PEDESTRIAN RADIATION MONITORS.

PEDESTRIAN RADIATION MONITORS.

PEDESTRIAN RADIATION MONITORS

V. Rudnichenko, S. Zvezhinsky

Modern Security Technologies No. 3, 2007

Currently, radioactive substances (RS), which are sources of ionizing radiation, are used in many areas of human practical activity. Numerous studies of their effects on living organisms have shown that any activity associated with such sources requires special safety measures. In our country, as in other civilized countries, a state system of radiation safety control and management has been created and is being continuously improved. The system is designed to ensure the protection of humans and their environment in possible scenarios of using RS and radiation, including protection against their use for terrorist purposes.

In recent years, threats of using radioactive weapons have increasingly become a tool of blackmail by terrorists. The terms «radiation» and «nuclear» terrorism have come into common use. Nuclear terrorism is understood as a set of intentions and actions of individuals (groups of individuals) to create, acquire a working nuclear explosive device (including a «dirty» one), with the threat or subsequent use to achieve declared political, social and other goals and intentions.

To implement such criminal intentions, nuclear materials (NM) or, better yet, fissile materials are needed. The term «NM» refers to uranium, plutonium, thorium or their compounds, as well as irradiated fuel for nuclear reactors. Uranium may contain natural isotopes, be depleted in the isotope 235 (less than 0.7%) or enriched in the isotopes 235 and 233. Highly enriched uranium (HEU) contains at least 20% of the isotope 235. Plutonium-239, which is produced artificially, and the isotope uranium-235 are the most common fissile materials; under the impact of thermal or fast neutrons, the nuclei of these elements split.

«Fissile materials» generally refer to plutonium and highly enriched uranium. The IAEA classifies plutonium (at least 80% of the isotope 239), highly enriched uranium (isotope 235), and uranium-233 as «direct-use» materials, i.e. suitable for making nuclear weapons. In addition to other components, only 12 kg of highly enriched uranium or 4 kg of plutonium are needed to make a nuclear device. Plutonium-239 and HEU are sources of gamma radiation and neutrons.

Unlike nuclear terrorism, radiation terrorism is based on the threat of using radiation materials (RM) to cause physical and/or economic damage due to the ability of the materials to emit ionizing radiation (alpha, beta, gamma or neutron), which is dangerous to human life and health. The term «RM» refers to those materials that cannot be used to create a nuclear explosive device (i.e. are not capable of maintaining a nuclear reaction), primarily sources of ionizing radiation, including those used in industry, science and medicine (americium, cobalt, radium, strontium, cesium, etc.).

Evaluating the vector of terrorist threats, counter-terrorism experts conclude that potential targets of nuclear and radiation terrorism may be:

• nuclear-hazardous and radiation-hazardous enterprises (e.g. nuclear cycle enterprises);
• radiation-sensitive enterprises (for example, those producing agricultural and food products, medicines, etc.);
• territories and objects of mass presence of people (airports, sea and river ports, railway and bus stations, metro, stadiums, concert halls, banks, shopping malls, hospitals, etc.).

The following may be subject to radioactive contamination: the human environment, various objects, materials, raw materials, air, water and food products, drinks, clothing, banknotes, securities, souvenirs, advertising products, etc. A terrorist act with radiation exposure may be carried out suddenly, quickly and covertly.

Considering these circumstances, one of the most important requirements of the Laws of the Russian Federation «On the Use of Atomic Energy» and «On the Radiation Safety of the Population» is the creation of a unified system of planning, coordination, control and implementation of a set of technical and organizational measures aimed at:

• preventing theft of radioactive and nuclear materials and their damage;
• preventing such materials from entering the human habitat.

A special place in the system of measures to prevent nuclear and radiation terrorism is occupied by measuring and information technologies aimed at preventing the unauthorized distribution of nuclear and radioactive materials. They are based on the targeted use of technical means of monitoring ionizing radiation, which allow for the timely detection and identification of sources of ionizing radiation. To achieve maximum effect, various means of monitoring are combined into a multi-level hardware system, forming several lines of defense to prevent possible criminal actions by terrorists.

An important component of such a system are stationary pedestrian radiation monitors — devices installed in places where people pass and monitoring the presence of RM and NM by registering ionizing radiation. The use of monitors allows the creation of the first line of defense, preventing the carrying of such materials from the protected object or into it.

The technical requirements that radiation monitors of nuclear materials must meet are regulated by GOST R 51635-2000 «Radiation Monitors of Nuclear Materials». According to it, pedestrian radiation monitors must be devices designed to detect NM and other RV by their gamma and/or neutron radiation. Depending on the detection threshold associated with the mass of the standard sample (SS) of NM, they are divided into the categories given in Table 1.

Table 1 Requirements for the mass of detectable NM according to GOST R 51635-2000

Notes:

1. CO from plutonium — plutonium standard sample, containing at least 98% plutonium by mass (239Pu content at least 93.5%)
2. Uranium CO — standard sample of uranium, the content of the mass fraction of uranium is not less than 99.75% (the content of 235U is not less than 89%)

The detection thresholds specified in the table correspond to the probability of detecting a radioactive sample of at least 0.50 (with a confidence probability of at least 0.95) when moving it through the minimum sensitivity zone of the monitor. As follows from Table 1, gamma radiation monitors are capable of detecting NM in smaller quantities, compared to neutron radiation monitors. In this regard, the main detection channel of the monitor is considered to be the gamma radiation registration channel.

Depending on the purpose and location of application of radiation monitors, various options for their construction are possible. It is necessary to take into account the fact that gamma radiation of the RV sample can be shielded by placing it in a protective metal container. The most probable container materials (due to availability) are lead and steel. When using a container with a sufficient wall thickness (for example, ~ 5 mm for lead), there is a significant loss in the quality of detection by the gamma radiation registration channel. Neutron radiation is not weakened by a metal container.

