Isotopic distributions were measured for the light fragment group in the transition region from asymmetric to symmetric fission for thermal neutron induced fission of Pu-239 and Pu-241 using the novel technology of calorimetric low temperature detectors in combination with the passive absorber method. Nuclear charge distributions were determined for 24 masses in the range A = 89 to A = 112 for Pu-241(nth , f) for the first time with the LOHENGRIN mass spectrometer. Moving from asymmetric to symmetric fission, known data were supplemented for masses from A = 110 to A = 112 for Pu-241(nth , f) and from A = 109 to A = 113 for Pu-239(nth, f). The isotopic yields were used to evaluate the charge polarization Delta Z and the proton odd-even effect delta p with emphasis on the borderline between asymmetric and symmetric fission. This highlighted a virtual shell effect near Z = 44 which mirrors the effect of the Z = 50 shell in the complementary heavy fragment group of the two Pu isotopes (Z = 94) studied.

The novel technology of calorimetric low-temperature detectors (CLTDs) was applied to determine isotopic yields of fission fragments using the passive absorber method for thermal-neutron-induced fission reactions at the LOHENGRIN mass spectrometer at the Institut Laue-Langevin in Grenoble, France. Precise yields were determined for( 92)Rb and Y-96. These fission products are the dominant contributors to the high-energy portion of the reactor antineutrino spectra. Our new measurements resolve inconsistencies between previous yield measurements and fission data libraries and reduce the nuclear data uncertainties in the computation of reactor antineutrino spectra by the summation method.

The novel technology of calorimetric low-temperature detectors (CLTDs) was applied to determine isotopic yields of fission fragments using the passive absorber method for thermal-neutron-induced fission reactions at the LOHENGRIN mass spectrometer at the Institut Laue-Langevin in Grenoble, France. Precise yields were determined for Rb92 and Y96. These fission products are the dominant contributors to the high-energy portion of the reactor antineutrino spectra. Our new measurements resolve inconsistencies between previous yield measurements and fission data libraries and reduce the nuclear data uncertainties in the computation of reactor antineutrino spectra by the summation method.

The novel technology of calorimetric low-temperature detectors (CLTDs) was applied to determine isotopic yields of fission fragments using the passive absorber method for thermal-neutron-induced fission reactions at the LOHENGRIN mass spectrometer at the Institut Laue-Langevin in Grenoble, France. Precise yields were determined for $^{92}\mathrm{Rb}$ and $^{96}\mathrm{Y}$. These fission products are the dominant contributors to the high-energy portion of the reactor antineutrino spectra. Our new measurements resolve inconsistencies between previous yield measurements and fission data libraries and reduce the nuclear data uncertainties in the computation of reactor antineutrino spectra by the summation method.

The first excited level of 96Mo was investigated in a high-statistics experiment using the 95Mo(n, γγ) cold neutron capture reaction. The measurements used the high cold neutron flux from the research reactor at Institut Laue-Langevin and employed the highly-efficient EXILL array to detect γ-ray coincidences. The recorded statistics allow identification of decay branches with only a small relative intensity including the E3 decay. With the knowledge of the newly measured branching ratio and the known transition probability, the lifetime of the level was determined and, subsequently, the strength of the other decay branches of the octupole phonon were calculated. The extracted electromagnetic decay strengths are compared to the systematics of the stable even–even molybdenum isotopes and values calculated in a Skyrme-force based quasiparticle random phase approximation and in a cluster approach. Additionally, the decay branch to the low-lying quadrupole isovector level was observed.

Nuclei in the neutron-rich region beyond 132Sn have been produced recently by various experiments using fission. Using isomer and β-decay studies nuclear structure data has been collected on the orbital evolution and collectivity in the region with both the increase of proton and neutron numbers. Examples on particular questions related to the g.s. of the A=136 odd-odd 136Sb nucleus and its heavier neighbours are given in the scope of expectations by shell-model theory.

FIPPS is the new nuclear physics instrument of ILL for the spectroscopy of nuclei produced in neutron-induced reactions. The performance of the first implementation of the setup will be shown, together with an overview of the first experimental campaign (December 2016-March 2017). Future perspectives and physics opportunities will then be discussed.

Radiative capture reactions on rare targets and prompt spectroscopy of neutron-rich nuclei produced in the neutron-capture induced-fission of 235U and 241Pu have been performed at the Institut Laue-Langevin (ILL). The high efficiency germanium (Ge) EXOGAM array has been used, complemented with Ge detectors from GASP and the ILL. LaBr3 detectors from the FATIMA collaboration have also been coupled to EXOGAM to measure lifetimes of lowlying excited states. The measured characteristics indicate very good performances of the overall setup. Some recent results will be discussed.

