Speaker
Description
The advantages of combining broadband bremsstrahlung beams and highly-brilliant gamma-ray beams from Compton back-scattering will be discussed on the basis of recent experimental programs. Due to the low angular momentum transfer through real photons, the sensitivity of such studies [1] is high for low multipoles, i.e., in particular for the electric and magnetic dipole response, but also for electric quadrupole excitations. Therefore, much experimental effort in recent years has gone into fostering a better understanding of the electric dipole response of atomic nuclei, which is dominated by the electric giant dipole resonance. Although this mode is known since the first days of photo-excitation experiments, some of its properties are yet to be determined, as well as the detailed structure on its low-energy tail, which eventually includes the so-called pygmy dipole resonance. The electric dipole response remains under active discussion, due to structural aspects and to better understand photon strength functions, which are one important ingredient in connection to nucleosynthesis [2]. Also important for photon strength functions is the magnetic dipole response, which includes fundamental modes such as the nuclear scissors mode and the isovector-M1 spin-flip resonance. The latter is of particular interest for astrophysical scenarios, due to the similarity between the spin-flip M1 and the Gamov-Teller operators. Data on the M1 response may, thus, constrain electron-capture rates for supernova scenarios [3], but is also important in view of coherent neutrino scattering, which excites in particular spin-flip M1 states [3]. Furthermore, real photons allow insight on E2 properties of nuclei, especially the first (collective) quadrupole excitations of even-even isotopes, hence, may give model-independent insight into the evolution of E2 strengths in nuclei [5,6].
This research is supported in part by the German DFG under contract no. 279384907-SFB 1245 and BMBF under grant no. 05P21RDEN9, as well as the State of Hesse within the LOEWE program “Nuclear Photonics”.
[1] A. Zilges, D.L. Balabanski, J. Isaak, and N. Pietralla, Prog. Part. Nucl. Phys. 122, 103903 (2022).
[2] S. Goriely, Phys. Lett. B 436, 10 (1998).
[3] K. Langanke, G. Martínez-Pinedo, and R.G.T. Zegers, Rep. Prog. Phys. 84, 066301 (2021).
[4] E. Ydrefors, K.G. Balasi, t.S. Kosmas, and J. Suhonen, Nucl. Phys. A 896, 1 (2012).
[5] T. Beck et al., Phys. Rev. Lett. 118, 212502 (2017).
[6] K.E. Ide et al., Phys. Rev. C 103, 054302 (2021).
