187 research outputs found
Silicon detectors for the n-TOF neutron beams monitoring
During 2014 the second experimental area EAR2 was completed at the n-TOF
neutron beam facility at CERN. As the neutrons are produced via spallation, by
means of a high-intensity 20 GeV pulsed proton beam impinging on a thick
target, the resulting neutron beam covers an enormous energy range, from
thermal to several GeV. In this paper we describe two beam diagnostic devices,
designed and built at INFN-LNS, both exploiting silicon detectors coupled with
neutron converter foils containing 6Li. The first one is based on four silicon
pads and allows to monitor the neutron beam flux as a function of the neutron
energy. The second one, based on position sensitive silicon detectors, is
intended for the reconstruction of the beam profile, again as a function of the
neutron energy. Several electronic setups have been explored in order to
overcome the issues related to the gamma flash, namely a huge pulse present at
the start of each neutron bunch which may blind the detectors for some time.
The two devices have been characterized with radioactive sources at INFN-LNS
and then tested at n-TOF. The wide energy and intensity range they proved
capable of sustaining, made them quite attractive and suitable to be used in
both EAR1 and EAR2 n-TOF experimental areas, where they became immediately
operational.Comment: submitted to Review of Scientific Instrument
Experimental setup and procedure for the measurement of the 7Be(n,p)7Li reaction at n_TOF
Following 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,) 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.Séptimo Programa Marco de la Comunidad Europea de la Energía Atómica (Euratom)-Proyecto CHANDA (No. 605203)Narodowe Centrum Nauki (NCN)-UMO-2012/04/M/ST2/00700-UMO-2016/22/M/ST2/00183Croatian Science Foundation-HRZZ 168
Exploring neutron channel solutions at CERN/n TOF for the Cosmological Lithium Problem
New measurements of the 7Be(n, α) 4He and 7Be(n, p)
7Li reaction cross sections from thermal to keV neutron energies have been performed at CERN/n TOF. High purity 7Be material was produced at the Paul Scherrer Institute (PSI) and implanted at CERN/ISOLDE, demonstrating the feasibility of neutron measurements on samples produced at radioactive beam facilities. The cross sections are significantly higher than previously determined, at low energies. The new estimate of the 7Be destruction rate based on these new results yields a decrease of the predicted cosmological 7Li abundance, insufficient to provide a viable solution to the Cosmological Lithium Problem (CLiP). The two n TOF measurements can finally rule out neutron-induced reactions as a potential explanation of the CLiP, leaving all alternative physics and astronomical scenarios still open
Experimental setup and procedure for the measurement of the 7Be(n,α)α reaction at n_TOF
The newly built second experimental area EAR2 of the n_TOF spallation neutron source at CERN allows to
perform (n, charged particles) experiments on short-lived highly radioactive targets. This paper describes
a detection apparatus and the experimental procedure for the determination of the cross-section of the
7Be(n,α)α reaction, which represents one of the focal points toward the solution of the cosmological
Lithium abundance problem, and whose only measurement, at thermal energy, dates back to 1963.
The apparently unsurmountable experimental difficulties stemming from the huge 7Be γ-activity,
along with the lack of a suitable neutron beam facility, had so far prevented further measurements. The
detection system is subject to considerable radiation damage, but is capable of disentangling the rare
reaction signals from the very high background. This newly developed setup could likely be useful also to
study other challenging reactions requiring the detectors to be installed directly in the neutron beam.European Atomic Energy Community (Euratom) Seventh Framework Programme FP7/2007-2011-UMO-2012/04/M/ST2/00700Croatian Science Foundation-HRZZ 168
Cross section measurements of 155,157Gd(n, γ) induced by thermal and epithermal neutrons
© SIF, Springer-Verlag GmbH Germany, part of Springer Nature 2019Neutron capture cross section measurements on 155Gd and 157Gd were performed using the time-of-flight technique at the n_TOF facility at CERN on isotopically enriched samples. The measurements were carried out in the n_TOF experimental area EAR1, at 185 m from the neutron source, with an array of 4 C6D6 liquid scintillation detectors. At a neutron kinetic energy of 0.0253 eV, capture cross sections of 62.2(2.2) and 239.8(8.4) kilobarn have been derived for 155Gd and 157Gd, respectively, with up to 6% deviation relative to values presently reported in nuclear data libraries, but consistent with those values within 1.6 standard deviations. A resonance shape analysis has been performed in the resolved resonance region up to 181 eV and 307 eV, respectively for 155Gd and 157Gd, where on average, resonance parameters have been found in good agreement with evaluations. Above these energies and up to 1 keV, the observed resonance-like structure of the cross section has been analysed and characterised. From a statistical analysis of the observed neutron resonances we deduced: neutron strength function of 2. 01 (28) × 10 - 4 and 2. 17 (41) × 10 - 4; average total radiative width of 106.8(14) meV and 101.1(20) meV and s-wave resonance spacing 1.6(2) eV and 4.8(5) eV for n + 155Gd and n + 157Gd systems, respectively.Peer reviewedFinal Accepted Versio
