Abstract: A current-mode detection system and an imaging detection system are designed according to the radiation level at 5.5 m from the target with shielding collimator. The two systems are simulated by Geant4. When the areal density is 10 mg/cm2 and the primary neutron yield is 1012, the current-mode detection system can be used for fuel areal density diagnosis with a signal to noise ratio(SNR) of 40∶1. When the areal density is 10 mg/cm2 and the primary neutron yield is 1011, the imaging detection system can be used for fuel areal density diagnosis with an SNR of 10∶1. If the areal density increases, the SNR is better. However, when the primary neutron yield is 1012, the neutron overlapping occurs. The overlapping probability can be reduced by shortening the exposure time or decreasing the thickness of the plastic scintillator.