nuclear methods for detecting explosives
Despite the emerging unfavorable criticism of the method of detecting explosives (HE) based on thermal neutron analysis (TNA), research into this method and its application possibilities, financed by the US Federal budget, continues.
This is explained by the high ability of the method to distinguish individual substances.
The Federal Aviation Administration (FAA) has not yet made a final decision on the method, its safety scientific adviser said.
ATN was the main method of detecting explosives in the late 1980s. At that time, six prototypes of systems capable of detecting explosives weighing more than 1 kg were manufactured. However, an analysis of the crash of Pan Am 103 over Lockerbie, Scotland, in 1989 showed that the cause of the accident was an explosion of explosives weighing less than 0.5 kg. Increasing the sensitivity of the ATN method to 0.7 kg led to an increase in the false alarm rate from 5-10 to 25-30%, which is unacceptable for airlines. This meant that 25-30% of air baggage items would be unreasonably recognized as suspicious. The disadvantages of these systems also included their large dimensions and high cost (from $750,000 to $1 million). Therefore, their operation was stopped in 1994. The negative attitude of the press towards the ATN-based systems increased after the conclusion of the commission investigating the aircraft crash near Lockerbie. Currently, the leading place among explosive detection systems is occupied by the CTX 5000 system of the In Vision company, which is the only system certified by the FAA. It uses the method of X-ray computed tomography with the reproduction of images on the monitor. This system also costs about 1 million dollars, and according to its use at US airports, the level of false alarms reaches 30%. The dimensions of the CTX 5000 system are approximately the same as the ATN-based systems. Airline officials have shown little enthusiasm for the CTX 5000 system.
SAIC (USA) is currently developing a smaller ATN-based system for screening briefcase-sized carry-on items, and the FAA is funding airports, airlines and agencies planning to test the system in early 1997.
To detect explosives, the ATN-based system scans a large number of volume elements occupied by a piece of luggage. In the previous ATN-based system, the size of one such volume element was 100 mm, which was too large to detect small dangerous items. Reducing this size to 50 mm in the new system means quadrupling the number of detectors in an already expensive system and reducing the signal strength reaching the detector.
However, reducing the size of the checked baggage items from checked to hand-carried and using the ATN method to detect electronic devices and fluids and other substances with low nitrogen content should reduce background noise and increase the probability of detecting explosives weighing less than 0.5 kg. The National Research Council's Civil Aviation Safety Committee recommended in 1993 that priority development of ATN-based systems continue until other systems with better performance become available.
SAIC's Small Parcel Explosive Detection System (SPEDS) can screen items up to 50 x 45 x 20 cm. Like earlier ATN-based systems, it uses a Californium 252 neutron source.
The neutrons it emits are slowed down to an energy of 0.025 eV, which is equivalent to the thermal effect of the environment. These thermal neutrons penetrate the inspected baggage item and are absorbed by the nuclei of nitrogen atoms. As a result, these nuclei emit gamma rays with an energy of 10.8 MeV. Explosives are distinguished by a high concentration of nitrogen in them. The resolution of the detectors located around the inspected item and perceiving partially collimated gamma radiation is equal to six volume elements, 410 allows distinguishing the less intense gamma radiation of substances with a lower nitrogen concentration, such as wool, leather and nylon, from the gamma radiation of explosives. The cost of SPEDS in serial production will be about $ 100,000. The system will automatically make a «pass-detain» decision and indicate which volume element belongs to the suspicious zone.
Another nuclear technology for detecting explosives is based on Pulsed Fast Neutron Analysis (PFNA). Research into the application of this technology was initiated by the FAA, and then the Defense Advanced Research Projects Agency (DARPA) awarded SAIC about $15 million to develop a system for detecting drugs in large containers from 1991 to 1996. DARPA support for this development ended in October 1996 when the agency withdrew from the drug enforcement effort, but was followed by an additional $6 million from the DARPA Technical Support Working Group.
In late 1995, the FAA awarded SAIC an $800,000 contract to modify existing PFNA equipment for screening air cargo. In 1992, the company tested the system by screening LD-3 containers loaded with suitcases. FAA testing was planned for late 1996.
