Excitation function shape and neutron spectrum of the Li 7 ( p , n ) Be 7 reaction near threshold

The forward-emitted low energy tail of the neutron spectrum generated by the $^{7}\mathrm{Li}(p,n)^{7}\mathrm{Be}$ reaction on a thick target at a proton energy of 1893.6 keV was measured by time-of-flight spectroscopy. The measurement was performed at BELINA (Beam Line for Nuclear Astrophysics) of the Laboratori Nazionali di Legnaro. Using the reaction kinematics and the proton on lithium stopping power the shape of the excitation function is calculated from the measured neutron spectrum. Good agreement with two reported measurements was found. Our data, along with the previous measurements, are well reproduced by the Breit-Wigner single-resonance formula for $s$-wave particles. The differential yield of the reaction is calculated and the widely used neutron spectrum at a proton energy of 1912 keV was reproduced. Possible causes regarding part of the 6.5% discrepancy between the ^{197}\mathrm{Au}(n,\ensuremath{\gamma}) cross section measured at this energy by Ratynski and Kappeler [Phys. Rev. C 37, 595 (1988)] and the one obtained using the Evaluated Nuclear Data File version B-VII.1 are given

Excitation function shape and neutron spectrum of the 7Li (p,n) 7Be reaction near threshold

et al.The forward-emitted low energy tail of the neutron spectrum generated by the Li7(p,n)Be7 reaction on a thick target at a proton energy of 1893.6 keV was measured by time-of-flight spectroscopy. The measurement was performed at BELINA (Beam Line for Nuclear Astrophysics) of the Laboratori Nazionali di Legnaro. Using the reaction kinematics and the proton on lithium stopping power the shape of the excitation function is calculated from the measured neutron spectrum. Good agreement with two reported measurements was found. Our data, along with the previous measurements, are well reproduced by the Breit-Wigner single-resonance formula for s-wave particles. The differential yield of the reaction is calculated and the widely used neutron spectrum at a proton energy of 1912 keV was reproduced. Possible causes regarding part of the 6.5% discrepancy between the Au197(n,γ) cross section measured at this energy by Ratynski and Kappeler [Phys. Rev. C 37, 595 (1988)PRVCAN0556-281310.1103/PhysRevC.37.595] and the one obtained using the Evaluated Nuclear Data File version B-VII.1 are given.This work was supported by the Istituto Nazionale di Fisica Nucleare and by the International Atomic Energy Agency (Austria), Contract No. 17883. G. M.-H. is indebted to Laboratori Nazionali di Legnaro staff for is help and support.Peer Reviewe

An ENDF-6 Compatible Evaluation for Neutron Induced Reactions of 232Th in the Unresolved Resonance Region

An evaluation for neutron induced reactions of 232Th has been performed in the unresolved resonance region from 4 to 100 keV. A generalized single-level representation compatible with the energy-dependent options of the ENDF-6 format has been used. The average partial cross sections have been expressed in terms of transmission coefficients by applying the Hauser-Feschbach statistical reaction theory including width-fluctuations. The evaluation is based on a combined analysis of experimental average cross section data (including the most recent capture cross section data obtained at the time of flight facilities GELINA and n_TOF), and results of optical model calculations. Experimental data on transmission and self-indication measurements as well as integral benchmark calculations have been used for validation. The evaluation also includes covariance information.JRC.D.5-Neutron physic

Measurement of the thulium stellar cross section at kT=30 keV by activation with an innovative method

