Using backscattered ionizing radiation to monitor objects.
Boris Aleksandrovich Vandyshev, Candidate of Technical Sciences
USE OF BACKSCATTERED IONIZING RADIATION
TO MONITOR OBJECTS
The article discusses some types of inspection equipment, the operating principle of which is based on the use of ionizing radiation reflected from contraband items, called backscattered ionizing radiation.
This principle allows you to obtain information about the internal contents of the controlled object during its inspection from one side.
In practice, there are often situations when monitoring objects using passing ionizing radiation is extremely difficult, and sometimes impossible, due to a number of reasons.
In such cases, they resort to the method of one-sided monitoring, using backscattered X-rays and gamma quanta and neutrons to obtain information about the internal content of the controlled object (CO). Backscattered radiation (BSR) is also used along with penetrating radiation (PR) to increase the information content of monitoring.
Information about the KO is presented to the operator either in digital form or as an image on the monitor screen.
The equipment for monitoring using the ORI is manufactured in stationary, mobile and portable versions.
The principle of its operation is based on the effects of the interaction of quanta and particles with the atoms of matter, so it makes sense to briefly recall some of them related to the equipment under consideration.
Interaction of ionizing radiation with matter
X-ray and gamma radiation have the same nature (they are short-wave electromagnetic radiation) and are subject to the same laws when interacting with matter.
The fundamental difference between these two types of radiation lies in the mechanism of their occurrence: X-ray radiation is of extranuclear origin and occurs when high-energy electrons are slowed down in matter; gamma radiation is a product of the transition of the nucleus of an atom from one energy state to another.
In the energy range of up to hundreds of kiloelectron volts (keV), commonly used in inspection equipment, X-ray and gamma quanta (hereinafter referred to as simply quanta) interact with the electrons of atomic shells as they pass through matter, being absorbed (photoelectric effect) or scattered (the so-called Compton scattering).
When scattered, the quantum transfers part of its energy to the electron, tearing it out of the atom and changing the direction of its original motion.
The change in direction of movement can occur in the range of angles from 0 to 180 degrees.
Quanta scattered at angles close to 180 degrees are called backscattered and carry information about the content of the KO.
When they enter the detector, they produce electrical signals in it, which are subject to electronic processing and presented to the operator in the form of an image or number.
The intensity of backscattered radiation for substances with lower density and lower atomic number (such as paper, explosives, drugs and other substances of organic origin) is greater than for substances with higher density and higher atomic number (for example, steel, brass, lead, etc.).
This fact is the fundamental principle of operation of the corresponding equipment.
Neutrons, which have zero electric charge, when propagating in a material environment interact mainly with the nuclei of atoms of substances.
One of the significant types of such interactions are elastic collision processes, as a result of which neutrons, transferring part of their kinetic energy to the nuclei of atoms, slow down and disperse, including at angles close to 180 degrees.
Neutron slowing down is a very important process, since neutron sources usually produce particles of fairly high energies — on the order of several megaelectronvolts (MeV).
The best moderators are nuclei of light elements (for example, hydrogen, carbon, etc.), which are part of organic substances, including explosives (HE).
Substances consisting of elements with a high atomic number (for example, iron, copper, lead, etc.) slow down and scatter neutrons much worse, due to which they easily penetrate thick metal barriers.
Thus, an organic substance placed behind a metal barrier will be a “reflector” of neutrons, and the number of backscattered neutrons can be used to judge the presence of an anomaly in this place (hidden paper, explosives, drugs, etc.). This principle underlies the operation of neutron inspection devices.
Portable devices
The American company “Campbell Security Equipment Company” (“CSECO”) has developed and is serially producing the K 910 B “Buster” portable contraband detector.
Its operating principle is based on recording the gamma quanta of the radioactive source barium-133 reflected from the surface being examined.
The source activity in different versions of the devices fluctuates between 100 and 500 microcuries.
Backscattered radiation is recorded by a solid-state detector made of cadmium telluride.
The device is designed to detect hidden drugs, explosives, currency and other contraband behind barriers made of metal, wood, plastic and fabric.
It can be used to inspect cars, trucks and vans, container walls, fuel tanks, car tires, boats and ships, etc. to detect hiding places. The device is very easy to operate.
At the beginning of the survey, it is automatically calibrated on the surface being surveyed, after which the device is manually moved along the surface and signals a change in the intensity of reflected quanta. The readings are displayed in digital form on the liquid crystal display, and when a cache is detected, an audible alarm is given.
