Transcript Diapositiva 1 - Istituto Nazionale di Fisica Nucleare

The gamma decay of the GDR at finite temperature
and of the pygmy resonance far from stability
G. Benzoni
INFN sezione di Milano (Italy)
Outline
1- Pygmy Dipole Resonance in 68Ni
• background
• experiment
• results
• perspectives
2. Preequilibrium Dipole Resonance
• Background
• Experiment G.AR.F.I.E.L.D.
• Results
• perspectives
Pygmy Dipole Resonance
Giant Dipole Resonance
30
N
Photoabsorption
cross section (a.u.)
Average Transition charge
densities
Collective oscillation of neutrons against protons
P
25
20
15
10
5
0
0
20
Gamma Energy [MeV]
Pygmy Dipole Resonance
Collective oscillation of neutron skin against the core
N
Photoabsorption
cross section (a.u.)
Average Transition charge
densities
16
14
12
10
8
6
4
2
0
0
20
Gamma Energy [MeV]
Physics Case:
How dipole response changes moving towards neutron rich nuclei ???
Dipole strength shifts at low energy
T. Hartmann PRL85(2000)274
48Ca
40Ca
0.29% EWSR
0.025% EWSR
Collective or non-collective
nature of the transitions?
How to excite this mode??
Stable nuclei  photoabsorption
132Sn
124Sn
4% EWSR
Exotic nuclei
Virtual photon breakup
LAND experiment
Virtual photon scattering
Adrich et al. PRL 95(2005)132501
RISING experiment
Two experiments performed:
• 400 MeV/u 68Ni (2004) + 197Au
• 600 MeV/u 68Ni (2005) + 197Au
Beam energy selected in
order to populate DIPOLE
modes more effectively
than other ones
Higher statistics dataset
68Ni
beam produced by
fragmentation of 86Kr @
900 MeV/u on thick Be
target (4g/cm2):
• 1010 ppspill 86Kr
•Spill length 6s, period 10 s
 (GDR)   (2 )
T.Aumann et al EPJ 26(2005)441
High resolution g-spectroscopy at the FRS
FRS provides secondary radioactive ion beams:
• fragmentation and fission of primary beams
• high secondary beam energies: 100 – 700 MeV/u
• fully stripped ions
FRS
2g/cm2 Au
RISING
RISING ARRAY
Euroball 15 Clusters
Located at 16.5°, 33°, 36° degrees
Energetic threshold ~ 100 keV
Hector 8 BaF2
Located at 142° and 90° degrees
Energetic threshold ~ 1.5 MeV
Miniball 7 HPGe segmented detectors
Located at 46°, 60°, 80°, 90° degrees
Energetic threshold ~ 100 keV
Beam identification and tracking detectors
Before and after the target
Calorimeter
Telescope
for beam identification
CATE
4 CsI
9 Si
Coulomb excitation of 68Ni @ 600 AMeV
Outgoing 68Ni
AoQ
Incoming 68Ni beam
E (CsI)
68Ni
1.2 %
4.4 %
DE (Si)
Z
40.00%
~ 6 Days of effective beam time
~ 400 GB of data recorded
~ 35%
35.00%
30.00%
~ 3.107 ‘ good
25.00%
68Ni
events ‘
20.00%
15.00%
Incoming+Outgoing 68Ni
10.00%
5.00%
0.00%
66Co
67Ni
68Ni
69Ni
70Cu
Coulomb excitation of
68Ni
(600 MeV A)
Pygmy Dipole Resonance
Baf2 Hector
60
60
25
68
Ni
60
25
25
20
20
20
50
50
50
15
15
15
10
40
10
40
10
40
5
5
0
5.0
7.5
10.0
12.5
30
20
20
10
10
0
5.0
7.5
10.0
12.5
Counts
30
Counts
Counts
5
30
8
10
12
Energy [MeV]
7.5
10.0
12.5
20
68
68
10
Ni
HPGe-Cluster
0
0
5.0
Ni
HPGe MiniBall
14
0
6
8
10 12
Energy [MeV]
14
0
6
8
10
12
Energy [MeV]
GEANT Simulations
A structure appears at 10-11 MeV
in all detectors
14
15.0
Coulomb excitation of
67
(600 MeV A)
The peak structure is roughly
2 MeV lower than in 68Ni
HPGe Cluster
40
67Ni
25
Ni
20
There is indication from a
more fragmented structure
15
30
10
Counts
5
20
0
5.0
7.5
10.0
