Fine Structure of the Pygmy Dipole Resonance in ${}^{136}\mathrm{Xe}$

The photoresponse of the semimagic $N=82$ nucleus ${}^{136}\mathrm{Xe}$ was measured up to the neutron separation energy $S_n$ using the $(\gamma ,{\gamma }^{\prime })$ 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.

Figure 1

Spectrum of one HPGe detector normalized to the incoming photon flux and the photopeak efficiency.

Figure 2

Measured and calculated $B(E1)\uparrow$ strength distributions. For details, see the text.

Figure 3

Measured integrated $B(E1)\uparrow$ up to 8 MeV in the $N=82$ isotones. The new value for ${}^{136}\mathrm{Xe}$ confirms the enhancement for higher neutron-to-proton ratios $N/Z$.

Figure 4

Comparison of the experimentally observed and in the QPM calculation predicted $E1$ strength with respect to its distribution (left) and fragmentation (right). For details, see the text.