The device is equipped with headphones for work in conditions of high noise. With dimensions of 140x64x64 mm, it weighs 1.1 kg. Power is supplied by rechargeable nickel-cadmium batteries.
A similar device for the same purposes is called “SEARCHER” and is offered to consumers by the company “Folien-Vogel”.
It has slightly larger, compared to “Buster”, weight and size characteristics (the weight of the sensor-probe is 2 kg, the separate electronic unit has dimensions of 170x120x50 mm) and signals the presence of hiding places with contraband by giving an audible signal.
The device is quite sensitive and allows detecting a small amount of ammonium nitrate behind an obstacle.
Power is supplied by 9V alkaline batteries, which provide continuous operation of the system for 8 hours.
A typical representative of portable neutron devices designed to detect contraband caches behind obstacles is the CINDI (Compact Integrated Narcotics Detection Instrument) detector, developed by the American company NOVA R&D, Inc.
It contains a source of fast neutrons californium-252 with an activity of about 50 microcuries. Backscattered neutrons are registered by a scintillation detector system, digital information is displayed on a liquid crystal display; there is a threshold sound indicator that signals the presence of anomalies in the object being examined.
Containers, tanks, vans, cars, ships, etc. serve as objects of examination.
The device has an autonomous battery power supply and is equipped with an automatic calibration system before measurements begin.
The procedure for conducting the inspection operation is the same as for gamma detectors, i.e. scanning the working part of the device along the surface of the KO.
A positive feature of the device is its ability to detect drug packages located behind the wall of both empty and filled with hydrogen-containing substances (gasoline, motor oil, etc.) containers.
For an empty tank (fuel tank, barrel), the detector signal at the location of the contraband will increase, for a full tank, it will decrease, since the content of the main scattering element — hydrogen in drugs is less than in the specified liquids. Tests of the device using an improvised drug simulator — sugar — showed that it is reliably detected behind barriers made of lead and iron.
The device complies with radiation safety standards in force in the USA.
As an example of domestic devices of the class under consideration, we can cite the neutron search device Sverchok”.
It is designed to detect hydrogen-containing caches (explosives, drugs, etc.) behind metal, brick, concrete, etc. barriers.
The device operates on the basis of the quantitative relationship between the recorded flow of reflected neutron radiation and the mass of the substance in the control zone. The neutron source is the isotope californium-252 with a flow of about 106 n/sec.
Power is supplied by nine type 373 elements (or batteries) with a continuous operation life of at least 30 hours. The weight of the device in the equipped state does not exceed 5 kg with the weight of the control panel 3.5 kg and the remote measuring unit with the rod 1.5 kg.
The device is serviced by one operator and complies with the radiation safety standards in force in Russia.
Foreign portable devices that use backscattered neutron and gamma radiation to monitor objects have proven their effectiveness in inspection practice; work is underway to create a combined device that uses both types of radiation simultaneously, which will expand the capabilities of portable search equipment.
Stationary and mobile equipment
The pioneer in the field of obtaining images of SO in backscattered X-rays is the American company American Science and Engineering, Inc.” (AS&E); it is also the world leader in the development and production of such equipment.
During the imaging process, the X-ray tube moving past the X-ray source (on a conveyor belt or by other means) is scanned vertically by a narrow X-ray beam (the so-called “pencil beam” or flying spot technology).
The narrow beam is formed from a wide beam of X-ray radiation emanating from the anode of the X-ray tube by passing it through rotating collimators, which are either a disk with a slot or a wheel with narrow radial channels.
The setup has two independent detector systems: an X-ray beam passing through the KO and quanta reflected from the KO.
One system is located behind the KO, the other in front of it.
The signals from the detectors, subjected to computer processing using a special program and linked to the coordinate of the beam on the KO, are the basis for the formation of black-and-white images on the screens of two monitors in transmitted and backscattered X-ray radiation.
Examples of such images are shown in Fig. 1.
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Fig. 1. Image of a suitcase in transmitted (a) and backscattered (b) X-ray radiation. The bright white rectangular spot in Fig. 1b is an imitation of a plastic explosive hidden in a radio receiver.
By analyzing them, the operator can detect a substance of organic origin hidden in the radio receiver (in this case, a plastic explosive simulant).
In the picture obtained in the transmitted beam, the image of this substance is shaded by denser layers that absorb X-ray radiation more strongly.
Similar inspection equipment, produced in a wide range, is installed at airports, customs offices and important facilities.
In radiation terminology, it is called “microdose”, since the dose load on the KO for one session is 20 microroentgen (normal natural radiation background is 10? 20 microroentgen per hour) and does not damage photographic materials.
