| dc.description.abstract | Above 1 MeV of incident neutron energy the fission fragment angular
distribution (FFAD) has generally a strong anisotropic behavior due to the combination of
the incident orbital momentum and the intrinsic spin of the fissioning nucleus. This effect
has to be taken into account for the efficiency estimation of devices used for fission cross
section measurements. In addition it bears information on the spin deposition mechanism
and on the structure of transitional states. We designed and constructed a detection device,
based on Parallel Plate Avalanche Counters (PPAC), for measuring the fission fragment
angular distributions of several isotopes, in particular 232Th. The measurement has been
performed at n_TOF at CERN taking advantage of the very broad energy spectrum of the
neutron beam. Fission events were recognized by back to back detection in coincidence in
two position-sensitive detectors surrounding the targets. The detection efficiency, depending
mostly on the stopping of fission fragments in backings and electrodes, has been computed
with a Geant4 simulation and validated by the comparison to the measured case of 235U
below 3 keV where the emission is isotropic. In the case of 232Th, the result is in good
agreement with previous data below 10 MeV, with a good reproduction of the structures
associated to vibrational states and the opening of second chance fission. In the 14 MeV
region our data are much more accurate than previous ones which are broadly scattered. | en_US |
