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The precise modeling of the de-excitation of Gd isotopes is of great interest for experimental studies of neutrinos using Gd-loaded organic liquid scintillators. The FIFRELIN code was recently used within the purposes of the STEREO experiment for the modeling of the Gd de-excitation after neutron capture in order to achieve a good control of the detection efficiency. In this work, we report on the recent additions in the FIFRELIN de-excitation model with the purpose of enhancing further the de-excitation description. Experimental transition intensities from the EGAF database are now included in the FIFRELIN cascades, in order to improve the description of the higher energy part of the spectrum. Furthermore, the angular correlations between γ rays are now implemented in FIFRELIN, to account for the relative anisotropies between them. In addition, conversion electrons are now treated more precisely in the whole spectrum range, while the subsequent emission of X rays is also accounted for. The impact of the aforementioned improvements in FIFRELIN is tested by simulating neutron captures in various positions inside the STEREO detector. A repository of up-to-date FIFRELIN simulations of the Gd isotopes is made available for the community, with the possibility of expanding for other isotopes which can be suitable for different applications. [ABSTRACT FROM AUTHOR]

Anomalies in past neutrino measurements have led to the discovery that these particles have non-zero mass and oscillate between their three flavours when they propagate. In the 2010s, similar anomalies observed in the antineutrino spectra emitted by nuclear reactors have triggered the hypothesis of the existence of a supplementary neutrino state that would be sterile, that is, not interacting by means of the weak interaction1. The STEREO experiment2–6 was designed to investigate this conjecture, which would potentially extend the standard model of particle physics. Here we present an analysis of the full set of data generated by STEREO, confirming observed anomalies while rejecting the hypothesis of a light sterile neutrino. Installed at the Institut Laue–Langevin (ILL) research reactor, STEREO accurately measures the antineutrino energy spectrum associated to the fission of 235U. The segmentation of the detector and its very short distance to the compact core are crucial properties of STEREO for our analysis. The measured antineutrino energy spectrum suggests that anomalies originate from biases in the nuclear experimental data used for the predictions7,8. Our result supports the neutrino content of the standard model and establishes a new reference for the 235U antineutrino energy spectrum. We anticipate that this result will allow progress towards finer tests of the fundamental properties of neutrinos but also to benchmark models and nuclear data of interest for reactor physics9,10 and for observations of astrophysical or geoneutrinos11,12.Accurate measurements of the antineutrino energy spectrum of 235U fission by the STEREO detector reject the sterile neutrino hypothesis and point to biases in the nuclear data to explain the discrepancies with the prediction. [ABSTRACT FROM AUTHOR]

Angular distribution and correlation measurements are an essential tool in nuclear structure experiments, especially when spectroscopic information on a specific nucleus is unknown. In most cases, the experimental determination of the spins and parities of the studied nuclear states, as well as the possible mixing between two electric/magnetic multipoles of a transition are determined using angular correlation measurements. In this work, the full effect of directional γ correlations is simulated, by using the formal theory of angular correlations. The statistical tensor formalism is employed, enabling to perform a full simulation of the angular correlation effects in a cascade of an arbitrary number of γ transitions. The present approach was coupled with the Monte Carlo code FIFRELIN, which can simulate the de-excitation of fission fragments or of excited nuclei after neutron capture. It provides a complete description of the spatial distributions of all the γ rays in the cascade, that can be used for simulation purposes in various applications both in nuclear and particle physics. The potential for a novel approach in data analysis of angular correlation measurements is discussed thoroughly. [ABSTRACT FROM AUTHOR]

Numerous measurements of fission product yields were performed since the discovery of the nuclear fission process. However, more precise and reliable fission product yields are requested. Lack of covariance matrices make difficult to use it for specific application purposes such as the propagation uncertainty of decay heat. In this work, we propose to measure independently the fission product mass yields for the whole heavy peak (including the symmetric mass region) for the 233 U (n th , f) reaction. Both average values and experimental covariance is provided. The fission product mass yields are measured with the LOHENGRIN recoil mass spectrometer of the ILL using an ionization chamber located at the focal plane. A new procedure of data taking has been developed in order to minimize the biases. Concretely several ionic charges and kinetic energy distributions have been measured for each mass. Particular attention has been considered in the monitoring of the target time evolution. Additional corrections were necessary in the symmetry mass region due to contaminants coming from the LOHENGRIN recoil mass spectrometer. A complex Monte Carlo analysis has been developed in order to better propagate all the uncertainties. The fission product mass yields of the 233 U (n th , f) and its associated covariance matrix has been produced. An overall good agreement has been observed with ENDF/B-VIII.0 in contrast with the JEFF-3.3 evaluation. A precision around 2% for the heavy peak has been measured. The experimental covariance matrix was also computed. In the symmetry mass region, two components were observed in the kinetic energy distribution. One of this component was considered as an artifact and was ruled-out. [ABSTRACT FROM AUTHOR]

