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The origin of the large angular momenta observed for fission fragments is still a question under discussion. To address this, we study isomeric yield ratios (IYRs), i.e., the relative population of two or more long-lived metastable states with different spins, of fission products. We report on IYRs of 17 isotopes produced in the 28-MeV 𝛼-induced fission of 232Th at the IGISOL facility of the University of Jyväskylä. The fissioning nuclei in this reaction are 233,234,235U*. We compare our data to IYRs from thermal neutron-induced fission of 233U and 235U, and we observe statistically significant larger IYRs in the 232Th(𝛼, 𝑓) reaction, where the average compound nucleus (CN) spin is 7.7 ℏ, than in 233,235U(𝑛th, 𝑓), with average spins of 2.6 and 3.6 ℏ, respectively. To assess the influence of the excitation energy, we study literature data of IYRs from photon-induced fission reactions, and find that, within current uncertainties, the IYRs indicate no dependency of the CN excitation energy. We conclude that the different IYRs seem to be due to the different CN spins alone. This would imply that the fission fragment angular momentum only partly comes from the fission process itself and is, in addition, influenced by the angular momentum present in the CN.

In this study, we applied the Total Monte Carlo (TMC) methodology in de-excitation simulations of primary fission fragments (FF) using the TALYS code. The goal was to develop and optimise a methodology to benchmark initial fission model assumptions on fission mass yield distributions, excitation energy sharing and angular momentum population. The study was performed on the thermal neutron induced fission of Pu-239(n(th),f). The work aimed at evaluating fission model deficiencies and parameter sensitivities. We systematically varied TALYS input data by generating 5000 random files through the GEF code, randomizing 94 model parameters that affect fission yields and energy distributions within 3% of their default values. This variation revealed significant changes in the fission observables, such as prompt neutron and gamma-ray multiplicities and energy spectra. The results indicate some systematic defects in the assumed excitation-energies and angular momenta. Another outcome from the study is the identification of a need for new correlation measurements on prompt neutrons and gamma-rays from the Pu-239(n(th),f) reaction, as well as an updated evaluation.

Isomeric yield ratios (IYR) are an important tool to study the angular momentum generation in nuclear fission and to investigate the possible influence on the spin of the compound nucleus formed in a fission reaction. One method to measure the IYR is to use the Phase-Imaging Ion-Cyclotron-Resonance (PI-ICR) technique. The PI-ICR is a direct ion counting technique based on the spatial separation of ions trapped in a circular motion and their projection onto a position-sensitive detector. Here the analysis routine is presented for the PI-ICR images produced to study 21 fission products formed by the 32 MeV α-induced fission of 232Th at the IGISOL facility.

The VERDI fission spectrometer is designed to measure fragment velocities and kinetic energies to achieve high-precision yield measurements. It consists of two time-of-flight (TOF) sections, each hosting a micro-channel plate (MCP) and up to 32 passivated implanted planar silicon (PIPS) detectors. The main challenge to achieve accurate fragment velocities is the so-called plasma delay time (PDT) phenomena in the PIPS detectors. In this work, we present a dedicated experimental campaign at the LOHENGRIN fission-fragment recoil separator, to solve the pending PDT challenges. The PDT effect was systematically investigated, as a function of mass and energy, using a dedicated time-of-flight setup. In addition, the pulse height defect (PHD) was determined simultaneously. The studies were conducted for five PIPS detectors, in energies and mass numbers ranging from 20 to 110 MeV and A = 85 to 149, respectively. Using digital signal processing, an excellent timing resolution was achieved, reaching as low as 60 ps (one σ) for the heavy ions. The PDT revealed a strong positive correlation with the ion energy and a weak negative correlation with the mass. The experimental PDT values determined from five detectors confirm a consistent systematic behavior with respect to mass and energy. Some systematic discrepancies were exhibited by two detectors, possibly due to the use of different pre-amplification chains. The PDT measurements ranged between 1 and 3.5 ns, for heavy ions relative to α-particles. The PHD values showed also a strong correlation with the ion energy, and moreover with the ion mass. The PHD for heavy ions was found to range between 2 and 8 MeV, relative to α-particles. Finally, a two-dimensional parameterisation was developed to model the experimental PDT data, as a function of mass and energy. This new model, which is valid in the fission fragment mass and energy regime, will be of benefit for heavy-ion velocity measurements, using silicon detectors, as done in VERDI.

