Population of the 2ms+ mixed-symmetry state of Ba 140 with the α -transfer reaction

Background: Identification of proton-neutron mixed-symmetric one-quadrupole phonon excitations (the 2ms+ states) of atomic nuclei provides information on the isovector part of the residual nucleon-nucleon interaction. It was predicted that the 2ms+ state of particular nuclei close to the U(5) limit of the interacting boson model, in particular Ba140, should be considerably populated by α-transfer reactions [C. E. Alonso et al., Phys. Rev. C 78, 017301 (2008)PRVCAN0556-281310.1103/PhysRevC.78.017301]. Purpose: We aim at the identification of the 2ms+ mixed-symmetry state (MSS) of radioactive Ba140 and investigate its population by the α-transfer reaction as a suitable tool to selectively populate MSSs and as a potential new signature for its mixed-symmetric character. Method: A γ-ray spectroscopy experiment was performed in inverse kinematics in order to populate the 2ms+ state of Ba140 by α-transfer from a natC target on Xe136 beam ions. The population of the candidate for the 2ms+ state of Ba140 was measured relative to the population of the 21+ state. Results: The candidate for the 2ms+ state of Ba140 was populated by α transfer three times weaker than predicted. Another 2+ state that can be ruled out as the MSS was in turn as strongly populated by the α transfer as predicted for the MSS. Conclusions: The relative population of 2+ states by α-transfer cannot serve as a new signature for MSSs, since other 2+ states are also strongly populated. Nevertheless, the substantial population of the MSS candidate of Ba140 by α transfer qualifies this type of reaction as suitable tool to excite MSSs and study their electromagnetic decay properties.