The invention provides a data processing device for processing an reference background spectrum and a measurement spectrum of a radioactive material represented by a multichannel spectrum to acquire energy region information of detected gamma rays comprises: energy region dividing means for degenerating multichannel spectrum into a degenerated spectrum of limited channels; degenerated spectrum calculating means for calculating a background and measurement degenerated spectrum corresponding to degenerated spectrum of limited channels respectively; energy ratio calculating means for calculating a energy ratio based on the calculated background and measurement degenerated spectrum; peak-detection means, for searching a peak value in the calculated energy ratios; energy region determining information for determining a corresponding energy region of gamma rays based on the searched peak value in the energy ratios. The invention also provides a radiation detection method and a radiation detection system employing the data processing device.