Fissile material detector having an array of active pixel sensors

A system and method detecting fissile material. According to one embodiment, a detector includes an array of active pixel sensors wherein each active pixel sensor operable to integrate a charge generated by radiation that is incident upon the active pixel sensor by using a charge-sensing element during an integration phase. Then, a voltage signal is generated that is based upon the integrated charge intensity during a readout phase. After reading out the voltage signal during the readout phase, the active pixel sensor is reset ready to integrate again. The integration phase is typically set to a time interval that is optimal for detecting radiation from fissile material, and the system is typically able to count individual events occurring in an integration period, and to digitally sum these event counts to measure rate of radiation events.