The VElocity foR Direct particle Identification (VERDI) is a spectrometer designed to determine fission-fragment mass distributions using the 2E-2v method, which measures both velocities and energies of the fission fragments. VERDI has two time-of-flight (TOF) sections, each with a Micro Channel Plate (MCP) Time-of-flight Start detector and up to 32 passivated implanted planar silicon (PIPS) detectors. Achieving the desired mass resolution is challenged by the need for high TOF and energy resolutions. Recent upgrades to the VERDI setup have improved its performance, achieving a TOF resolution as low as 355 ps (FWHM) and an energy resolution of 22 keV (FWHM) for alpha-particles. These enhancements in resolution demonstrate improved capabilities for achieving high-precision fission fragment yield measurements.

The experimental setup called Medley, has been installed at the Neutrons For Science (NFS) facility at GANIL to study the production of light-ions in neutron-induced reactions. The setup was first used to measure the neutron spectral flux from neutron-proton elastic scattering, allowing both an assessment of its performance, and providing an independent measurement of the neutron flux of the facility. The experimental setup, its performance, and the challenges encountered are presented and discussed. The deduced neutron flux is compared to the one obtained from a monitor based on position-sensitive parallel plate avalanche counters (PS-PPACs).

Challenging neutron-capture cross-section measurements of small cross sections and samples with a very limited number of atoms require high-flux time-of-flight facilities. In turn, such facilities need innovative detection setups that are fast, have low sensitivity to neutrons, can quickly recover from the so-called. gamma-flash, and offer the highest possible detection sensitivity. In this paper, we present several steps towards such advanced systems. Specifically, we describe the performance of a high-sensitivity experimental setupat CERN n_TOF EAR2. It consists of nine sTED detector modules in a compact cylindrical configuration, two conventional used large-volume C6D6 detectors, and one LaCl3(Ce) detector. The performance of these detection systems is compared using Nb-93(n, gamma) data. We also developed a detailed GEANT4 Monte Carlo model of the experimental EAR2 setup, which allows for a better understanding of the detector features, including their efficiency determination. This Monte Carlo model has been used for further optimization, thus leading to a new conceptual design of a gamma detector array, STAR, based on a deuterated-stilbene crystal array. Finally, the suitability of deuterated-stilbene crystals for the future STAR array is investigated experimentally utilizing a small stilbene-d12 prototype. The results suggest a similar or superior performance of STAR with respect to other setups based on liquid-scintillators, and allow for additional features such as neutron-gamma discrimination and a higher level of customization capability.

The neutron time-of-flight facility n_TOF at CERN is a spallation source dedicated to measurements of neutroninduced reaction cross-sections of interest in nuclear technologies, astrophysics, and other applications. Since 2014, Experimental ARea 2 (EAR2) is operational and delivers a neutron fluence of similar to 4 center dot 10(7) neutrons per nominal proton pulse, which is similar to 50 times higher than the one of Experimental ARea 1 (EAR1) of similar to 8 center dot 10(5) neutrons per pulse. The high neutron flux at EAR2 results in high counting rates in the detectors that challenged the previously existing capture detection systems. For this reason, a Segmented Total Energy Detector (sTED) has been developed to overcome the limitations in the detector's response, by reducing the active volume per module and by using a photo-multiplier (PMT) optimized for high counting rates. This paper presents the main characteristics of the sTED, including energy and time resolution, response to gamma-rays, and provides as well details of the use of the Pulse Height Weighting Technique (PHWT) with this detector. The sTED has been validated to perform neutron-capture cross-section measurements in EAR2 in the neutron energy range from thermal up to at least 400 keV. The detector has already been successfully used in several measurements at n_TOF EAR2.