Fig. 1. Schematic of detection positions

Fig. 2. Schematic of counting system

Fig. 3. Single pulse signal of plastic scintillator

Fig. 4. Energy spectrum fitting curve for source in large-area plastic scintillator at position 1 (a) ⁶⁰Co，(b) ¹³⁷Cs

Fig. 5. Background and source energy spectrum for different position of large area plastic scintillatorr (a) ⁶⁰Co，(b) ¹³⁷Cs

Fig. 6. Signal-amplification linear relationship

Fig. 7. Relationship between the threshold voltage and detector efficiency for the ⁶⁰Co source at different positions of the large-area plastic scintillator

Fig. 8. Relationship between threshold voltage and MDA and efficiency

Fig. 9. Relationship between threshold voltage and MDA and efficiency for ¹³³Ba source