On-line Mobile in situ Gamma Spectrometry

Sammanfattning: Mobile in situ gamma spectrometry is an important instrument in the emergency preparedness for nuclear accidents. This work contributes through the development of strategies and analysis methods for two applications: an on-line search for gamma-emitting orphan sources and on-line estimation of the depth distribution of 137Cs. A strategy and analysis method dedicated to the detection of orphan sources in the environment has been developed. The work has mainly been performed with 137Cs sources, but can be applied to any gamma-emitting radionuclide. The strategy is based on measurements with a short sampling time(approx. 1–5 s) at a constant velocity (~40–50 km/h). The analysis method is based on the ratio between an observed quantity from the most recently measured 15 seconds and the preceding 60 seconds. Hypothesis testing is applied to decide if a high ratio is likely to be caused by an orphan source present in the environment or by natural variation. Since consecutive measurements are serially correlated, the underlying statistical distribution of the natural variation was estimated experimentally. Several quantities have been evaluated as input to the analysis procedure. For 137Cs, the quantity showing the highest detection distance was the number of counts in the 137Cs window, corrected for the 40K in the spectrum (potassium-stripped counts). A 2 GBq 137Cs source was detected at a distance of 180 m from the road, using a 4129 cm3 NaI(Tl) detector. The depth distribution of 137Cs significantly affects the photon fluence distribution above ground, thus it is vital to know the depth distribution when estimating the true activity per unit area or unit mass by means of in situ gamma spectrometry. In this work, the peak-to-valley method (based on the ratio between the number of counts from primary and forward-scattered photons) was evaluated for on-line estimation of the depth distribution directly from the measured spectra. The depth distribution was described by a truncated Lorentzian distribution. The objectives were to evaluate the applicability of the peak-to-valley method in mobile in situ gamma spectrometry, and to determine a minimal radioactivity inventory giving reasonable statistical un-certainty within a counting time of less than ten minutes, for a large HPGe detector system (100% relative efficiency). The results show that the method can be applied when the activity concentrations of 137Cs is above ~100 kBq/m2 , accepting an intro-duced uncertainty of about 50% in the estimated true activity.