In April 2007, first cold neutrons flew through the new neutron guide dedicated to prompt gamma activation analysis at the research reactor FRM II. The cold neutron flux intensity of 6.0·109 cm−2·s−1 with mean neutron energy of 1.8 meV was measured at the exit window of the guide. The design of the neutron guide is flexible in order to fulfil various requirements at once: the last 7 m are elliptically tapered and the final 1 m of it is removable. The description of the PGAA instrument, first tests and experiments providing its characterization are the subjects of this paper. [ABSTRACT FROM AUTHOR]

Prompt gamma activation analysis (PGAA) is a well known tool for non-destructive bulk elemental analysis of objects. The measured concentrations are only representative of the whole sample if it is homogenous; otherwise it provides only a sort of average composition of the irradiated part. In this latter case one has to scan the sample to obtain the spatial distribution of the elements. To test this idea we have constructed a prompt gamma activation imaging — neutron tomograph (PGAI-NT) setup at the NIPS station of the Budapest Research Reactor, consisting of a high-resolution neutron tomograph and a germanium gamma-spectrometer. The samples are positioned relative to the intersection of the collimated neutron beam and the projection of the gamma-collimator (isocenter) by using an xyzω-moving table. [ABSTRACT FROM AUTHOR]

The European collaboration “ANCIENT CHARM” () aims to develop new non-destructive neutron techniques to image the internal composition of complex archaeological objects in order to answer various archaeological questions. Among these techniques, prompt gamma activation imaging (PGAI) and neutron tomography (NT) form a unique combination which can determine the 3D distribution of most elements in objects with a non-destructive procedure. A spatial resolution better than 2 mm has already been achieved in a moderately scattering matrix material. [ABSTRACT FROM AUTHOR]

The 5H system was produced in the 3H( t, p) 5H reaction studied at small CM angles with a 58MeV tritium ion beam. High statistics data were used to reconstruct the energy and angular correlations between the 5H decay fragments. A broad structure in the 5H missing-mass spectrum showing up above 2.5MeV was identified as a mixture of the 3/2+ and 5/2+ states. The data also present an evidence that the 1/2+ ground state of 5H is located at about 2MeV. Then, the 5H and 5He systems were explored by means of transfer reactions occurring in the interactions of 132MeV 6He beam nuclei with deuterium. In the 2H( 6He, 3H) reaction a T = 3/2 isobaric analog state of 5H in 5He was observed at an excitation energy of 22.0±0.3MeV with a width of 2.5±0.3MeV. [ABSTRACT FROM AUTHOR]

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z=82−122 formed in the reactions with 48Ca and 58Fe ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multi-detector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments, fission, quasi-fission and evaporation residues cross sections, multiplicities of neutrons and γ quanta and their dependence on the mechanism of formation and decay of compound systems have been studied. [ABSTRACT FROM AUTHOR]

The effects of the entrance channel and shell structure on the experimental evaporation residues have been studied by analyzing the 32S + 182W, 48Ti + 166Er and 60Ni + 154Sm reactions leading to 214Th*. the 40Ar + 181Ta reaction leading to 221Pa*; the 48 + 243Am, 248Cm, 249 reactions leading to the 291115, 296116 and 297118 superheavy compound nuclei, respectively. The fusion mechanism and the formation of evaporation residues of heavy and superheavy nuclei have been studied. In calculations of the excitation functions for capture, fusion and evaporation residues we used such characteristics as mass asymmetry of nuclei in the entrance channel, binding energies and shape of colliding nuclei, potential energy surface, driving potential, partial-fusion cross-sections and survival probability of the compound nucleus, Γn/ Γf ratio at each step along the de-excitation cascade of the compound nucleus. The calculations have allowed us to make useful conclusions about the mechanism of the fusion-fission process, which is in competition with the quasifission process, and the production of the evaporation residues. [ABSTRACT FROM AUTHOR]

To investigate [sup 5]H resonance states with a better instrumental resolution, we utilized the two-neutron transfer reaction [sup 3]H(t,p)[sup 5]H accomplished with the use of a cryogenic liquid-tritium target and 57.5-MeV triton beam. As a result of this study, a valuable fraction of protons detected at θ[sub lab]=18[sup °]–32[sup °] in ptn coincidence events was attributed to the states of the [sup 5]H nucleus. Two resonance states situated at 1.8±0.1 and 2.7±0.1 MeV above the t+n+n decay threshold were obtained in the missing mass energy spectrum of the [sup 5]H nucleus. The peak located close to E[sub [sup 5]H]=1.8 MeV was clearly seen in the [sup 5]H spectrum obtained from the energy distributions of [sup 3]H nuclei emitted in the reaction [sup 2]H([sup 6]He, [sup 5]H)[sup 3]He at θ[sub lab]=17[sup °]–32[sup °]. The width (Γ[sub obs]≤0.5 MeV) obtained for the two [sup 5]H resonance states is surprisingly small. A state of [sup 4]H with E[sub res]=3.3 MeV and γ[sup 2]=2.3 MeV was obtained in the reaction [sup 2]H(t, p)[sup 4]H from the spectra of protons leaving the target at θ[sub lab]=18[sup °]–32[sup °] and detected in coincidence with neutrons emitted in the decay of [sup 4]H nuclei.© 2003 MAIK “Nauka / Interperiodica”. [ABSTRACT FROM AUTHOR]

