Study of giant resonances with active targets

Jan 18, 2018, 10:20 AM
Aula Magna, Facultad de Física (Santiago de Compostela)

Aula Magna, Facultad de Física

Santiago de Compostela

Facultade de Física. Campus Vida s/n | E-15782, Santiago de Compostela | Galicia, Spain


Dr Simone Ceruti (KU Leuven)


The presence of coherent motions of particles in many-body systems, i.e. collective motions, is a common feature in several branches of physics. In atomic nuclei, a particular case of nuclear collective motion is represented by the giant resonances (GR) [1], which are the subject of this presentation. These resonance states play a key role in the understanding of the nuclear structure because of their connection with the bulk properties of atomic nuclei.
Giant resonances can be macroscopically viewed as a quantum oscillation of two fermionic liquids (neutron and protons) involving spatial (L), spin (S) and isospin (I) degree of freedom. In the case of an isoscalar oscillation (∆I=0) neutron and protons move together in phase. On the other hand, in an isovector oscillation (∆ I =1) neutron and protons move in opposite direction.
The isoscalar giant monopole resonance (ISGMR) measures the collective response of the nucleus to density fluctuations (∆I, ∆S, ∆L=0) [1]. The ISGMR is particularly interesting for its connection with the incom- pressibility of the nucleus KA, which, in turn, can be linked to the incompressibility of nuclear matter K∞, an important ingredient of the nuclear-matter equation-of-state (EOS). The EOS, essentially, describes the binding energy per nucleon as a function of nuclear density and it plays an important role in the description of heavy-ion nuclear collision, the collapse of the heavy stars in super novae explosion and the description of neutron stars [2]. In order to improve the understanding of this nuclear mechanism, new experimental data in unstable nuclei far from the stability are needed [3].
The reaction mechanism used to excite the ISGMR is the inelastic scattering of the nuclei of interest on an hadron isosclar probe, typically an α particle. The use of an active target coupled with silicon detectors allows to measure the α particles at forward angles (where the maximum of the cross section is located) and with a very small kinetic energy [4].
In addition, the (α,α’) reaction can be also used to excite isoscalar dipole states (L=1, I=0) around the neutron separation energy [5]. These states, also called pygmy dipole resonance (PDR), are of great interest for the impact on astrophysical phenomena, such as r-process nucleosynthesis [6]. The nature of the PDR is largely debated. SpecMAT [7], an active target placed in a high magnetic field and coupled with scintillation detectors, will be a powerful detector to observe PDR in unstable nuclei.
In this presentation the use of active targets to study ISGMR and PDR will be shown.
[1] M. N. Harakeh and A. van der Woude, Giant Resonances, Fundamental High-Frequency Modes of Nuclear Excitation, Oxford Science Publications, 2001.
[2] J.M. Lattimer and M. Prakash, Astrophys. J. 550, 426 (2001) and Science 304, 5670 (2004).
[3] E. Khan, J. Margueron and I. Vidana, Phys. Rev. Lett. 109, 092501 (2012).
[4] M. Vandebrouck, et al., Phys. Rev. Lett. 113, 032504 (2014).
[5] D. Savran, T. Aumann, and A. Zilges, Prog. Part. Nucl. Phys. 70, 210 (2013). [6] S. Goriely, Phys. Lett. B 436, 10 (1998).
[7] R. Raabe, SpecMAT ERC Consolidator Grant (2014).

Primary authors

Dr Simone Ceruti (KU Leuven) Mr Alex Raj (KU Leuven) Riccardo Raabe (KU Leuven) Dr Hilde De Witte (KU Leuven) Oleksii Poleshchuk (KU Leuven, Institute for Nuclear and Radiation Physics) Mr Maxim Renaud (KU Leuven) Mr Jiecheng Yang (KU Leuven)

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