Gamma-ray fast-timing measurements in Dysprosium nuclei using LaBr3 coincidence spectroscopy

This thesis presents measurements of the reduced electromagnetic transition probabilities, B(E2 : 2+ -> 0+), in the nuclei 164Dy, 166Dy and 178W, from experimental determination of the yrast 2+ excited state lifetimes. Experiments were performed using the accelerator at the ALTO facility at the IPN Orsay. Gamma-ray spectroscopy studies were performed with the HPGe-LaBr3(Ce) hybrid spectrometer, ν-Ball. In the first experiment of the campaign, excited states were populated using reactions between an 18O beam, at three separate primary beam energies of 71, 76 and 80 MeV on the same gold-backed, isotopically-enriched 164Dy target of thickness 6.3 mg/cm2. Coulomb excitation/inelastic nuclear scattering, two-neutron transfer reactions, and fusion-evaporation reactions produced excited states in 164Dy, 166Dy and 178;9W nuclei, respectively. A measurement of the yrast I{pi} = 2+ excited state lifetime in 166Dy was used to determine a first measurement for the quadrupole deformation of this nucleus. This work makes 166Dy the nucleus with the largest valence product, NpNn, with a measured electromagnetic transition rate from the yrast 2+ to the 0+ ground state, below A=208. The experimental methodology was validated through demonstrating consistency between previously determined excited state lifetimes in 164Dy and 178W. The methodology and effectiveness of multiplicity/sum-energy gating, for channel selection between fusion evaporation events and lower multiplicity/energy events from inelastic nuclear scattering and Coulomb excitation of the target, and from two-neutron transfer reactions to 166Dy, is also demonstrated. Following the selection of gamma rays emitted by 164Dy or 178W using multiplicity/sum-energy constraints, lifetime measurements of the yrast 2+, 4+ and 6+ excited states in these nuclei were made. Excited state lifetimes were deduced using the fast-timing coincidence method using LaBr3-LaBr3 coincident events. The yrast 2+ lifetimes in 164Dy, 166Dy and 178W were determined to be 2.35(6), 2.3(2) and 1.08(4) ns, respectively, which correspond to B(E2 : 2+ ! 0+) values of 214(6), 199(19) and 161(6) W.u., respectively. These can be used within the context of the rotational model to correspond to values of the quadrupole deformations for these nuclei of β2 = 0.352(5), 0.339(16) and 0.272(5), respectively. The dysprosium values are compared with theoretical predictions of the evolution of quadrupole deformations through the dysprosium isotopic chain as the N=104 valence maximum system, 170Dy is approached. The final part of this thesis reports first results on a separate study of the production yields of primary fission products produced following fast-neutron induced fission of 238U, performed at the LICORNE facility at the IPN-Orsay, also using the nu-Ball gamma-ray spectrometer.These data are compared with predictions of the GEF code

Probing nuclear cluster symmetries through the harmonic oscillator

The present contribution explores the symmetries of the deformed harmonic oscillator for both prolate and oblate deformations. It demonstrates the emergence of clustering from the degeneracies of the deformed harmonic oscillator and the appearance of the cluster structures in the associated densities. The universality of molecular structures is presented, demonstrating that molecular-like exchange of protons and neutrons is encoded into the mean-field. The nature of oblate-like structures is also explored

The

A γ-spectroscopy campaign named “ν-Ball” was perfomed at the ALTO facility. A large fraction of the beam time was dedicated to the fast neutron induced fission of two fissioning systems: 232Th and 238U. During the data analysis, it was noticed that the high activity of the natTh was heavily contaminating any coincidence matrices (or cubes) built. This caused the identification of weakly produced fission fragments identification to be almost impossible. It was decided to explore the opportunity opened by new analysis methods based on neural networks algorithms. In this paper, the methods to build an adequate neural network and the results obtained for fission event reconstruction are presented

The v-Ball campaign at ALTO: Study for neural network based trigger for fission process

International audienceA γ-spectroscopy campaign named “ν-Ball” was perfomed at the ALTO facility. A large fraction of the beam time was dedicated to the fast neutron induced fission of two fissioning systems: 232Th and 238U. During the data analysis, it was noticed that the high activity of the natTh was heavily contaminating any coincidence matrices (or cubes) built. This caused the identification of weakly produced fission fragments identification to be almost impossible. It was decided to explore the opportunity opened by new analysis methods based on neural networks algorithms. In this paper, the methods to build an adequate neural network and the results obtained for fission event reconstruction are presented

High resolution gamma-ray spectroscopy of fast-neutron induced fission

Experiments to perform precision spectroscopy of fast neutron induced fission were carried out during the ν-Ball experimental campaign at the ALTO facility of IJC Laboratory Orsay. Low energy fission of 232Th(n,f), 238U(n,f) and spontaneous fission of 252Cf were studied using this hybrid highresolution spectrometer and calorimeter. New observables such as γ-ray multiplicity distributions correlated with specific fission fragments are presented and discussed. A new method using fast-timing techniques to detect prompt fission neutrons in coincidence with prompt fission γ-rays is described

