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Measurement of the 58Ni neutron capture cross section

The (n, g) cross section of 58Ni – of importance for the nuclear astrophysics, as well as for nuclear technologies – was measured at the neutron time-of-flight facility n_TOF at CERN. For the measurement, two C6D6 liquid scintillation detectors were used, specifically optimized to provide as low neutron sensitivity as possible. At n_TOF, the highly luminous white neutron beam covers 12 orders of magnitude in energy – from 10 meV to 10 GeV. It is produced by a pulsed beam of 20 GeV protons impinging on a massive Pb spallation target, yielding a total of 2×10^15 neutrons per pulse. Through an evacuated beamline the neutrons are transported towards the experimental area at a distance of approximately 185 m from a spallation target. In the process the beam is collimated by the set of two collimators, while charged particles are removed by a sweeping magnet. The neutron flux is actively monitored during the measurement by means of a silicon based neutron beam monitor SiMon, relying on 6Li(n, t)a reaction. The measurement is performed by detecting the prompt capture g-rays from 58Ni(n, g) reaction. The kinetic energy of captured neutron is calculated from its time-of-flight, which is determined from a time-stamp of a detected g-ray, relative to the start-signal caused by an intense g-flash following every proton pulse hitting the spallation target. In order to calculate a capture yield from a measured data, the well-established Pulse Height Weighting Technique was applied. A capture yield was analyzed in the energy range between 27 meV and 400 keV. The resolved resonance region was analyzed up to 122 keV by means of a multilevel R-matrix code SAMMY. Within the resolved resonance region 51 capture resonances were identified and their parameters reported. By complementing these results with the data from the unresolved resonance region – analyzed by the specialized code SESH – the Maxwellian averaged cross sections (MACS) were calculated for the stellar temperatures of kT = 5–100 keV. The new results call for the revaluation of the cross section data presently available throughout the various evaluation libraries, while revealing a significant impact on the final 58Ni abundance in the massive stars. In calculating the capture yield a significant effort has been paid to the clear identification of the separate background components. While the environmental and the so called empty-frame background are easily and regularly measured, the neutron background – caused by the neutrons elastically scattered off the sample itself – has been identified for the first time at n_TOF by means of the high-precision GEANT4 simulations. The simulated results were extensively compared against the available experimental data, confirming that they may be used with high degree of confidence for reaching the new precision standards in analyzing the capture data from n_TOF.

n_TOF; neutron time of flight; 58Ni neutron capture cross section; GEANT4 simulations; neutron background

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