A Mobile System for Detecting Gamma-Radiation Sources: II. Design and First Experiments

Cv.N. Lazarov¹, Ch.B. Lenev³, M.G. Mitev², L.T. Tsankov¹

¹Dept of Nuclear Engineering, University of Sofia, Bulgaria
² Dept of Electronic Technique, Technical University of Sofia, Bulgaria
³ Institute of nuclear research and nuclear energy, Bulgarian academy of science, Sofia, Bulgaria

The problems concerning the building of a mobile system for searching lost or 'orphan' radioactive sources are discussed. A hardware design as well as methods for measurement, data acquisition and real time data processing are suggested aiming at a maximal approach to the theoretical detectable limits at a reasonable cost. The other limiting conditions are:

• use of a 3"x3" NaI(Tl) scintillation detector;
• covered energy range 60-3000keV;
• the need of applying adaptive estimates of the stationary signal component (i.e. the background) using the measurements history in order to minimise its fluctuations and hence to diminish the threshold detectable level;
• he possibility of operation in various vehicles, including a fully automated operation mode (i.e. without operator);
• saving the information about the coordinates, time and the measured values to allow a retrospective analysis (via transferring data to an external computer).

A resonant charge-code transducer [1] is used for shaping the signal coming from the scintillation detector and for its subsequent amplitude analysis. This method is based on a pulse excitation of a LC circuit and counting the maxima of its decreasing oscillations until a fixed threshold level is reached. The latter count is used as a channel number in a histogram memory. So, this spectrometer has a genuine logarithmic energy scale. A basic advantage of the logarithmic scale is the possibility to achieve a reasonable energy resolution in a wide energy range using a limited number of channels (128 or 256).
In order to diminish the energy consumption and hence to ensure a long time operation in independent mode it is suggested the data acquisition, processing and saving system to be based on microcontrollers of the series PIC16 of MICROCHIP. A time performance analysis is carried out proving that a real time signal acquisition and processing is possible.
A Global Position System (GPS) receiver is built in the system to fix coordinates and time during the survey. A channel for communication with an operating centre via the public celular net (GSM) is also provided. A prototype of the proposed system is realised and some first results of its tests are given.

[1] M. Mitev, Ch. Lenev. Charge to Digital Logaritmic Resonant Converter. National Scientific Conference “Electronics ‘99”, book III, 28 -33, Sozopol, September 23-25, 1999.