Sub-Barrier Coulomb Excitation ofSn110and Its Implications for theSn100Shell Closure

Abstract

The first excited 2 state of the unstable isotope 110 Sn has been studied in safe Coulomb excitation at 2:82 MeV=u using the MINIBALL array at the REX-ISOLDE post accelerator at CERN. This is the first measurement of the reduced transition probability of this state using this method for a neutron deficient Sn isotope. The strength of the approach lies in the excellent peak-to-background ratio that is achieved. The extracted reduced transition probability, BE2 : 0 ! 2 0:220 0:022e 2 b 2 , strengthens the observation of the evolution of the BE2 values of neutron deficient Sn isotopes that was observed recently in intermediate-energy Coulomb excitation of 108 Sn. It implies that the trend of these reduced transition probabilities in the even-even Sn isotopes is not symmetric with respect to the midshell mass number A 116 as 100 Sn is approached. DOI: 10.1103/PhysRevLett.98.172501 PACS numbers: 23.20.Js, 21.60.Cs, 25.70.De, 27.60.+j Substantial interest has recently arisen in the shell structure of atomic nuclei with only a few nucleons outside the double shell closure at 100 Sn. As an example, a series of experiments aiming at isotopes in this region has been carried out using fusion-evaporation reactions in the recent past [1]. With the advent of radioactive ion beams these studies are now taken further using sub-barrier and intermediate-energy Coulomb excitation [2,3]. In this Letter we present the only sub-barrier or ''safe'' Coulomb excitation experiment in this region to date. The study of the reduced transition probability -the BE2-of the first excited 2 state in an even-even nucleus gives a direct handle on the collectivity of that state. It can thus be used to measure systematic changes in the strengths of shell gaps. The general motivation for this kind of study goes back to our incomplete knowledge of the mechanisms that govern shell formation and their implications for the structure of nuclei far from stability. It is well known that a strong spinorbit force was introduced into the nuclear shell-model on Fermi's suggestion by Goeppert Mayer [4] and independently by Haxel, Jensen, and Suess [4] to explain the observed shell gaps. However, these papers were substantially predated by the consideration of a nuclear spin-orbit force by Inglis [5] who noted that the relativistic Thomas term which arises as a consequence of the noncommutation of Lorentz transformations should act also in atomic nuclei. This term, given by the vector product of the velocity and acceleration of the bound nucleon, gives rise to nuclear LS coupling, a result which can be derived from the Dirac equation [6]. In this picture, the acceleration is proportional to the derivative of the potential experienced by the bound particle, a notion still used in mean-field approaches today. As a consequence, the splitting of the shell gaps becomes density dependent and may change with the PRL 98,

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

“…In experiments on the light odd Sn isotopes, Di Julio et al [33,34] investigated the structure of 107,109 Sn and put constraints on the single-particle dominated orbits in the odd Sn isotopes. In experiments on 106,108,110 Sn Cederkall et al [29] and Ekstrom et al [30] addressed the strength of the shell closure at 100 Sn. The remaining part of this section will expand on this topic.…”

“…In contrast, the contaminant In is released within seconds. Yield measurements show that ca 85% of the In isotopes with half lifes above [29,30] are indicated with blue crosses and the results from high-energy CE from GSI [76,79], NSCL [80] and RIKEN [81] are given by green stars, and pink and brown triangles, respectively. The results from a typical LSSM calculation [76] and an RQRPA calculation [82] Sn.…”

“…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].…”