Speaker
Description
The study of nuclear fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. In the framework of a collaboration between the CEA, the LPSC and the ILL, a program of actinide fission yields measurements has been initiated for several years at the LOHENGRIN spectrometer. However, the measurement of very low fission yields in the symmetry region and the heavy wing of the distributions are difficult to achieve due to the strong contamination by other masses with much higher yields and require the development of a new experimental setup.
This talk will first present the results of an absolute measurement of the $^{235}$U(n$_{th}$,f) mass yields using an ionization chamber placed at the exit of the spectrometer. Although very well documented in the literature, these yields show uncertainties lying from 3% to 10% with large discrepancies between libraries and a lack of correlation matrices. New experimental data obtained at the LOHENGRIN spectrometer will be detailed, along with the measurement method and the production of experimental covariance matrices.
The second part will show the development of a Time of Flight (ToF) line in order to improve the background rejection in the mass yield measurements. In the symmetry region, the precision of the measurement is limited by the background estimation due to the charge exchanges with the residual gas of the separator. We foresee to analyze the events using a triple coincidence (ΔE x E) x ToF, whereas today only ΔE x E selection is available. The new ToF line is built using Si$_3$N$_4$ foils and electron detectors for the start and stop detectors. The talk will present the choices made for the electron detectors technology along with the progress achieved on the ToF line characterization.