Speaker
Description
Pairing correlations play a crucial role in defining the properties of atomic nuclei. The evolution of these correlations in exotic nuclei is a subject which has received much attention in recent years, as new accelerator facilities are providing unique radioactive beams for study. Of particular interest is the role of pairing in neutron-rich isotopes. In particular in the proton-magic Sn isotopes, theoretical calculations based on Skyrme-Hartree-Fock mean field and continuum RPA predict a significant increase in the neutron pair-transfer strength to low-lying excited 0+ states (pairing vibrations) for N = 82 – 90 nuclei [1].
The first excited 0+2 state can be regarded as a pairing vibrational mode built on the weakly bound p3/2 and p1/2 orbits, which shows a rather long tail in the transition density extending beyond the nuclear surface (>10 fm) resulting in a large strength, comparable to that populating the ground state. The large increase in the pair-transfer probability for N > 82 is almost independent of the volume/surface pairing parametrization used in the mean field calculations. Theoretical calculations for 138Xe (N=84) have been performed [2] and show that a similar phenomenon is also expected in the Xe isotopes. The enhanced tail in the pair transition density depends on the neutron Fermi energy and thus in 138Xe, with more strongly bound neutrons compared to 134Sn, the effect is less pronounced in the Xe isotopes but still noticeable cross N = 82.
Motivated by these predictions and the unique beam and instrumentation capabilities available at CERN, we propose to study (t,p) reactions in reverse kinematics using Xe beams from HIE ISOLDE impinging on a radioactive tritium target and the Solenoidal Spectrometer ISS to identify the outgoing protons, with improved resolving power over conventional setups. According to the anticipated yields [3] we could systematically compare the 2n-transfer cross sections as a function of the projectile mass using beams of 134,136,138,140Xe. Absolute and relative measurements of the L=0 pair transfer strength to the ground and excited 0+ states along the Xe isotopic chain will give us insights into the nature of this novel aspect of pairing collectivity.
References.
- H. Shimoyama and M. Matsuo. Phys. Rev. C, 84:044317, 2011.
- S. Tamaki. Master thesis, Niigata University, 2016.
- https://test-isoyields2-dev1.web.cern.ch/Yield_Home.aspx
