Speaker
Description
With its long chain of experimentally accessible isotopes, the Sn isotopes with their closed Z=50 proton shell are a fertile testing ground for nuclear structure models. Yet, there is debate on basic properties, even of stable Sn isotopes, namely, the B(E2) excitation strengths of their first-excited 2$^+$ states. Experiments toward the neutron-deficient isotopes revealed unexpectedly large B(E2) values, which had been related to enhanced quadrupole correlation due to the particular neutron orbitals filled at the beginning of the shell, whereas the structures of heavier isotopes are seniority dominated. This structural transition has been described as a phase transition between both regimes within a Monte Carlo shell model approach [1], yielding a local minimum in B(E2) values at $^{116}$Sn. The particulars of the structural transition, however, differ in other model approaches. Data, mostly obtained through Coulomb-excitation and Doppler-shift techniques (see, e.g., [2,3]) show conflicting trends and magnitudes of B(E2) strengths.
We used an alternative method, nuclear resonance fluorescence [4], exploiting the purely electromagnetic interactions between photons and nuclei, to probe the B(E2) strength across the predicted minimum at N=66, avoiding uncertainties of nuclear interactions or stopping powers of ions in materials. The isotopes $^{112,116,120}$Sn have been measured at the bremsstrahlung facility, DHIPS, at the S-DALINAC at TU Darmstadt in three experiments. Therein, we measured the E2 excitation strengths of the first 2+ states of $^{116,120}$Sn relative to that of $^{112}$Sn, hence, avoiding any potential systematic error from absolute scales. In addition, we measured the E2 excitation strength of the 2$_1^+$ state of $^{112}$Sn relative to well-known excitation cross sections in other isotopes, in order to determine the absolute scale of E2 strengths in the Sn isotopes. The new results indicate a rather smooth change-over between the quadrupole-collective and seniority schemes around N=66, and are in agreement with several sets of Coulomb-excitation data.
This research is supported in part by Deutsche Forschungsgemeinschaft – Project-ID 279384907 – SFB 1245, and by the State of Hesse within the LOEWE research project “Nuclear Photonics”.
[1] T. Togashi, Y. Tsunoda, T. Otsuka, N. Shimizu, and M. Honma, Phys. Rev. Lett. 121, 062501 (2018).
[2] A. Jungclaus et al., Phys. Lett. B 695, 110 (2011).
[3] M. Allmond et al., Phys. Rev. C 92, 041303 (2015).
[4] A. Zilges, D. Balabanski, J. Isaak, and N. Pietralla, Prog. Part. Nucl. Phys. 122, 103903 (2022).
