Li neutron drip line nucleus*

At the RIKEN Radioactive Isotope Beam Factory, the spin-isospin response of ¹¹Li was measured in charge-exchange (p,n) reaction at 181 MeV/nucleon beam energy. There is no available data for isovector spin-flip giant resonances in nuclei with large isospin asymmetry factors, where (N−Z)/A>0.25 [1]. Our work aims to investigate this unexplored region up to (N−Z)/A =0.5. 

    The charge-exchange (p,n) reactions in inverse kinematics combined with the missing-mass technique are powerful tools to extract the B(GT) strengths of unstable isotopes up to high excitation energies, without Q-value limitation of the β decay [1]. In our previous work on ¹³²Sn [2], we demonstrated that accurate information about giant resonances can be obtained for unstable nuclei by using this probe. The combined setup [3] of PANDORA low-energy neutron spectrometer [4] and SAMURAI large-acceptance magnetic spectrometer [5] together with a thick liquid hydrogen target allowed us to perform the experiment with high luminosity.

    The β decay of ¹¹Li is complex. The ¹¹Li β-decay involves the largest number of decay channels ever detected [6] and experimental results have been reported for cases, when the daughter breaks into fragments, and emission of one, two, and three neutrons, α particles and ⁶He, tritons, and deuterons has been observed in several β-decay studies [8]. However, the B(GT) values were not clearly deduced as these studies were effected by the Q value.

    In this talk, the obtained double differential cross section up to about 40 MeV, including the Gamow-Teller (GT) Giant Resonance region in ¹¹Li will be reported, as well as the B(GT) values. We will discuss the nature of several newly identified decay channels of ¹¹Be also. 

    Our observation, that the GT peak occurs below the Isobaric Analog State in ¹¹Li, will be discussed in connection with the variation of residual spin-isospin interaction in exotic nuclei. 

[1] K. Nakayama, et al., Phys. Lett. B 114, 217 (1982). 

[2] M. Sasano et al., Phys. Rev. Lett. 107, 202501 (2011).

[3] J. Yasuda et al., Phys. Rev. Lett. 121, 132501 (2018).

[4] L. Stuhl et al., Nucl. Instr. Meth. B 463, 189 (2020).

[5] L. Stuhl et al., Nucl. Instr. Meth. A 866, 164 (2017). 

[6] T. Kobayashi, et al., Nucl. Instr. Meth. B 317, 294 (2013). 

[7] M. Madurga et al., Nucl. Phys. A 810, 1 (2008).

[8] R. Raabe et al., Phys. Rev. Lett. 101, 212501 (2008).