Recent state-of-the-art Monte Carlo shell-model calculations predict shape coexistence in Zr isotopes. In this context, the 94Zr nucleus is particularly interesting since some experimental investigations have already suggested the possible coexistence of spherical and oblate shapes, however, no definitive conclusion on its

deformation has been reported to date. As such, a dedicated experiment to study collectivity and configuration coexistence in 94Zr by means of a low-energy Coulomb

excitation was performed. This study was performed at the INFN Legnaro National Laboratory with the GALILEO-SPIDER setup, which, in this instance, was further augmented with 6 Lanthanum Bromide scintillators (LaBr3:Ce) in order to to

maximize the γ-ray detection efficiency

Doherty, D. T.

Marchini, N.

Nannini, A.

Rocchini, M.

Zielińska, M.

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DOI 10.1393/ncc/i2019-19143-5Colloquia: EuNPC 2018IL NUOVO CIMENTO 42 C (2019) 143Low-energy Coulomb excitation of 94ZrN. Marchini(1)(2), M. Rocchini(2), A. Nannini(1)(2), D. T. Doherty(3),M. Zielińska(4), K. Hadyńska-Klȩk(3), D. Testov(5)(6), A. Goasduff(5)(6),G. Benzoni(7), F. Camera(7)(8), S. D. Bakes(3)(5)(9), D. Bazzacco(6),T. Berry(3), D. Brugnara(5)(9), M. H. V. Brunet(3), G. Carozzi(5)(9),W. N. Catford(3), M. De Rizzo(5), F. Galtarossa(9), P. Garrett(10),N. Gelli(2), A. Gottardo(9), A. Gozzelino(9), A. Illana(9), J. Keatings(11),A. R. L. Kennington(3), D. Mengoni(5)(6), L. Morrison(3), D. R. Napoli(9),M. Ottanelli(2), P. Ottanelli(1)(2), G. Pasqualato(5)(6), F. Recchia(5)(6),S. Riccetto(12), M. Scheck(11), M. Siciliano(9), G. Sighinolfi(5),J. Sinclair(11), J. J. Valiente Dobón(9) and I. Zanon(5)(9)(1) Università degli Studi di Firenze - Firenze, Italy(2) INFN, Sezione di Firenze - Firenze, Italy(3) University of Surrey - Guildford, UK(4) IRFU, CEA, Université Paris-Saclay - Paris, France(5) Università degli Studi di Padova - Padova, Italy(6) INFN, Sezione di Padova - Padova, Italy(7) INFN, Sezione di Milano - Milano, Italy(8) Università degli Studi di Milano - Milano, Italy(9) INFN, Laboratori Nazionali di Legnaro - Legnaro (Padova), Italy(10) University of Guelph - Guelph, Canada(11) University of the West of Scotland - Paisley, UK(12) Università degli Studi di Perugia and INFN, Sezione di Perugia - Perugia, Italyreceived 5 February 2019Summary. — Recent state-of-the-art Monte Carlo shell-model calculations predictshape coexistence in Zr isotopes. In this context, the 94Zr nucleus is particularly in-teresting since some experimental investigations have already suggested the possiblecoexistence of spherical and oblate shapes, however, no definitive conclusion on itsdeformation has been reported to date. As such, a dedicated experiment to studycollectivity and configuration coexistence in 94Zr by means of a low-energy Coulombexcitation was performed. This study was performed at the INFN Legnaro NationalLaboratory with the GALILEO-SPIDER setup, which, in this instance, was fur-ther augmented with 6 Lanthanum Bromide scintillators (LaBr3:Ce) in order to tomaximize the γ-ray detection efficiency.Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 N. MARCHINI et al.1. – IntroductionThe observation that an atomic nucleus may exhibit eigenstates with different shapesat similar excitation energies is a widespread phenomenon that is now thought to occurin nearly all nuclei [1]. However, a complete description of this characteristic feature,i.e. shape coexistence, is still missing. An additional motivation for the study of shapecoexistence arises due to its sensitivity to the proton-neutron interaction within thenucleus, the knowledge of which is of crucial importance for our understanding of nuclearstructure.The zirconium isotopes exhibit a variety of collective behaviour phenomena; in themid-shell region around N ∼ 40 they exhibit a deformed shape, progressing to a closedneutron shell configuration at N = 50, and then to a sudden reappearance of deformationat N ∼ 60. It is, therefore, not surprising that the zirconium region has been the subjectof intense experimental and theoretical work, in order to study how collectivity evolves inthese isotopes and to examine the coexistence observed between various configurations.The 94Zr (N = 54) isotope is particularly interesting, because it is thought to be a strongcandidate for displaying type-II shell evolution [2], as recently proposed for the zirconiumisotopes (around N = 56) by state-of-the-art Monte Carlo shell model calculations [3](see fig. 1).We have, therefore, performed a dedicated Coulomb-excitation experiment with theaim of investigating the degree of quadrupole deformation both in the ground stateand the low-lying excited states of 94Zr. “Safe” Coulomb excitation is a purely elec-tromagnetic process due to the Coulomb field acting between two colliding nuclei. The“safe” energy condition ensures that the contributions of the short-range nuclear force arenegligible and, therefore, the experimental results can be obtained in a nuclear-model-independent way. This is fulfilled by maintaining a minimum distance of 5 fm betweenthe nuclear surfaces [4]. The Coulomb-excitation technique preferentially excites low-lying collective nuclear states, offering the possibility to measure observables directlyrelated to the shape of the nucleus at low excitation energy.94Zr 94ZrN NFig. 1: Experimental (continuous line) and calculated excitation energies (symbols) ofthe 0+1,2 (panel a) and 2+1,2 (panel b) states in Zr isotopes. The symbols indicate thecalculated shape of each state. Figure adapted from ref. [3].LOW-ENERGY COULOMB EXCITATION OF 94Zr 3SPIDERLaBr3:CeGALILEOFig. 