Estudio de coincidencias ultrarrápidas en núcleos exóticos alrededor del ⁷⁸Ni: la cadena de desintegración β del ⁸¹Zn

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Atómica, Molecular y Nuclear, leída el 16/06/2016The evolution of single-particle states in neutron-rich nuclei provides a key information on their nuclear structure and is an important ingredient for the development of nuclear models that can be applied to predict the structure at the borderline of nuclear map. The role of neutron excitations across shell gaps and the evolution proton-neutron interaction can be studied in these exotic nuclei. In particular, magic nuclei are key players for the mapping of the single-particle degrees of freedom around closed cores. A special region of interest is found around the doubly-magic 78 28Ni50. In addition, gross properties of these nuclei play a role in the astrophysical rapid neutron capture process. Nuclei in the vicinity of 78Ni have motivated recent experimental and theoretical studies, aimed at the understanding of the nuclear structure in this region with a large neutron excess. In this thesis we investigate the nuclear structure of 81;80Ga, 81Ge and 81As, populated in the decay chain of 81Zn, which was produced at ISOLDE, CERN in the framework of a systematic fast-timing investigation of neutron-rich nuclei populated in the decay of Zn. The selectivity and efficiency of the production of Zn ion beams had been previously optimized in order to guarantee the most pure beam of 77{u100000}82Zn nuclei. The estimated yield of 81Zn was 600 ions/ C giving an average activity of particles during the experiment of about 10000 counts per second. The experimental setup included two HPGe detectors, two LaBr3(Ce) detectors and a NE111A plastic scintillator for particle detection. Coincidences with the detector were used for -ray background suppression, and - coincidences between the HPGe detectors to determine the level schemes. For half-life measurements the combination of LaBr3(Ce) scintillator crystals and Time-to- Amplitude Converters was employed. The signals from the detectors were processed by a digital data acquisition (DAQ) system composed by four Pixie-4 Digital Gamma Finder cards, specially designed for -ray spectroscopy which was used for decay level schemes and the Advanced Time Delayed (“fast-timing“) (t) method employed to measure the excited level lifetimes. From the structural point of view, the isotopes under study are relatively simple systems with a few particles and/or holes outside the doubly-magic core, and thus can be treated rather successfully within the nuclear shell model...El estudio de la evolucíon de los estados de particula independiente en ńucleos ricos en neutrones, adeḿas de proporcionar valiosa informacíon sobre la estructura nuclear, es de gran importancia para el desarrollo de modelos nucleares téoricos que predicen con exactitud la estructura nuclear en ńucleos elejados del valle de estabilidad. El estudio de estos ńucleos permite tambíen determinar el papel que desempe˜nan las excitaciones de neutrones a lo largo de las diferentes capas nucleares junto con la evolucíon de las interacciones que se producen entre los protones. Son de especial inteŕes las especies situadas en regiones cercanas a los nucleos doblemente ḿagicos, como el 78 28Ni50, ya que presentan el escenario perfecto para determinar los grados de libertad de las part́ıculas independientes en las capas cerradas. Asimismo, ciertas propiedades de estos ńucleos juegan un papel importante en las procesos de nucleośıntesis como el proceso r (captura ŕapida de neutrones). Debido a estos hechos, estos ńucleos han sido durante las ́ultimas d́ecadas la raźon de amplios estudios, tanto experimentales como téoricos, cuyo objetivo era entender las modificaciones en estrucutra nuclear en la regíon con gran exceso neutŕonico. Esta tesis describe el estudio de los ńucleos de 81;80Ga, 81Ge y 81As, poblados en la cadena de desintegracíon del 81Zn. El ńucleo padre, 81Zn, fue producido en ISOLDE, CERN aprovechando la gran pureza de los haces radiactivos de Zn, cuya selectividad y eficiencia habian sido optimizadas con anterioridad. El montaje experimental consta de dos detectores de HPGe, dos detectores de centelleo ultrarŕapidos de LaBr3(Ce) y un centelleador pĺastico (NE111A) que act́ua como el detector . Los esquemas de niveles se construýıan baśandose en las coincidencias - entre los HPGe aprovechando su buena resolucíon enerǵetica, mientras que las medidas de vidas medias se hicieron con el uso del centelleador pĺastico y los detectores de LaBr3(Ce), junto con los TACs (Time-to- Amplitude converters). El ḿetodo de coincidencias ultrarŕapidas o "fast-timing" (t)’ fue utilizado para determinar las vidas medias de los estados excitados. Las se˜nales de cada detector fueron procesadas por un sistema de acquisicíon digital formado por cuatro tarjetas Pixie-4 Digital Gamma Finder, dise˜nadas especialmente para la espectroscoṕıa...Depto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEunpu

Low-Z boundary of the N=88-90 shape phase transition: Ce-148 near the critical point

Excited states of the neutron-rich nucleus Ce-148 have been populated by neutron-induced fission of a U-235 target. Its electromagnetic decays were studied by means of gamma-ray coincidence spectroscopy with fast-timing capabilities. Lifetimes of the 2(1)(+) and 4(1)(+) states of Ce-148 were obtained and their E2 decay rates deduced. The B-4/2 = B(E2; 4(1)(+) -> 2(1)(+))/B(E2; 2(1)(+) -> 0(1)(+)) ratio indicates that Ce-148 is a transitional nucleus while the N = 88/90 shape phase transition evolves into a gradual change of nuclear deformation for proton numbers Z < 60

The (n,gamma) campaigns at EXILL

At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n,gamma) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns

