An overview of the past, present and future research activities at the CERN neutron time of flight facility n TOF is given, with special focus on the astrophysical aspects. During the first campaign (Phase I), neutron capture cross sections of relevance for several aspects of the s-process nucleosynthesis have been measured. A second campaign has recently started (Phase II), aiming at the study of the weak s-process component via accurate (n, γ) measurements of the Fe and Ni isotopes. Some changes in the facility will allow us to perform these measurements with improved experimental conditions.

The 241 Am(n,γ ) cross section has been measured at the n_TOF facility at CERN using deuterated benzene liquid scintillators, commonly known as C 6 D 6 detectors, and time-of-flight spectrometry. The results in the resolved resonance range bring new constraints to evaluations below 150 eV, and the energy upper limit was extended * Corresponding author: frank.gunsing@cea.fr † www.cern.ch/ntof 0556-2813/2014/89(4)/044609 044609-1 ©2014 American Physical Society K. FRAVAL et al.

The neutron sensitivity of the C 6 D 6 detector setup used at n TOF for capture measurements has been studied by means of detailed GEANT4 simulations. A realistic software replica of the entire n TOF experimental hall, including the neutron beam line, sample, detector supports and the walls of the experimental area has been implemented in the simulations. The simulations have been analyzed in the same manner as experimental data, in particular by applying the Pulse Height Weighting Technique. The simulations have been validated against a measurement of the neutron background performed with a nat C sample, showing an excellent agreement above 1 keV. At lower energies, an additional component in the measured nat C yield has been discovered, which prevents the use of nat C data for neutron background estimates at neutron energies below a few hundred eV. The origin and time structure of the neutron background have been derived from the simulations. Examples of the neutron background for two different samples are demonstrating the important role of accurate simulations of the neutron background in capture cross section measurements.

in the white spectrum neutron beam at the n TOF facility at CERN. A reaction chamber based on Parallel Plate Avalanche Counters (PPAC) was used, where the detectors and the targets have been tilted 45 • with respect to the neutron beam direction in order to cover the full angular range of the fission fragments. A GEANT4 simulation has been developed to study the setup efficiency. The data analysis and the preliminary results obtained for the 232 Th(n,f) between fission threshold and 100 MeV are presented here.

The 238 U to 235 U fission cross section ratio has been determined at n TOF up to ∼1 GeV, with two different detection systems, in different geometrical configurations. A total of four datasets have been collected and compared. They are all consistent to each other within the relative systematic uncertainty of 3-4%. The data collected at n TOF have been suitably combined to yield a unique fission cross section ratio as a function of the neutron energy. The result confirms current evaluations up to 200 MeV. A good agreement is also observed with theoretical calculations based on the INCL++/Gemini++ combination up to the highest measured energy. The n TOF results may help solving a long-standing discrepancy between the two most important experimental dataset available so far above 20 MeV, while extending the neutron energy range for the first time up to ∼1 GeV.

The decay of on-line mass-separated 98 Rb to 98 Sr is studied by ␥ spectroscopy. The revised decay scheme adds further evidence of the coexistence of very different shapes in 98 Sr. A set of levels is proposed to originate from particle-hole pair excitations across the Zϭ40 spherical gap in analogy with 96 Sr. A deformed Kϭ3 band with probable even parity is built on a 7.1-ns isomer at 1838 keV. It is interpreted as a two-quasineutron excitation in accordance with a quantum Monte Carlo pairing calculation based on a deformed shell model. Configurations of the calculated lowest-lying two-quasiparticle levels confirm the importance of the ͓404͔9/2 neutron orbital at the largest deformations in the neutron-rich AӍ100 region.

