Speaker
Mr
Andreas Jan Ekstrom
(Physics Department, University of Lund, Sweden)
Description
The advent of radioactive ion beams (RIBs) places exotic nuclei within reach of experimental study. In particular, RIBs enables a new investigation of the low energy structure of isotopes in the vicinity of the doubly magic 100Sn nucleus. We present here the first results from a series of measurements of the reduced transition probability - the B(E2) value - for the neutron deficient Sn-isototpes 110,108,106Sn [1]. All three experiments were performed using sub-barrier Coulomb excitation at 2.8 MeV/u at REX-ISOLDE with the MINIBALL germanium detector array. This technique provides a measurement accuracy of ~10%. The B(E2) value can presently difficult to obtain using any other method, due to a high lying 6+ isomer present in the neutron-defiecient even Sn isotopes.
Doubly-magic nuclei are of significant theoretical importance, since they provide a good testing ground for shell-model calculations. The Sn isotopic chain comprises nuclei between neutron numbers N=82 and N=50. Therefore it provides a unique opportunity to study the shell structure evolution as a function of the neutron degree of freedom. The spectroscopy of the low lying states in the even Sn isotopes have been explained within the generalized seniority scheme [2]. Large scale shell model calculations using a model space confining the neutrons to the $gdsh$ orbitals support this picture [3]. Our recent experiments together with results from refs. [3,4] indicate a deviation from theoretical predictions manifested in a stronger than expected collectivity towards the proton dripline. This might imply that further core-polarization effects and a refined effective interaction are needed. One perhaps important effect recently discussed in refs. [5,6,7] is that of an enhanced attractive neutron-proton interaction in spin- isospin-flip vertices contained in the monopole part of the tensor force. Hence, with a decrease of neutrons in the orbital 0g7/2, the proton 0g9/2 becomes less bound, implying an increased core-excitation probability. Thus, this effect could compete with a reduction of the B(E2) values originating in the decrease in neutron number. We will present the latest results and data analysis from 110,108,106Sn, and where they stand in comparison with present theoretical models.
[1] J. Cederkall et al., Phys. Rev. Lett. 98, 172501 (2007).
[2] I. Talmi, Nucl. Phys. bf A172, 1 (1971).
[3] A. Banu et al., Phys. Rev. C 72, 061305(R) (2005).
[4] C. Vaman et al., arXiv:nucl-ex/0612011v1.
[5] T. Otsuka et al., Phys. Rev. Lett. 87, 082502 (2001).
[6] T. Otsuka et al., Phys. Rev. Lett. 95, 232502 (2005).
[7] B. L. Cohen et al., Phys. Rev. 127, 597 (1962).
Author
Mr
Andreas Jan Ekstrom
(Physics Department, University of Lund, Sweden)
Co-authors
Prof.
Claes Fahlander
(Physics Department, University of Lund, Sweden)
Dr
Joakim Cederkall
(PH Department, CERN 1211, Geneva 23, Switzerland.)
Prof.
Morten Hjorth-Jensen
(Physics Department and Center of Mathematics for Applications, University of Oslo, Norway)
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