12.5
10
In all cases the measured
width is consistent with that
extracted from GEANT
simulations with a
monochromatic g source
Resonance width G < 1 MeV
0
4
6
8
10
Energy [MeV]
12
25
RPA
16
14
20
~10
MeV
12
10
mb
15
RMF
68Ni
~10 MeV
68Ni
8
6
10
4
5
2
0
0
5.0
7.5
10.0
0
12.5
Energy [MeV]
5 10 15 20 25 30 35 40
Energy (MeV)
G. Colo’ private communication
D. Vretnar et al. NPA 692(2001)496
Both RPA and RMF predict for 68Ni Pygmy strength at approximately
10 MeV for 68Ni. The degree of collectivity is still debated
Predictions are available only for
68Ni
In the case of 67Ni as it is a vibration of the neutron skin
the value of the neutron binding energy is important. As a
simple rule the localization in energy of the strength should
be linearly correlated to the neutron binding energy
Eb (68Ni)  7.8 MeV
Eb (67Ni)  5.8 MeV
Conclusions
 Measurement of high energy g-rays from Coulex of 68Ni at 600
MeV/u.
 First experiment of this type ever performed
 Strength at 10.5 MeV has been observed in all three kind
of detectors
 Peaks line-shape is consistent with GEANT simulations
(GPDR < 1 MeV)
 Low Energy Dipole strength has also been observed in 67Ni
and 69Ni
RISING Collaboration
A.Bracco, G. Benzoni, N. Blasi, S.Brambilla, F. Camera, F.Crespi, S. Leoni,
B. Million, M. Pignanelli, O. Wieland, P.F.Bortignon, G.Colo’
University of Milano, and INFN section of Milano, Italy
A.Maj, P.Bednarczyk, J.Greboz, M. Kmiecik, W. Meczynski, J. Styczen
Niewodnicaznski institute of Nuclear Physics, Kracow, Poland
T. Aumann, A.Banu, T.Beck, F.Becker, A.Burger, L.Cacieras, P.Doornenbal, H. Emling, J. Gerl, M.Gorska,
J.Grebozs, O.Kavatsyuk, M.Kavatsyuk, I. Kojouharov, N. Kurtz, R.Lozeva, N.Saito, T.Saito, H.Shaffner,
H. Wollersheim
and FRS collaboration
GSI
J.Jolie, P. Reiter, N.Ward
University of Koeln, Germany
G. de Angelis, A. Gadea, D. Napoli,
National Laboratory of Legnaro, INFN, Italy
S. Lenzi, F. Della Vedova, E. Farnea, S. Lunardi,
University of Padova and INFN section of Padova, Italy
D.Balabanski, G. Lo Bianco, C. Petrache, A.Saltarelli,
University of Camerino, Italy
M. Castoldi and A. Zucchiatti,
University of Genova, Italy
G. La Rana,
University of Napoli, Italy
J.Walker,
University of Surrey
Preequilibrium Dipole Resonance
The Preequilibrium Dipole Response
Charge NOT equilibrated
t=0 fm/c
CN
fusion
Dynamic
Dipole
Charge equilibration in N/Z
asymmetric heavy-ion collisions
All degree of freedom
EQUILIBRATED
GDR
A rapid re-arrangement of
charge accompanied by emission
of dipole radiation
The intensity of dipole emission
depends on the absolute value of
the charge that must be shifted
to restore the mass balance
(Dipole moment)
D(t=0)=18.2 fm
g
100Mo
32S
N
+
N/Z=1
N/Z=1.38
36S
96Mo
g
132Ce
g
Z
N/Z=1.28
g
132Ce
N
g
g
Z
R p  Rt
NZ
N N
D(t  0) 
( RZ (t  0)  RN (t  0)) 
Z p Zt     g 
N/Z=1.29
A
A
N/Z=1.25
 Z  p  Z N/Z=1.28
t
+
D(t=0)=1.7 fm
o information on the charge equilibration
in relation to the reaction mechanism;
o information on the damping of the
dipole mode;
o information on the symmetry energy of
the nuclear matter at lower densities
than the saturation one
g
THEORY:
Preequilibrium dipole radiation present in all N/Z asymmetric
reactions;
Dependence on charge asymmetry
mass asymmetry and
incident energy.