Lifetime measurements in the odd-A nucleus Hf-177

Lifetimes of low- and high-spin states in the odd-A nucleus Hf-177 were measured during the EXILL&FATIMA campaign using a spectrometer equipped with eight HPGe-Clover detectors and 16 fast-timing LaBr3(Ce) detectors. For the determination of lifetimes in the pico- to nanosecond regime, the well-established generalized centroid difference method was used. Lifetimes of the 9/2(-), 9/2(+), 11/2(-), 11/2(+) states were remeasured, the lifetimes of the 13/2(-), 17/2(-), 13/2(+), 15/2(+), and 17/2(+) states were determined for the first time and an upper limit for the 19/2(+) state has been established. From these lifetimes absolute reduced transition probabilities were extracted and compared to particle-rotor-model calculations and quasiparticle-phonon-model calculations showing the importance of including multipole-multipole interactions in the description of odd-A nuclei in the rare earth region

Lifetimes and shape-coexisting states of( 99)Zr

Lifetimes of intermediate-spin states in two rotational bands of Zr-99 have been measured. These states were populated following the neutron-induced fission of U-235 at the PF1B beamline of the Institut Laue-Langevin, Grenoble, during the EXILL-FATIMA campaign. The nucleus Zr-99(59) exhibits shape coexistence and lies precisely on the border of an abrupt change in ground-state deformation when going from N = 58 to N = 60, making its study interesting for understanding the mechanisms involved in the rapid onset of deformation here. The B(E2) values extracted for decays in the nu 3/2[541] band allow quadrupole deformations of beta(2) = 0.34(1) and 0.26(3) to be determined for the 821.6- and 1236.6-keV members, whereas beta(2) = 0.32(3) was found for the 850.5-keV member of the nu 3/2[411] band. Some of the excited states known in Zr-99 have been reasonably described with interacting boson-fermion model (IBFM) calculations. Type-II shell evolution is proposed to play a major role in modifying single-particle energies in Zr-99

Low-spin states in Ge-80 populated in the beta decay of the Ga-80 3(-) isomer

The structure of Ge-80 has been investigated at the ISOLDE facility at CERN. A previous study reported for the first time a low-lying 0(2)(+) intruder state at 639 keV, based on the coincidence with a previously unobserved 1764-keV gamma ray, and suggested it as evidence for shape coexistence in Ge-80. We used the beta decay from the 3(-) 22.4-keV state in Ga-80 to enhance the population of low-spin states in Ge-80, including any excited 0(+) level, and gamma gamma coincidences to investigate it. We observed a 1764-keV gamma ray in coincidence with strong transitions in Ge-80, thus not feeding the proposed 639-keV 0(2)(+). No connecting transitions from previously known levels to the 639-keV and 2403-keV 2(3)(+) states could be established either. Shell-model calculations for Ge isotopes and N = 48 isotones were performed. They succeed to explain most of the experimental levels, but fail to reproduce the presence of a 0(2)(+) state below approximate to 1200 keV in Ge-80. Our experimental findings and shell-model calculations are difficult to reconcile with a very low-lying 0(2)(+) state in Ge-80

Low-lying isomeric states in 80Ga from the β- decay of 80Zn

A new level scheme of 80Ga has been determined. This nucleus was populated following the β− decay of 80Zn at ISOLDE, CERN. The proposed level scheme is significantly different compared to the previously reported one and contains 26 levels up to 3.4 MeV in excitation energy. The present study establishes that the previously identified 1.9-s β− -decaying 6− isomer is the ground state of 80Ga and the 1.3-s β− -decaying 3− isomer lies at an excitation energy of 22.4 keV. A new isomeric level was identified at 707.8 keV and its half-life was measured to be 18.3(5) ns, allowing the 685.4-keV transition de-exciting this state to be assigned anM2 multipolarity. The newly measured spectroscopic observables are compared with shell-model calculations using the jj44bpn and JUN45 interactions

The (n,gamma) campaigns at EXILL

© Owned by the authors, published by EDP Sciences, 2015. ESSN: 2100-014X Artículo firmado por 67 autores. This work was supported by NUPNET by contract 05P12PKNUF, 05P12RDNUP and DNC7RP01/4, by the Science and Technology Facilities Councils (UK), by the Spanish MINECO via the FPA2010-17142 and PRIPIMNUP-2011-1338 projects, and by the DFG by cluster of Excellence Origin and Structure of the Universe, grant KR 1796/2-1 and SFB634. This material is based in part upon work supported by the U.S.D.O.E., SC, Office of nuclear physics, under Award No. DE-FG02-91ER40609. The EXILL campaign would not have been possible without the support of several services at the ILL and the LPSC. We are grateful to the EXOGAM collaboration for the loan of the detectors, to GANIL for assistance during installation and dismantling, to the INFN Legnaro laboratory for the loan of the GASP detectors and to the FATIMA collaboration for the use of the LaBr3(Ce) scintillators. International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15)(15. 2014. Dresden, Alemania)At the PF1B cold neutron beam line at the Institut Laue Langevin, the EXILL array consisting of EXOGAM, GASP and ILL-Clover detectors was used to perform (n,gamma) measurements at very high coincidence rates. About ten different reactions were measured in autumn 2012 using a highly collimated cold neutron beam. In spring 2013, the EXOGAM array was combined with 16 LaBr3(Ce) scintillators in the EXILL&FATIMA campaign for the measurement of lifetimes using the generalised centroid difference method. We report on the properties of the set-ups and present first results from both campaigns.NUPNETScience and Technology Facilities Councils, UKMINECO, SpainDFGU.S.D.O.E., SC, Office of nuclear physicsILLLPSCDepto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEpu