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Direct neutron measurements

The ACEN group has been formally consitued after a first experiment performed at the end of the last century at the 4-MV Van de Graaff facility of CENBG. We measured the radiative capture cross section of 232Th for neutrons in the energy range of 60 keV to 2 MeV.

The group then started a program of high precision measurements for fast neutron induced fission cross sections. One of the main difficulties for these measurements consists in the precise determination of the monoenergetic neutron flux on the fission target. This is done using a known standard cross section as reference, and by measuring the wanted cross section relative to this reference. The uncertainity on the cross section data for this standard reaction is an important part of the one of the measured one. Usual standards are 10B, 235U or 238U. But for the neutron energies of interest, their uncertainities are rather important. We decided to developp an experimental setup for neutron flux determination using another standard, the neutron elastic scattering on protons, which is known with a high precisison in the neutron energy range of interest. The setup consists mainly in a thin foil of polypropylene and a silicium telescope for recoil proton measurement, both aligned with the fission setup at 0°.

This program covered fission cross section measurements of nuclei involved in the thorium based fuel cycle (232Th, 233U), and the minor actinide 243Am. We extended this program to the non fissile isotopes of plutonium 240Pu and 242Pu.

These experiment took place at the 4-MV Van de Graaff facility of CENBG, then at the new AIFIRA facility, at the 7-MV Van de Graaff of CEA/DAM at Bruyères-le-Châtel, and at the Van de Graaff facility of IRMM, at Geel.

These experiments were part of the PhD of Cedric Grosjean (233U, 232Th) and Grégoire Kessedjian (243Am).

Recently, we started a RD program to develop a recoil proton gazeous detector (DPR) to measure the neutron flux at energies lower than 500 keV.

Our group is also involved in a program of measurement of the so called alpha yield ratio of 233U, i.e. the ratio of radiative capture to fission cross sections, in the énergy range of the resolved resonance region (0.1 eV to 100 eV neutron energy). The test experiments have been performed at the GELINA neutron time of flight facility at IRMM, Geel.

Most of the difficulty arise from the gamma. The setup must discriminate gamma coming from a capture event from the ones due to the deexcitation of the fission fragments. This is done using a fission chamber as a veto detector. The very precise determination of the detection efficiency of this fission chamber is the main difficuly of the experiment. We developped a fission ioniszation chamber at CENBG, and we performed careful studies of the efficiency and developped sa strong program of simulation using an upgraded MCNP code.

This program is part of the PhD of Iulia Companis.

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