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Abstract In counting experiments associated with pulsed sources, a high data collection rate can lead to considerably large counting losses, especially in the case of spallation Time-of-Flight facilities equipped with medium and short flight paths where the research interest is focused on higher neutron energies where counting losses can be quite large due to the higher neutron flux, the more compressed time frame compared to the one on lower energies and the higher cross-section depending on the reaction. Examples of such measurements are the neutron induced fission experiments at the new experimental area EAR-2 at the n_TOF facility at CERN. Although analytical expressions to account for this inefficiency exist in literature, the introduced corrections are not always sufficient to retrieve the true reaction rate, therefore a different approach is mandatory. This work explores the possibility to quantify the counting losses using detector emulation devices and exponential fits in waiting time distributions. The methodology is benchmarked in the test case of the standard 238U(n,f) cross-section with reference to 235U(n,f) for bandwidths up to 1.9 MHz and counting losses that exceed 60%. [ABSTRACT FROM AUTHOR]

Abstract A novel MicroMegas detector based on microbulk technology with an embedded XY strip structure was developed, obtained by segmenting both the mesh and the anode in perpendicular directions. This results in a very low-mass device with good energy and spatial resolution capabilities. Such a detector is practically “transparent” to neutrons, being ideal for in-beam neutron measurements and can be used as a quasi-online neutron beam profiler at neutron time-of-flight facilities. A dedicated front end electronics and acquisition system has been developed and used. The first studies of this new detection system are presented and discussed. [ABSTRACT FROM AUTHOR]

The measurement of the 236U(n, f) cross section was attempted with a new MicroMegas detector, based on the Microbulk technology. The incident quasi-monoenergetic neutron beams with energies in the range of 4-10 MeV were produced via the 2H(d, n) reaction at the neutron beam facility of the Institute of Nuclear and Particle Physics at the NCSR "Demokritos" and first results of the analysis are presented. [ABSTRACT FROM AUTHOR]

Cross sections for the 174Hf(n, 2n)173 Hf and 176Hf(n, 2n)175Hf reactions have been measured on the 5.5-MV Van de Graaff tandem accelerator of National Centre for Scientific Research "Demokritos" in Athens, in the neutron energy region from 8.8 to 11.0 MeV, using the activation technique. An experimental method to account for the contamination of the 176Hf(n, 2n) reaction by the 176Hf(n, λ) reaction activated by the parasitic neutrons of the beam is presented. Statistical model calculations have also been performed using the nuclear-reaction codes EMPIRE-II and TALYS. The results and effects of the nuclear input parameters as well as pre-equilibrium emission are discussed in detail. [ABSTRACT FROM AUTHOR]

In the present work, the 197Au(n,2n) reaction cross section is experimentally determined relative to the 27Al(n,α)24Na reaction at incident neutron energies of 9.0-10.5 MeV by means of the activation technique. The quasimonoenergetic fast neutron beam was produced via the 2H(d,n)3He reaction at the 5.5-MV Tandem Van de Graaff accelerator at the National Centre of Scientific Research (NCSR) "Demokritos" and was studied to determine the contribution of background "parasitic" neutrons using the multiple foil activation technique and the SULSA unfolding code. The cross sections for the population of the second isomeric state (12-) of 196Au and the sum of the ground (2-) and first isomeric state (5-) population cross sections were independently determined. Auxiliary Monte Carlo simulations were performed with the MCNP code. Theoretical calculations of the above cross sections in the 8-25 MeV region were carried out with the use of the STAPRE-F, EMPIRE, and TALYS codes, which were also compared in their implementation of the generalized superfluid model. The theoretical results are compared with previous work in the same mass region, and the strong dependence on the level scheme of the nuclei involved was revealed. [ABSTRACT FROM AUTHOR]