The 33S(n,α)30Si cross section measurement at n-TOF-EAR2 (CERN) : From 0.01 eV to the resonance region
The 33S(n,α)30Si cross section measurement, using 10B(n,α) as reference, at the n-TOF Experimental Area 2 (EAR2) facility at CERN is presented. Data from 0.01 eV to 100 keV are provided and, for the first time, the cross section is measured in the range from 0.01 eV to 10 keV. These data may be used for a future evaluation of the cross section because present evaluations exhibit large discrepancies. The 33S(n,α)30Si reaction is of interest in medical physics because of its possible use as a cooperative target to boron in Neutron Capture Therapy (NCT)
Measurement of 73 Ge(n,γ) cross sections and implications for stellar nucleosynthesis
© 2019 The Author(s). Published by Elsevier B.V.73 Ge(n,γ) cross sections were measured at the neutron time-of-flight facility n_TOF at CERN up to neutron energies of 300 keV, providing for the first time experimental data above 8 keV. Results indicate that the stellar cross section at kT=30 keV is 1.5 to 1.7 times higher than most theoretical predictions. The new cross sections result in a substantial decrease of 73 Ge produced in stars, which would explain the low isotopic abundance of 73 Ge in the solar system.Peer reviewe
Time-of-flight and activation experiments on 147Pm and 171Tm for astrophysics
The neutron capture cross section of several key unstable isotopes acting as branching points in the s-process are crucial for stellar nucleosynthesis studies, but they are very challenging to measure due to the difficult production of sufficient sample material, the high activity of the resulting samples, and the actual (n,γ) measurement, for which high neutron fluxes and effective background rejection capabilities are required. As part of a new program to measure some of these important branching points, radioactive targets of 147Pm and 171Tm have been produced by irradiation of stable isotopes at the ILL high flux reactor. Neutron capture on 146Nd and 170Er at the reactor was followed by beta decay and the resulting matrix was purified via radiochemical separation at PSI. The radioactive targets have been used for time-of-flight measurements at the CERN n-TOF facility using the 19 and 185 m beam lines during 2014 and 2015. The capture cascades were detected using a set of four C6D6 scintillators, allowing to observe the associated neutron capture resonances. The results presented in this work are the first ever determination of the resonance capture cross section of 147Pm and 171Tm. Activation experiments on the same 147Pm and 171Tm targets with a high-intensity 30 keV quasi-Maxwellian flux of neutrons will be performed using the SARAF accelerator and the Liquid-Lithium Target (LiLiT) in order to extract the corresponding Maxwellian Average Cross Section (MACS). The status of these experiments and preliminary results will be presented and discussed as well
New measurement of the 242Pu(n,γ) cross section at n-TOF-EAR1 for MOX fuels : Preliminary results in the RRR
The spent fuel of current nuclear reactors contains fissile plutonium isotopes that can be combined with 238U to make mixed oxide (MOX) fuel. In this way the Pu from spent fuel is used in a new reactor cycle, contributing to the long-term sustainability of nuclear energy. The use of MOX fuels in thermal and fast reactors requires accurate capture and fission cross sections. For the particular case of 242Pu, the previous neutron capture cross section measurements were made in the 70's, providing an uncertainty of about 35% in the keV region. In this context, the Nuclear Energy Agency recommends in its "High Priority Request List" and its report WPEC-26 that the capture cross section of 242Pu should be measured with an accuracy of at least 7-12% in the neutron energy range between 500 eV and 500 keV. This work presents a brief description of the measurement performed at n-TOF-EAR1, the data reduction process and the first ToF capture measurement on this isotope in the last 40 years, providing preliminary individual resonance parameters beyond the current energy limits in the evaluations, as well as a preliminary set of average resonance parameters
Observation of TeV gamma-rays from the unidentified source HESS J1841-055 with the ARGO-YBJ experiment
We report the observation of a very high energy \gamma-ray source, whose
position is coincident with HESS J1841-055. This source has been observed for
4.5 years by the ARGO-YBJ experiment from November 2007 to July 2012. Its
emission is detected with a statistical significance of 5.3 standard
deviations. Parameterizing the source shape with a two-dimensional Gaussian
function we estimate an extension \sigma=(0.40(+0.32,-0.22}) degree, consistent
with the HESS measurement. The observed energy spectrum is dN/dE =(9.0-+1.6) x
10^{-13}(E/5 TeV)^{-2.32-+0.23} photons cm^{-2} s^{-1} TeV^{-1}, in the energy
range 0.9-50 TeV. The integral \gamma-ray flux above 1 TeV is 1.3-+0.4 Crab
units, which is 3.2-+1.0 times the flux derived by HESS. The differences in the
flux determination between HESS and ARGO-YBJ, and possible counterparts at
other wavelengths are discussed.Comment: 17 pages, 4 figures, have been accepted for publication in Ap
- …