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 the fact that terrorist threats may involve the use of radiation dispersal devices (RDDs, dirty bombs). One of the problems that customs officers and security forces experience is a high frequency of "innocent" radiation alarms caused by airport passengers who have undergone a medical treatment. Since the half-life of the isotopes that are used for medical treatment ranges from hours to several days, the dose-rate for days and even weeks after the treatment is high enough to trigger a radiation alarm of a border monitor. In this paper we describe the development and testing of a real time gamma spectrometer based on a commercially available large volume NaI-detector and a computer-coupled multichannel analyzer (MCA) with fast data collection, stabilization 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 in the radiation ward of hospitals. This work has been performed under IAEA Research Agreements with the Atom Institute of the Austrian Universities, the Austrian Research Center Seibersdorf, and the International Atomic Energy Agency (IAEA)