A method to determine the distance, shielding and activity of orphan 137Cs sources in mobile gamma spectrometric search

A mobile gamma spectrometer’s series of pulse height distributions (gamma spectra) acquired when passing a point source contains enough information to derive the distance to the source, its shielding and activity. Two Excel routines, SODAC and SSC, have been developed to determine these source parameters. SODAC determines the source-road distance by finding the best fit of the measured series of the primary photon peak count rates to a count rate function characteristic of the distance to the source. A possible source shield reduces the primary count rate and raises the count rate from Compton scattered photons in the shield. SSC calculates the shield mass thickness for common building material using ratio functions of scattered to primary count rates obtained from measurements on known shielding geometries. SODAC then calculates the source activity. The technique was practically tested for 137Cs point sources but can also be applied to other gamma-emitting radionuclides.