Nuclear dissipation in capture reactions is investigated using backtracing, a new analysis protocol. Combining analysis procedure with dynamical models, the difficult and long-standing problem of competition and mixing between quasifission and fusion-fission is solved for the first time. The nature of the relevant dissipation is determined as one-body dissipation. At low excitation energy where shell e3ects are strongly e3ective, the shape of the mass distribution could be a powerful check of the nature and the magnitude of the dissipation. [ABSTRACT FROM AUTHOR]

A resonance state situated at 1.8±0.1 and, most likely, another state positioned at 2.7±0.1 MeV above the t+n+n decay threshold were observed in the missing mass energy spectrum of the 5H nucleus produced in the reaction 3H(t,p)5H. The peak located close to $$E_{^5 H} = 1.8$$ MeV also was seen in the 5H spectrum obtained from the energy distributions of 3H nuclei emitted in the reaction 2H(6He,5H)3He. The width (Γobs ≤ 0.5 MeV) obtained for the two 5H resonance states is surprisingly small. A state of 4H with E res = 3.3 MeV and γ2 = 2.3 MeV was obtained in the reaction 2H(t,p)4H from the proton spectrum. [ABSTRACT FROM AUTHOR]

By comparing theoretical and experimental excitation functions of evaporation residues resulting from the same compound nucleus or heavy and superheavy nuclei, it is possible to understand the effect of the entrance channel and the shell structure of reacting nuclei on the fusion mechanism. The competition of complete fusion with the quasifission process is strongly related to the intrinsic fusion barrier B*[SUBfus] and the quasifission barrier B[SUBqf], as well as the size of the well in the nucleus - nucleus potential. In our calculations of the excitation functions for capture, fusion, and evaporation residues, we use the relevant variables such as mass asymmetry of nuclei in the entrance channel, potential energy surface, driving potential, spin distribution, and surviving probability of compound nucleus that are responsible for the mechanism of the fusion-fission process. As a result, we obtain a beam energy window for the capture of the nuclei before the system fuses and the Λn/Λf ratio at each step along the deexcitation cascade of the compound nucleus. Calculations performed in the framework of the model taking into account the nuclear shell effect and shape of colliding nuclei allow us to reach useful conclusions about the mechanism of the fusion-fission process and the production of the evaporation residues. We analyze the [SUP40]Ar + [SUP176]Hf, [SUP86]Kr + [SUP130]Xe, and [SUP124]Sn + [SUP92]Zr reactions leading to [SUP216]Th*; the [SUP32]S + [SUP182]W and [SUP60]Ni + [SUP154]Sm reactions leading to [SUP214]Th*; the [SUP48]Ca + [SUP248]Cm reaction leading to the [SUP296]116 compound nucleus; and the [SUP48]Ca + [SUP249]Cf reaction leading to the [SUP297]118 compound nucleus. [ABSTRACT FROM AUTHOR]

The process of fusion-fission of heavy and superheavy nuclei (SHE) with Z = 82-122 formed in the reactions with [SUP48]Ca and [SUP58]Fe ions at energies near and below the Coulomb barrier has been studied. The experiments were carried out at the U-400 accelerator of the Flerov Laboratory of Nuclear Reactions (JINR) and at the XTU Tandem accelerator of the National Laboratory of Legnaro (LNL) using the time-of-flight spectrometer of fission fragments CORSET and the neutron multidetector DEMON. As a result of the experiments, mass and energy distributions (MED) of fission fragments; fission, quasifission, and evaporation residue cross sections; and multiplicities of neutrons and γ-quanta and their dependences on the mechanism of formation and decay of compound systems have been studied. [ABSTRACT FROM AUTHOR]

The STEREO experiment measures the electron antineutrino spectrum emitted in a research reactor using the inverse beta decay reaction on H nuclei in a gadolinium loaded liquid scintillator. The detection is based on a signal coincidence of a prompt positron and a delayed neutron capture event. The simulated response of the neutron capture on gadolinium is crucial for the comparison with data, in particular in the case of the detection efficiency. Among all stable isotopes, 155Gd and 157Gd have the highest cross sections for thermal neutron capture. The excited nuclei after the neutron capture emit gamma rays with a total energy of about 8MeV. The complex level schemes of 156Gd and 158Gd are a challenge for the modeling and prediction of the deexcitation spectrum, especially for compact detectors where gamma rays can escape the active volume. With a new description of the Gd (n, γ ) cascades obtained using the FIFRELIN code, the agreement between simulation and measurements with a neutron calibration source was significantly improved in the STEREO experiment. A database of ten millions of deexcitation cascades for each isotope has been generated and is now available for the user. [ABSTRACT FROM AUTHOR]