Study of the tensor force contribution in the O isotopic chain using QFS reactions

by Antoine BARRIERE, 3^rd year PhD student at GANIL

In the shell model framework, the two-body nuclear force can be divided into a central, spin-orbit (SO) and tensor parts.

In this work, we focus on the evolution of the proton 0p_1/2-0p_3/2 SO splitting in the O chain, when the neutron 0d_5/2 orbital is filled by 6 neutrons, from ¹⁶O to ²²O. The vast majority of studies performed so far in stable nuclei of the chart of nuclides shows that the amplitude of the SO splitting scales with approximately A^2/3 [1], due to the surface-dominant term of the spin-orbit force. The present study, that goes well beyond stability, should demonstrate if such a decrease of the SO splitting between ¹⁶O and ²²O is found and if it is larger than expected. Indeed, the action of tensor forces should lead to a further decrease of the SO splitting, added to the role of SO force.

An experiment has been recently performed using the R³B setup at GSI, within the FAIR Phase-0 program. One of the associated scientific goals is to study the role of the tensor force when approaching the neutron drip line. During this experiment a “cocktail” of nuclei, among which ²²O, was sent on a 5 cm LH2 target surrounded by tracking detectors and the CALIFA calorimeter [2]. This calorimeter allows to detect gamma-rays and light particles from the QFS reactions in inverse kinematics. To study the spectroscopy of unbound states with an unprecedented energy resolution, this new setup includes the high resolution and granularity neutron detector NeuLAND [3].

The ²²O(p,2p) QFS knockout reaction provides information on the tensor force contribution to the 0p_1/2-0p_3/2 SO splitting in the O isotope chain, from N=8 to N=14 shell closure, when the neutron 0d_5/2 orbital is filled. The SO orbit splitting amplitude is planned to be obtained from the energies and spectroscopic factors of the 1/2^– and 3/2^– states in ²¹N. In addition, the ²²O(p,pn) reaction has also been studied in order to determine if 6 neutrons are indeed added in the 0d_5/2 orbital from ¹⁶O to ²²O, or if a fraction of them are occupying the nearby 1s_1/2 orbital.

Preliminary results from this studies will be presented.

[1] G. Mairle, Phys. Lett. B , (1993) 39.
[2] A. Knyazev et al., Nucl. Instr. and Meth. A , (2019) 393-404.
[3] K. Boretzky et al., Nucl. Instr. and Meth. A , (2021) 165701.