Speaker
Summary
The nuclear mesoscopic system is based on the subtle interplay between single
and collective degrees of freedom which manifest themselves depending on the nu-
clear configuration and excitation. In recent years many efforts have been devoted
to investigate evidences for this manifestation and to clarify when one description
holds at the expense of the other, when the two coexist and finally when the other
takes over.
One of the still disputed phenomenon is the single-particle/collective origin of
strength accumulated at the low energy tail of the giant-resonance coherent excitation, commonly referred to as a pygmy resonance.
Experimental observations started in the 50’s and 60’s, yet only recently NRF
focused on the photo-response of nuclei in the region of the particle threshold. The
field received a boost with the advent of high-energy radioactive beams and the
possibility to study their properties in reaction experiments, see [1] and references
therein. Started from 11 Li the predicted and observed large E1 transition probabilities for halo nuclei at the threshold has triggered the idea that a dipole vibrational
mode of loosely bound neutrons against a core nucleus should appear and might
be observable in neutron-rich nuclei. The gross feature of the E1 response, which
is mainly given by the IVGDR, is reproduced quite consistently in many models,
the fine structure and especially the energetically low-lying part of the E1 strength
often differs more drastically between different calculations, since it is much more
dependent on the details of the nuclear forces and theoretical models.
An extensive work has been recently deployed to describe the low-energy electric
dipole strength. Various approaches are being used, from HartreeFockBogoliubov
plus (quasi-particle) random- phase approximations (Q)RPA based on different interactions [2]
to the algebraic Interacting Boson Model [3]. Many of these models
predict the E1 strength located at low excitation energies as a signature of the
neutron-skin oscillation. While the low-lying component of the E1 strength is ob-
served in almost all calculations, the degree of collectivity is under debate.
In parallel with the neutron skin phenomenon, expected to take place mostly in
the neutron-rich nuclei, a proton skin might exist. Calculations have predicted the
existence of such evidence in neutron deficient nuclei [4], which are nowadays within
reach at the frontier nuclear physics facilities with an adequate intensity.
This LoI aims at investigation of the pygmy strength in neutron-deficient isotopes
to establish the degree of collectivity and the possible evidence of a proton skin.
To this end, we plan to populate 42Ti and the completely unknown 44Cr by
fragmenting a 78Kr beam, at an energy of 345 MeV/u, onto a thick Be target. The
predicted intensity of the 78Kr beam is about 50 pnA. A LISE++ simulation with a
1 g/cm 9 Be target gives a transmission efficiency of about 50% up to the secondary
target. In the case of 42Ti more than 10E3 pps are expected at the secondary target,
assuming a cross section of about 1 μb. The fragmentation cross section used for
the 44Cr yield estimate is 1.1 nb, from EPAX 3.1. The resulting 44 Cr intensity at
the position of the secondary target is therefore 12 pps. The use of the DALI2
gamma-ray array is also envisaged to detect the decay out of the pygmy, populated
at about 10MeV via a Coulex on a 197Au secondary target.
References
[1] D. Savran, T. Aumann, A. Zilges, Progr. in Part. and Nuc. Phys. 70 (2013) 210-245.
[2] N. Paar, D. Vretenar, E. Khan, G. Col`, Rep. Progr. Phys. 70 (2007) 691.
[3] S. Pascu, J. Endres, N.V. Zamfir, A. Zilges, Phys. Rev. C 85 (2012) 064315.
[4] N. Paar, D. Vretenar, P. Ring, Phys. Rev. Lett. 94 (2005) 182501.