The neutron time-of-flight facility n_TOF at CERN, fully operational since 2002, combines a high instantaneous neutron flux with high energy resolution. The wide energy range and the high neutron flux per time-of-flight burst result in a much enhanced signal to background ratio for neutron capture of radioactive isotopes and makes this facility well suited for the measurement of high quality neutron-induced reaction cross-sections. Neutrons are created by spallation reactions induced by a pulsed 20 GeV/c proton beam impinging on a lead target. A 5 cm water slab surrounding the lead target serves as a coolant and at the same time as a moderator of the spallation neutron spectrum, providing a wide energy spectrum from 0.1 eV to about 250 MeV. By the end of 2005, a first phase of data taking has been successfully terminated. Fission and capture experiments have been performed on a variety of isotopes of interest for nuclear astrophysics, advanced nuclear technologies and for basic nuclear physics. The instrumentation developed for this facility consists of parallel plate avalanche counter and fission ionization chamber detectors for the fission experiments and of low mass C 6 D 6 detectors and a 4 pi BaF 2 total absorption calorimeter for capture experiments. A new data acquisition system, based on sampling of the detector signals, has been developed to cope with the high count rates and to minimize the effective dead time to only a few tens of nanosecond. A second phase of data taking is planned to start in 2007, after an upgrade of the spallation target. On the longer term, the construction of a flight path at 20 m resulting in an increased neutron flux of about a factor of 100 opens new possibilities.

Ionization tracks on CMOS circuits produce so called Single Event Effects. Measuring the absorbed energy per event in the micro-structures of an integrated circuit is difficult, therefore a simulation can be useful. In this work, we present Geant4 applications to simulate the incidence of charged particles on a CMOS flip-flop designed according to AMISC5 rules. The energy per event absorbed in the flip-flop transistors is calculated for the panoply of beams available at CNA (Spanish National Accelerator Center): protons (18 MeV) and deuterons (9 MeV) from the Cyclotron, and ion beams from the 3-MV Pelletron (alpha, carbon, oxygen and silicon).

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The GEANT4 toolkit is widely used for simulation of high energy physics (HEP) experiments, in particular, those at the Large Hadron Collider (LHC). The requirements of robustness, stability and quality of simulation for the LHC are demanding. This requires an accurate description of hadronic interactions for a wide range of targets over a large energy range, from stopped particle reactions to low energy nuclear interactions to interactions at the TeV energy scale. This is achieved within the GEANT4 toolkit by combining a number of models, each of which are valid within a certain energy domain. Comparison of these models to thin target data over a large energy range indicates the strengths and weaknesses of the model descriptions and the energy range over which each model is valid. We will discuss improvements in the pre-compound, de-excitation, Bertini Cascade and Fritiof string models brought about by these validation tests. The pre-compound and de-excitation models get input from comparisons with low energy data on inclusive proton and neutron production. Several new features were added to the Bertini Cascade model in order to improve agreement with low and intermediate energy data. The Fritiof string model was compared to intermediate energy data to determine the low energy edge of its validity range. Software has been developed to handle the large number of validation tests required to provide the feedback needed to improve the models. We will discuss the validation suites developed for the above work, including plans for an improved automated system which will encompass hadronic interactions at all energies below a few TeV.

A general numerical solution of the dispersion integral relation between the real and the imaginary parts of the nuclear optical potential is presented. Fast convergence is achieved by means of the Gauss-Legendre integration method, which offers accuracy, easiness of implementation and generality for dispersive optical model calculations. The use of this numerical integration method in the optical-model parameter search codes allows for a fast and accurate dispersive analysis.

The neutron capture cross section of 206 Pb has been measured at the CERN n TOF spectrometer using a setup of two C 6 D 6 detectors. In the energy interval from 1 eV to 600 keV the cross section is dominated by resonances, which were analyzed via the R-matrix analysis code SAMMY. In the relevant energy ranges for stellar nucleosynthesis, i.e., at thermal energies of kT = 8 keV and kT = 23 keV, the present Maxwellian average cross section differs by 20% and 9% from the recommended values of Bao et al respectively. From the new cross section the s-abundance of 206 Pb could be reliably determined as 70(4)%. This result is of importance in order to test and constrain r-process abundance calculations in the actinide region, because the r-process portion of 206 Pb is dominated by α-back decays of short-lived transbismuth isotopes.

A fission reaction chamber based on Parallel Plate Avalanche Counters (PPAC) was built for measuring angular distributions of fragments emitted in neutron-induced fission of actinides at the neutron beam available at the Neutron Time-Of-Flight (n TOF) facility at CERN. The detectors and the samples were tilted 45 • with respect to the neutron beam direction to cover all the possible values of the emission angle of the fission fragments. The main features of this setup are discussed and results on the fission fragment angular distribution are provided for the 232 Th(n,f) reaction around the fission threshold. The results are compared with the available data in the literature, demonstrating the good capabilities of this setup.