The Two-Photon Decay of the 11-/2 Isomer of 137Ba and Mixed-Symmetry States of 92,94Zr and 94Mo

Abstract

The first part of this thesis covers the two-photon decay of the 11 2 − isomer in 137 Ba. Clear evidence of this exotic decay mode is found, which opens up the possibility to use the twophoton decay probability as a general tool to investigate the structure of atomic nuclei. Details on the experiment are given, and the major challenges to successfully perform this difficult experiment are outlined. The two-photon decay experiment was conducted with the new LaBr 3 -detector ball which was setup from scratch in the framework of this thesis. The properties and performance of this powerful detector system -consisting of 18 large LaBr 3 -detectors -are discussed.Furthermore, proton scattering at 200 MeV has been performed on the nuclei 92,94 Zr, 94,96 Mo and 70 Zn. The measured cross sections are compared to calculations in the framework of the Quasi-Particle Phonon Model (QPM). Conclusions about the validity of the QPM results are drawn. A comparison of the excitation cross sections of the 2 + 1 states and the one-phonon quadrupole states with mixed neutron and proton symmetry (2 + ms ) allows to prove the special character of the 2 + ms states independently of electromagnetic transition strengths. The possible existence of 3 − and 4 + states with mixed proton and neutron symmetry is discussed. Furthermore the origin of collectivity of low-lying states in spherical nuclei is investigated in the QPM. Zusammenfassung

First observation of the competitive double-gamma (“γγ/γ”) decay process

“…In particular, we have set out to firmly observe γγ/γ-decay events, to study the energy distribution of the simultaneously emitted γ quanta and their angular correlation, to possibly determine their multipole order and radiation character, to measure the total double-γ-to-single-γ decay branching ratio, and thereby finally the partial γγ/γ-decay rate. Our experimental results have initially been published elsewhere [17] and this conference contribution is predominantly based on the previously published material which at the time of the oral presentation has been under review, and on the material presented in the doctoral thesis of Dr. Christopher Walz [18]. The following section provides a brief description of the experimental set-up.…”

Electromagnetic character of the competitive γγ/γ-decay from 137mBa

“…From the two angular data points as well as the energy spectrum of the individual γ rays at 72 • angle, the off-diagonal polarizabilities α M2E2 = 33.9(2.8) e 2 fm 4 /MeV and the α E3M1 = 10.1(4.2) e 2 fm 4 /MeV polarizabilities were extracted. While the observation of the peak associated with γγ/γ decay was statistically clear, the nature of this decay was more uncertain, having the two dominating multipolarity combinations separated only by a small statistical difference, favouring the α M2E2 component 12 . The decay diagram of this process is shown in Figure 1.…”