Multipole excitations in hot nuclei within the finite temperature quasiparticle random phase approximation framework

The effect of temperature on the evolution of the isovector dipole and isoscalar quadrupole excitations in ${}^{68}\mathrm{Ni}$ and ${}^{120}\mathrm{Sn}$ nuclei is studied within the fully self-consistent finite temperature quasiparticle random phase approximation framework, based on the Skyrme-type SLy5 energy density functional. The new low-energy excitations emerge due to the transitions from thermally occupied states to the discretized continuum at finite temperatures, whereas the isovector giant dipole resonance is not strongly impacted by the increase of temperature. The radiative dipole strength at low energies is also investigated for the ${}^{122}\mathrm{Sn}$ nucleus, becoming compatible with the available experimental data when the temperature is included. In addition, both the isoscalar giant quadrupole resonance and low-energy quadrupole states are sensitive to the temperature effect: while the centroid energies decrease in the case of the isoscalar giant quadrupole resonance, the collectivity of the first $2^{+}$ state is quenched and the opening of new excitation channels fragments the low-energy strength at finite temperatures.

Figure 1

Upper panel: The isovector dipole strength function in ${}^{68}\mathrm{Ni}$ calculated with FT-QRPA and the Skyrme-type SLy5 interaction at $T=0$, 1, and 2 MeV. Lower panel: The reduced transition probabilities for the low-energy dipole region at $T=0$ MeV and $T=2$ MeV.

Figure 2

Same as Fig. 1 but for the ${}^{120}\mathrm{Sn}$ nucleus.

Figure 3

The $\gamma$-ray strength function in ${}^{122}\mathrm{Sn}$ at $T=0$, 1.02, 1.17, and 1.55 MeV using the Skyrme-type SLy5 interaction. The discrete spectrum is broadened by using Lorentzian functions with 0.1 MeV (a) and 0.3 MeV (b) width. The experimental data are taken from Ref. [56].

Figure 4

The isoscalar quadrupole strength function in ${}^{68}\mathrm{Ni}$ calculated with the FT-QRPA and the Skyrme-type SLy5 interaction at $T=0$, 1, and 2 MeV.

Figure 5

Same as in Fig. 4 but for the ${}^{120}\mathrm{Sn}$ nucleus.

Figure 6

The isoscalar quadrupole strength function in ${}^{120}\mathrm{Sn}$ calculated with QRPA ($T=0$ MeV), FT-RPA, and FT-QRPA at $T=0.5$ (a), 0.75 (b), and 1 (c) MeV.