O+Mo @ 4-8-14-20 MeV/u
V. Baran et al . PRL87(2001)182501
How to distinguish preequilibrium from thermalized emission?
reaction
40Ca
36S
+
+
100Mo
40Ca
36S
104Pd
+
100Mo
+ 104Pd
CN
Proj N/Z
Target N/Z
CN N/Z
140Sm*
1
1.38
1.258
140Sm*
1.25
1.26
1.258
Compare the high energy gamma-ray emission from a
N/Z SYMMETRIC and a N/Z ASYMMETRIC reaction.
Evidence of a prompt dipole emission: EXCESS of the
high energy gamma-ray yield in the asymmetric
reaction
Linearized spectra
40Ca
+
36S
100Mo
+
104Pd
Flibotte et al. PRL77(1996)1448
•The intensity is never higher
than 25% of the total yield with
stable beams
• The measurements need to be
extremely clean and exclusive
Garfield + Hector @ Laboratori
Nazionali di Legnaro
• g + LCP + residues
• comparison btw. N/Z Asymmetric
and Symmetric reactions leading to
same CN at same E* and I
• two different bombarding
energies
 8.1 AMeV and 15.6 AMEV
Garfield
BaF2
PPAC
Reaction studied to populate
N
  1
 Z t
N
 16   1.32
from ZO pinduced reaction
10
3
10
2
10
1
10
0
4
64Ni+68Zn
statistical
Model
10
E*
(MeV)
D (fm)
16O+116Sn
8.1
100
8.6
64Ni+68Zn
4.7
100
1.2
200
8.6
200
1.2
16O+116Sn
15.6
will be discussed in a
64Ni+68Zn
7.8
64Ni+68Zn
statistical
Model
3
10
2
10
1
10
0
10
*
*
E =200MeV
E =150MeV
5
5
10 15 20 25 30
64Ni+68 Zn
0,12
4
10
3
10
2
10
1
10
0
5
68
Ni+ Zn
statistical
Model
64Ni+68Zn
statistical
Model
*
E =
100MeV
10 15 20 25 30
Eg [MeV]
0,20
64
64
68
Ni+ Zn
statistical
Model
O.Wieland et
0,10al. PRL 97 (2006) 012501
statistical
Model
ield [a.u.]
0,08
10 15 20 25 30
10
Eg [MeV]
Eg [MeV]
ield [a.u.]
Ebeam
(MeV/u)
Yield [a.u.]
4
0,08
CN
reaction
ield [a.u.]
10
Yield [a.u.]
Yield [a.u.]
following paper [ref]
132Ce*
0,15
The analysis of the N/Z
symmetric reaction lead
to important results on
Temperature dependence
of different
contributions to GDR
width
Counts [a.u.]
