Abstract
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 time-of-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.
Highlights
The VERDI time-of-flight spectrometer is dedicated to measurements of fission product yields and of prompt neutron emission data
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
Summary
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. It will be used as the start detector in a 2E-V configuration, i.e. that the time of flight is measured for only one of the fragments In this configuration only the post-neutron mass of fission fragment can be determined directly. Since it is not possible yet to make very thin diamond detector that would allow fragment to cross them we have to use a different detector for the 2E-2V configuration This second detector will be composed of an electrostatic mirror and a micro-channel plate used to detect secondary electrons emitted by the target when fission fragments are emitted.
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