Neutron dose assessment by unfolding spectra from neutron activation analysis of a smartphone

Smartphones are widely available and can serve as opportunistic dosimeters in nuclear emergencies. However, the complex internal structures of smartphones lead to neutron moderation, scattering, absorption and gamma-ray self-absorption. These effects may induce substantial errors when conducting activation analysis in a neutron field. This study developed a neutron spectrum unfolding method using Geant4 Monte Carlo simulation with the SAND-II iterative algorithm. An equivalent smartphone model was established to approximate the real sample in both neutron transport and induced gamma-ray emission. The neutron spectrum simulated inside the smartphone was used as the initial input spectrum for unfolding. To verify this method, a smartphone sample was irradiated by a²⁵²Cf spontaneous fission source, and induced gamma rays from elements such as Au, Ni, Ti, and Al in the smartphone were measured by a high-purity germanium detector. The calculated reaction rates of the corresponding activation reactions agree with experimental values within 5%, and the unfolded neutron spectrum shows less than 2.1% deviation from the reference spectrum in the dose-dominant fast-neutron region. Based on the unfolded spectrum, the derived neutron effective dose rate was 0.761 Sv h^-1, representing a difference of just 2.2% from the reference value.