16O+116Sn
@ 250 MeV
Large pre-equilibrium
component
Only statistical decay
64Ni+68Zn
@ 500 MeV
Ea [MeV]
Comparison with statistical model
using the GDR parameters:
EGDR= 14 MeV
GGDR= 7.3 (8 MeV/u)
12.9 (15 MeV/u)
Determination of the
preequilibrium
component thanks to the
measurement of the
particles
reaction
Ebeam
(MeV/u)
E*
(MeV)
ECN
(MeV)
16O+116Sn
8.1
100
95
16O+116Sn
15.6
200
165
S. Barlini et al., to be published in PRC
Comparison with statistical model calculations to extract extra yield
1,0E-01
1,0E-01
16O+116Sn@8
1,0E-02
AMeV
16O+116Sn@15
1,0E-02
1,0E-03
AMeV
mg
mg
1,0E-03
1,0E-04
1,0E-04
1,0E-05
1,0E-05
1,0E-06
1,0E-06
5
10
15
20
25
5
E (MeV)
Extra gamma multiplicity
2,0E-04
10
15
20
25
E (MeV)
@ 15 AMeV
1,5E-04
mg
16O+116Sn
•Preequilibrium emission is
enhanced at high beam energy
•Same centroid as the GDR
1,0E-04
5,0E-05
16O+116Sn
@ 8 AMeV
0,0E+00
5
10
15
20
E (MeV)
25
30
Simulation of reaction dynamics allows the evaluation of the
DIPOLE MOMENT during the preequilibrium phase
(dinuclear system before CN equilibration)
Excitation of collective
dipole mode ~85 fm/c
16O+116Sn
8 MeV/u
b=0 fm
CN formation
(equilibration)
Bremsstrahlung formula
gives directly the g emission
of the dynamical dipole
M. Di Toro, V. Baran, C. Rizzo
dP
2e2
2



D ( )
3
dEg 3c Eg
V.Baran et al., PRL87 (2001) 182501-1
FGDR(Eg) (arb. units)
6 MeV/nucleon
(E*=117 MeV)
[1]
S + Mo
Eg (MeV)
16 MeV/nucleon
(E*=304 MeV)
9 MeV/nucleon
(E*=173.5 MeV)
[2]
S + Mo
Eg (MeV)
[3]
Ar + Zr
Eg (MeV)
D. Pierroutsakou . et al., PRC 71 (2005) 054605
16O+116Sn
2,0E-04
15 AMeV
1,0E-03
BNVdip
8,0E-04
1,0E-04
exp. data
6,0E-04
mg
mg
1,5E-04
5,0E-05
8 AMeV
4,0E-04
2,0E-04
0,0E+00
5
10
15
20
25
30
0,0E+00
E (MeV)
0
2,0E-04
Centroid energy different from exp.
5
10
E (MeV/u)
mg
1,5E-04
1,0E-04
15 MeV/u
5,0E-05
Yield increasing with energy
even if absolute value is not
well reproduced
8 MeV/u
0,0E+00
0
5
10
15
20
Eg (MeV)
25
30
15
20
Perspectives
Stable beams:
measure angular distributions
Measure an asymmetric reaction WITHOUT dipole moment in entrance channel
Exotic beams
Increase further the N/Z asymmetry
Proj.
Energy
MeV/u
Ta
CN
E*
fusion
mb
Spin
D(T=0)
fm
D
(mass)
P.D.
Yield*
132Sn
4.9
40Ca
172Yb
98.22
855
77
38
0.18
14
77Ni
3.9
95Mo
172Yb
100
728
88
35
0.03
12
140Xe
5.4
32S
172Yb
100
1016
74.8
29
0.22
9
94Kr
4.2
78Se
172Yb
100
672
88
23
0.03
5
22Ne
6.4
150Nd
172Yb
100
1443
71.51
10
0.29
2
48Ca
5
124Sn
172Yb
99.8
857.8
88
5
0.14
1
The isotopes has been selected from the list of
page II-8 of spiral documentation
* C. Simenel et al PRL 86 (2001) 2971
Garfield + Hector collaboration
A.Corsi, O.Wieland, A.Bracco, F.Camera, S.Brambilla, G.Benzoni,
M.Casanova, F.Crespi, S.Leoni,
A.Giussani, P.Mason, B.Million, D.Montanari, A.Moroni, N.Blasi,
Dipartimento di Fisica, Universitá di Milano and I.N.F.N. Section of Milano,
Milano Italy
A.Maj, M.Kmiecik,M.Brekiesz, W.Meczynski, J.Styczen, M.Zieblinski,
K.Zuber
Niewodniczanski Institute of Nuclear Physics Krackow Poland
F.Gramegna, V.L.Kravchuk S.Barlini, A.Lanchais, P.F.Mastinu and
L.Vannucci
INFN, Laboratori Nazionali di Legnaro, Legnaro, Italy
G.Casini, M.Chiari, and A.Nannini
INFN, Sezione di Firenze, Firenze, Italy
A.Ordine
INFN sez di Napoli, Napoli
M. Di Toro, V. Baran, C. Rizzo
Conclusions
 Measurement of high energy g-rays from Coulex of 68Ni at 600 MeV/u.
