A fast algorithm for real-time monitoring of artificial radioisotopes in presence of variable natural radioactivity

The Catalan Environmental Radiological Surveillance Network consists of different types of scintillation gamma spectrometry detectors for the continuous and real-time automatic measurement of environmental radiation. The Network has a total of 35 monitors that obtain spectra every 10 minutes: 23 direct measurement monitors (13 with LaBr₃(Ce), 5 SrI₂(Eu) and 5 NaI (Tl) detectors), 10 particulate filter monitors (9 with LaBr₃(Ce) detectors and 1 with SrI₂(Eu)) and 2 river water monitors.

A method for the automatic and real-time quantification of the activity concentration of artificial, natural and NORM isotopes was developed and tested in the laboratory. The uncertainties in the activity concentrations, as well as the corresponding detection limits, were calculated applying the ISO-11929 standard. The developed and validated method that is exposed in the present study will be implemented shortly in all the stations of the Network.

The method is based on the analysis by spectral regions or ROIs (Regions of Interest). The method eliminates from the ROIs of the artificial (or NORM) isotopes of study the contributions due to emissions of natural isotopes (overlapping and Compton radiation), the ambient background, the possible intrinsic background of the detector and the contributions of other possible isotopes. As a result, an equation is generated for each isotope that allows us to obtain its net activity concentration (Bq/m3). This procedure is applied to determine the activity concentration of isotopes of natural origin (²¹²Pb, ²¹⁴Pb and ²¹⁴Bi), artificial (¹³¹I, ¹³⁷Cs and ⁶⁰Co) and NORM (²³⁴Th).

The method successfully eliminates the contribution of natural elements, intrinsic background and Compton contribution, both in situations with high and low activity of natural isotopes. Therefore, it allows obtaining the net activity concentration of the artificial isotopes of interest and eliminates the presence of false positives that could be produced by the presence of natural isotopes.