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Two digital spectroscopy systems have been developed to handle high rate events in active interrogation applications. In the first system, signal from a PMT is directly digitized by a digitizer and then processed on a PC offline, using deconvolution method. By bypassing the preamplifier, this system can handle extremely high count rate without sacrificing energy resolution very much. To reduce the amount of data needed to be transferred between the DAQ system and the PC, another system is developed based on fast ADC and FPGA. In addition to the merits of the first system, real-time DSP is implemented on the FPGA chip and energy spectrum is generated simultaneously during the measurement. Systems are used in photofission experiments utilizing a 6 MeV pulse mode radiation therapy LINAC and nuclear resonance fluorescence (NRF) experiments utilizing a 3 MeV Van de Graff accelerator. Delayed gamma rays from photofission products are measured in between interrogation pulses. Resonant peaks from NRF reaction are observed in various samples.