Session 4WHA: Frontiers of Spin-Isospin Excitations I

Gamow-Teller Giant Resonance in 11Li 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).   

Quantum many-body techniques for the description of spin-isospin excitations and beta-decay rates for the r process

Modeling the r-process nucleosynthesis requires a huge amount of nuclear physics input, and in particular, the knowledge of beta-decay rates for thousands of neutron-rich nuclei. Due to their very short lifetimes, most of these nuclei are not accessible experimentally and astrophysical simulations thus heavily rely on theoretical methods, which should be as universal and reliable as possible.

In this context, microscopic methods based on the self-consistent mean field, such as the quasiparticle random phase approximation (QRPA), are good candidates as they are applicable to arbitrarily heavy nuclei at relatively low numerical cost. QRPA, however, suffers from a lack of correlations that typically leads to an imprecise description of the nuclear response, in particular the low-energy behavior, which determines beta decay. Extending this approach is thus crucial to improve the precision and reliability of the calculations and ultimately decrease the resulting uncertainties in the predictions of elemental abundances.

In this talk I will discuss how accounting for the coupling between nucleons and collective nuclear vibrations provides a way to include the relevant correlations needed to achieve this goal. I will present calculations of spin-isospin modes of excitations that can be compared to data, as well as new calculations of beta-decay rates of neutron-rich nuclei, focusing on those around neutron-shell closures which are of particular importance for the r-process. I will examine the impact of particle-vibration coupling on the rates, and the contribution of both Gamow-Teller and first-forbidden transitions.   

Constraining Electron Capture Rates with (d,2He) Reactions in Inverse Kinematics

Electron-capture (EC) rates play a decisive role in core-collapse and thermonuclear supernovae, the crust of accreting neutron stars in binary systems, and the final core evolution of intermediate mass stars. Charge-exchange reactions (CERs) at intermediate energies (~100 MeV) are crucial in extracting information for neutron-rich nuclei as the EC Q-values are positive for such nuclei. The differential cross sections in CERs at zero momentum transfer are proportional to the Gamow-Teller strength, B(GT), from which the EC rates can be calculated. In a first of a kind experiment, the S800 spectrometer at National Superconducting Cyclotron Laboratory (NSCL) along with Active-Target Time Projection Chamber (AT-TPC) setup was used to run an experiment with (d,²He) probe in inverse kinematics to study unstable nuclei. Data from the experiment for the ¹⁴O(d,²He)¹⁴N and ¹³N(d,²He)¹³C reactions have been analyzed to extract the differential cross section for ground and excited states and the extracted B(GT) is in good agreement with theory and existing data.