High resolution gamma-ray spectroscopy of fast-neutron induced fission

International audienceExperiments to perform precision spectroscopy of fast neutron induced fission were carried out during the ν-Ball experimental campaign at the ALTO facility of IJC Laboratory Orsay. Low energy fission of 232Th(n,f), 238U(n,f) and spontaneous fission of 252Cf were studied using this hybrid highresolution spectrometer and calorimeter. New observables such as γ-ray multiplicity distributions correlated with specific fission fragments are presented and discussed. A new method using fast-timing techniques to detect prompt fission neutrons in coincidence with prompt fission γ-rays is described

Nuclear Data Sheets for A=218

The evaluated data are presented for 11 known A=218 nulcides (Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa and U). For 218Pb, 218Bi, 218At and 218Pa, data are available only for the ground states. For 218U, only the g.s. and a high-spin isomer are known, with no data on γ-ray transitions available. For 218Po, ten excited states are known from 218Bi decay, with no knowledge on the multipolarities of gamma-ray transitions. For 218Th, five excited states in the g.s. band are known from an in-beam γ-ray study. Data on level half-lives, multipolarities and mixing ratios of gamma transitions is generally lacking for A=218 nuclei. The static magnetic dipole moment has been measured for only an isomer in 218Fr. With the exception of a new nuclide, 218Pb, and measurements of half-lives of ground states of a few nuclides of A=218 and A=222, no substantial structure information has become available since the previous evaluation in 2006. Q values are adopted from 2017Wa10 (AME-2016). The present evaluation supersedes the previous A=218 ENSDF evaluations, 2006Ja03, 1995El08, 1987El12 and 1977To13. This evaluation was carried out as part of a joint IAEA-ICTP workshop for Nuclear Structure and Decay Data, organized and hosted by the IAEA, Vienna, and ICTP, Trieste, October 15-26, 2018.Work supported by the IAEA, Vienna; ICTP, Trieste; and the Office of Science, U.S. Department of Energy under contracts: DE-AC02-98CH10946 (BNL), DE-AC02-05CH11231 (LBNL), and DE-AC02-98CH10886 (ORNL)

Lifetime measurement around 132^{132}Sn with the ν-Ball array

International audienceNeutron-rich nuclei were studied after fast neutron-induced fission of 238 U and 232 Th at the ALTO facility in Orsay, France. The neutrons were produced with the LICORNE directional neutron source using a pulsed 7 Li beam provided by the Tandem accelerator. The actinide targets were surrounded by the ν-Ball array to measure the de-excitation of fission fragments. The ν-Ball array is a hybrid spectrometer which consists of HPGe detectors (equipped with BGO shielding) and LaBr3(Ce) scintillation detectors allowing for high-resolution γ-ray spectroscopy and lifetime measurements in the ns/sub-ns range. This contribution presents preliminary results on 134 Te and 137 I populated in the 238 U(n,f) reaction

Nuclear Data Sheets for A=218A=218

International audienceThe evaluated data are presented for 11 known A=218 nulcides (Pb, Bi, Po, At, Rn, Fr, Ra, Ac, Th, Pa and U). For 218 Pb, 218 Bi, 218 At and 218 Pa, data are available only for the ground states. For 218 U, only the g.s. and a high-spin isomer are known, with no data on γ -ray transitions available. For 218 Po, ten excited states are known from 218 Bi decay, with no knowledge on the multipolarities of gamma-ray transitions. For 218 Th, five excited states in the g.s. band are known from an in-beam γ -ray study. Data on level half-lives, multipolarities and mixing ratios of gamma transitions is generally lacking for A=218 nuclei. The static magnetic dipole moment has been measured for only an isomer in 218 Fr. With the exception of a new nuclide, 218 Pb, and measurements of half-lives of ground states of a few nuclides of A=218 and A=222, no substantial structure information has become available since the previous evaluation in 2006. Q values are adopted from 2017Wa10 (AME-2016). The present evaluation supersedes the previous A=218 ENSDF evaluations, 2006Ja03, 1995El08, 1987El12 and 1977To13. This evaluation was carried out as part of a joint IAEA-ICTP workshop for Nuclear Structure and Decay Data, organized and hosted by the IAEA, Vienna, and ICTP, Trieste, October 15–26, 2018

Multi-quasiparticle sub-nanosecond isomers in 178W

We report on the first measurement of the half-lives of K(pi) = 11- and 12+ four-quasiparticle states in the even-even nucleus W-178. The sub-nanosecond half-lives were measured by applying the centroid shift method to data taken with LaBr3 scintillator detectors of the NuBall array at the ALTO facility in Orsay, France. The half-lives of these states only became accessible by the combination of several experimental techniques - scintillator fast timing, isomer spectroscopy with a pulsed beam, and the event-by-event calorimetry information provided by the NuBall array as a whole. The measured half-lives are 476(44)ps and 275(65)ps for the I(pi) = 11- and 12+, respectively. The decay transitions include weakly hindered E1 and E2 branches directly to the ground-state band, bypassing the two-quasiparticle states. This is the first such observation for an E1 transition. The interpretation of the small hindrance hinges on mixing between the ground-state band and the t-band.JRC.G.2-Standards for Nuclear Safety, Security and Safeguard