Separation of contributions of isovector E 2 and E 1 giant resonances in direct and inverse reactions with real and virt
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paration of Contributions of Isovector E2 and E1 Giant Resonances in Direct and Inverse Reactions with Real and Virtual Photons L. Z. Dzhilavyan*, A. M. Lapik, V. G. Nedorezov, and B. A. Tulupov Institute for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia *e-mail: [email protected] Abstract—A brief overview of the methods for separating the contributions of isovector electric quadrupole (E2) and dominant dipole (E1) giant resonances in atomic nuclei, which are excited in direct and inverse reactions with photons (real and virtual), is given. The basic parameters of isovector giant resonance E2, which were declared to date by applying some of these methods to the results with 208Pb, are also presented. DOI: 10.1134/S106377961701004X
INTRODUCTION Multipole giant resonances (GRs) are the universal responses of atomic nuclei to various external perturbations. GRs are governed by the nature of interactions of nucleons within a nucleus. Electric dipole (E1) isovector GRs have been studied the most extensively (see, for example, [1]). The available data on the key properties (centroid position, width, and intensity) of electric quadrupole (E2) isovector GRs are rather contradictory and sketchy (see, for example, [2]), although these data are of prime importance for compiling a quantitative description of GRs as coherent collective motions in nuclei. The identification of E2 GR is a challenging task, since it is “immersed” into the considerably more intense isovector E1 GR. One needs to choose such experimental conditions in which the ratio of contributions of these GRs is altered in favor of E2 GR. The studies into E2 GR should be focused primarily on heavy spherical nuclei (e.g., 208Pb), which are expected to have more compact GR strength functions at various nuclear excitation energies E*. A brief overview of the methods for separating the contributions of isovector E2 and E1 GRs in direct and inverse nuclear reactions with photons (real and virtual), is given below. 1. SCATTERING OF ELECTRONS BY ATOMIC NUCLEI The most detailed studies to date on isovector E2 GR have been performed for the reactions of inelastic scattering of electrons by nuclei, where energy E* and momentum q is transferred to a nucleus. Such reactions may be regarded as reactions induced by virtual photons with an unconstrained relation between E* and q (in contrast, this ratio is fixed in reactions
induced by real photons). If one can change experimental conditions (kinetic energy Ee of incident electrons and angle θ of detection of scattered electrons by a magnetic spectrometer), this method appears to be a flexible tool for the measurement of ratios between the contributions of GRs of different multipolarities and their identification. The results of early inclusive (e, e') experiments at pulsed electron accelerators (see, for example, [3] for 208Pb and 197Au and [4] for 208Pb) were used to compile a database of positions, widths, and intensities of isovector E2 GRs [5]. However, it is rather hard to retrieve the data on GRs
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