probing the character of the pygmy dipole...

V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments

Investigation of the Pygmy Dipole Resonance

in particle- coincidence experiments

V. Derya1*, J. Endres1, M. N. Harakeh2,3, D. Savran4,5,

M. Spieker1*, H. J. Wörtche2, and A. Zilges1

1Institute for Nuclear Physics, University of Cologne, Germany 2KVI, Rijksuniversiteit Groningen, The Netherlands

3GANIL, CEA/DSM-CNRS/IN2P3, Caen, France4ExtreMe Matter Institute EMMI and Research Division, GSI, Darmstadt, Germany

5Frankfurt Institute for Advanced Studies FIAS, Frankfurt a.M., Germany

4th Workshop on Nuclear Level Density and Gamma Strength

Oslo, May 27 – 31, 2013

Supported by the DFG (ZI 510/4-2 and SFB 634), by the EU under EURONS Contract No. RII3-CT-2004-506065

in the 6th framework programme, and by the Alliance Program of the Helmholtz Association (HA216/EMMI)

*Supported by the Bonn-Cologne Graduate School of Physics and Astronomy


The PDR in particle- coincidence experiments

Introduction

The particle- coincidence method

Systematic study in (,) and (,)

First results 140Ce(p,p)

Summary

Outlook


Introduction

Isovectorial electric Giant

Dipole Resonance

Low-lying E1 strength

2-Phonon (2+3-)1-

Pygmy Dipole

Resonance (PDR)

(,) (,Xn)A. Leprêtre et al., Nucl.

Phys. A258 (1976) 350

A. Zilges et al., Phys.

Lett. B 542 (2002) 43


Relevance of the PDR

Symmetry energy of the

equation of state (EOS)

neutron-skin thickness

E1 strength

Nucleosynthesis

neutron-capture rates

-ray strength function

photo-absorption cross section

S. G

orie

ly, Ph

ys. L

ett. B

43

6 (1

99

8) 1

0


PDR studied with (,) in N=82 isotones

Real-photon scattering for stable nuclei

below the particle thresholds

Strongly fragmented E1 strength

Accessible quantities:

transition energy E

multipole character

reduced transition strength B(E1)

fragmentation

integrated strength

A. Zilges et al., Phys. Lett. B 542 (2002) 43

S. Volz et al., Nucl. Phys. A779 (2006) 1

D. Savran et al., Phys. Rev. Lett 100 (2008) 232501


ProbeInteraction

(dominant)

Location of

interaction

Character of

interaction

(dominant)

photon Electromagnetic whole nucleus isovector

particle* hadronic surface isoscalar

proton* hadronic surfaceisoscalar and

isovector

Probes for scattering experiments

Insight into the structure of the (dipole) excitations

•at medium particle energies

(≈ 50-150 MeV)


The particle- coincidence method

Reaction: inelastic particle scattering

Coincident detection of particles and rays

Performed with particles at Kernfysisch

Versneller Instituut in Groningen, The Netherlands

beam at E = 136 MeV

– AGOR cyclotron

– Cyclotron frequency: 28 MHz

– Particle current: 0.4-1 pnA

particle

particle


HPGe-detector array

beam

BBS

20 cm

Photo by S.G.Pickstone


Big-Bite Spectrometer

QQD-type spectrometer

Two quadrupole magnets

One dipole magnet

Maximum solid angle: 9.2 msr

Ph

oto

by

S.G

. Pic

ksto

ne

50 cm


Big-Bite Spectrometer

EUROSUPERNOVA System

for particle detection

– Focal Plane Detector System

2 Vertical Drift Chambers

energy and particle

trajectories

– scintillator layer trigger

Ph

oto

by

S.G

. Pic

ksto

ne

50 cm


12

2,4,31

PDR region

Sn

Excitation spectrum in coincidence

94Mo (,)

Typical energy resolution: ≈ 300 keV


The - coincidence matrix

EX E-E [MeV]

Inte

nsity

48Ca(,)

E[M

eV

]

Excited states

Detector response

Decay branching


The - coincidence matrix

EX E-E [MeV]

Inte

nsity

48Ca(,)

Energy spectra through projection

Selecting transitions by setting gates

E[M

eV

]


Selecting transitions – Projected spectra

Gate on EX≈E +1Gate on EX≈E+E2

12

10

48Ca(,)


- angular correlation from DWBA

spin sequence BBS @ 3.5°

beam


Experimental method: (,’)

Selective excitation

– Isoscalar probe

– Mainly low spin from ground state

– Natural parities

Powerful data analysis

– HPGe detectors with high energy resolution

– Selection of transitions

– Cross sections

– Branching ratios

– Angular distributions Spin assignments


PDR in 140Ce

D. Savran et al.,

Phys. Rev. Lett. 97 (2006) 172502

Splitting:

low-energy part

(,) and (,)

