SYSTEMS AND METHODOLOGIES AVAILABLE 1. Installed g or neutron radiation monitors

Installed g or neutron radiation monitors working in real time are designed to automatically detect the presence of radioactive materials or sources being carried by pedestrians or transported in vehicles, in order to keep a check on any unauthorized movement of nuclear materials or sources [4, 5]. These types of radiation monitors are often known as portal monitors

and typically consist of an array of detectors in one or two vertical pillars with associated electronics. The instrument sensitivity is strongly dependent upon the distance, volume of the detector and monitoring time [6]. The detection of sources in vehicles is difficult owing to the inherent shielding of the vehicle structure and its components. Careful consideration should therefore be given to selecting the optimum location to install fixed radiation portal monitors so that they can be most effective. It is necessary to place the instrument at a location where the speed of the vehicle is controlled and reduced.

The cost of equipment is negligible considering the seriousness of the problem, and the State must balance the amount it is prepared to invest against the objectives it is aiming at. The following should be considered by the authorities in charge to identify the best location from the point of view of the efficiency of the detection of orphan sources:

— When a radioactive source is no longer in use and is not under proper, qualified supervision, it could be abandoned without any special warn-ing signs on it and treated as waste. In that case, and dependwarn-ing on its size or its appearance, it might enter into a recycling process, with the risk that it will be burnt or melted and then dispersed. Of particular interest in this connection are scrapyards, garbage areas, foundries, steel plants, etc.

— When radioactive sources are lost from normal control, they can either remain on the territory of the State or can be moved out without any malevolent intention. If the probability of these events is significant and if the locations of the potential border crossing points can be identified, it would be useful to consider the detection of such traffic, in which case not only road and railway border crossings should be considered but also rail-way stations, seaports and airports.

When a programme is formulated to detect orphan sources, it is essential to try to detect them at places where sources could be inadvertently disposed of, such as scrapyards, foundries and garbage areas. The entrance gates of such places should be equipped with the appropriate detectors.

3.2. Transportable gradiation monitors

Three types of gradiation detectors can be used [7, 8]:

(a) Surface stationary installations.This is the same type of equipment as the installed system but the detectors are generally smaller and they can be

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put close to the sample for control. For example, the equipment can be accommodated in a crane and the display and operation can be controlled by a portable computer. The advantage of this is that a smaller source can be detected, as the shielding within a load of ferrous scrap is much less than in a full vehicle load.

(b) Vehicle mounted systems.Generally large detectors are used together with automatic data logging of gross count rate and dose rate, along with positions derived from the Global Positioning System (GPS). One such system developed in India is the Environmental Radiation Monitor with Navigational Aid (ERMNA), which can be used for mapping the environmental radiation background [9]. ERMNA is mounted in a suitable vehicle and the survey route is planned according to whether the survey is to be conducted to search for an orphan source or for large area contamination. The radiation mapping of the route followed by the vehicle is achieved by recording the g dose rate data tagged with positional co-ordinates on a real time basis. Since the railway system has a very large network in India, mapping along the rail routes will cover a significant portion of the Indian landscape [10]. The Aerial Radiation Counting System (ARCS) made in the United States of America has multiple detectors of various sizes, including a 16 L Geoline NaI(Tl) detector, and the system records gross count rate along with positional co-ordinates. The g count rate profile of the route followed by the vehicle helps to locate the orphan sources.

(c) Airborne systems. Airborne systems use larger detectors than other systems. They can be used to search for distributed activity due to accidents and also for single sources (see the example of airborne detection systems described in Sections 4.4 and 4.5). Their advantages include the ability to cover a larger ground area quickly, but in an urban area, where structures are higher than two floors, the effects of shielding and geometry can significantly limit the detection level.

3.3. Portable gradiation monitors

Handheld gradiation monitors use very small detectors; they generally indicate counts per second and have an audio output to identify changes in count rate by varying the pitch of the note generated. These detectors are very sensitive and can be used to locate a source in a large volume.

4. OPERATIONAL EXPERIENCE: THE CASE STUDY OF GEORGIA