Resumen del trabajo presentado a la: "International Conference on Nuclear Data for Science and Technology" celebrada en New York (US) del 4 al 8 de marzo de 2013.-- et al.The production of Maxwell-Boltzmann neutron spectra is of interest in several fields. Maxwellian-averaged cross sections (MACS) ranging from kT=5 keV to 120 keV are key parameters in the calculation of the astrophysical reaction rates of nucleon, photon and charged-particle interactions with the different elements and their isotopes needed among other things for the description of observed elemental abundances. Spectrum average (SPA) cross sections measured in the well-characterized maxwellian neutron spectrum with high accuracy could be used for the validation of nuclear data libraries in the above-mentioned energy range. Both the library cross-section value and corresponding covariances could be challenged. The estimated uncertainty from the nuclear data library of SPA cross sections is directly linked to off-diagonal elements of the uncertainty matrix; therefore the uncertainty of the calculated SPA cross-section is sensitive to librarian covariances. The energy range ~100-200 keV is also extremely important in the description of fast neutron systems (e.g. for fast Generation-IV reactors). The MACS of the different elements can be measured directly by activation providing a maxwellian neutron spectrum at the sample position. Beer and Käppeler is considered the classical work in this field, they showed the possibility to produce a quasi maxwellian neutron spectra at kT=25 keV by the Li-7(p,n) reaction at proton energy near-threshold. Then the MACS at the reference temperature (30 keV) can be obtained by a correction of the spectrum and an extrapolation from 25 to 30 keV. Mastinu et al proposed a new method for producing maxwellian neutron spectra at different temperatures, in particular a very accurate maxwellian spectrum at kT=30 keV can be achieved. While the classical method uses monochromatic proton energy, the key point of the Mastinu et al method is to shape the proton beam to a particular distribution near-threshold the Li-7(p,n) reaction by means of an energy degrader that can be made of different material and thicknesses. The Mastinu et al method is used to measure by activation the MACS of the Tm169(n,g) reaction at kT=30 keV using Au-197(n,g) as a reference. The available experimental data for this reaction is rather limited and there is only one available data for the MACS at kT=30 keV obtained by the TOF technique. The method will be explained and the results will be shown. Around 13% of the solar abundance of Thulium is made by the slow neutron capture process. Astrophysical implications will be discussed for nucleosynthesis in stars driven by s-process.Peer reviewe

The LENOS Project at Laboratori Nazionali di Legnaro of INFN-LNL

Resumen del trabajo presentado a la: "International Conference on Nuclear Data for Science and Technology" celebrada en New York (US) del 4 al 8 de marzo de 2013.-- et al.LENOS (Legnaro NeutrOn Source) project at the Laboratori Nazionali di Legnaro of INFN (Italy) is a neutron irradiation facility for nuclear astrophysics studies and validation of evaluated nuclear data libraries. It is based on a high current low energy RFQ. The facility under construction will use the 5 MeV, 50 mA proton beam of RFQ under test at LNL to produce an unprecedented neutron flux, precisely shaped to a Maxwell- Boltzmann energy distribution at variable temperature kT. A new method has been proposed to obtain the desired neutron spectra at different stellar energies and a dedicated target, able to sustain a very high specific power, has been developed. We will present the facility, the method used to shape the neutron beam, the preliminary results of the high power test of the micro-channel water cooled target and the preliminary results of the validation measurement. Waiting for LENOS facility, currently we are carrying out measurements at existing low power electrostatic accelerators, using other degrees of freedom available in order to shape accurately the proton beam to a desired distribution. An overview of recent measurements together with the capability of using additional degrees of freedom to achieve more accurate Maxwellian distributions at temperatures higher than 50 keV will be presented.Peer reviewe

Activation Cross Section Measurements for Neutron-induced Reactions on Cr, Ni, Cu, Zr, Ta and W Isotopes from the Threshold to 20 MeV

Studies of excitation functions of fast neutron-induced reactions are important for many fields of nuclear science, nuclear technology development. In this work, recent results are reported on neutron-induced reaction cross section measurements for different isotopes of Cr, Ni, Cu, Ta and W obtained with the activation technique. Quasi-monoenergetic neutrons were produced via the 3H(d,n)4He reaction. Both natural and enriched samples were used to account for the interference between reactions leading to the same nuclide. Activities of the samples were determined by gamma-spectrometry using high-purity germanium detector. The 27Al(n,a)24Na standard cross section was used for neutron fluence rate determination. The present results are compared with other measurements, evaluated data and nuclear model calculations.JRC.D.5-Neutron physic