In this regard, combined detection methods are widely used. One of them is the simultaneous use of gamma and neutron radiation detectors in monitors. The second method is the creation of a combined monitor that has a gamma radiation recording channel and a metal object detection channel. With both combination methods, the decision to detect RV is made when any of the recording channels is triggered (an alarm is issued).

As gamma radiation detectors in modern radiation monitors, scintillation detectors are usually used, either based on an inorganic scintillator Nal (T1), or based on a plastic scintillator. Each of these types of detectors has its own advantages and disadvantages.

The advantages of detectors based on inorganic scintillators include smaller dimensions and weight. The sensitivity of these detectors is maximum in the low-energy gamma radiation region, which includes gamma radiation of nuclear materials:

• for uranium-235 — energy 143 keV and 204 keV,
• for plutonium-239 — energy 129 keV and 414 keV.

In this energy range, the sensitivity of both types of detectors is comparable. At high energies, which are characteristic of gamma sources such as cesium-137, cobalt-60 and others, detectors based on plastic scintillators have an advantage in sensitivity.

One of the main characteristics of radiation monitors is the frequency of false alarms. With a continuous monitoring method, it should be no more than one false alarm per 8 hours of operation in the absence of pedestrians passing through the monitored space. For a monitor operating in a continuously changing radiation background (Earth), this requirement is quite strict.

In order to improve the efficiency of operation, an operating mode is used with the launch of the processing algorithm from the presence sensor, which determines the presence of a person in the controlled space. In this case, the issuance of a trigger signal is impossible if there are no passages. For the operating mode of the monitor with a presence sensor, in accordance with the GOST requirement, the frequency of false alarms should be no more than one per 1000 passages. This requirement is less stringent than the requirement for the frequency of false alarms in the continuous monitoring method. However, it should be taken into account that when working with a presence sensor, the monitor is not able to detect the passage of a sample of the radioactive substance through the controlled area.

The determining factor for meeting the requirements for detection probability and false alarm rate is the set time parameters for the movement of the radioactive sample. These parameters are the speed of movement and the duration of the time interval during which the sample is in the controlled space. The slower the RV sample moves, the more time (exposure) can be allocated for processing the information. The required detection threshold P0 and exposure time T are related by the relationship:

P0 ~ T -0.5 ,

and the false alarm rate Fл decreases approximately linearly with increasing T:

Fл ~ T.

Therefore, the exposure time T of the monitor is selected as long as possible.

The following options are used for organizing the inspection:

1. inspection of people in free passage mode without stopping in the control zone;
2. inspection of people using a forced stop in the control zone.

In the first variant, the exposure time, as a rule, does not exceed -1 s. According to the second variant with a stop in the control zone, the exposure time is chosen to be 2-3 s, while the gain is -1.5 times in the detection threshold (it can be increased), which leads to an increase in noise immunity.

The tactics of using monitors are determined by the conditions at a specific object and the control tasks. The diversity of objects and tasks has led to the appearance on the market of various options of radiation monitors, differing in design, configuration and method of implementing recording channels.

According to the tactics of application, pedestrian radiation monitors can be divided into the following groups:

• portal, made in the form of arches or racks, and representing autonomous radiation monitoring equipment;
• built into various barrier devices (turnstiles, cabins, gateways, doors).

The latter usually work in conjunction with the access control and management system (ACS), and monitor local areas such as doorways, passages in corridors.

For a complete presentation of monitors as a separate group, it is necessary to single out ACS devices — turnstiles and cabins, which include a radiation channel that performs the functions of a radiation monitor. Table 2 presents information on the characteristics of the most well-known domestic pedestrian radiation monitors (taken from the manufacturers' websites).

Figure 1 shows the appearance of the combined radiation monitor «Spectrum» manufactured by FSUE «Dedal» (Dubna) since 2000 (various modifications), Figure 2 shows the radiation monitor «Arka-1P» manufactured by ZAO «Intra» (Moscow), Figure 3 shows the products of the Scientific and Production Center «Aspect» (Dubna): the monitor stand «RM-1SM» and the portal «Yantar-2P». Figure 4 shows the monitors «TMKP-111» (left) and «TSRM-61» (as part of the ACS) manufactured by VNIIA (Moscow).

Figure 1. External appearance of the combined radiation monitor «Spectrum»

Figure 2. External appearance of the Arka-1P radiation monitor

Figure 3. External appearance of the radiation monitors: RM-1SM stands (left) and Yantar-2P portal.

Figure 4. External appearance of the monitors: TMCP-111 (left) and TSRM-61.

Foreign radiation monitors by several companies, the most famous of which is «Canberra» (USA). The main technical characteristics of the manufactured products correspond to the best domestic samples, surpassing them in design and user interface. Figure 5 shows a pedestrian combined radiation monitor «IRM-22A» (Canberra), its cost is approximately 1.5-2 times higher than the domestic analogue «Spectrum». RV detectors are plastic scintillators with a volume of 45 liters.

Figure 4. External appearance of the radiation monitor manufactured by Canberra (USA)

Thus, currently advanced technologies in the field of monitoring radioactive and nuclear materials make it possible to build a reliable registration system on the path of their unauthorized movement. Radiation monitors produced by domestic manufacturers meet modern requirements for equipment for preventing terrorist acts. A wide range of equipment capabilities allows you to choose the most suitable version of the radiation monitor in a particular case, depending on the tactics of use and operating conditions.

The characteristics of pedestrian radiation monitors can be found on page 26 of the journal «Modern Security Technologies» No. 3(22), 2007.