Measurement of 183 W(n, n’ γ ) and (n, 2n γ ) cross-sections (preliminary)

. The necessary improvement of evaluated nuclear databases for appplication will be achieved with improvement of models and new, precise data. In particular, the e ff ect of inelastic neutrons scattering can be of importance for reactors. In order to test the models, we performed measurement of (n, n’ γ ) and (n, 2n γ ) cross-sections on 183 W. These data will help constrain the calculation codes and ensure a better evaluation of the total (n, x n) cross section. The experimental setup and the data analysis method will be presented. The preliminary experimental results for the 183 W isotope will be compared to predictions from Talys nuclear reaction code


Motivation
The development of nuclear reactor project relies essentially on evaluated nuclear reaction databases for numerical simulations, to optimize and predict performance and control parameters. However, these databases still have significant uncertainties, which makes it impossible to reach the required precision in calculations. New measurements and more accurate theoretical descriptions of the involved reactions are needed to improve the evaluated databases. Inelastic neutron scattering reactions, noted (n, xn), are among the reactions of interest as they modify the neutron spectrum, the neutron population, and produce radioactive species.
Tungsten is not an active element in nuclear reactors, but, because of its chemical and mechanical properties, it is used in many alloys. The interaction of neutrons with tungsten is therefore of importance for reactor physics, in particular for fusion reactors, in which tungsten is one of * e-mail: ghenning@iphc.cnrs.fr * * Currently, Univ. of Helsinki the most exposed materials to high energy neutrons. From a theoretical point of view, a better description of (n, xn) reactions on tungsten nuclei allows an improvement of models for other key nuclei in reactor fuel. Indeed, tungsten isotopes are deformed like actinides, but also easier to describe as they do not present a neutron-induced fission channel. Still, there are very few measurements available today to test evaluations [11]. Our new experimental data will provide an extensive and constraining test to the predictability of models

Experimental setup
The measurement of 183 W(n, xnγ) cross-section have been performed in 2012 and analysis started recently. As of today, there is no (n, n') cross-section data available -only some (n, 2n) and some (n, n')-level are registered in [11]. The (n, xn γ) cross-section measurements are performed with the GRAPhEME setup at the JRC-Geel. It is made up of four High Purity Germanium detectors surrounding the target of interest (here, 183 W) and a fission chamber upstream. The neutron flux is determined using the fission chamber. The energy of the reacting neutrons are calculated from their time of flight. The data analysis was performed by methods that were recently detailed in reference [12].
In the following, only the preliminary γ yields (i.e. non-normalized γ to neutron counts ratio) will be presented. The results are compared to default TALYS-1.8 calculations [13].
We also present two transitions in the 183 W(n, 2n γ) 182 W channel. The 4 + 329.4 keV to 2 + 100.1 keV ,  We see in the figures 1, 2 3 and 4 that the shapes of the yields are compatible with Talys predictions. In particular, the (n n' γ) yields fall once the neutron energy gets above the neutron separation energy (6.2 MeV [14]). Likewise, the (n 2n) channel opens up above S n and grows until slightly (≈ 2 MeV) above S 2n (14.25 MeV). The overall shapes (position of the maximums, slopes, ...) are similar.

Conclusion
We extracted some preliminary (n, n' γ) and (n, 2n γ) yields from the data has been recorded with GRAPhEME. They present similar shapes than the one observed in TALYS calculations and in accordance with expected physics (thresholds, . . . ). Therefore, we are confident that once the full analysis is performed, it will produce high quality cross-section data for several (n, n' γ) and (n, 2n γ) channels.