Experimental work on Nuclear Astrophysics at JRC GELINA facility

. JRC Geel operates a neutron time-of-flight facility based on an electron accelerator, GELINA. Experimental setups to determine total and reaction cross sections (capture, elastic and inelastic scattering, fission and charged particle reactions) are available at measurement stations different flight-path length. While most of the experimental work is focussed on nuclear energy applications, regularly cross section of interest for astrophysical problems are measured. Examples of those are capture experiments of nuclei relevant for the s-process such as 89 Y or the 16 O (n,  ) reaction which is the inverse of the 13 C(  ,n) reaction, an important source of neutrons feeding the s-process.


Introduction
Joint Research Centre (JRC) is the science and knowledge service of the European Commission, providing advice and support to European policies. JRC-Geel hosts the GELINA facility [1], an intense pulsed white neutron source driven by a linear electron accelerator which is dedicated to high-resolution time-of-flight measurements. GELINA combines four specially designed and distinct units: a high-power pulsed linear electron accelerator, a post-accelerating beam compression magnet, a mercury-cooled uranium target, and measurement stations at flight path lengths ranging from 10 m to 400 m. Thanks to the multiple neutron beam lines, as many as 10 experiments can be carried out simultaneously.
Research at GELINA is focused on the following priority topics: -Neutron cross sections measurements for energy and non-energy nuclear applications -Measurements of nuclear data standards -Integral experiments for the validation of nuclear data libraries and testing of nuclear transport codes -Better understanding of the nuclear fission process -Development of advanced detection methods and scientific concepts in nuclear technologies -Basic physics such as nuclear reaction theory or nuclear astrophysics Since the early 90's, an extensive investigation on neutron cross sections relevant for nuclear astrophysics was performed at IRMM (former JRC-Geel), led by Franco Corvi in the field of neutron capture cross sections and by Cyriel Wagemans with neutron-induced charge particle reactions. Their work can be summarized by the contributions to Nuclei in Cosmos proceedings, the reference conference in the field. An overview of these contributions is detailed in Table 1.

Capture cross sections in the vicinity of N=50: the 89 Y(n,) reaction
The slow neutron capture processes (s-process) is responsible for the abundances of a large fraction of the nuclides of masses between 50 and 210 and, in particular, for the presence of two main peaks in the elemental abundance distribution. The first s-process peak is due to 88 Sr, 89 Y and 90 Zr having a magic number of neutrons equal to 50. The cross section of these nuclides strongly influences the production of all heavier elements, at least up to the second s-process peak corresponding to the next bottleneck at Ba, La and Ce with a neutron magic number of 82. In particular, for the yttrium case its stellar abundance can be derived from high-resolution spectra thanks to the large number of optical lines, being extensively used to constrain stellar models. An accurate determination of the 89 Y capture cross section is a primordial input for these models. At GELINA, transmission measurements on yttrium samples of different thicknesses between 1 and 8 mm and neutron capture measurement with a disk of 80 mm diameter and 2 mm thickness were carried out by researchers from INFN Bari. Capture measurements with a Y disk of 30 mm diameter and 1 mm thickness were also carried out at the n_TOF facility.
The transmission measurements were performed at the 50 m transmission station of FP4. A 10 B overlap filter was used to reduce the contribution of slow neutrons coming from previous accelerator bursts and Na, Co and W black resonance filters were used to determine the background contribution. The neutron beam passing through the sample and filters was detected by a cylindrical NE912 Li-glass scintillator of 6.35 mm thickness, placed at 47 m from the neutron target. The capture measurements at n_TOF and GELINA were performed by applying the total energy detection principle using C6D6-based liquid scintillators which are place at 125 with respect to the beam direction. The GELINA setup, which is presented in Figure 1, was located at the 60 m station of FP14. The energy distribution of the incident neutron beam was measured with a 10 B ionisation chamber placed 80 cm in front of the sample.
Resonance parameters were derived from a combined analysis of capture yields and transmission data, following the prescription given in Ref [7]. The yields were analysed by means of the R-matrix codes SAMMY [8] and REFIT [9] in the Reich-Moore approximation. The Doppler broadening, neutron self-shielding and multiple interactions and the facility TOF response function, were taken into account. The whole set of measurements enabled the identification of 112 resonances belonging to the 89 Y for neutron incident energies up to 95 keV. Thanks to the combination of experimental data obtained in different experimental conditions bias effect due to the TOF response function, the normalization of the capture data and the background contributions are largely reduced.
2.2 Neutron-induced charge particle reactions: 16  for measuring the reaction in 16 O, while the well characterized 235 U chamber H19 [10] from PTB Braunschweig, was used for an absolute determination of the incident neutron spectrum using the 235 U standard. The DFGIC was operated with a gas mixture of 95% Kr and 5% CO2 at 2 bar. The oxygen of the CO2 between the cathode and the grid was the effective target nuclei.
The signals from the H19 and the FGIC were digitized and processed off-line to determine the time-of-flight and the deposited energy. For the FGIC, the drift time of the particle was used to only select events with full energy deposition between the cathode and the grid and accurately provide the number of oxygen nuclei acting as a target. The data analysis allowed to discriminate 16 O(n,0) events from scattering or other reactions in the gas and electrodes. The e method to determine the absolute cross section was validated with the determination of the elastic scattering cross section on C nuclei. The results obtained for neutron energies between 3 and 9 MeV suggests a bias of -9% for the cross section recommended in ENDF/B-VIII.0 and of +26% for the one in JEFF-3.3. For the 13 C(,n)reaction, a correction factor is proposed which is very close to one derived from the thick target yields obtained by West and Sherwood [11] between 3.5 and 5 MeV. Pigni and Croft [12] Ciani et al. [13] Bair and Hass [14] 0

Summary and perspectives
In this work, we have provided an overview of different experimental work carried out at JRC GELINA facility on neutron-induced reactions of relevance for nuclear astrophysics, detailing two examples on neutron capture and neutron-induced charge particle reactions. This experimental program performed in collaboration with external partners is pursued with capture measurements on 90,91 Zr and 94,95,96 Mo isotopes. The EUFRAT open access program provides the framework to researchers from EU Member States to propose further research on nuclear astrophysics using the neutron facilities at JRC-Geel.