Search for the Pygmy Dipole Resonance in 64Fe

Riccardo Avigo

Outlines

Introduction

Resonances in nucleiPygmy Dipole Resonance

The experimental tecnique for PDR measurement

PDR in 64Fe measurement

Perspectives

Previous measurements in the same mass regionExperimental setupAims of my activities

Resonances in nuclei

Collective motion of nucleons in nucleus

Perturbation Force:External nuclear interactionExternal coulomb interaction

Restoration Force:Internal nuclear interaction

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

Resonance properties have connections with important features of nuclear structure (compressibility, nuclear deformations, isospin mixing …)

Nucleus can be seen like an elastic system

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?

Neutron Skins Neutron stars

Pygmy Resonance

EOS?

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]

Experimental technique to induce PDR in nuclei

PDR induced by virtual photon scattering (coulomb excitation)

TARGET

PROJECTILE

TARGET

PROJECTILE

VIRTUAL PHOTONS

TARGET

PROJECTILE

TARGET

PROJECTILE

γ RAY

NEUTRON

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

In our case we measure gamma decay of the collective state

γ RAYS DETECTOR

Pygmy in 68Ni

An experiment was performed in GSI with RISING setup to study PDR in 68NiGood agreement with previsions on photoabsorption cross section was achived

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)]

Comparison of experimetal data and teorethical model[PRL 102, 092502 (2009)]

The stregth related to PDR, extarpolated by this experiment was 0 5% [PRL 102, 092502 (2009)]

The neutron skin thickness obtained ΔR = 0.200 ± 0.015 fm

[A.Carbone PRC 81, 041301(R) (2010)]

Pygmy in 64Fe

64Fe

68Ni

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

1n

Teoretical calculations show where searching PDR in 64Fe

Measurement of Pygmy in 64Fe

An experiment at GSI laboratories was performed to measure PDR in 64Fe

64Fe was produced by fragmentation of a 86Kr beam

Coulomb excitation of 64Fe was performed making 64Fe nuclei colliding on a 208Pb target

A magnetic separator was used to select 64Fe between all the fragments produced by fragmentation of 86Kr

Magnetic Dipole

Scheme of a magnetic fragment separator

Experimental procedure to induce PDR

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)

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

γ ray decay of 64Fe was measured with scintillators (LaBr3:Ce) and semiconductor detectors (HPGe)

Measurement of Pygmy in 64Fe

Experimental procedure to measure PDR γ decay

γ RAYS DETECTOR

TRACKING and TIME OF FLIGHT DETECTORS

ARRAY OF E-ΔE TELESCOPES

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

After calibration it is possible to have a good selection of 64Fe nuclei, colliding on the target

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

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

A/Q

Z

E

ΔEE [keV]

64Fe

64Fe

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 correctionIt 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

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

γ raystarget

What’s next?

64Fe

68Ni

70Ni 72Ni

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.

Thanks for the attention !

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

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

[A.Carbone PRC 81, 041301(R) (2010)] [Furnsthal NPA 706 (2002) 85–110]

[C. J. Horowitz, J. Piekarewicz PRC 64, 062802(R)]

3ρ0L (MeV/fm3)

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