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[en] The FIFRELIN code developed by CEA (Cadarache) includes the RIPL3 database of nuclear level schemes and is able to estimate the intensities of gamma-ray transitions in all the fission fragments. In the EXILL experiment conducted in 2012 and 2013 at ILL (Institut Laue-Langevin), a target made of 235U (and also 241Pu, which was not investigated in this work) was surrounded by an array of high-resolution, germanium gamma-ray detectors and irradiated by an intense cold neutron beam. We have extracted the intensities of the main discrete gamma-ray transitions in a set of fission fragments, using the triple gamma-ray coincidence technique, and we have compared our results to FIFRELIN outputs. The result of our study on the gamma-ray cascades in Kr and Ba fragment pairs will be presented, in particular the evolution of main transition intensities in Ba (Kr) isotope as a function of the fission partner Kr (Ba) isotope. This document is made up of an abstract and the slides of the presentation. (authors)
[en] A fast-timing experiment was performed at the Argonne National Laboratory between December 2015 and January 2016, with the purpose of measuring the lifetimes of the lowest lying states of nuclei belonging to the deformed regions around mass number A ∼ 110 and A ∼ 150. The lifetimes of the first excited states are the basic ingredients for quadrupole moment calculations, which give information about the structural evolution that occurs in the deformed areas of the nuclear chart. The nuclei of interest were populated in the spontaneous fission of 252Cf since the maxima of the fragment mass distribution lie approximately in the centre of the regions of interest. The 252Cf source was placed at the focus of the Gamma-sphere array (51 HPGe) which was coupled with a fast timing array, comprising 25 LaBr3:Ce scintillator detectors. Each sub-array covered ∼ 2π of the solid angle. Quadruple events (two gamma rays from Gamma-sphere and two from the LaBr3:Ce array) were collected for a period of 30 days. LaBr3:Ce scintillator detectors have been extensively used in the past to perform fast-timing measurements thanks to their capability to access the subnanosecond regime. The high energy resolution of HPGe detectors together with the excellent time resolution of LaBr3:Ce detectors, conferred to this hybrid array the right features to perform both fast-timing and spectroscopy measurements. This was the first time that the Gamma-sphere array was successfully coupled with an array containing such a large number of LaBr3:Ce detectors. Details of the setup and the digital acquisition system will be given, focusing on the signal processing and the coincidence mechanism. Some preliminary results obtained from 152Eu and 166Ho sources will be shown in order to prove the effectiveness of this arrangement. The status of the analysis for the nuclei 103,105Zr, lying in the first peak of the 252Cf fragment mass distribution, will be presented. This document is made up of an abstract, a poster and the slides of the presentation. (authors)
[en] Full text: The neutron-rich Xe isotopes (Z=54) are located north-east of the doubly magic shell closures at Z=50 and N=82. In this region of the nuclear chart, the onset of octupole collectivity towards the 'magic' octupole numbers Z=56 and N=88 is expected too. The data presented here originate from two experimental campaigns performed at the Institut Laue-Langevin (ILL) in 2013 and the Argonne National Laboratory (ANL) at the turn of the year 2015/2016. The investigated nuclei were populated by neutron-induced fission of 235U and 241Pu (ILL) or spontaneous fission of 252Cf (ANL). Prompt gamma-ray spectroscopy of the fission fragments was performed with combined arrays of high-resolution HPGe detectors and fast LaBr3(Ce) detectors. The Xe isotopes of interest were tagged by gating in the HPGe detectors on one or more of its characteristic transitions. The lifetimes of excited states in the ps regime were determined from the LaBr3(Ce) detectors applying the 'Generalized Centroid Difference' (GCD) method . At ILL, the set-up consisted of 8 CLOVER detectors of EXILL combined with 16 LaBr3(Ce) detectors from the FATIMA collaboration . Several lifetimes in the yrast bands of 138,140,142Xe were determined, most of them for the first time . For the first time and more challenging, also two lifetimes in the odd isotopes 139,141Xe were measured. At ANL, the set-up consisted of half of GammaSphere (51 HPGe detectors) and 25 LaBr3(Ce) detectors from the NuSTAR-FATIMA collaboration . The status of the ongoing analyses and selected results will be presented. References:  J.-M- Regis et al., NIM A 726, 191 (2013);  J.-M- Regis et al., NIM A 763, 210 (2014);  S. Ilieva et al., Phys. Rev. C 94, 034302 (2016);  E. Gamba et al., contribution to this workshop. This work is supported by the German BMBF under grant no. 05P12RDNUP (NuPNET), ILL, the EXILL and FATIMA collaborations, the TU Darmstadt - GSI cooperation contract and HIC for FAIR; the work at ANL is funded by the U.S. DOE contract n. DE-AC02-06CH11357 and used resources of the DOE's ATLAS facility. (authors)
[en] This series of slides present the excited states that have been identified in 116,118,120Sn using the (nth, γ) reactions on natural Sn. A 85-keV transition has been identified in 116Sn using the β decay of the I=5+, T = 54.29 mn state of 116m1In. Partial energy level schemes of Sn isotopes are given.