Abstract: The differential cross-sections of the 11B(d, α 0, α 2,p0) reactions for the determination of the depth distribution of boron in near-surface layers of materials have been determined in the projectile energy region E d,lab =900−1200keV. The experiment was carried out in energy steps of 25keV and for four detector angles between 140° and 170° (in steps of 10°). The obtained experimental data are suitable for Nuclear Reaction Analysis (NRA) studies. A qualitative discussion of the observed cross-section variations through the influence of broad resonances in the d+ 11B system is also presented. The results are also compared to data from literature, when present, and are validated through a benchmarking experiment using a high purity thick B4C target. [Copyright &y& Elsevier]

The neutron-induced fission cross section of 237Np was experimentally determined at the high-resolution and high-intensity facility n_TOF, at CERN, in the energy range 100 keV to 9 MeV, using the 235U(n,ƒ) and 238U(n,ƒ) cross section standards below and above 2 MeV, respectively. A fast ionization chamber was used in order to detect the fission fragments from the reactions and the targets were characterized as far as their mass and homogeneity are concerned by means of a spectroscopy and Rutherford backscattering spectroscopy respectively. Theoretical calculations within the Hauser-Feshbach formalism have been performed, employing the empire code, and the model parameters were tuned in order to successfully reproduce the experimental fission cross-sectional data and simultaneously all the competing reaction channels. [ABSTRACT FROM AUTHOR]

Recent accurate data obtained for the isomeric cross section of the 197Au(n, In) reaction provide a valuable opportunity to consider the question of the effective moment of inertia of the nucleus within a local consistent model analysis of all available reaction data for the 197Au target nucleus. Thus, a definite proof of a moment of inertia equal to that of the rigid-body has been obtained for the 196Au nucleus, while indications infer about half the rigid-body for 194Ir. [ABSTRACT FROM AUTHOR]

Abstract: Quasi-monoenergetic neutron beams, in the energy range 7–11.5MeV, produced via the 2H(d,n) reaction, have been used at the 5.5MV tandem T11/25 Accelerator Laboratory of NCSR “Demokritos”. The flux variation of the neutron beam is monitored with a BF3 detector, while the absolute flux is obtained with respect to reference reactions. An investigation of the energy dependence of the neutron fluence has been carried out using two independent techniques: by a liquid scintillator BC501A detector and deconvolution of its recoil energy spectra performed by means of the DIFBAS code, as well as via the multiple foil activation technique in combination with the SULSA unfolding code. The neutron facility has also been characterized by means of Monte Carlo simulations with MCNP5. [Copyright &y& Elsevier]

Several cross-section measurements of neutron-induced reactions on Ge found in literature, are performed utilizing natGe targets. The production of the same residual nucleus as the measured one might occur as a result of the unavoidable presence of neighboring isotopes in the same target, acting as a contamination. Corrections must be made based on theoretical calculations and models in order to resolve this problem. The accuracy and limits of a methodology for these "theoretical corrections" are investigated in this work using isotopically enriched targets, which can produce very accurate results without the need for such corrections. Experimental cross-section measurements have been made for the 76Ge(n,2n)75Ge, 72Ge(n, α)69mZn and 72Ge(n,p)72Ga reactions, via the activation technique, with the 27Al(n, α)24Na reaction used as reference, employing both a natGe and isotopically enriched Ge targets. The 3H(d,n)4He (D–T) reaction was used for producing the quasi-monoenergetic neutron beam in the 5.5 MV Tandem Accelerator Laboratory of the National Centre for Scientific Research "Demokritos" in Athens, Greece, at an incident deuteron beam energy of 2.9 MeV. Using HPGe detectors, γ -ray spectroscopy was applied to determine the induced γ -ray activity of the residual nuclei. • Cross section measurements with natural Ge targets need to be corrected for contaminations from neighboring isotopes • A methodology is proposed for the correction of the contribution from parasitic channels • The accuracy and limits of the methodology are investigated with two sets of measurements • Enriched and natural Ge targets were employed for cross-section measurements via the activation technique [ABSTRACT FROM AUTHOR]

