Large-area plastic scintillators are widely used in radiation monitoring. They are typically employed as gamma radiation counters through signal over-threshold detection. The value of the threshold voltage affects the detector efficiency and minimum detectable activity. When rays are incident at different positions of the large-area plastic, the photon collection efficiencies generated through collecting end pair differ, leading to differences in the detector efficiency.

This study aims to explore appropriate threshold voltage to reduce the differences and maintain a low minimum detectable activity (MDA), hence achieving high detection efficiency for large-area plastic scintillator.

Firstly, an energy spectra acquisition system was designed using STM32F429 produced by STM semiconductor as the main control chip and a counter for detector efficiency testing of large-area plastic scintillator in size of 400 mm×300 mm×50 mm. Then, the reasonable threshold voltage was determined according to the background energy spectra of the large-area plastic scintillator, the energy spectrum of the ⁶⁰Co and ¹³⁷Cs sources at different positions of the large-area plastic scintillator with acquisition time of 3 min, and signal-amplification relationship.

The determined threshold voltage using above mentioned method for ⁶⁰Co source is 93.7 mV. To achieve high detector efficiency and low MDA for the large-area plastic scintillator, the best determined threshold voltage for ⁶⁰Co source is 95 mV. At this voltage, the detector efficiency for ⁶⁰Co source is 22%, the MDA is 78 Bq.

The proposed method has reference value for large-area plastic scintillation designing and manufacturing detectors.