136 research outputs found
Quantification of spin alignment in fission by simultaneous treatment of gamma and conversion electron angular distributions
The study of the angular momentum properties of fission fragments can shed
light about the complex mechanisms that characterize the fission process. One
quantity that is of significant interest, and has not yet been studied
adequately, is the alignment of the fragments, which is the cause of anisotropy
of the {\gamma} rays along the fission axis and has been observed in various
past and recent experiments. In this work, we have performed calculations using
the FIFRELIN code, in an attempt to quantify the alignment of the nuclear spins
after neutron-emission. Under the statistical tensor formalism of angular
distributions, the conversion-electron and the {\gamma}-ray angular
distributions can be treated simultaneously in an event-by-event calculation.
This enables a first prediction of the conversion-electron angular distribution
with respect to the fission axis. An average value for the alignment of fission
fragments is deduced for 252Cf, with the use of recent experimental data. The
method used for the present work can serve as a starting point for future
theoretical and experimental studies in terms of {\gamma} and
conversion-electron spectroscopy in view of studying the spin alignment of
individual fission fragments, which could further improve our understanding on
the process of fission
Improved FIFRELIN de-excitation model for neutrino applications
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 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 {\gamma}
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.Comment: Corrected typos on author names on arXiv metadat
The joint evaluated fission and fusion nuclear data library, JEFF-3.3
The joint evaluated fission and fusion nuclear data library 3.3 is described. New evaluations for neutron-induced interactions with the major actinides U, U and Pu, on Am and Na, Ni, Cr, Cu, Zr, Cd, Hf, W, Au, Pb and Bi are presented. It includes new fission yields, prompt fission neutron spectra and average number of neutrons per fission. In addition, new data for radioactive decay, thermal neutron scattering, gamma-ray emission, neutron activation, delayed neutrons and displacement damage are presented. JEFF-3.3 was complemented by files from the TENDL project. The libraries for photon, proton, deuteron, triton, helion and alpha-particle induced reactions are from TENDL-2017. The demands for uncertainty quantification in modeling led to many new covariance data for the evaluations. A comparison between results from model calculations using the JEFF-3.3 library and those from benchmark experiments for criticality, delayed neutron yields, shielding and decay heat, reveals that JEFF-3.3 performes very well for a wide range of nuclear technology applications, in particular nuclear energy
Observation of a nuclear recoil peak at the 100 eV scale induced by neutron capture
Coherent elastic neutrino-nucleus scattering and low-mass Dark Matter
detectors rely crucially on the understanding of their response to nuclear
recoils. We report the first observation of a nuclear recoil peak at around 112
eV induced by neutron capture. The measurement was performed with a CaWO
cryogenic detector from the NUCLEUS experiment exposed to a Cf source
placed in a compact moderator. The measured spectrum is found in agreement with
simulations and the expected peak structure from the single-
de-excitation of W is identified with 3 significance. This
result demonstrates a new method for precise, in-situ, and non-intrusive
calibration of low-threshold experiments
Constraints on the dipole photon strength for the odd uranium isotopes
Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies.
Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest.
Methods: The γ-ray spectra from neutron-capture reactions on the U, U, and U nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV.
Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths.
Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section
Study of the photon strength functions and level density in the gamma decay of the n+U-234 reaction
The accurate calculations of neutron-induced reaction cross sections are relevant for many nuclear applications. The photon strength functions and nuclear level densities are essential inputs for such calculations. These quantities for U-235 are studied using the measurement of the gamma de-excitation cascades in radiative capture on U-234 with the Total Absorption Calorimeter at n_TOF at CERN. This segmented 4 pi gamma calorimeter is designed to detect gamma rays emitted from the nucleus with high efficiency. This experiment provides information on gamma multiplicity and gamma spectra that can be compared with numerical simulations. The code DICEBOXC is used to simulate the gamma cascades while GEANT4 is used for the simulation of the interaction of these gammas with the TAC materials. Available models and their parameters are being tested using the present data. Some preliminary results of this ongoing study are presented and discussed
Constraints on the dipole photon strength for the odd uranium isotopes
Background: The photon strength functions (PSFs) and nuclear level density (NLD) are key ingredients for calculation of the photon interaction with nuclei, in particular the reaction cross sections. These cross sections are important especially in nuclear astrophysics and in the development of advanced nuclear technologies. Purpose: The role of the scissors mode in the M1 PSF of (well-deformed) actinides was investigated by several experimental techniques. The analyses of different experiments result in significant differences, especially on the strength of the mode. The shape of the low-energy tail of the giant electric dipole resonance is uncertain as well. In particular, some works proposed a presence of the E1 pygmy resonance just above 7 MeV. Because of these inconsistencies additional information on PSFs in this region is of great interest. Methods: The ?-ray spectra from neutron-capture reactions on the U234, U236, and U238 nuclei have been measured with the total absorption calorimeter of the n_TOF facility at CERN. The background-corrected sum-energy and multi-step-cascade spectra were extracted for several isolated s-wave resonances up to about 140 eV. Results: The experimental spectra were compared to statistical model predictions coming from a large selection of models of photon strength functions and nuclear level density. No combination of PSF and NLD models from literature is able to globally describe our spectra. After extensive search we were able to find model combinations with modified generalized Lorentzian (MGLO) E1 PSF, which match the experimental spectra as well as the total radiative widths. Conclusions: The constant temperature energy dependence is favored for a NLD. The tail of giant electric dipole resonance is well described by the MGLO model of the E1 PSF with no hint of pygmy resonance. The M1 PSF must contain a very strong, relatively wide, and likely double-resonance scissors mode. The mode is responsible for about a half of the total radiative width of neutron resonances and significantly affects the radiative cross section
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