Implementing an Optimization Algorithm to Amend Spectra-Dosimeter Conversion Coefficients

Using conversion coefficients ensures that an ambient dose equivalent is obtained from the pulse-height spectra. In spectrometric dosimetry systems, the curve of the conversion coefficient may be obtained by the subdivision of the measured gamma spectra into multiple energy bands. In certain techniques, namely the Gaussian elimination (GE) technique, the calculated lower energy band conversion coefficient should be taken into account to find the higher ones. Thus, uncertainty in calculating the conversion coefficient in the initial step may influence the next step of the calculation. This study intends to obtain the conversion coefficient of the determined energy band for each radiation source individually in order to reduce the uncertainties of the higher energy band conversion coefficients. In this proposed technique, the measured gamma spectrum subdivides into multiple energy bands. Unlike the GE technique, conversion coefficients of each energy band for all monoenergetic radiation sources are calculated with the help of an optimization algorithm. Then, the mean value of the calculated conversion coefficients of that determined energy band is considered to be a conversion coefficient for that assumed energy band. Since the number of energy bands become more than one for higher energy radiation sources, the particle swarm optimization algorithm is used to calculate and optimize the conversion coefficients by solving a system of linear equations with m unknown variables. In this work, the sodium iodide doped with thallium (NaI(Tl)) scintillation detector, which is an affordable and widely used nuclear radiation detector at room temperature, is applied as the spectra-dosimeter instrument. To determine the NaI(Tl) scintillation detector readout and response function, the Geant4 simulation code is used. A relatively good agreement was observed between the gamma energy spectrum and the dose obtained from our configurations and these specifications from the radioisotopic gamma source.