Dual gamma-ray and thermal neutron counter based on BGO and GAGG scintillators coupled to silicon photomultipliers

A dual radiation monitor capable of independently measuring gamma‑ray and thermal‑neutron count rates is presented. The instrument employs two inorganic scintillators with complementary sensitivities: a BGO crystal for efficient photon detection with negligible neutron response, and a GAGG crystal with high thermal‑neutron sensitivity due to its gadolinium content. Both crystals are coupled to silicon photomultipliers, enabling low‑voltage operation and a mechanically robust assembly. A photon‑subtraction method was developed to extract the neutron rate from the low‑energy window of the GAGG spectrum by modeling its photon contribution as a linear function of the total and low‑energy count rates measured with the BGO detector. Measurements performed near a research reactor, together with comparisons against a reference 3-He detector and a commercial Geiger counter, demonstrate linear response over a wide dynamic range and clear discrimination between photon‑only and mixed photon–neutron fields. The results confirm the suitability of the proposed system as a mixed‑field monitor, with potential extension to dose-rate monitoring applications after appropriate calibration.