CNRS Université Bordeaux

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The work performed at CENBG took place in a revaluation campaign of the fission cross-section 233U(n,f) in the range of fast neutrons (1-6.5 MeV). The cross-section is deduced from the number of detected fission fragments, the incident neutron flux on the target and the number of target atoms.

The fission fragments were detected by a set of two photovoltaic cells. The number of target nuclei was determined from α activity of the target measured with a Si detector.

The estimation of the neutron flux is then the most delicate measurement. The neutron flux can be deducted from the activity of monitoring targets (such as gold foils or fission rate of 235,238U). Here, we choose as the reference neutron cross -section the neutron elastic scattering on protons in a polypropylene film of several microns in thickness. In this neutron energy range, the (n,p) scattering cross-section is known with a much higher precision (1%) than for the other above-mentioned reactions. The recoil protons were detected with a Si surface barrier detector.

An opened view of the chamber with the experimental setu is shown in the following figure. The proton beam delivered by the 4-MV Van de Graaff comes from the right side on the neutron production target (TiD or TiT).

The monoenergetic neutron flux at 0° forward angle successively crosses the fission and the neutron setup.

The fission setup is constituted by a 233U target and a 238U target placed back to back between two sets of fission detectors, each of two photovoltaic cells. The 238U sample is used as a second neutron flux monitoring.

The neutron setup is made of a ploypropylene foil (PP) and the Si telescope to detect recoil scattered protons at 0° and measure their kinetic energy. A tantalum screen can be inserted between the PP and the Si for background measurements to substract the contribution of scattered neutrons and parasitic reactions in the Si.

The results of this campain are shown in the next figures. We also measured 232Th fission in this experiment, in order to confirm the method and to study systematical uncertainities.



Some of the experiments of this program have been performed at the Van de Graaff facility of CEA/DAM at Bruyères-le-Châtel.

This program is part of the PhD of Cédric Grosjean.


A statistical model calculation (Hauser-Feshbach) described in ref. [a] has been used to interpret from 0.01 to 10 MeV neutron energy, the fission cross section of the 233U(n,f) reaction up to the second chance. Furthermore, the inelastic scattering cross section 233U(n,n’) and the neutron capture 233U(n,γ) which cannot be measured directly in neutron induced reactions have been extracted from the calculations as well as the 233U(n,2n) cross section.


[a] M. Petit et al., Nuclear Physics A 735, 345 (2004)



Related publications:

Mesure de la section efficace de fission induite par neutrons rapides des noyaux 232Th / 233U dans le cadre des cycles de combustibles innovants
C. Grosjean, PhD, Université Bordeaux 1, Mar. 2005
Neutron measurements for innovative fuel cycle and transmutation performed at the CEN Bordeaux-Gradignan
M. Aïche et al., 2nd workshop on Neutron Measurements, Evaluations and Applications, NEMEA-2, Bucharest, Oct. 2004, 49-51