Neutron-induced fission reactions play a crucial role in a variety of fields of fundamental and applied nuclear science. In basic nuclear physics they provide important information on properties of nuclear matter, while in nuclear technology they are at the basis of present and future reactor designs. Finally, there is a renewed interest in fission reactions in nuclear astrophysics due to the multi-messenger observation of neutron star mergers and the important role played by fission recycling in r-process nucleosynthesis. Although studied for several decades, many fundamental questions still remain on fission reactions, while modern applications and the development of more reliable nuclear models require high-accuracy and consistent experimental data on fission cross sections and other fission observables. To address these needs, an extensive fission research programme has been carried out at the n_TOF neutron time-of-flight facility at CERN during the last 18 years, taking advantage of the high energy resolution, high luminosity and wide energy range of the neutron beam, as well as of the detection and data acquisition systems designed for this purpose. While long-lived isotopes are studied on the 185 m long flight-path, the recent construction of a second experimental area at a distance of about 19 m has opened the way to challenging measurements of short-lived actinides. This article provides an overview of the n_TOF experimental programme on neutron-induced fission reactions along with the main characteristics of the facility, the various detection systems and data analysis techniques used. The most important results on several major and minor actinides obtained so far and the future perspectives of fission measurements at n_TOF are presented and discussed. [ABSTRACT FROM AUTHOR]

The U neutron-induced fission cross-section has been measured at incident neutron energies of 452, 550, 651 keV and 7.5, 8.7, 10 MeV using the Li ( p, n) and the H( d, n) reactions, respectively, relative to the U( n, f ) and U( n, f ) reference reactions. The measurement was performed at the neutron beam facility of the National Center for Scientific Research 'Demokritos', using a set-up based on Micromegas detectors. The active mass of the actinide samples and the corresponding impurities were determined via $ \alpha$ -spectroscopy using a surface barrier silicon detector. The neutron spectra intercepted by the actinide samples have been thoroughly studied by coupling the NeuSDesc and MCNP5 codes, taking into account the energy and angular straggling of the primary ion beams in the neutron source targets in addition to contributions from competing reactions ( e.g. deuteron break-up) and neutron scattering in the surrounding materials. Auxiliary Monte Carlo simulations were performed making combined use of the FLUKA and GEF codes, focusing particularly on the determination of the fission fragment detection efficiency. The developed methodology and the final results are presented. [ABSTRACT FROM AUTHOR]

A new high flux experimental area has recently become operational at the n_TOF facility at CERN. This new measuring station, n_TOF-EAR2, is placed at the end of a vertical beam line at a distance of approximately 20m from the spallation target. The characterization of the neutron beam, in terms of flux, spatial profile and resolution function, is of crucial importance for the feasibility study and data analysis of all measurements to be performed in the new area. In this paper, the measurement of the neutron flux, performed with different solid-state and gaseous detection systems, and using three neutron-converting reactions considered standard in different energy regions is reported. The results of the various measurements have been combined, yielding an evaluated neutron energy distribution in a wide energy range, from 2meV to 100MeV, with an accuracy ranging from 2%, at low energy, to 6% in the high-energy region. In addition, an absolute normalization of the n_TOF-EAR2 neutron flux has been obtained by means of an activation measurement performed with Au foils in the beam. [ABSTRACT FROM AUTHOR]

The 241Am(n,2n) 240Am reaction cross section has been measured at four energies, 10.0, 10.4, 10.8, and 17.1 MeV, by means of the activation technique, relative to the 27Al(n,α) 24Na reaction reference cross section. Quasi-monoenergetic neutron beams were produced via the ²H(d,n) ³He and the ³H(d,n) 4He reactions at the 5.5 MV Tandem T11/25 accelerator laboratory of NCSR "Demokritos". The high purity 241Am targets were provided by JRC-IRMM, Geel, Belgium. The induced γ -ray activity of 240Amwasmeasured with high-resolution high-purity germanium (HPGe) detectors. Auxiliary Monte Carlo simulations were performed with the MCNP code. The present results are in agreement with data obtained earlier and predictions obtained with the EMPIRE code. [ABSTRACT FROM AUTHOR]

