A real time, isotope identifying gamma spectrometer for monitoring of pedestrians

The demand for installation and use of radiation monitors at border crossing points and other locations in a country has significantly increased due to fact that terrorist threats may also involve the use of radiation dispersal devices (RDDs, dirty bombs). One of the problems, customs officers and security forces run into is caused by passengers at airports having undergone a medical treatment, as they cause "innocent" radiation alarms with a high frequency. Since half-life of the isotopes, which are used for medical treatment, ranges from hours to several days, the dose-rate for several days and weeks after the treatment is high enough to trigger a radiation alarm of a border monitor. In this paper we describe the development and test of a real time gamma spectrometer, based on a commercially available large volume NaI detectors and a computer-coupled multi channel analyser (MCA) with fast data collection, stabilisation of the energy scale and isotope identification software. The system is capable of measuring a burst of gamma spectra in second intervals, to identify the isotopes and to produce a "green" alarm in real time when a medical isotope is present and a "red" alarm in other cases. The system has successfully been tested under laboratory conditions, as well as at an international airport and on patients of the radiation ward in hospitals. This work has been performed under IAEA Research Agreements between the Atom Institute of the Austrian Universities, the Austrian Research Center Seibersdorf and the International Atomic Energy Agency.