Measurement of the MACS of 159Tb(n, γ) at kT=30 keV by activation

The measurement of the Maxwellian-Averaged Cross-Section (MACS) of the 159Tb(n, γ) reaction at kT=30 keV by the activation technique is presented. An innovative method for the generation of Maxwellian neutron spectra at kT=30 keV is used. An experimental value of 2166±181 mb agrees well with the MACS value derived from the ENDF/B-VII.1 evaluation, but is higher than KADoNiS recommended value of 1580±150 mb. Astrophysical implications are studied. © 2014 Elsevier Inc.This work was funded by the Spanish projects FPA2009-08848, FPA2011-28770-C03-02, P07-FQM-02894, and the funding agencies of the participating institutes. M. Pignatari acknowledges support from NuGrid via the Ambizione grant of the SNSF (Switzerland), the NSF grants PHY 02-16783 and PHY 09-22648 (JINA) and EU MIRG-CT-2006-046520, and from Eurogenesis (MASCHE).Peer Reviewe

Temperature-tuned Maxwell-Boltzmann neutron spectra for kT ranging from 30 up to 50keV for nuclear astrophysics studies

The need of neutron capture cross section measurements for astrophysics motivates present work, where calculations to generate stellar neutron spectra at different temperatures are performed. The accelerator-based 7Li(p,n)7Be reaction is used. Shaping the proton beam energy and the sample covering a specific solid angle, neutron activation for measuring stellar-averaged capture cross section can be done. High-quality Maxwell–Boltzmann neutron spectra are predicted. Assuming a general behavior of the neutron capture cross section a weighted fit of the spectrum to Maxwell–Boltzmann distributions is successfully introduced.G. Martín-Hernández acknowledges support from Laboratori Nazionali di Legnaro, Instituto Nazionale di Fisica Nucleare and M. Pignatari from Ambizione grant of the SNSF, NSF grants PHY 02-16783 and PHY 09-22648 (Joint Institute for Nuclear Astro physics, JINA), EU MIRG-CT-2006-046520, and Eurogenesis (MASCHE).Peer reviewe

New Cross Section Measurements for Neutron-Induced Reactions on Cr, Ni, Cu, Ta and W Isotopes Obtained with the Activation Technique

Herein we report on 50Cr(n,x)48V, 58Ni(n,pa)54Mn, 58Ni(n,x)56Co, 63Cu(n,pa)59Fe, 181Ta(n,a)178mLu, 181Ta(n,a)178gLu, 181Ta(n,x)180mHf, 181Ta(n,p)181Hf, 181Ta(n,2n)180gTa, 182W(n,p)182Ta, 183W(n,x)182Ta, 183W(n,p)183Ta, 184W(n,x)183Ta, 184W(n,a)181Hf, 184W(n,p)184Ta, 186W(n,a)183Hf, 186W(n,x)185Ta, 186W(n,p)186Ta, and 186W(n,2n)185mW reaction cross section measurements using the activation technique. The irradiations were carried out at the 7-MV Van de Graaff accelerator at IRMM, Geel. Quasi monoenergetic neutrons with energies between 13.8 and 20.5MeV were produced via the 3H(d,n)4He reaction at Ed = 1, 2, 3, and 4MeV. Both natural and samples enriched in 182W, 183W, 184W, and 186W were used to facilitate correction for interference between reactions leading to the same product. Standard gamma-ray spectrometry was employed for the measurement of the radioactivity. In addition to the standard detector efficiency calibration a Monte Carlo simulation of the coaxial HPGe detector was performed with the MCNP5 code in order to achieve higher geometry flexibility and better accuracy. The measured results are compared with work by other authors, current evaluated data files, TALYS and EMPIRE calculations using consistent parameter sets.JRC.D.5-Neutron physic