This work reports high-precision cumulative yield measurements of key isotopes from 235U(nth,f) reactions using the FIPPS (FIssion Product Prompt γ-ray Spectrometer) at ILL, representing the first dedicated cumulative fission yield campaign in the facility. In this work, advanced spectroscopic techniques were employed to reduce nuclear data uncertainties, while evaluating FIPPS capabilities for fission yield measurements. A pre-irradiated 235U target was exposed to a high neutron flux, and the resulting γ-rays were recorded using a 16-element HPGe Clover array. A 7-day irradiation at a thermal neutron flux of ~3.3 x 107 n/s/cm2 was followed by a 23-day decay period. The multi-parameter FIPPS setup, providing data from 64 detector channels, enabled detailed reconstruction of the entire fission process from irradiation to decay. Our analysis framework combined machine learning-based spectral analysis with γ-emission simulations, establishing new benchmarks for fission yield measurements. The approach demonstrates the strength of integrating high-precision γ-spectroscopy with advanced computational methods. Cumulative yields for key isotopes were determined with small uncertainties and showed good agreement with evaluated libraries. The developed methodology offers a robust foundation for future fission product studies and nuclear data improvements, applicable to both short- and long-lived isotopes of nuclear relevance.

The low-spin structure of the 206Tl nucleus was studied in the thermal neutron capture reaction 205Tl(n, gamma ) 206Tl at the Institut Laue-Langevin in Grenoble making use of the multidetector HPGe array Fission Product Prompt gamma -ray Spectrometer and gamma gamma -coincidence techniques. The information on discrete structures located below the neutron binding energy in 206Tl was extended: a total number of 99 gamma rays (75 new) were observed and 21 excited states (8 new) were located. The analysis of the angular correlations of gamma rays was used to extract information on transitions multipolarities, which helped with spin-parity assignments for the located levels. The obtained experimental results were compared to shell-model calculations involving one-proton-hole, oneneutron-hole excitations below the 208Pb core. The two-body nucleon-nucleon realistic interactions derived from CD-Bonn free nucleon-nucleon potential were used. Reasonable agreement is obtained for the excitation energies of the states which, according to calculations, have highly fragmented wave functions, particularly in the highenergy region where the density of levels increases. The observed discrepancies are interpreted as a consequence of the large uncertainties in the determination of the off-diagonal matrix elements of the realistic shell-model interaction, which are mainly responsible for the fragmentation of the wave functions.

We present the detailed design and performance simulations of BIFROST, a multiplexing indirect neutron time-of-flight spectrometer at the European Spallation Source. The instrument allows a neutron bandwidth of Δλ = 1.74 Å to reach the sample. The polychromatic flux can reach 6 × 109 n/s/cm2 while retaining a relative energy resolution, δEi/Ei, of around 3.5% at Ei = 5 meV. A fast pulse-shaping chopper allows for flexibility in the primary spectrometer resolution. For an opening time of 0.1 ms, a relative energy resolution down to 0.2% is achieved for Ei = 12 meV. The secondary spectrometer consists of 45 analyzer arrays, populating 9 scattering angle channels in the horizontal scattering plane, each of which covers 5.2° in scattering angle. Each channel hosts 5 analyzer arrays reflecting neutrons of fixed energies Ef equal to 2.7, 3.2, 3.8, 4.4, and 5.0 meV. Utilizing the prismatic analyzer concept, the back-end geometry allows a secondary spectrometer energy resolution in the range of 0.02-0.05 meV. The unique design of BIFROST offers not only an unprecedented neutron flux but also the ability to adjust the energy resolution by more than one order of magnitude. Focusing on the horizontal scattering plane, the spectrometer is ideally suited for extreme environment studies and for studying samples much smaller than 1 cm3. The drastic increase in measurement efficiency, compared to current high-flux spectrometers, has a particularly high impact on the fields of quantum magnetism, unconventional superconductivity, and functional materials.

Fission yields are one of the most used observables to describe the fission process. They are also mandatory for nuclear fuel cycle studies or nuclear reactor calculations for instance. In this paper we will show how we can extract independent fission yields and the fission product angular momentum by the combined use of γ-ray measurements and the LOHENGRIN spectrometer. In particular, the importance of nuclear structure data is presented.

