In high resolution photon scattering experiments on the actinide nucleus U, three dipole excitations have been found in the energy region around 1.8 MeV. The experiments yielded model independent information about the energies, spins, gamma decay branching ratios, cross sections, and the lifetimes of these J = 1 states which lie in the energy region of the electron-positron lines observed by the EPOS and ORANGE Collaborations at the GSI Darmstadt. PACS number(s): 21.10.Re, 21.10.Tg, 23.20.Lv, 25.20.Dc Throughout the past decade the emission of electronpositron pairs has been studied intensively in heavy ion collisions with energies close to and slightly above the Coulomb barrier. The e+e sum spectra measured at the GSI Darmstadt by the EPOS and ORANGE Collaborations indicated narrow lines superimposed on a broad bump for different nuclei [1 -5]. In the U+U, U+Th, U+Pb, and U+Ta systems a number of discrete lines appear at sum energies between 555 and 815 keV, a review of the results can be found in Ref. [5]. Recent experiments at Argonne with the APEX setup at the ATLAS linac give no evidence for such lines [6].In the inverse fundamental process of Bhabha scattering no evidence for a resonance structure around the corresponding energies has been found [7 -11]. One possible explanation for the observation of an e+e li.ne may be the existence of a strong nuclear excitation which decays partly via internal pair conversion (IPC). The electron-positron line at, e.g. , 634 keV should then stem from an excitation at 634 keV +2mo(e ) = 1658 keV if the line is from a source at rest. We want to stress, however, that IPC from a moving source does not in general give rise to a sharp sum energy line. Internal pair conversion favors dipole transitions. For a detailed interpretation of the results from the heavy ion reactions it is therefore useful to know all strong dipole excitations of the nucleus in this excitation energy range, i.e. , between 1.5 and 1.9 MeV. It is the aim of this paper to give a "complete" survey of the dipole states in this energy region in
The branching ratios of the collective levels in ' Xe were discussed in the frame&work of the proton-neutron interacting boson model . It is shown that the experiment is only consistent with rather small M1 admixtures among the low-lying collective levels. These small Ml matrix elements imply strong constraints on the proton-neutron interacting boson model Hamiltonian.The various collective models have focused hitherto mostly on the excitation energies and E2 transitions.With the discovery of the 3.08 MeV isovector (1+) excitation in ' Gd and of similar excitations in neighboring nuclei by the Darmstadt group, ' there has been a surge of interest in a theoretical description of the magnetic properties of collective nuclei, viz. , g factors and Ml transitions.Clearly, besides the experimental evidence on the isovector excitations there exists in the literature a large body of information on Ml transitions in the form of intensity branching ratios and less accurately in the form of E2jM1 mixing ratios of transitions. These data clearly involve both E2 and Ml matrix elements, and in order to obtain reliable information on the Ml matrix elements one should consider only those transitions which have strong E2 matrix elements in the collective model considered. In the following we will consider as an example the nucleus ' Xe, because there exist particularly detailed data for this nucleus from recent experiments at Koln and Jyvaskyla, and also because this nucleus belongs to a large class of nuclei in the Xe-Ba region which have been shown to belong to the O(6) limit in the interacting boson model (IBM-1). Thus we have a rather good understanding of the energies and of the E2 transitions of the collective levels of ' Xe. In order to describe Ml transitions we will use the IBM-2 version of the interacting boson model which comprises both proton and neutron bosons, which has been introduced by Arima et al. 3 In the IBM-2 model one can describe Ml transitions with the operator T (Ml), T(M1 ) =g pL p+g"L" =(Np+N") '[(N~p+N"g")(L p+L") +(gp -gn)(NBLp -NpLn)l in nuclear Bohr magneton units, where gp and g"are the g factors for the proton and neutron bosons. The values of these g factors of the bosons are not completely known at the moment. The experimental evidence comes from the M1 transitions to the mixed symmetry 1+ state usually called the isovector state' which leads to 0.6 &gp -g"&1. ' Similar values are also found from systematic fits to the g factors of the 2+ states which have been performed by Wolf et al. which yield gp M =(spd"-s"dp)(spd"-s"dp)
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