1. Pygmy Dipole Resonance in 64Fe
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Pygmy Dipole Resonance in 64Fe
Riccardo Avigo

2. Outlines Introduction The experimental tecnique for PDR measurement
Resonances in nuclei
Pygmy Dipole Resonance
The experimental tecnique for PDR measurement
PDR in 64Fe measurement
Previous measurements in the same mass region
Experimental setup
Aims of my activities
Perspectives

3. Resonances in nuclei Collective motion of nucleons in nucleus
Nucleus can be seen like an elastic system
Perturbation Force:
External nuclear interaction
External coulomb interaction
Restoration Force:
Internal nuclear interaction
Resonance properties have connections with important features of nuclear structure (compressibility, nuclear deformations, isospin mixing …)
Most famous example: Giant Dipole Resonance
Perturbation: coulomb excitation
Restoration Force: Nuclear interaction between neutrons and protons
Collective Motion: an antiphase oscillation of protons against neutrons

4. Pygmy Dipole Resonance
We can describe neutron rich nuclei as a N=Z core and a neutron skin.
The oscillation of the neutron skin against the core is called Pygmy Dipole Resonance (PDR)
PDR can be induced in a nucleus by an E1 coulomb excitation (like GDR)
The name pygmy is due to the lower stregth in respect GDR

5. Why studying PDR in neutron rich nuclei?
The study of the pygmy strength is expected to provide information on the neutron skin and symmetry energy of the equation of state .[A.Carbone PRC 81, (R) (2010)]
Information about neutron skin and symmetry energy is extremely relevant for the modelling of neutron stars: in particular the radius of neutron star is related to the symmetry energy [J. Piekarewicz Jour. of Phys. 420 (2013) ]
The existance of pygmy resonance could have an important role in nucleosynthesis by R-process: the strength of pygmy resonances could be able to change (n,γ) reaction rate [Goriely Phys. Let. B –18]
Neutron Skins
Neutron stars
Pygmy Resonance
EOS ?

6. Experimental technique to induce PDR in nuclei
PDR induced by virtual photon scattering (coulomb excitation)
In our case we measure gamma decay of the collective state
TARGET
TARGET
TARGET
TARGET
γ RAYS DETECTOR
VIRTUAL PHOTONS
PROJECTILE
NEUTRON
PROJECTILE
PROJECTILE
PROJECTILE
γ RAY
Nucleus of interest colliding on a Target
Coulomb interaction with the target
PDR induced in nucleus of interest
PDR decay by emission of gammas and netrons

7. Pygmy in 68Ni
An experiment was performed in GSI with RISING setup to study PDR in 68Ni
Good agreement with previsions on photoabsorption cross section was achived
Comparison of experimetal data and teorethical model
[PRL 102, (2009)]
The stregth related to PDR, extarpolated by this experiment was ̴ 5%
[PRL 102, (2009)]
The neutron skin thickness obtained
ΔR = ± fm
[A.Carbone PRC 81, (R) (2010)]
Upper panel-68Ni photoabsorption cross section (total black, virtual photon method blue, virtual photon method taking in account branching ratio red)
Bottom panel – comparison of photoabsortion cross section (including response function) and experimental data [PRL 102, (2009)]

8. Pygmy in 64Fe
It is important to have more measurements in the mass region of 68Ni to fix the models describing PDR
Teoretical calculations show where searching PDR in 64Fe
68Ni
64Fe 1n

9. Measurement of Pygmy in 64Fe
Experimental procedure to induce PDR
An experiment at GSI laboratories was performed to measure PDR in 64Fe
64Fe was produced by fragmentation of a 86Kr beam
Magnetic Dipole
A magnetic separator was used to select 64Fe between all the fragments produced by fragmentation of 86Kr
Scheme of a magnetic fragment separator
Coulomb excitation of 64Fe was performed making 64Fe nuclei colliding on a 208Pb target

10. Measurement of Pygmy in 64Fe
Experimental procedure to measure PDR γ decay
γ ray decay of 64Fe was measured with scintillators (LaBr3:Ce) and semiconductor detectors (HPGe)
It is important to be sure that gammas detected are related to coulomb excitation of 64Fe (and not other reactions such as fragmentation, fission..).
For this reason it is important to identify the nuclei outcoming from the target: A and Z measurement (E-ΔE telescopes)
ARRAY OF E-ΔE TELESCOPES
Gammas are emitted by a source (64Fe) in flight
 (v/c) and direction of the nuclei were measured (tracking Si-detectors) to apply Doppler correction
TRACKING and TIME OF FLIGHT DETECTORS
γ RAYS DETECTOR

11. Aims of my activities
The aim of my research plan is the measurement of PDR γ decay to ground state in 64Fe. In particular this could allow to have an experimental evaluation of the strength related to it.
The first step is the calibration of detectors involved in the experimental setup Z
After calibration it is possible to have a good selection of 64Fe nuclei, colliding on the target
64Fe
A/Q ΔE
E [keV]
The evaluation of β and direction of nuclei is important for doppler correction but also to insert correct gates to have energy γ spectra as cleanest as possible
The selection of correct nuclei coming out by the target is essential
64Fe E

12. Aims of my activities
γ decay of PDR was measured with scintillators (LaBr3:Ce) and AGATA, an array of HPGe segmented detectors
Reconstruction of γ direction is important for doppler correction
It is also important to be able to clean the spectra by background radiation.
Segmented detectors allow a good recontrution of γ ray direction
The main difficulty to suppres background is due to the fact that γ rays don’t release energy in a continuous way
target
γ rays
An algorithm to correlate correctly interaction points with the correct γ ray to suppress background is avaible with AGATA
This tracking algorithm needs improovements to have good performances at high energies (>15 MeV). A significant effort is needed to achive the aim of studying PDR γ ray spectra

13. What’s next? Thanks for the attention ! 70Ni 72Ni 68Ni 64Fe
PDR in 70,72Ni mesurement was approved in RIKEN laboratories. In these nuclei the neutron skin is expected to be thicker than in 68Ni and 64Fe due to the more excess of neutrons.
70Ni
72Ni
68Ni
64Fe
Thanks for the attention !

14. nuclear and astrophysiscal features connected to PDR
The energy per particle in a nuclear system characterized by a total density ρ (sum of the neutron and proton densities ρn and ρp) and by a local asymmetry δ ≡ (ρn − ρp)/ρ
S(ρ) is the symmetry energy and its slope can be written as
It was shown not only that PDR strength is related to L parameter but also that a connection exists between L and neutron skin thickness
Strength [%]
3ρ0L (MeV/fm3)
[A.Carbone PRC 81, (R) (2010)]
[Furnsthal NPA 706 (2002) 85–110]
Moreover nuclear structure parameters can be fixed by the netron skin radius: this has consequences not only on structure of nuclei but also on netron stars radii
[C. J. Horowitz, J. Piekarewicz PRC 64, (R)]