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

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