high-energy part

(,) only


Systematic study in (,) and (,) experiments

J. Endres et al.,

Phys. Rev. C 80 (2009) 034302

D. Savran et al.,

Phys. Rev. Lett. 97 (2006) 172502

J. Endres, E. Litvinova et al.,

Phys. Rev. Lett. 105 (2010) 212503

Neutron magic (N=82) isotones 140Ce and 138Ba

Proton magic (Z=50) isotope 124Sn



J. Endres et al.,

Phys. Rev. C 80 (2009) 034302

D. Savran et al.,

Phys. Rev. Lett. 97 (2006) 172502


Phys. Rev. Lett. 105 (2010) 212503



Splitting is a common feature of the low-lying dipole

response in semi-magic heavy neutron-rich nuclei


Transition densities for two RQTBA states in 124Sn

Low-lying state: Typical PDR state

High-lying state: Transitional region towards the GDR

Interpretation of the splitting

J. Endres et al., Phys. Rev. C 85 (2012) 064331

- In phase

- Large neutron

contribution at the

surface

- Slightly out of phase

- Enhanced proton

contribution

Similar conclusions in N. Tsoneva et al., Phys. Rev. C 77 (2008) 024321


(,) and (,) in 94Mo

C. Romig,

private communication

Non-magic (N=52, Z=42) isotope 94Mo

Near to (sub) shell closure

V. Derya et al.,

Nucl. Phys. A906 (2013) 94



J. Endres et al.,

Phys. Rev. C 80 (2009) 034302

D. Savran et al.,

Phys. Rev. Lett. 97 (2006) 172502


Phys. Rev. Lett. 105 (2010) 212503



Splitting is a common feature of the low-lying dipole

response in semi-magic heavy neutron-rich nuclei


Light-mass nuclei: halo nuclei, single-particle

character excitations

Medium-mass nuclei: development of a more

collective electric-dipole excitation mode?

Dependence on N/Z ratio in the calcium chain

Low-lying dipole strength in lighter nuclei


Low-lying dipole strength in lighter nuclei

T.D. Poelhekken et al., Phys. Lett. B 278 (1992) 423

T. Hartmann et al., Phys. Rev. C 65 (2002) 034301

V. Derya et al., to be published

T. Hartmann et al., Phys. Rev. C 65 (2002) 034301


J=1- states in (,) and (,)


Strongest state in (,) at 7.3 MeV is missing in (,)


?

d/d < 0.15 mb/sr


Strongest state in (,) at 7.3 MeV is missing in (,)

Strongest state in (,) at 7.6 MeV is weak in (,)


?


High Intensity -Ray Source

(HIS) at the Duke Free Electron

Laser Laboratory (DFELL)

100% linear polarized and nearly

mono-energetic intense -ray

beam (I ~ 107 photons/sec)

6 HPGe detectors:

– One for beam monitoring

– One at backward angle

– Four at = 90° in the horizontal and

vertical plane Parity assignment

Target: 48Ca

– Amount: 1 g

(≈$250,000)

Parity Measurement at HIS

direction of polarization 2 c

m


The State at 7.298 MeV

-ray beam energy of 7.3 MeV

Measured for 1.5 h

Experimental asymmetry:

SE

DE

spectrum of the vertical detectors spectrum of the horizontal detectors


9 dipole excitations were observed

The excited dipole states do have negative parity

Parity assignments in 48Ca


QRPA calculations:

strong IS LED states present in all even-even

Ca isotopes (N=14-40)P. Papakonstantinou et al. PLB 709 (2012) 270

Character changes

proton-skin oscillation

pure IS oscillation

neutron-skin oscillation

Experimental results on 48Ca:

– separated strong IS LED state at 7.6 MeV

– weak in IV channel pure IS oscillation

character changes at N > 28

Comparison with theoretical results for 48Ca

Nat N=30

(Gogny interaction)


ProbeInteraction

(dominant)

Location of

interaction

Character of

interaction

(dominant)

photon Electromagnetic whole nucleus isovector

particle* hadronic surface isoscalar

proton* hadronic surfaceisoscalar and

isovector

Probes for scattering experiments

Insight into the structure of the (dipole) excitations

•at medium particle energies

(≈ 50-150 MeV)


performed at KVI

beam energy: 80 MeV

central BBS angle: 6°

8 HPGe detectors

target enrichment: 99.72 %

First results 140Ce(p,p)


Excitation spectrum in coincidence


spectrum with gate on Ex=E


Comparison of the probes


Systematic study of E1 excitations in (,) and

(,) experiments in stable and spherical nuclei

– A = 48-140

– N/Z = 1.23-1.48

– semi-, doubly-, and non-magic

(,) method is appropriate tool for identifying

the contribution of the PDR to the total E1

strength and strength function

First results of 140Ce(p,p) show qualitatively

different behavior for the proton probe

Summary


Outlook: iThemba LABS

K600 spectrometer at 0°

Continuation of particle- coincidence experiments

at medium particle (p, , …) energies

First feasibility test in December 2012

– Performed by R. Neveling et al.

– beam of 160 MeV energy

– 2 Clover detectors, 1 big NaI detector

R. Neveling et al., Progress Report: PR194 (2013)


Outlook: &HORUS in Cologne

HORUS: spectrometer

14 HPGe detectors

6 BGO shields

Photopeak efficiencywith SONIC: 2% at 1332 keV

Energy resolutionwith digital DAQ: ≤2.5 keV at 1332 keV, 6 kcps

SONIC: particle spectrometer

Silicon Identification Chamber

8 E-E detector tubes

Solid angle coverage ≈ 4%


Combining and particle spectroscopy

various probes (p, d, , …)

particle- coincidence



Combining and particle spectroscopy

various probes (p, d, , …)

particle- coincidence

Excitation-energy spectrum from

a very recent experiment:92Mo(p,p‘) at 10.5 MeV

Si detector at 131°

E = 79 keV @ 10.5 MeV



University of Cologne (Cologne, Germany)

V. Derya, J. Endres, A. Hennig, J. Mayer, L. Netterdon,

S. G. Pickstone, P. Scholz, M. Spieker, T.-M. Streit,

M. Weinert, and A. Zilges

Kernfysisch VersnelIer Instituut (Groningen, The Netherlands)

S. Bagchi, M. N. Harakeh, N. Kalantar, A. Najafi, C. Rigollet,

and H. J. Wörtche

ExtreMe Matter Institute (Darmstadt, Germany)

E. Fiori, J. Isaak, B. Löher, D. Savran, and J. Silva

TU Darmstadt (Darmstadt, Germany)

N. Pietralla, C. Romig, and M. Scheck

Supported by:

probing the character of the pygmy dipole...

Documents