Search for the Pygmy Dipole Resonance in 64 Fe

1. Search for the Pygmy Dipole Resonance in 64Fe Riccardo Avigo

2. Outlines Introduction 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. Neutron Skins PygmyResonance 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, 041301(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) 012143] 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 436 1998. 10–18] Neutron stars ? 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, 092502 (2009)] The stregth related to PDR, extarpolated by this experiment was ̴ 5% [PRL 102, 092502 (2009)] The neutron skin thickness obtained ΔR = 0.200 ± 0.015 fm [A.Carbone PRC 81, 041301(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, 092502 (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? 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 densityρ (sum of the neutron and proton densitiesρn andρp) and by a local asymmetryδ ≡ (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, 041301(R) (2010)] [FurnsthalNPA 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, 062802(R)]