In recent experiments, the new concept of calorimetric low-temperature detectors (CLTDs) was applied for the first time for the investigation of isotopic yields of fission fragments. Fragments from neutron-induced fission sources were mass-separated by the LOHENGRIN spectrometer at the ILL Grenoble and, after passing silicon nitride membranes used as degraders, detected in a CLTD array. The new detector concept of a thermal detector provides a fundamental advantage over conventional ionization-mediated detectors, in particular for heavier particle masses at low energies. Using fissile targets of 235 U, 239 Pu and 241 Pu, nuclear-charge separation was studied in the mass region 82 ≤ A ≤ 139. For light fragments, the Z resolution matches historically best values with conventional techniques, while for heavier masses substantial improvement was attained. We have gained first LOHENGRIN data on the isotopic yields in the light-mass group of 241 Pu. Towards mass-symmetry, known Z-yield data were supplemented in the range A = 110 to 113 for 241 Pu and 239 Pu. Extended data sets were cumulated for A = 92 and 96 due to a recent request from studies on the reactor anti-neutrino spectrum. Furthermore, considerable progress was achieved to extend isotopic yield measurements up to the heavy-mass region, hardly accessible until now.

The γ-coincidence studies of low-spin structures of 210Bi and 206Tl are presented. The 210Bi nucleus, populated in thermal neutron capture reaction, was investigated using EXILL HPGe array at Institut Laue-Langevin in Grenoble. The experimental results were compared to the shell-model calculations allowing to draw the conclusions on the nature of the low-spin excitations populated below the neutron binding energy in 210Bi (4.6 MeV). It has been found that some levels cannot be described by the valence proton and neutron couplings, but may arise from couplings of valence particles to the 3- octupole phonon of the doubly magic 208Pb core. Moreover, preliminary results of a low-spin structure measurements of 206Tl by the γ-coincidence technique, making use of the 205Tl(n,γ)206Tl reaction at the FIPPS prompt γ-ray spectroscopy facility of ILL are shown. The population of a large number of excited states of 206Tl above the ground state up to the neutron binding energy (at 6.5 MeV), within a few units of spin is expected. The analysis involving double and triple γ-coincidences and γγ-angular correlations will allow to significantly extend the experimental information on the energy and spin-parity of the levels in 206Tl. This will help shedding light on the proton-hole and neutron-hole couplings near the doubly magic core 208Pb.

Nuclear fission yields data measurements for thermal neutron induced fission of 241Pu have been carried out at the Institut Laue Langevin (ILL) in Grenoble, using the Lohengrin mass spectrometer. Mass, isotopic and isomeric yields have been extracted for the last measurements. A focus is given in this document to the mass yield results which are obtained for almost the entire heavy peak and most of the light high yields masses, along with the covariance matrix. The mean kinetic energy as a function of the fission product mass has also been extracted from the measurements. The total mean kinetic energy pre and post neutron emission have been assessed and compared to other works showing a rather good agreement.

Fission yields are essential for nuclear reactor studies (decay heat, fuel inventory...) and constitute also one of the main observables needed to improve our understanding of the fission process. The symmetric mass region is of particular interest due to various intriguing properties of the fission fragments already reported in the literature : inversion of the nuclear charge polarization, large width of the fission fragment kinetic energy distribution, strong change of the prompt neutron multiplicity, etc. Recently, measurements of fission yields and kinetic energy distributions in the symmetric mass region were achieved at the LOHENGRIN mass spectrometer of the Institut Laue-Langevin (ILL). This experimental work is challenging due to the low counting rate and the appearance of contaminant masses, leading to pronounced components in the fission fragment kinetic energy distribution. Despite removing the undesirable contributions, the fission fragment kinetic energy distributions still show two components, indicating that the fission process could be modal. To go further and better characterize these components a comparison between our experimental data and Monte Carlo calculations (FIFRELIN code) simulating the de-excitation of the fission fragments for different fission channels will be presented and discussed.

A large array of Ge detectors installed at ILL, around a 235U target irradiated with cold neutrons, (EXILL) allowed measurement of prompt gamma-ray cascades occurring in fission fragments with an unambiguous determination of fragments. Here we present preliminary results of a systematic comparison between experimental γ-ray intensities and those obtained from the Monte-Carlo simulation code FIFRELIN, which is dedicated to the de-excitation of fission fragments. Major γ-ray intensities in the 142Ba and 92Kr fission products, extracted from EXILL data, were compared to FIFRELIN, as well as to reported values (when available) obtained with EUROGAM2 in the spontaneous fission of 248Cm. The evolution of γ-ray intensities in 92Kr versus the complementary partner in fission (i.e. versus the total number of evaporated neutrons by the fission pair) was then extracted and compared to FIFRELIN.

The study of fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. In the past with the LOHENGRIN spectrometer of the ILL, priority has been given for the studies in the light fission fragment mass range. The LPSC in collaboration with ILL and CEA has developed a measurement program on symmetric and heavy mass fission fragment distributions. The combination of measurements with ionisation chamber and Ge detectors is necessary to describe precisely the heavy fission fragment region in mass and charge. Recently, new measurements of fission yields and kinetic energy distributions are has been made on the 233U(nth,f) reaction. The focus of this work has been on the new optical and statistical methodology and the self-normalization of the data to provide new absolute measurements, independently of any libraries, and the associated experimental covariance matrix.

Received 22 February 2017DOI:https://doi.org/10.1103/PhysRevC.95.039901©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsNuclear structure & decaysNuclear Physics

The mixing ratio of the main 320-keV, M1 + E2 ground-state γ transition in 210Bi has been more precisely quantified, allowing a significant reduction in the uncertainty of measurements of the neutron-capture cross section to the ground state of 210Bi from 25% to 0.9%. Accurate values for neutron-capture cross sections to both the ground and long-lived 9− isomeric state at 271 keV in 210Bi are of particular importance as Pb-Bi finds increased usage in Accelerator Driven Systems.

Low-spin excited levels in $_{35}^{86}\mathrm{Br}_{51}$ and $_{36}^{86}\mathrm{Kr}_{50}$, populated following ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay and the neutron-induced fission of $^{235}\mathrm{U}$, were measured using the Lohengrin fission-fragment separator and the EXILL array of Ge detectors at the PF1B cold-neutron facility of the Institute Laue-Langevin Grenoble. Improved populations of excited levels in $^{86}\mathrm{Br}$ remove inconsistencies existing in the literature on this nucleus. Directional-linear-polarization correlations, analyzed using newly developed formulas, as well as precise angular correlations allowed the unique ${1}^{\ensuremath{-}}$ and ${2}^{\ensuremath{-}}$ spin and parity assignments to the ground state of $^{86}\mathrm{Br}$ and the 4016.3-keV level in $^{86}\mathrm{Kr}$, respectively. Based on these results we propose that the Gamow-Teller ${\ensuremath{\beta}}^{\ensuremath{-}}$ decays of $^{86}\mathrm{Se}$ and $^{86}\mathrm{Br}$ involve the $\ensuremath{\nu}{g}_{7/2}\ensuremath{\rightarrow}\ensuremath{\pi}{g}_{9/2}$ transition in addition to the $\ensuremath{\nu}{p}_{3/2}\ensuremath{\rightarrow}\ensuremath{\pi}{p}_{3/2}$ transition proposed earlier. In $^{86}\mathrm{Kr}$ we have identified ${1}_{1}^{+}, {2}_{3}^{+}$, and ${3}_{1}^{+}$ levels, analogous to the mixed-symmetry states in $^{94}\mathrm{Mo}$, which in $^{86}\mathrm{Kr}$ are from proton excitations, only. Large-scale, shell-model calculations with refined interactions reproduce well excitations in $^{86}\mathrm{Br}$ and $^{86}\mathrm{Kr}$ and support our interpretations.

Nuclear fission consists of splitting a nucleus into smaller nuclei. Several observables are available to study the fission process such as fission yields or fission fragment angular momentum. Currently, fission models cannot predict all the observables with an acceptable accuracy for nuclear fuel cycle studies for instance. Improvement of fission models is an important issue for the knowledge of the process itself and for the applications. In this work, we take an interest in fission fragment angular momentum distribution. Isomeric ratios (IRs) are a common observable giving access to investigate these distributions. We measured accurate IRs for 88Br, 132Sn and 132Te with the fission fragment separator LOHENGRIN and developed a new analysis method to assess the mean value and uncertainty of the IR. An evaluation of the angular momentum distribution of 132Sn was also performed with the FIFFRELIN code.

Preliminary results are presented from a γ-spectroscopy study of low-spin states of several Ca isotopes, produced by neutron capture on a Ca target, at very high coincedence rates. The experiment was performed at the PF1B cold-neutron facility at ILL (Grenoble, France), using the array of HpGe detectors, named EXILL