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Over the years there have been many efforts put in trying to understand the electric dipole (E1) strength of atomic nuclei. It is known that the nuclear E1 response is mostly dominated by the IsoVector electric Giant Dipole Resonance (IVGDR), which can be understood as a collective harmonic motion of protons against neutrons [1].
In neutron rich-nuclei, part of the E1 strength is redistributed around the neutron separation energy, producing a concentration of low energy dipole excitations known as a Pygmy Dipole Resonance (PDR), which instead consists in an oscillation of a neutron skin against an isospin symmetric core. The closer we are to the neutron drip-line, the more complex the PDR becomes, heavily affecting its properties [2].
This study focuses in the double neutron halo nuclei 11Li. The PDR in 11Li is significantly different from a regular PDR due to the very low neutron separation energy of 11Li, which produces a large imbalance of neutrons in the neutron skin with pairing energy playing an important role in it. Although the PDR for 11Li was initially observed in [3], this observation only accounts for a small part of the total E1 response in 11Li. Recent theoretical studies have predicted the presence of an IVGDR in 11Li that was not observed before, which accounts for most of its E1 strength [4].
In order to experimentally study the complete E1 stregnth of 11Li, an inelastic scattering experiment in inverse kinematics was performed at the Facility for Rare Isotope Beams (FRIB) in July 2024. A 53.4 MeV/u 11Li beam was sent into the Active Target Time Projection Chamber (AT-TPC) [5] which acted as the proton active target, as well as the tracking detector for the scattered protons from the reaction. Additionally, the S800 spectrometer [6] was used at the end of the beam line in order to study the decay products of the excited 11Li.
Although the PDR in 11Li was already observed previously [3, 7, 8], the results from this experiment provide a preliminary measurement of an IVGDR in 11Li, which to our knowledge is a first for double halo nuclei. These results are of importance to fully understand the E1 response of 11Li, and may provide useful insight into the E1 properties of halo nuclei in general.
References:
[1] M. N. Harakeh and A. van der Woude. Giant Resonances: Fundamental High-frequency Modes of Nuclear Excitation. Oxford science publications. Oxford University Press, 2001.
[2] T. Aumann. Low-energy dipole response of exotic nuclei. Eur. Phys. J. A, 55:234, 2019.
[3] T. Nakamura, et al. Observation of strong low-lying E1 strength in the two-neutron halo nucleus 11Li. Phys. Rev. Lett., 96:252502, Jun 2006.
[4] R. A. Broglia et al. Pygmy resonances: what’s in a name? Physica Scripta, 94(11):114002, 2019.
[5] J. Bradt et al. Commissioning of the Active-Target Time Projection Chamber. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 875, Pages 65-79, 2017.
[6] D. Bazin et al. The S800 spectrograph. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Volume 204, Pages 629-633, 2003.
[7] R. Kanungo et al. Evidence of soft dipole resonance in 11Li with isoscalar character. Phys. Rev. Lett., 114:192502, May 2015.
[8] J. Tanaka et al. Halo-induced large enhancement of soft dipole excitation of 11Li observed via proton inelastic scattering. Physics Letters B, 774:268–272, 2017.
