Improvement in neutronics codes jointly with the advent of high performance computing systems made the calculations more sensitive to the nuclear data. The latter are used both for solving the neutron transport equation together and for nuclear instrumentation validation and operation. Hence, it becomes relevant to improve the knowledge of the fission cross section of fertile secondary actinides as the 242Pu one, which is fissile in a fast neutron flux. This isotope has been chosen as a deposit for the fission chamber for the online monitoring of the fast flux in the experimental irradiation reactor Jules Horowitz (RJH) at CEA Cadarache. As any nuclear thermal or fast neutron reactor, it has a high neutron flux around 1 MeV. This motivates the improvement of the fission cross section for the fertile 242Pu isotope, for which the various experimental data show a dispersion of 10 to 15% around 1 MeV.
The standard measuring technique of a fission cross section is based on simultaneous comparison between the target nucleus and another one so-called reference nucleus. Usually secondary standards are used as reference reactions, known within a few percent, and calibrated to a primary standard of very high precision. The classical isotope chosen as reference for fission cross sections is the 235U whose fission cross section is known with an accuracy of 0.5 to 5%. The use of the same reaction as reference leads to correlations between the different measurements. Our approach aims to produce an independent measurement and to bypass the secondary standard by performing the measurement directly with the primary standard. Thereby, the obtained measurements are completely uncorrelated to any other. The reaction cross section 1H(n,n)p was chosen to achieve our goal since the latter is known from 0.2 to 0.5% over the energy range 0-20 MeV, allowing very accurate measurements. Quantifying the neutron flux with the 1H(n,n)p reaction requires a precise count of the number of recoil protons emitted by a hydrogenated sample of chosen thickness irradiated by this neutron flux. It is therefore essential to use a recoil proton detector having a perfectly known intrinsic efficiency in all operating regimes and a linear response with respect to the input signal. Above 1 MeV, the use of a silicon junction is fully adequate. However, this device is unsuitable at lower energies when a large number of gamma and electrons generate a crippling background noise. This presentation will therefore focus on the recent development and validation of the Gaseous Proton Recoil Telescope (GPRT), insensitive to gamma/electrons noise. This detector uses the Micromegas technology for the detection plane, contains a small time-projection chamber and will be used for the 242Pu fission cross section measurement from 200 keV to a few MeV. During this work, the optimal conditions and the intrinsic efficiency of this detector have been investigated and will be presented. The track reconstruction and the background rejection will also be shown.