Speaker
Description
A concentration of electric dipole strength below the neutron separation threshold, often unified under the name Pygmy Dipole Resonance (PDR), is known to be common in medium to heavy mass nuclei and has been studied intensively in the past. As a part of the general $\gamma$-ray strength function, its generating mechanisms are of genuine interest for nuclear structure and nuclear astrophysics alike, and several complementary experiments have uncovered intricate details of this strength accumulation.
An isoscalar surface-mode character was identified in the lower-energy part of the PDR in $^{124}$Sn based on the different responses observed in $(\alpha,\alpha'\gamma)$ or $(^{17}$O$,^{17}$O$'\gamma)$ experiments compared to $(\gamma,\gamma')$ data [1,2]. Conversely, the higher-energy part was attributed to a more isovector excitation taking place deeper inside the nucleus. In $^{120}$Sn, access to the single-particle character of the PDR was recently gained by comparing $^{119}$Sn$(d,p\gamma)$ and $^{120}$Sn$(\gamma,\gamma')$ reactions in both experiment and theory [3,4]. Although visually similar to the case of $^{124}$Sn$(\alpha,\alpha'\gamma)$, the $^{119}$Sn$(d,p\gamma)$ experiment is sensitive to a very different type of nuclear structure information.
This contribution gives an overview of the current experimental knowledge on the PDR in $^{120}$Sn and discusses the structural comprehension based on Quasiparticle-Phonon-Model calculations. Combined with dedicated reaction theory for each population mechanism, consistent observables from experiments and theory are presented and the impact of the information gained in $^{120}$Sn on the genuine low-energy E1 strength throughout the chart of nuclides is discussed.
Supported by the DFG (ZI 510/10-1).
[1] J. Endres et al., Phys. Rev. Lett. 105, 212503 (2010)
[2] L. Pellegri et al., Phys. Lett. B 738, 519 (2014)
[3] M. Weinert et al., Phys. Rev. Lett. 127, 242501 (2021)
[4] M. Müscher et al., Phys. Rev. C 102, 014317 (2020)
