NUMERICAL EVALUATION OF THREE-STEP SELECTIVE PHOTO-IONIZATION OF GD157 AND GD155 ISOTOPES

Abstract

A three-step selective photo-ionization for Gd155 and Gd157 isotopes by pulsed laser wasevaluated numerically. Due to large thermal neutron absorption cross sections, these two isotopes innatural gadolinium are good candidates as burnable poison to control reactivity early in the fuel cycleand allow extended fuel burn up. Rate equations have been used to calculate the dynamics of the isotopepopulations, ionization rate, and selection factor. Doppler broadening, as well as power broadening foratomic energy levels, has been considered in the calculations. The effects of laser intensity, frequencybandwidth, and laser detuning in the ionization process have also been assessed. By applying a variety ofconditions, the sensitivity of different parameters in the ionization process was investigated. The resultsshow that the ionization yield increases with the intensities of lasers, and becomes saturated at somespecific values. Depending on the energy bandwidth of each state, laser bandwidth and the detuning oflasers greater than particular values can affect the ionization yield. Due to small and finite isotope shiftsof energy states, the selection factor can be influenced by detuning and laser bandwidth. From theresults, it can be inferred that for Gd155 and Gd157 isotopes, ionization is more sensitive to the first andsecond laser bandwidth and detuning. From these results, optimum selection factors and ionization byusing appropriate laser intensities, bandwidth, and detuning can be achieved.