Speaker
Description
The tin isotopic chain with its magic 50 proton closed shell is a benchmark for models of nuclear structure. While the neutron-rich tin nuclei around the magic 82 neutron shell play an important role in the rapid-capture nuclear process, the mid-shell region of the tin isotopes can display collective phenomena known as shape coexistence [1]; for example, in $^{116}$Sn$_{66}$ deformed bands based on 2 particle – 2 hole excitations across the proton 50 shell gap exist [2,3]. Furthermore, at energies below the particle threshold, a new phenomenon called Pygmy Quadrupole Resonance (PQR) have been recently observed in $^{124}$Sn below 5 MeV [4]. Coupled with theoretical calculations, the new excitation mode was interpreted as a quadrupole-type oscillation of the neutron skin. This study prompted investigations for corresponding states in the neighboring $^{118,120}$Sn nuclei populated using thermal neutron capture of $^{117,119}$Sn(n,g).
Thermal neutron capture of $^{117,119}$Sn populates states in $^{118,120}$Sn at the neutron separation energy of about 9 MeV. The capture states in these experiments consist of 0$^+$ and 1$^+$ spins, ideal for populating subsequent 2$^+$ states which could be attributed to the PQR predicted to exist in the 3-5 MeV range.
In the experiments performed at the Institut Laue-Langevin in Grenoble, France, a continuous high-flux of thermal neutrons of 10$^8$ s$^{-1}$ cm$^{-2}$ from the 57 MW research reactor was used for capture reactions on enriched odd-A Sn targets. Gamma-ray transitions from excited states in nuclei of interest were detected by the Fission Product Prompt gamma-ray Spectrometer (FIPPS) [5] consisting of eight large n-type high purity germanium (HPGe) clover detectors and augmented with eight additional Compton-suppressed HPGe clovers from INFN Horia Hulubei, in Bucharest, Romania for enhanced gamma-ray efficiency and additional angular coverage used to produce angular correlations for spin assignments. In addition, 15 fast response LaBr$_3$(Ce) were used to allow for fast timing measurements of nuclear states using the centroid-shift method as described in [3].
Preliminary results from the $^{117,119}$Sn(n,g)$^{118,120}$Sn experiments will be presented highlighting the newly observed levels within the 3-5 MeV energy range of interest for PQR and lifetimes of excited states in $^{120}$Sn.
[1] K. Heyde and J. L. Wood. Rev. Mod. Phys., 83, (2011).
[2] J.L. Pore et al., Eur. Phys. J A 53, 27, (2017).
[3] C. M. Petrache et al., Phys. Rev. C 99, 024303 (2019).
[4] M. Spieker et al., Phys. Lett. B 752, 102 (2016).
[5]. C. Michelagnoli et al., EPJ A 193, 04009, (2018).
| Keyword-1 | Nuclear Structure |
|---|---|
| Keyword-2 | Gamma-ray spectroscopy |
