1,721,956 research outputs found
Preparation and characterization of 33S samples for 33S(n,α)30Si cross-section measurements at the n_TOF facility at CERN
No full textThin 33S samples for the study of the 33S(n,α)30Si cross-section at the n_TOF facility at CERN were made by thermal evaporation of 33S powder onto a dedicated substrate made of kapton covered with thin layers of copper, chromium and titanium. This method has provided for the first time bare sulfur samples a few centimeters in diameter. The samples have shown an excellent adherence with no mass loss after few years and no sublimation in vacuum at room temperature. The determination of the mass thickness of 33S has been performed by means of Rutherford backscattering spectrometry. The samples have been successfully tested under neutron irradiation. © 2018 The Author
Be 7 (n,p) Li 7 Reaction and the Cosmological Lithium Problem: Measurement of the Cross Section in a Wide Energy Range at n-TOF at CERN
No full textWe report on the measurement of the Be7(n,p)Li7 cross section from thermal to approximately 325 keV neutron energy, performed in the high-flux experimental area (EAR2) of the n-TOF facility at CERN. This reaction plays a key role in the lithium yield of the big bang nucleosynthesis (BBN) for standard cosmology. The only two previous time-of-flight measurements performed on this reaction did not cover the energy window of interest for BBN, and they showed a large discrepancy between each other. The measurement was performed with a Si telescope and a high-purity sample produced by implantation of a Be7 ion beam at the ISOLDE facility at CERN. While a significantly higher cross section is found at low energy, relative to current evaluations, in the region of BBN interest, the present results are consistent with the values inferred from the time-reversal Li7(p,n)Be7 reaction, thus yielding only a relatively minor improvement on the so-called cosmological lithium problem. The relevance of these results on the near-threshold neutron production in the p+Li7 reaction is also discussed. © 2018 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the »https://creativecommons.org/licenses/by/4.0/» Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI
The new KADoNiS v1.0 and its influence on the weak s-process nucleosynthesis
No full textFive years after the release of KADoNiS v0.3 (www.kadonis.org), a major update on the s-process database has been completed and is presently available for testing at HTTP://EXP-ASTRO.PHYSIK.UNI-FRANKFURT.DE/KADONIS1.0/. All available datasets between 1H and 210Bi and their energy dependence for kT= 5 - 100 keV have been reviewed, updated, and evaluated. The main difference to previous releases of KADoNiS is the intensive work that has been put in the inclusion of new experimental energy-dependent cross sections which should make the extrapolations to higher and lower energies more reliable. Also a revision of the 197Au(n,γ) cross section has been applied which is widely used as reference cross section for time-of-flight and activation measurements. Several recent measurements in the astrophysical energy range have led to a new recommended value used throughout KADoNiS v1.0 which is 3-5.4% higher than the previously adopted value. Datasets for the up to now missing light stable isotopes 6Li, 10,11B, and 17O were added and recommendations based on experimental data given. The total number of datasets in the KADoNiS database increased from 370 to more than 440 which is mainly due to the inclusion of radioactive isotopes 1-2 mass units away from stability. Although no measurements are available for these cases, they play an important role in modern s-process calculations with higher neutron densities. To be able to use a consistent database for s-process simulations it was decided to include these isotopes in the new update and calculate semi-empirical recommended cross sections. In this update also indirect measurements, namely (γ,n) measurements complemented by Hauser-Feshbach predictions with constrained input, have been included which yielded new (n,γ) recommended values for e.g. the branchings at 85Kr, 185W, and 186Re. For other branching isotopes, e.g. 63Ni, first direct experimental time-of-flight data yielded a factor of 2 higher cross section compared to the previously used theoretical prediction. The influences of the changes of the new (n,γ) cross sections on the weak s-process have been investigated. Whereas the differences after core He burning are within 20%, the impact of the new set of MACS on the s-process abundance distribution is much larger at the end of the shell C burning which leads to a "weaker" s-process efficiency. © Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence
Performance of the neutron time-of-flight facility n_TOF at CERN
No full textThe neutron time-of-flight facility n_TOF features a white neutron source produced by spallation through 20 GeV/c protons impinging on a lead target. The facility, aiming primarily at the measurement of neutron-induced reaction cross sections, was operating at CERN between 2001 and 2004, and then underwent a major upgrade in 2008. This paper presents in detail all the characteristics of the new neutron beam in the currently available configurations, which correspond to two different collimation systems and two choices of neutron moderator. The characteristics discussed include the intensity and energy dependence of the neutron flux, the spatial profile of the beam, the in-beam background components and the energy resolution/broadening. The discussion of these features is based on dedicated measurements and Monte Carlo simulations, and includes estimations of the systematic uncertainties of the mentioned quantities. © The Author(s) 2013
Recent results in nuclear astrophysics at the n-TOF facility at CERN