[en] Full text: The region of nuclear deformation around neutron-rich A ∼ 100 has attracted considerable attention since its prediction  and especially after its first experimental confirmation . It witnesses one of the most impressive onset of deformation throughout the nuclear chart right across N=60. Nuclear ground-state deformation in the region was experimentally observed between Rb (Z=37) and Mo (Z=42). In addition, the shape coexistence right below (N ≤ 59) is not readily explained in the standard interpretation . The experimental studies in the region have been facilitated due to the possibility to populate those nuclei as fission products of heavy elements. Some important recent contribution to those experimental investigations have come for the Kr isotopes through mass measurements  and Coulomb-excitation  studies. Those both reveal that the ground-state deformation in the krypton isotopes develops only gradually. In an attempt to shed light on the underlying single-particle structure, responsible for the peculiarity of the region, we performed a Coulomb excitation study of a chain of rubidium isotopes at REX-ISOLDE. The nuclei of interest, 93,95,97,99Rb, were produced using an UCx target and were post-accelerated to about 2.8 MeV/u. They were further sent to a secondary target in the centre of the Miniball array, which was used for the detection of the γ-rays, in coincidence with the beam particles, detected by a position-sensitive Si strip detector. Prior to the present study information on excited states was only available for 93Rb. In our measurement, the previously known low-energy level scheme of 93Rb was confirmed. Very similar single-particle like pattern was observed as well for 95Rb. The sudden change in the structure appears at 97Rb, as could be anticipated from previous nuclear moments and charge-radii measurements . This was experimentally observed through the identification of rotational bands, built on the ground states of 97Rb and 99Rb, from multi-step Coulomb excitation. The B(E2) transition probabilities between a number of excited states in these isotopes were determined as well. The results from the present study  suggest that a fine balance between a spherical shell gap at Z=38, for N ≤ 58, and a deformed one, at N=60 and above, might be one of the main reasons for the sudden onset of the deformation. The practical absence of any mixing between these well-defined spherical and deformed structures allows for the appearance of this most-sudden onset of deformation throughout the nuclear chart and defines 97Rb as the corner stone of the region of deformation around A ∼ 100.
[en] This series of slides details the first observation of excited states in 88Se. The level scheme is interpreted with the shell model. The structure appears to be consistent with a γ-soft or transitional nucleus and collectivity seems to develop from pseudo-SU(3) scheme.