 First experiment of this type ever performed
 Strength at 10.5 MeV has been observed in all three kind of detectors
 Peaks line-shape is consistent with GEANT simulations (GPDR < 1 MeV)
 Low Energy Dipole strength has also been observed in 67Ni and 69Ni
 Dynamical dipole mode measured in N/Z asymmetric reaction 16O+116Sn
 Complete treatment of preequilibrium particle emission
 Extra yield increasing with energy (8 MeV/u  15 MeV/u)
Accordance with simulation based on bremsstrahlung emission
G.AR.F.I.E.L.D.
Thank you
t=0 fm/c
t  10-22s
t  10-21 s
CN formation
GDR oscillation
If (N/Z)proj≠(N/Z)targ
the system displays
dipole oscillation due
to charge equilibration
NZ
D
RCM , Z  RCM , N
A
Simulation code developed by Baran et al. based on
BNV model applyed to 16O(@8MeV/u)+116Sn
This
oscillation:
-displays
collective
features like
GDR
-produces g
emission
Simulation code developed by Baran et al. based
16
116
on BNV model applyed to O(@8MeV/u)+ Sn
This g emission can be calculated with the
bremsstrahlung formula once known D:
2
2
dP
2e
''

D
(

)
3
dEg 3c Eg
The Dynamical Dipole Mode – Prompt dipole
REARRANGMENT of p and n to establish the charge to mass equilibrium
Density plots, projected on the reaction plane,
for central collisions
Simenel et al. PRL86(2001)2971
Pre-equilibrium
Time evolution of the dipole moment
parallel to the deformation axis for b=0.
Thermal Equilibrium
3.10-21 s
A rapid re-arrangement of charge accompanied by emission of dipole radiation
(timescale of the order of 10-21 s)
The intensity of dipole emission depends on the absolute value of the charge
that must be shifted to restore the mass balance
From Centroid energy E0 one can estimate the value of
• bsym vs nuclear density
• bsym vs neutron number
E0  bsym
From the FWHM extract information on the damping mechanism
• Time needed to damp the dipole oscillation
• Understand how nucleons move in the fusion process
Ganil October 2005
Virtual photon scattering technique first experiment with a relativistic beam
GDR - PYGMY Excitation
•
400 MeV/u
197Au
68Ni
(2004) +
•
600 MeV/u
197Au
68Ni
(2005) +
16
O 208Pb
 (GDR)   (2 )
T.Aumann et al EPJ 26(2005)441
Why Pygmy Resonance is important ?
Pygmy Resonance has an
important
impact on the r-process
nucleosynthesis
Nupecc long range plan 2004
Goriely et al PLB436(1998)10
Different mean field approaches give different predictions in terms of collectivity,
strength and line-shape of the pygmy resonance
Isovector properties of nuclear force
If (N/Z)proj≠(N/Z)targthe system displays
dipole oscillation due to charge equilibration
displays collective features like GDR
-produces g emission
NZ
D
RCM , Z  RCM , N
A
2
dP
2e2
''

D ()
3
dEg 3c Eg
Simulation code developed by Baran et al. based on
BNV model applyed to 16O(@8MeV/u)+116Sn
Experimental problems:
• the CN has to be produced at the same
E* and I
• The intensity is never higher than 25%
of the total yield with stable beams
• The measurements need to be
extremely clean and exclusive
Open questions:
• Centroid is at lower energies
 not yet seen exp.
• Still to study angular distribution
 check if pure dipole
• The dinamical contribution will show a clear
anisotropic gemission
• The higher N/Z asymmetry the stronger
the effect
• few combination proj-target
can produce the same CN
• fusion reaction selection
• spin selection
• No contaminations from
target impurities
O+Mo @ 4-8-14-20 MeV/u
Baran et al . PRL87(2001)182501
Conclusions
 We have measured high energy g-rays from Coulex of
68Ni
at 600 MeV/u.
 First experiment of this type ever performed
 Strength at 10.5 MeV has been observed in all three kind of detectors
 Peaks line-shape is consistent with GEANT simulations (GPDR < 1 MeV)
 Low Energy Dipole strength has also been observed in
67Ni
and
69Ni