Background: gamma -ray spectroscopy studies of the Sn isotopes provide important information on nuclear structure and shell evolution across the long isotopic chain between the doubly-magic Sn-100 and Sn-132 nuclei. These studies also offer great value to test and tune nuclear models which can then be applied to other regions of the nuclear chart. Purpose: We aim to expand the level scheme of Sn-118 by populating low-spin states in the range of 3-5 MeV and determine their angular momentum for the possible connection of these states to pygmy quadrupole Resonances, a new phenomenon observed in the neighboring Sn-112,Sn-114,Sn-124 isotopes as a resonance-like structure in the 3-5 MeV range. Method: Excited states in Sn-118 were populated via the Sn-117(n, gamma)Sn-118 reaction at the Institut Laue-Langevin in Grenoble, France. The FIssion Product Prompt gamma-ray Spectrometer (FIPPS), an array of eight n -type high purity Germanium clover detectors augmented with eight additional clover detectors from IFIN "Horia Hulubei" were used to detect gamma rays from excited states in Sn-118. The array provides a superior efficiency for gamma -ray detection and nearly 4 pi coverage for the measurements of angular correlations for spin assignment of excited nuclear levels. Results: Through gamma -gamma coincidences, 112 excited states were identified with 57 being newly placed in the level scheme. From these states, 567 gamma -ray transitions were observed with 501 being newly identified. Many levels were identified in the 3-5 MeV region. Further, an indirect measurement of the E0 transition which decays from the 0(3)(+) state to the 2p -2h, 0(2)(+) state was performed and the q(K)(2)(E0/E 2) and X(E0/E2) for this transition were determined to be 12.7(11) and 6.3(5), respectively. The 10(3) x rho(2)(E0) was determined to be >38 based on a half-life limit of <200 ps of the 2057-keV, 0(3)(+) level. Conclusions: The abundant spectroscopic information on Sn-118 obtained in the present experiment is an important input to the theoretical description of nuclei in the region and highlights the capabilities of the FIPPS array at ILL in conjunction with neutron capture reactions. Many states identified in the 3-5 MeV region could very likely have J = 2(+) and contribute to the pygmy quadrupole resonances.

Large inconsistencies still exist in nuclear data libraries regarding the kinetic parameters of delayed neutron (DN) precursors. As an example, there is a 17 T_1/2 = (8.87 ± 0.10) s. Those results are consistent with the values recommended by the IAEA/CRP work and they come with reduced uncertainties compared with previously published results.

This article presents an experimental effort to provide high-quality data to improve the evaluation of the 239 Pu delayed neutron yield in the thermal energy range. The set-up is composed of a long counter with sixteen 3 He tubes, a fast shutter system to produce irradiation cycles with short rising/falling times, and a miniature fission chamber containing 114μg of 239 Pu. The whole system was installed in the PF1B experimental zone of the Institut Laue-Langevin, which provides a cold neutron beam. The repetition of irradiation/decay cycles enables to saturate the delayed neutron precursors and to measure their yield through the observed activity, shortly after the beam-stop. The innovation of our measurement technique relies on the clear distinction between prompt and delayed neutron counting, thanks to boron absorbers, without the necessity to move the sample. In such a way, it is possible to normalize the counting of delayed neutron emission to the one of total neutron emission, based on the well-known value of the prompt neutron multiplicity. The present work provides a delayed neutron yield value of v d = 0.642(5)%. The latter is in 1σ agreement with the IAEA recommendation of 0.628(38)%, with a strongly reduced uncertainty thanks to our normalization technique.

The low-spin structure of the 95Zr nucleus was investigated at Institut Laue-Langevin (Grenoble) by employing three complementary methods: Either cold-neutron capture on 94Zr or cold-neutron-induced fission of 235U, both using the highly efficient EXILL array of high-purity germanium (HPGe) detectors, or & beta; decay of mass separated 95Y ions at the focal point of the Lohengrin fission-fragment separator. Together 21 new levels and 44 new transitions were placed in the level scheme. Spin-parity assignments to most of the levels in 95Zr were significantly improved.

The study of nuclear fission yields has a major impact on the characterization and understanding of the fission process and is mandatory for reactor applications. In the framework of a collaboration between the CEA, the LPSC and the ILL, a program of actinide fission yield measurements has been ongoing for several years at the LOHENGRIN spectrometer. However, the measurement of very low fission yields in the symmetry region and the heavy wing of the distributions is difficult to achieve due to the strong contamination by other masses with much higher yields and requires the development of a new experimental setup. This paper will first present the results of a new absolute measurement of the 235 U(n th ,f) mass yields using an ionization chamber placed at the exit of the spectrometer. Although very well documented in the literature, these yields show uncertainties lying from 3% to 10% with large discrepancies between libraries and a lack of correlation matrices. New experimental data obtained at the LOHENGRIN spectrometer will be detailed, along with the measurement method and the production of the experimental covariance matrix. The second part will show the development of a Time of Flight (ToF) line in order to improve the background rejection in the mass yield measurements. In the symmetry region, the precision of the measurement is limited by the background estimation due to the charge exchanges with the residual gas of the separator. We plan to analyze the events using a triple coincidence (∆E x E) x ToF, whereas today only (∆E x E) selection is available. The new ToF line is built using Si 3 N 4 foils and secondary electron detectors (SED) for the start and stop detectors. We will present the choices made for the SED technology along with the progress achieved on the ToF line characterization.

Eight EXOGAM-type, Ge-clover detectors of the EXILL array, arranged on an octagon plan and acting as Compton polarimeters, were used to measure directional-polarization correlations of γ rays populated in various processes. Measurements of γ radiation following neutron-induced fission of 235 U and 241 Pu targets; β - -decay following fission; (n, γ ) reactions on various stable targets and measurements using radioactive sources of 60 Co, 133 Ba and 152 Eu were performed at the PF1B cold-neutron facility of the Institut Laue-Langevin in Grenoble. Digital electronics and a trigger-less acquisition system allowed the collection of signals from Ge detectors down to about 20 keV, providing measurements of linear polarization down to 120 keV. The precise sensitivity calibration was determined for the set of eight EXILL clover polarimeters in a range from 100 keV to 10000 keV. A new formula for calculating the directional-polarization correlations for the upper transition in a γγ cascade is derived from first principles. Directional-polarization correlations were measured for a few dozens of transitions in a number of nuclei, including values for twenty transitions in the 152 Sm nucleus measured for the first time, and compared against the calculated values, to illustrate the quality of the technique. The combined analysis of angular and directional-polarization correlations is presented, which helps the determination of spin-parity assignments to excited levels.

Excited levels in 92Kr, 96Zr, and 98Mo nuclei were reinvestigated using high-statistics multiple-& gamma; coincidence data measured with the EXILL and FIPPS Ge arrays, following neutron-induced fission of 235U and neutron capture on a 97Mo target, respectively. The experimental goal was to search for new levels, especially with low spins, as well as to firm up spin-parity assignments to known levels. In total of 16 new levels with 64 new or corrected decays and 35 new or improved spin-parity assignments were observed in the three nuclei. We also performed large-scale shell-model calculations to learn more about the microscopic structure of levels in these nuclei. The evolution of collectivity in N = 56 isotones is discussed, stressing the important role of various single-particle excitations, in particular of the & pi;g9/2 orbital, in the shape evolution in the region.

Background: Nuclear spectroscopy of neutron-rich isotopes provides important information on their nuclear structure and has a valuable impact on the modeling of the r-process path. Particularly interesting are nuclei close to doubly-magic species, e.g., Sn-132, with only several valence particles. Such is the barely explored I-137 nucleus, investigated here in detail. Purpose: To establish excited states in I-137, ss decay of the Te-137 ground state is studied. In addition, the unknown ss-delayed neutron-emission channel of Te-137 to I-136 is inspected. Search for levels and for candidates for Gamow-Teller and first-forbidden transitions between the mother nucleus and excited states in the daughter nucleus is conducted within the experimental observations. Methods: ss-delayed gamma-ray spectroscopy is employed to study excited states in I-137. The nucleus is populated in the decay of a mass-separated beam of Te-137, produced in neutron-induced fission of U-235. Results: The new level scheme of I-137 populated in ss decay is established. The half-life T-1/2 of Te-137 is determined to be 2.46(5) s. The ss-delayed neutron-emission probability P-n value of Te-137 is deduced as a lower limit to be 2.63(85)%. Conclusions: The experimental results are an important input to the theoretical description of nuclei in the region, being well interpreted within large-scale shell model calculations, and provide essential information on the first-forbidden transitions beyond N = 82 and Z = 50.

A novel, witness-sample-based technique for detection of isotope-specific material signatures excited by quasi-mono-energetic gamma sources is presented. Proof of principle demonstrations involving the detection, assay, and imaging of 7Li are reviewed.

We demonstrate the possibility to directly detect microgram amounts of the isotope 7 L i using a quasi-monochromatic high-energy photon beam. The isotope selective detection is based on a witness scatterer absorbing and re-emitting photons via nuclear resonance fluorescence. This enables the detection of isotopes with microgram accuracy at long distances from the actual sample. Further, we demonstrate that the technique can deliver quantitative information without specific knowledge of the photon flux and no spectral capabilities or knowledge of the resonance fluorescence cross section. Detection of low-atomic-weight isotopes screened by heavy shielding is also shown. The techniques described are applicable to all next-generation, ultrahigh brilliance, laser-Compton light sources currently under construction.

B. M. Nyakó, J. Timár, ∗ M. Csatlós, Zs. Dombrádi, A. Krasznahorkay, I. Kuti, D. Sohler, T. G. Tornyi, M. Czerwiński, T. Rza̧ca-Urban, W. Urban, P. Ba̧czyk, L. Atanasova, D. L. Balabanski, K. Sieja, A. Blanc, M. Jentschel, U. Köster, P. Mutti, T. Soldner, G. de France, G. S. Simpson, and C. A. Ur Institute for Nuclear Research (Atomki), Pf. 51, 4001 Debrecen, Hungary Faculty of Physics, University of Warsaw, ul. Pasteura 5, PL-02-093 Warsaw, Poland Department of Med. Physics and Biophysics, Medical University Sofia, 1431, Sofia, Bulgaria ELI-NP, Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering IFIN-HH, 077125 Bucharest-Magurele, Romania Université de Strasbourg, IPHC, Strasbourg, France; CNRS, UMR7178, 67037 Strasbourg, France Institut Laue-Langevin, 71 avenue des Martyrs, 38042 Grenoble Cedex 9, France GANIL, CEA/DSM-CNRS/IN2P3, Bd Henri Becquerel, BP 55027, F-14076 Caen Cedex 5, France LPSC, Université Joseph Fourier Grenoble 1, CNRS/IN2P3, Institut National Polytechnique de Grenoble, F-38026 Grenoble Cedex, France Extreme Light Infrastructure-Nuclear Physics (ELI-NP)/IFIN-HH, 077125 Bucharest-Magurele, Romania (Dated: March 10, 2021)

Detailed studies of the low-spin structures of neutron-rich Ni isotopes may help shedding light on the shape coexistence phenomenon. Of particular interest is the Ni-65 nucleus, since it lies between Ni-64 and Ni-66, where shape coexistence has been reported earlier. A spectroscopic investigation of Ni-65 has been performed at Institut Laue-Langevin via the reaction Ni-64(n, gamma) Ni-65, using the FIPPS HPGe array. Several new gamma transitions have been observed and angular correlation analyses have been performed. A comparison with Monte Carlo shell-model calculations pointed to a dominance of spherical states up to 1.5 MeV excitation energy, together with the appearance of two states of oblate character.

The execution and analysis of complex experiments are challenged by the vast dimensionality of the underlying parameter spaces. Although an increase in data-acquisition rates should allow broader querying of the parameter space, the complexity of experiments and the subtle dependence of the model function on input parameters remains daunting owing to the sheer number of variables. New strategies for autonomous data acquisition are being developed, with one promising direction being the use of Gaussian process regression (GPR). GPR is a quick, non-parametric and robust approximation and uncertainty quantification method that can be applied directly to autonomous data acquisition. We review GPR-driven autonomous experimentation and illustrate its functionality using real-world examples from large experimental facilities in the USA and France. We introduce the basics of a GPR-driven autonomous loop with a focus on Gaussian processes, and then shift the focus to the infrastructure that needs to be built around GPR to create a closed loop. Finally, the case studies we discuss show that Gaussian-process-based autonomous data acquisition is a widely applicable method that can facilitate the optimal use of instruments and facilities by enabling the efficient acquisition of high-value datasets. Gaussian process regression (GPR) is a powerful, non-parametric and robust technique for uncertainty quantification and function approximation that can be applied to optimal and autonomous data acquisition. This Review introduces the basics of GPR and discusses several use cases from different fields.

Excited levels in 90Sr, 92Sr, 94Sr, and 96Sr nuclei were reinvestigated using high-statistics multiple-gamma coincidence data from neutron-induced fission of 235U and spontaneous fission of 252Cf, measured using Exogam at Institut Laue Langevin and Gammasphere arrays, respectively. The experimental goal was the search for new excited levels and firm spin-parity assignments to known levels. A total of 23 new levels with 30 new or corrected decays and 39 new or improved spin-parity assignments were obtained in the four nuclei. Negative-parity structures on top of 3- excitation were firmly identified and extended to higher spins. New positive-parity structures in 94Sr and 96Sr were observed with 3+ excitations characteristic of gamma collectivity. The 277.7-keV, E2 decay from the 1507.0-keV level to the second 0+ level in 96Sr, found in this paper, completes the coexisting deformed band in this nucleus. To learn about the microscopic structure of levels in the 88-96Sr nuclei, we performed large-scale shell-model calculations. The calculations compared to the experiment, helped the discussion of the evolution of collectivity in strontium isotopes, highlighting the important role of various single-particle excitations in phase transitions and shape coexistence in the region. The special role of the neutron 9/2+[404] extruder as a catalyst of the deformation change in the region is highlighted.

Excited states of the neutron-rich 89Br have been observed for the first time. They were populated in coldneutron induced fission of 235U at the PF1B facility of the Institut Laue-Langevin, Grenoble. The measurement of gamma radiation following fission has been performed using the EXILL array of Ge detectors. The observed level structure looks similar to the yrast level structure of 87Br. Large valence space shell model calculations performed for 89Br confirmed this similarity. Comparison of the observed pi g9/2 band with the results of the shell model calculations provides information on the evolution of collectivity in this region.