Speaker
Description
Independent yield distributions of high-energy neutron-induced fission is of importance to achieve a good understanding of the fission process. Further more, these data also plays an important role in the development of Generation IV nuclear reactors. While the mass and charge yield distributions of thermal neutron-induced fission is well known, there is a lack of experiment data for high-energy neutron-induced fission.
To facilitate such measurements, a dedicated ion guide and a proton-to-neutron converter were developed and put to use in experiments at the IGISOL facility in Jyväskylä. In parallel, a simulation model based on GEANT4, MCNPX, and GEF was developed to optimize the setup. To benchmark the simulation model, foils located in the ion guide during the irradiation were taken out from the ion guide and analysed using $\gamma$-ray spectrometry. The measurements were conducted after the beam was turned off and after several days of cooling using a HPGe detector under shielding. Based on the identified $\gamma$-ray transitions, the productions of corresponding fission products and neutron activation products could be calculated and used to benchmark the transportation and collection of fission products in the simulations, as well as the simulated neutron flux. The conclusion is that the transportation of fission products in the helium gas simulated by GEANT4 agrees to the measurement very well, while the transportation of fission products in the uranium targets deviates by about 10~%. Furthermore, the neutron flux at high-energy seems to be overestimated by about 40~%.
From the benchmark, it has been proven that the predictive power of the model is remarkable and sufficient for the purpose of modeling the ion guide. Furthermore, the parameters involved in the simulations, such as neutron production, distance between the neutron source and the ion guide, the volume of the ion guide and so on, play an important role in the optimization of the setup. However the lower fission rate than expected suggests the optimization on these parameters may not be enough to achieve sufficient intensity of fission products, especially for nuclei far from the line of stability. To further improve the setup, we are considering to implement electric field guidance, similar to the RF structure of the CARIBU gas catcher~[1], to more efficiently collect the fission products.
[1] G. Savard, A.F. Levand, and B.J. Zabransky. The CARIBU gas catcher".
In: Nucl. Inst. Meth. B 376 (2016), pp. 246-250.
