Program - Brookhaven National Laboratory

direction was recently used at n TOF. This new configuration makes possible to detect fission fragments
at any angle between 0 ◦ and 90 ◦ so that the full angular distribution can be measured. Apart from the
interest on a precise knowledge of the angular distributions for developing nuclear models, this method
will make possible to properly correct the detection efficiency in the cross section measurements. The first
measurement with this experimental setup has been done at n TOF to measure the angular distribution
of fragments emitted in Th-232(n,f) reaction. These results represent the first measurement of the angular
distribution in neutron-induced fission covering such a wide neutron energy range, from fission threshold
up to 1 GeV. In addition, our results also confirm the anisotropic behaviour observed recently by Ryzhov
et al. [3] for Th-232, that was the unique measurement of the angular distribution of the fragments in
fission induced by neutrons in the range 20-100 MeV.
[1] C. Paradela et al., Phys. Rev. C 82, 034601 (2010) [2] D. Tarrío et al., Phys. Rev. C 83, 044620
(2011) [3] I. V. Ryzhov et al., Nucl. Phys. A 760 19 (2005)
HA 8 5:45 PM
Measurement of the 233U(n,g) and 233U(n,f) Cross Sections in the Resonance Region
I. Companis, M. Aiche, L. Mathieu, G. Barreau, G. Boutoux, S. Czajkowski, B. Haas, B. Jurado, J.
Matarranz, V. Simutkin, I. Tsekhanovich, d’Etudes Nucléaires Bordeaux Gradignan, CNRS/IN2P3,
Univ. Bordeaux 1, Chemin du Solarium, 33175 GRADIGNAN, France. G. Kessedjian, Laboratoire de
Physique Subatomique et de Cosmologie, CNRS/IN2P3, Univ. Joseph Fourier, INPG, 53 avenue des
Martyrs, FR - 38026 Grenoble Cedex, France. J. Heyse, P. Schillebeeckx, A. J. M. Plompen, EC-JRC,
Institute for Reference Materials and Measurements, Retieseweg 111, B-2440 Geel, Belgium. F. Gunsing,
CEA, DSM-Saclay, IRFU/SPHN, F-91191 Gif-sur-Yvette, France. G. Noguere, CEA, DEN-Cadarache,
F-13108 Saint-Paul-lez-Durance, France.
The 232 Th/ 233 U cycle is an alternative to the classical 238 U/ 239 Pu cycle, with the advantage of a lower
production of highly radiotoxic nuclear wastes. But as 233 U does not exist, it has to be produced in
nuclear plants. A reliable development of thorium-based implies a good knowledge of the breeding ratio
of such reactor. It puts severe requests to the accuracy of the capture cross section of 233 U, especially in
the resonance region. Since the available experimental data are scarce and sometimes contradictory, new
experimental studies are required. A new simultaneous measurement of σ(n,f) and σ(n,γ) will shortly be
performed at the neutron time-of-flight facility GELINA at Geel (Belgium). The fission events are detected
by a multi-targets high efficiency ionization chamber (IC). An efficient array of C6D6 scintillators is used for
the detection of gamma-rays and neutrons produced in capture and fission reactions. The disentanglement
between fission and capture gamma-rays can be achieved by the demand of anticoincidence between events
in the IC and the C6D6 detectors, whereas the pulse-shape analysis technique allows for the gamma/neutron
discrimination. Seen the difference in the fission and capture cross sections, the assignment of a gamma-ray
to one or the other reaction type has to be very effective and reliable. With the IC efficiency not absolute,
a correction should be applied to take into account the undetected fission events. To keep this correction
factor low and reliable, the efficiency parameter of the IC should be high and known with a high degree of
accuracy. The IC efficiency towards fission can be defined as a ratio between numbers of detected neutrons
with and without coincidence with fission fragments. It is therefore a value directly extractable from the
experimental data. MCNPX simulations will be presented and discussed, along with the results from test
experiments of the IC.
Corresponding author: L. Mathieu
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