Status and outlook of the neutron time-of-flight facility n_TOF at CERN

Abstract

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 C6D6 detectors and a 4 pi BaF2 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

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

“…Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for 24 Mg, 25 Mg, and 26 Mg are 3.8±0.2 mb, 4.1±0.6 mb, and 0.14±0.01 mb, respectively. …”

“…The sensitivity study of Iliadis et al [5] has demonstrated that the cross section of the 24 Mg(n, γ ) reaction is important for the origin of 26 Al, because its main production mechanism in massive stars is strongly affected by the uncertainties of several cross sections, including that of 24 Mg(n, γ ). The capture cross sections of the Mg isotopes in current nuclear data libraries exhibit deficiencies in the resolved resonance region (RRR), in particular concerning the assignment of resonance spins.…”

“…Typically, these experiments covered only a limited energy region, and capture data were taken with large detectors, less suited for capture cross-section studies on isotopes in the mass region of Mg, where the cross sections are by far dominated by the elastic-scattering channel. While the most abundant isotope 24 Mg has been investigated several times [12][13][14][15], there are few neutron data for 25,26 Mg. Because the resonance-dominated capture cross sections of all three Mg isotopes are relatively small, the measurements can be strongly affected by various kinds of background and exhibit, therefore, rather large discrepancies. For instance, spin and parity of the first neutron resonance in 25 in the JENDL evaluation, although the parity assignment of Ref.…”

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We have measured the neutron capture cross sections of the stable magnesium isotopes 24,25,26 Mg in the energy range of interest to the s process using the neutron time-of-flight facility n_TOF at CERN. Capture events from a natural metal sample and from samples enriched in 25 Mg and 26 Mg were recorded using the total energy method based on C 6 2 H 6 detectors.

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Measurement and resonance analysis of the237Np neutron capture cross section

“…The n_TOF (Phase-1) facility [18,19] is part of the fixed target experimental program at CERN. At n_TOF a highintensity neutron pulse is produced every 2.4 s from spallation reactions induced by a 20-GeV/c proton beam incident on a 80 × 80 × 60 cm 3 lead target.…”