No full textThe neutron time of flight (n-TOF) facility at CERN is a spallation source characterized by a white neutron spectrum. The innovative features of the facility, in the two experimental areas, (20 m and 185 m), allow for an accurate determination of the neutron cross section for radioactive samples or for isotopes with small neutron capture cross section, of interest for Nuclear Astrophysics. The recent results obtained at n-TOF facility are presented. © Copyright owned by the author(s)
High-accuracy determination of the neutron flux at n_TOF
No full textThe neutron flux of the n TOF facility at CERN was measured, after installation of the new spallation target, with four different systems based on three neutron-converting reactions, which represent accepted cross sections standards in different energy regions. A careful comparison and combination of the different measurements allowed us to reach an unprecedented accuracy on the energy dependence of the neutron flux in the very wide range (thermal to 1 GeV) that characterizes the n TOF neutron beam. This is a pre-requisite for the high accuracy of cross section measurements at n TOF. An unexpected anomaly in the neutron-induced fission cross section of 235U is observed in the energy region between 10 and 30 keV, hinting at a possible overestimation of this important cross section, well above currently assigned uncertainties. © Società Italiana di Fisica / Springer-Verlag 2013
RE-EVALUATION of the 16O(N, γ)17O CROSS SECTION at ASTROPHYSICAL ENERGIES and ITS ROLE AS A NEUTRON POISON in the s-PROCESS
No full textThe doubly magic nucleus 16O has a small neutron-capture cross section of just a few tens of microbarns in the astrophysical energy region. Despite this, 16O plays an important role as a neutron poison in the astrophysical slow neutron capture (s) process due to its high abundance. We present in this paper a re-evaluation of the available experimental data for 16O(n, γ)17O and derive a new recommendation for the Maxwellian-averaged cross sections between kT = 5 and 100 keV. Our new recommendations are lower up to kT = 60 keV compared to the previously recommended values but up to 14% higher at kT = 100 keV. We explore the impact of this different energy dependence on the weak s-process during core helium burning (kT = 26 keV) and shell carbon burning (kT = 90 keV) in massive stars where 16O is the most abundant isotope. © 2016. The American Astronomical Society. All rights reserved
The n-TOF facility: Neutron beams for challenging future measurements at CERN
No full textThe CERN n-TOF neutron beam facility is characterized by a very high instantaneous neutron flux, excellent TOF resolution at the 185 m long flight path (EAR-1), low intrinsic background and coverage of a wide range of neutron energies, from thermal to a few GeV. These characteristics provide a unique possibility to perform high-accuracy measurements of neutron-induced reaction cross-sections and angular distributions of interest for fundamental and applied Nuclear Physics. Since 2001, the n-TOF Collaboration has collected a wealth of high quality nuclear data relevant for nuclear astrophysics, nuclear reactor technology, nuclear medicine, etc. The overall efficiency of the experimental program and the range of possible measurements has been expanded with the construction of a second experimental area (EAR-2), located 20 m on the vertical of the n-TOF spallation target. This upgrade, which benefits from a neutron flux 30 times higher than in EAR-1, provides a substantial extension in measurement capabilities, opening the possibility to collect data on neutron cross-section of isotopes with short half-lives or available in very small amounts. This contribution will outline the main characteristics of the n-TOF facility, with special emphasis on the new experimental area. In particular, we will discuss the innovative features of the EAR-2 neutron beam that make possible to perform very challenging measurements on short-lived radioisotopes or sub-mg samples, out of reach up to now at other neutron facilities around the world. Finally, the future perspectives of the facility will be presented. © The Authors, published by EDP Sciences, 2017
Measurement of the 241Am neutron capture cross section at the n-TOF facility at CERN
No full textNew neutron cross section measurements of minor actinides have been performed recently in order to reduce the uncertainties in the evaluated data, which is important for the design of advanced nuclear reactors and, in particular, for determining their performance in the transmutation of nuclear waste. We have measured the 241Am(n,γ) cross section at the n-TOF facility between 0.2 eV and 10 keV with a BaF2 Total Absorption Calorimeter, and the analysis of the measurement has been recently concluded. Our results are in reasonable agreement below 20 eV with the ones published by C. Lampoudis et al. in 2013, who reported a 22% larger capture cross section up to 110 eV compared to experimental and evaluated data published before. Our results also indicate that the 241Am(n,γ) cross section is underestimated in the present evaluated libraries between 20 eV and 2 keV by 25%, on average, and up to 35% for certain evaluations and energy ranges. © The Authors, published by EDP Sciences, 2017
Experimental setup and procedure for the measurement of the 7Be(n,p)7Li reaction at n_TOF
No full textFollowing the completion of the second neutron beam line and the related experimental area (EAR2) at the n_TOF spallation neutron source at CERN, several experiments were planned and performed. The high instantaneous neutron flux available in EAR2 allows to investigate neutron induced reactions with charged particles in the exit channel even employing targets made out of small amounts of short-lived radioactive isotopes. After the successful measurement of the 7Be(n,α)α cross section, the 7Be(n,p)7Li reaction was studied in order to provide still missing cross section data of relevance for Big Bang Nucleosynthesis (BBN), in an attempt to find a solution to the cosmological Lithium abundance problem. This paper describes the experimental setup employed in such a measurement and its characterization. © 2017 The Author(s
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