The neutron Time of Flight (n_TOF) facility at CERN is a source of a wide-range (1 eV

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A series of neutron capture cross section measurements of interest to nuclear astrophysics have been recently performed at n TOF, the neutron spallation source operating at CERN. The low repetition frequency of the proton beam driver, the extremely high instantaneous neutron flux, and the low background conditions in the experimental area are optimal for capture cross section measurements on low-mass or radioactive samples. An overview of the measurements performed during the two experimental campaigns in 2002 and 2003 is presented with special emphasis on the measurement of the capture cross sections of the Os isotopes relevant for the cosmochronology based on the Re/Os clock.

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Abstract.A series of measurements of neutron induced fission cross section of various TRU isotopes have been performed at the CERN n TOF spallation neutron facility, in the energy range from thermal to nearly 250 MeV. The experimental apparatus consists in a fast ionization chamber (FIC), used as a fission fragment detector with a high efficiency. Good discrimination between alphas and fission fragments can be obtained with a simple amplitude threshold. In order to allow the monitoring of the neutron beam and to extract the n TOF neutron flux, the well known cross section of the 235 U(n,f) reaction, considered as a fission standard, has been used. Preliminary results for the cross section are shown for some selected isotopes such as 235 U, 233 U and 245 Cm in the energy range from 0.050 eV to about 2 MeV.