Application of the spectral determination method to unified β-, γ- and X-ray spectra

Abstract

ABSTRACT So far, we have developed the innovative radioactivity quantification technique of spectral determination method (SDM) based on the first principle that a γ-ray spectrum obtained for a sample is a linear superposition of individual spectra of the radioactive nuclides included in the sample. We have applied this method to spectra measured with Ge, NaI detectors and liquid scintillation counter (LSC) and demonstrated that the determination uncertainty of the SDM was better than those of previous peak analysis methods for the spectra obtained from a sample including 8 or 9 radionuclides. In the present study we extended the SDM to apply to a unified spectrum composed of LSC and Ge detector, and the number of nuclides has been increased to 40. The new SDM code was named as SDM-LG. We selected 40 radionuclides from possible radionuclides included in nuclear debris and radioactive wastes in the environment which were produced by the nuclear accident in Fukushima in March. 2011. We carried out LSC and Ge measurements for 17 and 7 nuclides, respectively. The rest of the standard spectra among the 40 nuclides have been derived by simulation calculations utilizing the Geant 4.10.3 Monte Carlo simulation tool kit. The derived LSC and Ge spectra for each nuclide were unified to a single spectrum and the 40 sets of the composed spectra were completed as a unified database. We studied the determination accuracy of the SDM-LG analysis by examining a composed spectrum made of 40 radionuclides with equal intensities (Each has a radioactivity fraction of 2.5%.). The SDM result shows that the relative determination uncertainties of 35 nuclides are below 20%. It is also indicated that by removing 3 interfering nuclides the determination accuracy of the other 37 nuclides could be improved.