The VERDI fission fragment spectrometer

The VERDI time-of-flight spectrometer is dedicated to measurements of fission product yields and of prompt neutron emission data. Pre-neutron fission-fragment masses will be determined by the double time-of-flight (TOF) technique. For this purpose an excellent time resolution is required. The time of flight of the fragments will be measured by electrostatic mirrors located near the target and the time signal coming from silicon detectors located at 50 cm on both sides of the target. This configuration, where the stop detector will provide us simultaneously with the kinetic energy of the fragment and timing information, significantly limits energy straggling in comparison to legacy experimental setup where a thin foil was usually used as a stop detector. In order to improve timing resolution, neutron transmutation doped silicon will be used. The high resistivity homogeneity of this material should significantly improve resolution in comparison to standard silicon detectors. Post-neutron fission fragment masses are obtained form the timeof-flight and the energy signal in the silicon detector. As an intermediary step a diamond detector will also be used as start detector located very close to the target. Previous tests have shown that poly-crystalline chemical vapour deposition (pCVD) diamonds provides a coincidence time resolution of 150 ps not allowing complete separation between very low-energy fission fragments, alpha particles and noise. New results from using artificial single-crystal diamonds (sCVD) show similar time resolution as from pCVD diamonds but also sufficiently good energy resolution. 1. Prompt neutron multiplicities and mass distributions The last years have seen the development of many nuclear applications, such as nuclear reactors of 4th generation, spallation source etc. requiring precise nuclear data for reactions induced by neutrons with energy going up to 20MeV. Pre-neutron mass and total kinetic energy (TKE) of fragments produced in neutron-induced fission are routinely measured at the European Commission Joint Research Center (EC JRC-IRMM) by means of twin Frisch-grid ionization chambers, where the energy of both fragments is measured. Thanks to the momentum conservation it is possible to infer the pre-neutron mass, but it is necessary to correct for prompt neutron emission. To do this correction, it is assumed that fragments are ae-mail: [email protected] This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20136205003 EPJ Web of Conferences at an equal temperature. The number of neutrons emitted can then be inferred based on existing data. However, it was suggested that in some case the equal temperature approximation does not hold: for 237Np [1] and 235U [2] most of available energy coming from the incident neutron will lead to enhanced neutron emission from the heavy fragment. It was also shown that, following the chosen hypothesis variations in the fission yield could reach up to 30% for very asymmetric mass division [3]. Considering the important consequences that these corrections have on the mass yields it is very important to do systematic measurements on many systems and at different excitation energies to better understand prompt neutron emission from fission fragments. Another motivation to measure accurately mass and energy distributions of fission fragments emerges from the need to more efficiently model the fission process for evaluation of nuclear data. Many models that can predict neutron multiplicities for different systems are in development but for the moments they all require experimental fission-fragment mass and kinetic energy data as input parameter [4–6]. Unfortunately, for the moment, input data with sufficient accuracy are scarce and the excitation energy range is limited. With the help of these models and good input data it will be possible to provide more accurate evaluated data where experimental data are missing.