Identification of alpha-emitting radionuclides in ambient measurements using unfolding method based on iterative and maximum likelihood algorithms.

An airborne particulate radioactivity monitor measures atmospheric radioactivity at decommissioning sites and nuclear facilities. Analyzing radionuclides utilizing alpha particle energy spectra is challenging with the monitor due to measurement conditions, such as ambient pressure and the presence of protective sheets on the detector. We applied an unfolding method to the measured spectra to enable high-energy resolution analysis within the monitor's detector structure. The performance of the unfolding method was evaluated through the acquisition of alpha particle spectra with various sources, including an 241Am source, a 239Pu/241Am/244Cm composite source, and a 148Gd/241Am/244Cm composite source. The unfolding spectra are derived from the detector response created from radiation simulation and the measured spectrum utilizing the iterative successive approximation method and non-negative least squares as the unfolding methods. A spectrum of an 241Am source acquired with a Si-PIN photodiode with protective sheets at ambient pressure has an energy resolution of 0.5 MeV at full width at half maximum. The energy resolution of the unfolding spectra is better than 0.2 MeV for both unfolding methods, and the unfolding spectra are appropriate to identify alpha particle sources. The radioactivity determined from the unfolding spectra agrees with that of the source within a range of uncertainty. The unfolding method significantly enhances the performance of nuclide analysis and facilitates the quantitative determination of radioactivity in the ambient environment.