Fine Structure of the Pygmy Dipole Resonance inXe136

Abstract

The photoresponse of the semimagic N=82 nucleus (136)Xe was measured up to the neutron separation energy S(n) using the (gamma, gamma') reaction. A concentration of strong dipole excitations is observed well below S(n) showing a fragmented resonancelike structure. Microscopic calculations in the quasiparticle phonon model including complex configurations of up to three phonons agree well with the experimental data in the total integrated strength, in the shape and the fragmentation of the resonance, which allows us to draw conclusions on the damping mechanism of the pygmy dipole resonance.

Fragmentation of electric dipole strength inN=82isotones

“…The E1 strengths, B(E1) ↓, below 10 MeV in 140 Ce, [15]. However, this is different from the experimental findings [1][2][3][4]. In the SRPA calculation, the coupling to the 2p2h configurations leads to a considerable E1 strength in this energy region.…”

“…The disagreement suggests that the coupling to complex configurations, such as multi-particle-multi-hole states, are required to study the PDR in these nuclei. In fact, the quasiparticle-phonon model (QPM), which takes into account coupling to multi-phonon states, successfully reproduces the low-energy dipole strengths in the N = 82 nuclei [2,4]. A similar approach based on the relativistic mean-field model has also been used to study the PDR in the tin and nickel isotopes [16].…”

“…Jz, 24.30.Cz, 27.60.+j The low-energy dipole states, often referred to as the pygmy dipole resonance (PDR), have attracted recent experimental [1][2][3][4][5][6] and theoretical interests [7][8][9][10][11][12] (see also the recent review [13] and references therein). It is also of significant astrophysical interest, since the low-energy dipole strengths close to the neutron threshold strongly affect the astrophysical r-process nucleosynthesis [14].…”

Breaking of axial symmetry in excited nuclei as identified in experimental data

“…mainly narrow peaks, which were interpreted as strength apparently of other character than isovector electric dipole [8,9,[21][22][23]. Even if it has minor importance for total photon absorption and the sum rules, it may influence the decay of the compound nucleus and consequently also the cross sections of capture reactions.…”

Decay pattern of the Pygmy Dipole Resonance in140Ce

“…The properties of this phenomenon have been studied in stable (see e.g. [3][4][5][6][7][8][9][10][11]) and in a few radioactive nuclei [12][13][14], in particular in the Z=50 and N=82 mass region. The underlying nature of the PDR is still unclear, and strong experimental and theoretical effort is put into its investigation.…”