The “101 Van” system was created for work in field conditions.
The X-ray inspection unit is mounted in a mini-van, as shown in Fig. 2.
Fig. 2. External view of the “101 Van” system.
The unit is equipped with a conveyor system and an operator console; power is supplied either from an on-board electric generator running on gasoline or liquefied gas, or from an external power grid.
The X-ray machine operates at two voltages on the X-ray tube, selected by the operator — 140 kV and 90 kV.
The radiation penetration is 12 mm on steel.
Tests conducted using 1000 packages, some of which contained contraband imitators, showed 83% correct analysis and 17% incorrect (missing and false positives by the operator).
The average analysis time for one package was 1 min 7 sec.
For comparison, the results of tests of the X-ray tomography unit STX 5000 SP of the American company «In Vision» are given: when examining 864 packages, correct analyses were obtained in 90% of cases, and incorrect ones in 10%. The analysis time for one package was 1 min. 30 sec.
That is, the control equipment using transmitted and backscattered X-ray radiation is comparable in its efficiency to the best of the existing, but much more complex and stationary system of similar purpose.
Both stationary and mobile systems have been developed for the control of large-sized objects (for example, cars and heavy-duty automobile trailers).
The stationary system, called CARGOSEARCH, can control heavy-duty trailers up to 19.5m long, 2.4m wide, 4.2m high and weighing up to 36 tonnes.
The system includes two X-ray scanning units (450 kV, 10 mA) located on either side of the truck to obtain the most complete information about the KO; images in transmitted and backscattered radiation are displayed on four monitors. The emitter and detector systems are positioned at an angle of 80 degrees to the longitudinal axis of the vehicle to obtain an isometric image of the KO.
This reveals the presence of double walls with contraband hidden between them, as well as undeclared cargo and illegal immigrants hidden in the trailer van. The radiation dose during the examination of the KO does not exceed 2 milliroentgen.
Three inspection speeds are used: 8; 4 and 1 meter per minute. Along with the stationary system, a mobile vehicle monitoring system (including heavy-duty trailers) has also been developed, operating only with backscattered radiation (“MOBILSEARCH”).
The system is based on a truck (Fig. 3); mobility allows for increased efficiency in detecting contraband due to the surprise effect when deployed in non-traditional locations.
Fig. 3. External appearance of the “MOBILSEARCH” system
The unit is equipped with a 450 kV X-ray emitter with a radial interrupter, providing a “flying spot” for scanning the KO, and a block of backscattered radiation detectors.
Power is supplied from an autonomous diesel generator with a capacity of 22 kilowatts.
The movement of the KO during scanning is provided by special electromechanical devices. The unit is serviced by three operators, one of whom is a truck driver.
Figure 4 shows an image of a passenger car in backscattered X-rays; drug-imitating sugar packets hidden in the trunk of the car are clearly visible.
Fig. 4. Image of a passenger car in backscattered X-rays obtained using the “MOBILSEARCH” system.
Backscattered X-rays are also used to detect contraband and sabotage and terrorist weapons hidden under a person’s clothing.
The “BODYSEARCH” system was created for this purpose (Fig. 5).
Fig. 5. External appearance of the “BODYSEARCH” system
The examination of a stationary subject is carried out by moving the X-ray scanning system; the operator analyzes the image obtained on the monitor screen, having the ability to control the brightness and contrast of the image, increase it by 2; 4 and 8 times and document it if necessary.
The examination time is 8-10 seconds, the dose load does not exceed 5 microroentgen.
Figure 6 shows a frontal demonstration image of a dressed “terrorist”, obtained using the “BODYSEARCH” installation.
In the middle part of the photo — a block of plastic explosive under the right armpit, a Smith & Wesson 9mm automatic pistol in the crotch area, a bunch of keys on the right hip, a wallet on the left hip; in the lower part of the photo — a folding Swiss Army knife on the right ankle; the upper part of the photo does not arouse suspicion.
Fig. 6. Frontal backscattered X-ray image of a clothed terrorist obtained using the BODYSEARCH system.
The material presented in the article clearly illustrates the role of the operator in analyzing the information provided by backscattered ionizing radiation during the examination of the KO.
Firstly, for the effective use of such equipment, despite all the measures taken to simplify its use, a qualified and well-trained operator who has undergone an internship in conditions as close to real as possible is required.
Secondly, the operator's efficiency will continuously improve as he gains practical experience, so it is very important to create conditions for the retention of such personnel and their periodic training in order to improve their qualifications.
It is in this aspect that the human factor is considered by specialists involved in the fight against smuggling and terrorism.