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SURVEILLANCE OF RADIOACTIVITY

What is called HOMELAND SECURITY in the United States is becoming more and more relevant for European countries: control of incoming goods for radioactive materials, which can be caused by faults or because of intentionally wasted radioactive sources.

The radioactive material must be emitting Gamma radiation to be detected. Nearly all radioactive elements emit these in one or the other way.

RGI manufactures scintillation detectors with the highest sensitivity for this type of radiation, normal size of the sensitive part being 1.8m height, 300mm width and 50mm thick. An evaluation computer measures the detector signal which is then displayed on a screen as well as being stored for documentation. Any incoming truck is checked for speed prior to starting documentation. The signal is constantly evaluated, which means the program detects all changes, including background. Speed of incoming trucks is measured by light barriers and later by a speed radar, with a photograph taken in case of alarm.

In addition to this radiation alarm system, another kind of detector can measure the energy level of the radiation and transmit a spectrum of the radiation to the evaluation computer. The spectrum is shown graphically on the screen and the kind of radioactive source/ radioactive element can be determined according to a peak in this spectrum.

Detector principle The detector, also called a scintillation counter uses a special material which generates very low light flashes, dark blue, that can be measured from outside by a light amplifier, a so called photomultiplier. The bigger the scintillator, the more sensitive the detector is. For the sensitivity it is important to have a big entrace, less important is the thickness of the scintillation material. Most of the radiation is absorbed in 50mm.

The window must be big enough to “see” the total loading of the truck. The schintillator material is a plastic material called PVT. It is very sensitive but cannot determine the energy of the radiation. Therefore a determination of the isotope is not possible. If this is necessary, an additional detector must be used. So, if necessary, after a normal alarm the truck has to be checked again to find out the type of radioactive material.

Evaluation of the signals

 For visualisation of the measuring signals, an industrial PC is used with all its features: Storage, printer connection etc. Also it has some electronics for outputs ( e.g. red light ). The measurement is continuous but only when a truck arrives does a special routine start. In the meantime, the measurment is able to do a correction for variations of background (sin some areas the detector is able to see whether it is raining ). The background correction avoids failure alarms and therefore the threshold for a real alarm can be closer to the background.

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