The IABN technology uses an accelerator to generate a pulsed stream of fast neutrons with an energy of 7-9 MeV, moving at a speed equal to 1/10 the speed of light. Inelastic collisions of neutrons with the nuclei of atoms of a substance lead to the conversion of energy. The nuclei of atoms of a substance emit energy in the form of gamma rays of varying intensity. Oxygen and carbon are almost undetectable when irradiated with a stream of thermal neutrons, but collisions of their atomic nuclei with fast neutrons cause gamma radiation with an energy of 6.13 and 4.44 MeV, respectively, which provides additional information for discrimination of various substances. HEs are characterized by a high concentration of oxygen and a relatively low concentration of carbon. Nitrogen is detected by gamma radiation with an energy of 1.6; 2.3 and 5.1 MeV.
A beam of fast neutrons scans the container, and the results of the irradiation are calculated from the time of arrival of the gamma radiation at the detectors at each pulse, since the front of the neutron pulse moves slowly compared to the gamma radiation, which propagates at the speed of light. As a result, a three-dimensional measurement is made with a resolution equal to the size of the volume element, i.e. 50 mm.
Fast neutrons easily penetrate heavy elements, but are slowed down by light elements. But at a low scanning speed, explosives can be detected even in a container filled with potatoes.
The first application of the IABN-based system will be on airlines for cargo screening, but in the future they will be used for the final screening of checked-in air baggage. The size and cost of the system depend on the type of neutron accelerator used. Currently, an accelerator based on the Van de Graaff electrostatic generator is used. The cost of a system capable of screening ten LD-3 containers per hour will be $5 million.
According to experts, the IABN method is the most promising atomic method for detecting explosives, as it makes it possible to identify atoms of various elements that make up explosives. However, the potential disadvantages of this technology are its high cost and complexity. Therefore, it is recommended to assign medium priority to testing and developing prototypes of systems based on this technology.
Fast neutrons are also used in a technique called transmissive radiography, which is being developed in two FAA-funded projects: one at the University of Oregon's Physics Department and one at Tensor Technology. The technology has not yet advanced to the point where the system can be field-tested. The FAA has already spent $3.5 million developing the systems, and officials hope that they will be used to screen cargo and baggage in containers. The cost, complexity, and shielding requirements of transmissive radiography systems are about the same as those using the ANSI technique.
In transmission radiography, a fan-shaped beam of neutrons with a continuous energy spectrum up to 8.2 MeV scans a piece of baggage or container, and a detector array located behind the object measures the energy spectra of the neutrons that pass through the object. Nitrogen, oxygen, carbon, and hydrogen attenuate the energy of neutrons to varying degrees, and spectral analysis must determine the elemental composition of the substances through which the scanning beam passes. The result is a two-dimensional image of the energy spectra, which serves as information for detecting explosives.
The Oregon State University prototype contains a linear array of 16 detectors that forms an image with the dimensions of its element on the surface of the inspected object being 30 mm. The object moves 30 mm during the time it crosses the neutron beam. Inspection of a 62 cm long suitcase takes 26.7 minutes. But the developers of the system believe that this time can be reduced to 8 seconds if a more powerful neutron beam, fast electronics and a flat array of 10×16 detectors are used.
The system was tested by screening 50 suitcases and performing 130 measurements. Half of the suitcases contained ten different types of explosives in various quantities and shapes. The explosives weighed between 150 and 650 g and were approximately 6 mm thick. The researchers developed an algorithm to overcome the identified deficiencies and limitations, in particular the use of only 2D images. Thresholds were calculated based on the screening of three pieces of luggage, 61 of which contained explosives. The following test results were obtained: one false alarm (2%), seven non-detections (9.5%), and 54 correct detections (88.5%).
Several suitcases were screened at different x-ray angles, and plastic explosives were reliably detected when the 3D images were reproduced.
FAA may resume funding for research and development based on the nuclear resonance absorption method and its applications to container screening. Previously, it funded research into the use of this method for screening checked-in airline baggage. The nuclear resonance absorption of gamma radiation method involves generating and analyzing gamma radiation with an energy of 9.17 MeV, which is predominantly absorbed by nitrogen. The method pays off if it allows the screening of a full LD-3 container. But this requires accelerators and shielding of the entire system. The effect of high-energy radiation on food products also remains an open question.