Octupole vibrational states in the lead region

Wollersheim, HJ

Hj. Wollersheim, Octupole vibrational states in the lead region, NUOV CIM A, 111(6-7), 1998, pp. 691-696

7

INGLESE

Article

Physics

NUOVO CIMENTO DELLA SOCIETA ITALIANA DI FISICA A-NUCLEI PARTICLES AND FIELDS

1124-1861 → ACNP

111

6-7

1998

691 - 696

ISI

1124-1861(199806/07)111:6-7<691:OVSITL>2.0.ZU;2-C

Collective properties of Pb-206, Pb-207, Pb-208 and Bi-209 have been inves tigated With gamma-ray spectroscopy using electromagnetic excitation by Pb- 208 projectiles. By measuring the absolute yields of the de-excitation gamm a-rays, B(E3) values for the 1-phonon octupole vibrational states were obta ined. While the present data for Pb-208 and 209Bi are consistent with earli er publications, much smaller values have been found for Pb-206 and Pb-207. Only for the doubly magic nucleus Pb-208 One observes the largest octupole collectivity of 34 Weisskopf units (W.u.), which is reduced to 60-70% by t he particle-vibration coupling in nuclei which consist of the Pb-208 core p lus (or minus) a few nucleons. As a consequence of the phonon vibration cha racter of the 3(-) state, one expects a quadruplet of 2-phonon octupole sta tes with spins and parities 0(+), 2(+), 4(+) and 6(+) at about twice the en ergy of the 1-phonon state, i.e. around 5.23 MeV. A first seat; tering expe riment suggesting the observation of at least one member of the 2-phonon mu ltiplet was performed via the reaction Pb-208+Pb-208 at an incident energy well above the Coulomb barrier In the present experiment the system Pb-208Pb-208 has been reinvestigated via Coulomb excitation at a safe bombarding energy of 5.0 MeV/u. The reduced cross-section for the excitation of the oc tupole states has been compensated by the higher full energy efficiency of five EUROBALL CLUSTER detectors. With the present experimental set-up trans ition probabilities down to 10(-5)-10(-6) can be measured.