The one-neutron removal cross sections of neutron-rich Al isotopes and longitudinal momentum distributions of the residues have been measured for A = 33 to 36 at relativistic energies (≈900 MeV/u). The inclusive data have been interpreted within the eikonal approximation. The evolution of the single-particle occupancy in the ground state of 33,34,35A1 has been studied and compared with shell model predictions. The inferred 2s1/2 neutron occupancy in the 33A1 ground-state wave function is 20% to 40% lower than the predicted one. The inclusive data do not exclude the presence of intruder states. Some intruder / = 1 occupancy is found in 34A1, similarly to 33Mg. The single-particle 1 f7/2 occupancy shows a gradual increase at N = 22. Correspondingly, a decrease of the lrf3/2 strength has been observed. [ABSTRACT FROM AUTHOR]

New measurements of the interaction cross sections of 22,230 at 900A MeV performed at the GSI, Darmstadt are reported that address the unsolved puzzle of the large cross section previously observed for 230. The matter radii for these oxygen isotopes extracted through a Glauber model analysis are in good agreement with the new predictions of the ab initio coupled-cluster theory reported here. They are consistent with a 220 + neutron description of 230 as well. [ABSTRACT FROM AUTHOR]

The interaction cross sections of 32-35Mg at 900A MeV have been measurmed using the fragment separator at GSI. The deviation from the r0A1/3 trend is slightly larger for 35Mg, signaling the possible formation of a longer tail in the neutron distribution for 35Mg. The radii extracted from a Glauber model analysis with Fermi densities are consistent with models predicting the development of neutron skins. [ABSTRACT FROM AUTHOR]

Abstract: The first reaction spectroscopy on the ground state structure of 33Mg through the measurement of the longitudinal momentum distribution from the one-neutron removal reaction using a C target at is reported. The experiment was performed at the FRS, GSI. The distribution has a relatively narrow width ( (FWHM)) and the one-neutron removal cross-section is . An increased contribution from the orbital is required to explain the observation showing its lowering compared to existing model predictions. This provides new information regarding the configuration of 33Mg and the island of inversion. [Copyright &y& Elsevier]

In the measurement of neutron capture cross-sections of fissile isotopes, the fission channel is a source of background which can be removed efficiently using the so-called fission-tagging or fission-veto technique. For this purpose a new compact and fast fission chamber has been developed. The design criteria and technical description of the chamber are given within the context of a measurement of the 233U(n, γ) cross-section at the n_TOF facility at CERN, where it was coupled to the n_TOF Total Absorption Calorimeter. For this measurement the fission detector was optimized for time resolution, minimization of material in the neutron beam and for alpha-fission discrimination. The performance of the fission chamber and its application as a fission tagging detector are discussed. [ABSTRACT FROM AUTHOR]

The 162Er(n,2n)161Er reaction cross section (Eth=9.26MeV) was measured at six incident neutron beam energies by means of the activation technique. Two energy regions were covered in the present work: the near-threshold energies between 10.7 and 11.3 MeV, as well as the higher energies from 17.1 up to 19.0 MeV. In this way, the energy range from the reaction threshold up to 19 MeV was experimentally mapped, considering also the existing experimental information around 14 MeV. The quasi-monoenergetic neutron beam at near-threshold energies was produced via the ²H(d,n)³He reaction, while the higher neutron beam energies were achieved by using the ³H(d,n)4He reaction. The primary deuteron beam was delivered in both cases by the 5.5 MV Tandem Van de Graaff accelerator of NSCR "Demokritos." Statistical model calculations were performed and compared with all the available experimental data. [ABSTRACT FROM AUTHOR]