Artificial diamonds as radiation-hard detectors for ultra-fast fission-fragment timing

In the framework of the construction of the double time-of-flight spectrometer VERDI, where we aim at measuring pre- and post-neutron masses directly and simultaneously, ultra-fast time pick-up detectors based on artificial diamond material were investigated for the first time with fission fragments from 252Cf ( 0.5 MeV / u < E / A < 2 MeV / u ). Signal stability under a high radiation fluence was determined up to at least 10 9 fission-fragments/cm2 together with more than 3.5×109 neutrons/cm2 and 3×1010 α - particles / cm 2 . This fluence is characteristic for fission experiments. The pre-requisite for the observed signal stability is the application of priming of the diamond material with a strong β - source for about 48 h. The intrinsic timing resolution of a 100 μ m thick polycrystalline CVD diamond detector with a size of 1×1 cm2 was determined to σ int = ( 283 ± 41 ) ps by comparison with Monte-Carlo simulations. Using broadband pre-amplifiers, 4-fold segmented detectors of same total size and with a thickness of 180 μ m show an intrinsic timing resolution of σ int = ( 106 ± 21 ) ps . This is highly competitive with the best micro-channel plate detectors. Due to the limited and batch-dependent charge collection efficiency of poly-crystalline diamond material, the detection efficiency for fission fragments may be smaller than 100%.