A (g,g') experiment on ⁸⁸Sr was performed at the S-DALINAC accelerator of the TU Darmstadt with bremsstrahlung of 6.7 MeV endpoint energy using two EUROBALL CLUSTER detectors. In the search for the 1^- two-phonon state in the N=50 nucleus ⁸⁸Sr we found surprisingly J^p = 1⁺ for the 4.742 MeV state [1] and a relatively large value of B(M1)=0.07 W.u. for the 4.742 MeV transition (Fig. 1) which is not reproduced by the quasiparticle-phonon model [2].

Fig. 1 Comparison of  ⁹⁴Mo and ⁸⁸Sr.

In order to give an interpretation for the 1⁺ state at 4.742 MeV, the picture of a new excitation mode called dynamic scissors mode is proposed: a superposition of the normal scissors mode (scissors-like isovector vibration of the deformed proton and neutron systems) with the dynamically deformed quadrupole vibrational state in a spherical nucleus. This idea is suported by the following two arguments: (i) For the first 2⁺ state in ⁸⁸Sr a deformation of d = 0.11 is obtained for the maximum elongation. Based on empirical estimates (est) [3] the normal scissors mode in ⁸⁸Sr would occur for d = 0.11 at the centroid of energies of E^est(1_sc⁺) = 3.17 MeV with a total strength of B^est(M1;1_sc⁺®0₁⁺) = 0.03 W.u.. For the dynamic scissors mode state E(1_dsc⁺) in ⁸⁸Sr one gets the estimate E^est(1_dsc⁺) = E^exp(2₁⁺)+ E^est(1_sc⁺) = 5.01 MeV. The estimates of 5.01 MeV and 0.03 W.u. are in fair agreement with the experimental results given above. (ii) There are many similarities of the deexcitation of the 4.742 MeV state in the spherical nucleus ⁸⁸Sr with that of the mixed symmetry scissors-like 1⁺_ms state in the deformed N=52 nucleus ⁹⁴Mo [4], and also for the first three 2⁺ states (Fig. 1). The 1⁺ state in ⁹⁴Mo is considered to be the scissors mode in this nucleus having the structure (2⁺₁ Ä 2⁺_ms), where 2⁺₁ corresponds to the phonon of the isoscalar quadrupole vibration and 2⁺_ms is the phonon of the isovector quadrupole vibration. For ⁸⁸Sr the energy sum of the 2⁺ isoscalar quadrupole vibrational state (1.836 MeV) and the newly observed possible 2⁺_ms isovector quadrupole vibrational state (2.935 MeV) results to 4.771 MeV in well accordance to 4.742 MeV for the 1⁺ state. In summary, the 1⁺ state at 4.742 MeV in the spherical N = 50 nucleus ⁸⁸Sr is a fingerprint for the existence of a scissors-like state. The picture of the dynamic scissors mode could be a schematic approach for this excitation giving a rough approximation to the real situation. The reality may be a complicated superposition of an isoscalar and isovector quadrupole vibration.

¹ Institut für Kernphysik, Universität zu Köln, 50937 Köln
² Institut für Kernphysik, TU Darmstadt, D-64289 Darmstadt
³ Institute of Nuclear Research and Nuclear Energy, Sofia, BG-1784 Sofia

References

[1] L. Käubler et al., Eur. Phys. J. A 7 (2000) 15
[2] M. Grinberg and Ch. Stoyanov, Nucl. Phys. A 573 (1994) 231
[3] N. Pietralla et al., Phys. Rev. C 58 (1998) 184
[4] N. Pietralla et al., Phys. Rev. Lett. 83 (1999) 1303  IKH 06/21/01 © L. Käubler