Recently a quantitative description of the two-neutron transfer reaction 12 C( 18 O , 16 O) 14 C was performed and the measured cross sections were successfully reproduced [M. Cavallaro et al., Phys. Rev. C 88, 054601 (2013)]. This task was accomplished by combining nuclear structure calculations of spectroscopic amplitudes and a full quantum description of the reaction mechanism. Verification of such a theoretical approach to other heavy nuclear systems is mandatory in order to use ( 18 O , 16 O) reactions to assess pair configurations in nuclear states. In this work we apply this methodology to the 16 O( 18 O , 16 O) 18 O reaction at 84 MeV. Experimental angular distributions for the two-neutron transfer to the ground state and 2 + 1 state of 18 O were obtained using the MAGNEX spectrometer at INFN-LNS. The roles of one-and two-step processes are analyzed under the exact finite range coupled reaction channel and the second order distorted wave Born approximation. We conclude that the one-step transfer mechanism is dominant in this system.

Background: In our recent work [M. J. Ermamatov et al., Phys. Rev. C 94, 024610 (2016)], the two-neutron transfer induced by the ( 18 O, 16 O) reaction was studied for 16 O nucleus. Theoretical analysis of the low-lying states of 18 O indicates that the transfer to the ground state proceeds predominantly through simultaneous transfer of the two-neutron system. Purpose: In this work, we extend our previous theoretical analysis towards high-lying states of the 18 O nucleus. In order to achieve a comprehensive picture, we revisit the experimental data for the 16 O(t,p) 18 O reaction at 15 MeV bombarding energy. We also include new experimental cross sections for the high-lying states of the 18 O residual nucleus, populated in the 16 O( 18 O, 16 O) 18 O reaction at 84 MeV. Method:The same spectroscopic parameters of the target nucleus were used as input in the coupled channel calculations for the transfer induced by triton and 18 O projectiles. Simultaneous two-neutron transfer is calculated within the coupled reaction channel approach, using the extreme cluster and independent coordinate models. The sequential process is calculated within the distorted-wave Born approximation. Results: Theoretical calculations reproduce the 16 O(t,p) 18 O data well, leading to natural parity states in the 18 O nucleus without the need of adjustable parameters. The same methods are applied to the 16 O( 18 O, 16 O) 18 O data and a good agreement is observed. Conclusions: Detailed analyses show the importance of a simultaneous mechanism for the two-neutron transfer reactions. In transferring two neutrons, the pairing correlation plays an important role.

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