Abstract: For critical nuclear cases including radioactive material leakage, illegal freight of special nuclear material, this paper proposes a research on an energy spectrometer with neutron/gamma discrimination via a single probe to achieve detection and monitor of special nuclear materials and radioactive materials. Several difficult research points have been triggered and expanded to tackle the challenging studies on FPGA and EJ299-33A including the data acquisition circuit design of nuclear pulses, nuclear signal process, pulse discrimination and offline analysis of energy spectra. The host machine software is able to process the real-time energy spectra for offering reference information in making a correct decision of nuclear emergency response. The application of novel detector and the fractural theory based processing method enables rays discrimination via FPGA, which effectively solves discrimination difficulties of particles via traditional detectors and analog circuits and also justifies the viability of the fractural-frequency spectrum based method application to nuclear signal processing. The n/ detector is developed with versatile capacities in particle discrimination, energy spectrum measurement, and radioactive element identification. In a mixed radioactive field of neutron and gamma, the proposed detector not only can perform ray discrimination and energy spectra measurement, but also handles the challenges in energy spectrum analysis of different particle and nuclei identification.