Search for the Pygmy Dipole Resonance in

64

Fe

Riccardo Avigo

Outlines Introduction

Resonances in nuclei Pygmy Dipole Resonance

The experimental tecnique for PDR measurement PDR in

64

Fe measurement

Previous measurements in the same mass region Experimental setup Aims of my activities

Perspectives

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

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

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 Skins

Pygmy Resonance

Neutron stars ?

EOS

PROJECTILE

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 PROJECTILE NEUTRON 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

Pygmy in

68

Ni

An experiment was performed in GSI with RISING setup to study PDR in 68 Ni 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

68 Ni 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)]

Pygmy in

64

Fe

It is important to have more measurements in the mass region of 68 Ni to fix the models describing PDR 64

Fe

68

Ni

Teoretical calculations show where searching PDR in 64 Fe

1n

Measurement of Pygmy in

64

Fe

Experimental procedure to induce PDR

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

TRACKING and TIME OF FLIGHT DETECTORS

Measurement of Pygmy in

64

Fe

Experimental procedure to measure PDR γ decay

ARRAY OF E-ΔE TELESCOPES 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 64 Fe (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) Gammas are emitted by a source ( 64 Fe) in flight  (v/c) and direction of the nuclei were measured (tracking Si-detectors) to apply Doppler correction γ RAYS DETECTOR

Aims of my activities

The aim of my research plan is the measurement of PDR γ decay to ground state in 64 Fe. 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 64 Fe nuclei, colliding on the target

64 Fe A/Q

The selection of correct nuclei coming out by the target is essential

ΔE 64 Fe E 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

Aims of my activities

γ decay of PDR was measured with scintillators (LaBr3:Ce) and AGATA, an array of HPGe segmented detectors target Reconstruction of γ direction is important for doppler correction It is also important to be able to clean the spectra by background radiation.

γ rays 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 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

What’s next?

PDR in 70,72 Ni mesurement was approved in RIKEN laboratories. In these nuclei the neutron skin is expected to be thicker than in 68 Ni and 64 Fe due to the more excess of neutrons. 70

Ni

72

Ni

68

Ni

64

Fe

Thanks for the attention !

nuclear and astrophysiscal features connected to PDR

The energy per particle in a nuclear system characterized by a total density and proton densities

ρ n

and

ρ p

) and by a local asymmetry

δ

≡ (

ρ n

−

ρ p

)

/ρ ρ

(sum of the neutron 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 [A.Carbone PRC 81, 041301(R) (2010)] 3 ρ 0 L (MeV/fm 3 ) [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, 062802(R)]