The improvement of neutronics simulations makes their accuracy more sensitive to the precision of the input data, such as neutron-induced cross sections. Among the most important ones, fertile actinides fission cross sections play a key role as they become fissile in a fast neutron flux, above 1 to 2 MeV, which is the peak of neutron population in a thermal, as well as fast, reactor core.
In order to obtain a precise fission cross section, high accuracy neutron flux measurements are essential. The standard technique is a comparison between the nucleus of interest and a reference. The later can be the 1H(n,n)p cross section, known with an accuracy of 0.2 to 0.5% over the energy range 0-20 MeV, using the proton recoil technique. The neutron flux is then converted through a hydrogenated sample of chosen thickness into a more easily countable proton flux that a proton recoil detector detects. An accurate cross section measurement requires an accurate count of recoil protons. The use of a recoil proton detector having a perfectly known intrinsic efficiency in all operating regimes and a linear response with respect to the input signal is therefore mandatory.
Above 1 MeV, a silicon junction is fully adequate and commonly used. However, at lower energies, the large number of gamma and electrons generated by the neutron source creates a crippling background noise, making this detector not suitable anymore. The other already-existing detectors not fitting all the requirements, the Gaseous Proton Recoil Telescope (GPRT) is developed for accurate measurement below 1 MeV and down to 200 keV. Its aim is to provide a quasi-absolute neutron flux measurement with an accuracy better than 2%, achievable because insensitive to gamma and electrons noise.
This presentation will thus focus on the description and characterization of this detector, composed of a double ionization chamber and using a segmented Micromegas detection plane. The pressure of the gas in the chamber can be adjusted to protons stopping range – and therefore to neutrons energy. The use of a slow gas enables the access to the third dimension. The possibility to reconstruct tracks and to reject background will hence be presented.
A particular attention has been paid to the optimal operation conditions of the detector, and especially to its intrinsic efficiency. The works carried out successively with an alpha source, a proton micro-beam and a test bench to verify a 100% intrinsic efficiency will be highlighted and the quantification of a dead time of 7.3 ms will be explained.