Strong dipole excitations around 1.8 MeV inU238

Abstract

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

Dipole strength distribution and specific properties of excitations in the second minimum in actinide nuclei

“…This total strength has been correctly described in the RPA [20] and in the QPNM [19]. Recently, strong dipole excitations around 1.8 MeV in 238 U have been found in [7]. It is shown [19] that we have described this strong dipole excitation in 238 U.…”

“…In measurements, several doubly even deformed nuclei in the rare-earth and actinide regions have been studied [3,4,5,6,7]. The electric-dipole excitation modes of deformed nuclei have been simultaneously observed in these and other measurements [4,5,7].…”

Nuclear resonance fluorescence in240Pu

“…No transitions to states other than the ground and first excited states were observed in this work. Therefore, as typical of NRF measurements on even-even nuclei [15][16][17], the total integrated cross section of the NRF state is assumed to be equal to the sum of the integrated cross sections for transitions to the ground and first excited states (i.e., for each observed state, Γ = Γ 0 + Γ 1 ).…”

Nuclear resonance fluorescence for materials assay

“…The spectrum has a drop-off of 3 orders of magnitude above 1. [18], [19] and an experiment to measure the NRF response of 237 Np has recently been conducted [7]. The strongest 235 U resonance measured is at 1.733 MeV with a cross section of 30 eV⋅b.…”