[en] The Daya Bay collaboration has recently published a precise measurement of the inverse beta decay cross section folded antineutrino spectrum, revealing a 5.4% antineutrino deficit beyond the three-flavor neutrino oscillation as well as a spectral distortion. This deficit is compatible with earlier nuclear reactor antineutrino experiments as analyzed by Mention et al, who showed a systematic deficit of antineutrinos at short distances, an effect referred to as the 'reactor neutrino anomaly'. The obvious question is whether this deficit is due to the presence of one (or more) sterile neutrinos, or simply an unknown component in the underlying nuclear physics used in the predictions. In this work, we explore three possibilities in the latter group. First, we study the feasibility of the 238U contribution and the effective Z used in the conversion method as possible sources of the disagreement by adjusting them to match the measured Daya Bay spectrum. Both scenarios are considered unlikely: the adjusted 238U antineutrino spectrum has an unphysical shape and smaller integral than expected from systematic trends, while the fitted Z effective would correspond to fission products with negligible fission yield. Motivated by the fact that 75% of a reactor antineutrino spectrum in the region relevant to the anomaly is accounted for by the decay of fewer than 50 nuclides, it is then possible that individual shape corrections arising from first forbidden transitions or second order terms such as weak magnetism may not cancel out, which led us to explore the sensitivity of the Daya Bay spectrum to experimental shape factor corrections. We find that using a linear term equal to +6%/MeV in the conversion method results in a much closer agreement with the Daya Bay spectrum. Unlike the two previous cases, this scenario cannot be ruled out as there is currently an absence of precisely measured electron spectra from the relevant fission products. Previously, it was noted how important beta intensities and fission yields are for the summation method. This sensitivity analysis highlights the importance of experimental shape factors for both conversion and summation calculations. One possible way forward to confirm the existence of the anomaly, or even quantify it, would be to precisely measure the electron spectra for the relevant nuclides, incorporate these data in summation calculations to understand their effect, and finally, through an average procedure, include them in a conversion calculation. This document is made up of an abstract and the slides of the presentation. (authors)
[en] Properties of fission in Th232 and U233 were studied at the Los Alamos Neutron Science Center (LANSCE) at incident neutron energies from sub-thermal to 40 MeV. Fission fragments are observed in coincidence using a twin ionization chamber with Frisch grids. The average total kinetic energy released from fission and fragment mass distributions are calculated from observations of energy deposited and conservation of mass and momentum. Accurate experimental measurements of these parameters are necessary to better understand the fission process in isotopes relevant to the thorium fuel cycle, in which Th232 is used as a fertile material to generate the fissile isotope of U233. This process mirrors the uranium breeder process used to produce Pu239 with several potential advantages including the comparative greater abundance of thorium, inherent nuclear weapons proliferation resistance, and reduced actinide production. For these reasons, there is increased interest in the thorium fuel cycle to meet future energy demands and improve safety and security while increasing profitability for the nuclear power industry. This research is ongoing and preliminary results concerning Th232 are presented. A first curve gives the average total kinetic energy as a function of the incident neutron energy between 1 and 40 MeV. A second curve gives the emission angles in function of the fragment masses and a third curve represents the fragment mass distribution. This document is made up of an abstract and the slides of the presentation.
[en] The surrogate-reaction method is an indirect method for determining neutron-induced cross sections of short lived nuclei, which are key quantities both in astrophysics and nuclear data applications. Fission and gamma decay probabilities of 237U, 238Np and 239Np have been measured, for the first time simultaneously in dedicated experiments, via the surrogate reactions 238U(3He,4He), 238U(3He,t) and 238U(3He,d), respectively. While a good agreement between our data and neutron-induced data is found for fission probabilities, gamma decay probabilities are several times higher than the corresponding neutron-induced data for the studied 237U and 238Np nuclei. We study the role of the different spin distributions populated in the surrogate and neutron induced reactions. Our results indicate a strong sensitivity of the gamma probability to the compound nucleus angular momentum distribution and, contrary to statistical model predictions, a much weaker sensitivity of the fission probability. Preliminary results from a Hauser-Feshbach calculation, coupled with a DWBA-deduced spin and parity distribution for the 238U(3He,4He) reaction, well reproduce, for the very first time, both measured fission and decay probabilities, helping to gain an insight into the origin of the weaker sensibility of the fission probability to the angular momentum. This finding is highly relevant because it implies that the Hauser-Feshbach calculation, tuned on decay probabilities measured in surrogate reactions, can provide reliable predictions for neutron-induced cross sections of short-lived nuclei that cannot be directly measured. In the contribution we will present the mentioned results and possible future developments. This document is made up of an abstract and the slides of the presentation. (authors)
[en] The Liquid Scintillation Spectrometry technique has advantages that can be used in nuclear physics experiments. Compared to ionization chambers for example, liquid scintillation provides very high detection efficiencies. In the context of studies on the nuclei fission process this is a definitive advantage where count losses have to be known very precisely. We will present the general features of a scintillating target used in nuclear physics experiments, and the special developments that were performed. Particularly the response of a few homemade and commercial scintillators to fission events will be presented. We will detail simulations that were performed, in order to precisely determine the detection efficiency to fission events and also to α decays. Finally we will present preliminary results obtained in the measurement of the spontaneous fission (SF) of 240Pu and 242Pu. It will be shown that a precise count of SF and alpha decay events can be made, together with a precise count of pile-up events. It will be shown that very precise ratio of the SF over α-decay half-lives can be made, and that the main limitation is due to the sample impurities knowledge. This document is made up of an abstract and the slides of the presentation. (authors)