Upload
others
View
0
Download
0
Embed Size (px)
Citation preview
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
The PDR in particle- coincidence experiments
Introduction
The particle- coincidence method
Systematic study in (,) and (,)
First results 140Ce(p,p)
Summary
Outlook
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
HPGe-detector array
beam
BBS
20 cm
Photo by S.G.Pickstone
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
12
2,4,31
PDR region
Sn
Excitation spectrum in coincidence
94Mo (,)
Typical energy resolution: ≈ 300 keV
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
The - coincidence matrix
EX E-E [MeV]
Inte
nsity
48Ca(,)
E[M
eV
]
Excited states
Detector response
Decay branching
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
The - coincidence matrix
EX E-E [MeV]
Inte
nsity
48Ca(,)
Energy spectra through projection
Selecting transitions by setting gates
E[M
eV
]
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Selecting transitions – Projected spectra
Gate on EX≈E +1Gate on EX≈E+E2
12
10
48Ca(,)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
- angular correlation from DWBA
spin sequence BBS @ 3.5°
beam
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
PDR in 140Ce
D. Savran et al.,
Phys. Rev. Lett. 97 (2006) 172502
Splitting:
low-energy part
(,) and (,)
high-energy part
(,) only
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
Splitting is a common feature of the low-lying dipole
response in semi-magic heavy neutron-rich nuclei
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
(,) 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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
Splitting is a common feature of the low-lying dipole
response in semi-magic heavy neutron-rich nuclei
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
J=1- states in (,) and (,)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Strongest state in (,) at 7.3 MeV is missing in (,)
J=1- states in (,) and (,)
?
d/d < 0.15 mb/sr
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Strongest state in (,) at 7.3 MeV is missing in (,)
Strongest state in (,) at 7.6 MeV is weak in (,)
J=1- states in (,) and (,)
?
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
9 dipole excitations were observed
The excited dipole states do have negative parity
Parity assignments in 48Ca
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
performed at KVI
beam energy: 80 MeV
central BBS angle: 6°
8 HPGe detectors
target enrichment: 99.72 %
First results 140Ce(p,p)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Excitation spectrum in coincidence
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
spectrum with gate on Ex=E
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Comparison of the probes
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Comparison of the probes
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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)
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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%
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
Combining and particle spectroscopy
various probes (p, d, , …)
particle- coincidence
Outlook: &HORUS in Cologne
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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
Outlook: &HORUS in Cologne
V. Derya, University of Cologne, AG Zilges The PDR in particle- coincidence experiments
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: