Breaking of axial symmetry in excited heavy nuclei as identified in giant dipole resonance data

Abstract

Abstract. A recent theoretical prediction of a breaking of axial symmetry in quasi all heavy nuclei is confronted to a new critical analysis of photon strength functions of nuclei in the valley of stability. For the photon strength in the isovector giant dipole resonance (IVGDR) regime a parameterization of GDR shapes by the sum of three Lorentzians (TLO) is extrapolated to energies below and above the IVGDR. The impact of non-GDR modes adding to the low energy slope of photon strength is discussed including recent data on photon scattering and other radiative processes. These are shown to be concentrated in energy regions where various model calculations predict intermediate collective strength; thus they are obviously separate from the IVGDR tail. The triple Lorentzian (TLO) ansatz for giant dipole resonances is normalized in accordance to the dipole sum rule. The nuclear droplet model with surface dissipation accounts well for positions and widths without local, nuclide specific, parameters. Very few and only global parameters are needed when a breaking of axial symmetry already in the valley of stability is admitted and hence a reliable prediction for electric dipole strength functions also outside of it is expected.

Description of nuclear photoexcitation by Lorentzian expressions for electric dipole photon strength function

“…The radius parameters R j , which are inversely proportional to the harmonic oscillator constants, were used in ref. [12], but it was indicated in this references that the differences in the results using (17) were not significant.…”

“…It can be seen from figs. 4,5 that, if Γ (ε γ ) is permanently increasing, the integrals I α (ε max ) of the Lorentzianlike function over energy with energy dependent width are very close to SLO values for gamma-ray till ∼30 MeV but for the gamma-ray energies 30 MeV they can be more than unity due to overestimation of the GDR component of the photoabsorption cross-section [12]. It leads to violation of the EWSR.…”

“…with fitting results: c(SMLO) = 0.42 ± 0.05, d(SMLO) = 0.90 ± 0.04; and c(SLO) = 0.32 ± 0.03, d(SLO) = 0.98 ± 0.03; c(GLO) = c(SLO); d(GLO) = d(SLO). Approximation of a triaxial ellipsoid is used in the TLO model for nuclear shape in deformed nuclei [11,12], and the nuclear shapes are determined in terms of β and the triaxiality angle γ, where β represents the extent of quadrupole deformation (describes the deviation from sphere) and γ gives the degree of axial asymmetry. In this approach, the GDR splits into three components and the E1 PSF is described by the expression (3) with j m = 3.…”

“…For the input parameters of the TLO PSF, the theoretical expressions were taken from different theoretical models. For the resonance energies of the normal modes, the expressions (13) of the nuclear hydrodynamics model were adopted [11,12]:…”

“…The width Γ TLO j was independent of gamma-ray energy and the expression with a power law dependence on the resonance energy was used [11,12,38]: , and within TLO model (19), Γ TLO r . In the calculations the neutron-proton asymmetry factor was taken as corresponding to Green's approximation [26] to the line of β stability.…”