2: The experimental setup used for the Coulomb-excitation experiment of 94Zr. TheGALILEO array, the SPIDER array and six LaBr3:Ce detectors are shown.2. – ExperimentThe experiment was performed at the Legnaro National Laboratory (LNL) of the Na-tional Institute of Nuclear Physics (INFN). A 370-MeV beam of 94Zr ions was impingedonto a 1 mg/cm2-thick 208Pb target. The GALILEO detector array [5] in conjunctionwith the recently commissioned SPIDER (Silicon PIe DetectoR) charged-particle detec-tor [6], was used to measure γ rays in coincidence with backscattered 94Zr ions. TheGALILEO γ-ray spectrometer consists of 25 Compton-suppressed HPGe detectors ar-ranged into 4 rings at 152◦, 129◦, 119◦ and 90◦ with respect to the beam direction. TheSPIDER charged-particle array is composed of 7 segmented Si detectors (each dividedinto 8 independent strips) assembled in a cone-like shaped configuration. SPIDER waspositioned within the GALILEO scattering chamber at backward angles, in order toavoid excessive radiation damage. As such, the polar angular coverage ranges from 123to 163 degrees in the laboratory frame. Six 3”×3” LaBr3:Ce scintillators were also cou-pled to the GALILEO-SPIDER setup (as shown in fig. 2), in order to increase the γ-raydetection efficiency. The resolution and efficiency of these LaBr3:Ce scintillators, whenimplemented in the setup, has been studied in detail, providing important informationfor the planning of future experiments with these detectors [7].The detection of γ-particle coincidences offers the possibility to have clean, Doppler-corrected γ-ray spectra, to study the properties of the projectile (and/or target) nu-cleus. As an example, a preliminary γ-ray energy spectrum acquired in coincidence withbackscattered 94Zr ions is shown in fig. 3 (about 50% of the total statistics is considered).The observed 94Zr transitions are labelled, together with peaks related to the excitationof the target and its contaminants. Direct and multi-step excitation of the 2+1 , 4+1 , 0+2 ,2+2 , 2+3 , 4+2 , 2+4 and 3−1 states of94Zr has been observed. For the 2+4 −→ 2+2 , 2+4 −→ 0+2, 4+2 −→ 2+2 transitions it is only possible to consider the sum of all angles covered by4 N. MARCHINI et al.CountsEnergy [keV]Pb x-rayscontaminants0 2+   21+4 1+   21+2 2+   21+2 1+   01+3 1—   21+2 3+   21+4 2+   21+2 2+   01+2 4+   21+PreliminaryFig. 3: Spectrum of γ-rays detected by the GALILEO array (without considering theLaBr3:Ce detectors) in coincidence with backscattered 94Zr ions. About 50% of the totalstatistics is presented.the SPIDER array in the analysis. However, for other transitions, the observed numberof counts is sufficient to divide the statistics into eight different scattering angle ranges,thus exploiting the full segmentation of SPIDER.All transitions needed to obtain the spectroscopic quadrupole moments of the first two2+ states have been observed, as well as those necessary to determine the deformation ofthe ground state and of the 0+2 state. The experimental data are currently being analysedwith the least-squares fitting code GOSIA [8]. The code fits a set of reduced matrixelements to the measured γ-ray yields (75 in this experiment) taking into account knownspectroscopic data related to electromagnetic matrix elements: branching ratios (10),lifetimes (8), E2/M1 mixing ratios (5). All data have equal weight in the χ2 function asthe γ-ray yields observed in the Coulomb-excitation experiment. A preliminary analysisseems to qualitatively confirm the predictions of Monte Carlo shell model, shown in fig.1. The present results (even if preliminary) are in agreement with the reduced transitionprobabilities available in the literature, e.g. [9]. This provides confidence that the on-going analysis will give clear and definitive information on the deformation and shapecoexistence in 94Zr.REFERENCES[1] HEYDE K. and WOOD J.L., Rev. Mod. Phys., 84 (2011) 1467.[2] OTSUKA T. and TSUNODA Y., Jour. Phys. G, 43 (2016) 024009.[3] TOGASHI T., TSUNODA Y., OTSUKA T. and SHIMIZU N., Phys. Rev. Lett. 117 (2016)172502.[4] CLINE D., Annu. Rev. Nucl. Part. Sci., 36 (1986) 683.[5] VALIENTE-DOBON J.J et al., LNL-INFN Annual Report 2014, (2015) 95.[6] ROCCHINI M. et al., Physica Scripta, 92 (2017) 074001.[7] MARCHINI N. et al., LNL-INFN Annual Report 2017, (2018) 88.[8] CZOSNYKA T., CLINE D. and WU C. Y. , Bull. Am. Phys. Soc. 28 (1983) 745.[9] CHAKRABORTY A. et al., Phys. Rev. Lett., 110 (2013) 022504.

Low-energy Coulomb excitation of 94Zr

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Low-energy Coulomb excitation of 94Zr

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