Mass yield, isotopic yield and kinetic energy distributions for heavy masses in thermal neutron induced fission

A more accurate knowledge of heavy fission product yields is needed to improve our understanding of the fission process and to increase the efficiency in nuclear reactor operation. High resolution measurements of fission observables can be done with the recoil mass spectrometer Lohengrin at the Institut Laue-Langevin in Grenoble, France, which was designed to measure fission fragment characteristics from neutron induced fission. The mass separator is situated at the research reactor of the institute and permits the placement of an actinide layer in a high thermal neutron flux. It separates fragments according to atomic mass, kinetic energy and ionic charge state by the action of magnetic and electric fields, and allows to determine these distributions, isotopic yields, and the fragment gamma-decay characteristics. Almost all fissile isotopes rangings from Th to Cf have been investigated with Lohengrin, and in particular mass and nuclear charge distributions for light fission products have been determined. The experimental set-up used to investigate the mass yield and the fission fragment kinetic energy distributions in the heavy mass region, or in the light mass region, is done by coupling a high resolution ionization chamber to the spectrometer. For low mass fission products a double anode ionization chamber is used to determine furthermore isotopic yields within a mass line. For fission fragments of higher mass this separation is no longer possible, and gamma-spectrometry is used instead to identify the contributing nuclear charges. The experimental set-up is shortly described in the present paper, and we present preliminary results for measurements of thermal neutron induced fission in 235U(nth, f), 239Pu(nth, f) and 241Pu(nth, f)