0gs+→21+Transition Strengths inSn106and

Andreas Ekström

¹

,

J. Cederkäll

²

,

C. Fahlander

³

et al.

Abstract

The reduced transition probabilities, B E2; 0 gs ! 2 1 , have been measured in the radioactive isotopes 108;106 Sn using subbarrier Coulomb excitation at the REX-ISOLDE facility at CERN. Deexcitation rays were detected by the highly segmented MINIBALL Ge-detector array. The results, B E2; 0 gs ! 2 1 0:222 19 e 2 b 2 for 108 Sn and B E2; 0 gs ! 2 1 0:195 39 e 2 b 2 for 106 Sn were determined relative to a stable 58 Ni target. The resulting B E2 values are 30% larger than shell-model predictions and deviate from the generalized seniority model. This experimental result may point towards a weakening of the N Z 50 shell closure. DOI: 10.1103/PhysRevLett.101.012502 PACS numbers: 23.20.Js, 21.60.Cs, 25.70.De, 27.60.+j Precision measurements in unstable nuclei together with recently developed models of the nucleon-nucleon interaction, stemming from many-body techniques and QCD, show promise to improve our understanding of the finer aspects of the dynamics of the atomic nucleus. One approach to this question is to measure reduced transition probabilities -B E2; 0 gs ! 2 1 -for specific nuclei in the vicinity of a shell closure and to compare these results with calculations based on such models. In particular, one of the pressing questions in nuclear physics today is whether the shell closures, that are well established close to stability, remain so also for isotopes with a more extreme proton-toneutron ratio. Intuitive models, such as the generalized seniority scheme [1], predict that these B E2 values follow a parabolic trend, that peaks at midshell, for a sequence of isotopes between two shell closures. In the following we address the 100 Sn shell closure and consequently present results from measurements in the sequence of neutron-deficient even-mass Sn isotopes. This approach has been made possible by newly developed facilities that produce high-quality radioactive ion beams. Recent measurements in 110;108 Sn [2 -4] consistently deviate from the broken-pair model as given by the generalized seniority scheme and from current large-scale shell-model calculations [2]. Parallel work [4], using intermediate energy Coulomb excitation, suggests a constant trend of the reduced transition probabilities extending to 106 Sn. In this Letter we report results from the first measurements of 108;106 Sn using subbarrier Coulomb excitation. This is the only experiment so far for 106 Sn that has permitted for complete control of the scattering process and thus explicitly fulfills the conditions for safe Coulomb excitation. Our result still deviates significantly from theoretical predictions but indicates a decreasing trend of the B E2 with a decreasing number of valence particles outside of the 100 Sn core. Note that with this Letter three different isotopes have been used for normalization as 112 Sn [2] and 197 Au [4] have been used previously. All three experiments yield similar PRL 101, 012502 (2008)

Z=50core stability inSn110from magnetic-moment and lifetime measurements

“…The reduced transition probability data in 104−134 Sn ( [1][2][3][4][5][6][7][8][9][10] and references therein), follow only above midshell (A = 116) the * Electronic address: kum@physics.rutgers.edu parabola-like curve of the shell model expectations [6]. For nuclei below midshell the B(E2) values are larger than expected but finally decrease at N = 54, as the doubly-closed shell Z = N = 50 is approached.…”

Nuclear-structure studies of exotic nuclei with MINIBALL

“…This is related to the reduction of the N=20 shell gap and quadrupole correlations enabling low-lying deformed p h 2 2 intruder states from the higher fp shell to compete with spherical normal neutron p h 0 0 states of the sd-shell. The promotion of a neutron pair across the N=20 gap can result in deformed intruder ground states and the two competing configurations can lead to the coexistence of spherical and deformed + 0 states in the neutron rich nuclei 30,32 Mg.…”

“…These findings were exceeding expectations for a semi-magic nucleus. Later on the ( ) B E2 values for the neutron-rich Mg isotopes 30,32,34 Mg were measured by different groups at the intermediate-energy facilities MSU [59,60] and GANIL [61] showing a wide range of results (see figure 3).…”

“…Moreover, the first g-factor measurement of excited states was successfully performed with a radioactive ion beam at ISOLDE in this region [24]. A shell-model study was dedicated to the neutron deficient Sn isotopes, where unexpected high values for CE of the first excited + 2 1 state were observed [29][30][31][32][33][34][35]. Isotopes around the doubly-magic nucleus 132 Sn were subject of CE experiment [36].…”

Shell-model configuration-interaction description of quadrupole collectivity in Te isotopes

“…) values of neutron deficient semi-magic Sn isotopes have triggered extensive experimental [2][3][4][5][6][7][8][9][10][11][12] and theoretical [13][14][15][16][17][18][19][20][21] activities, in particular regarding the fundamental roles played by core excitations and the nuclear pairing correlation (or seniority coupling). The study of transition rates in isotopic chains just above Z = 50 may provide further information on the role of core excitations [22,23].…”