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Some aspects of reaction mechanism study in
collisions induced by Radioactive Beams
Alessia Di Pietro
Outline of the talk
 Effects on fusion and elastic scattering cross-sections at
energies around the Coulomb barrier in reactions induced by
halo and weakly bound Radioactive Beams.
What have we learned from the existing experimental data?
The reaction 6He+64Zn.
The reaction
13N+9Be.
Summary and conclusions.
The EXCYT facility at LNS.
Effects of Halo structure on fusion
below the Coulomb barrier?
Different theoretical models predicts enhancement or
hindering of fusion cross-section in reaction induced by
Halo nuclei depending on how the break-up is treated in the
calculations.
Break-up removes flux from the fusion channel.
Strong coupling with break-up enhances fusion crosssections.
6He+209Bi
Fusion excitation function
Enhancement of fusion crosssection below the Coulomb
barrier is observed.
Total reaction excitation function
J.J. Kolata Phys.Rev.Lett.81(1998)4580
An even larger enhancement
of the total reaction crosssection was measured.
E.F. Aguilera Phys.Rev.C 63(2001)061603
The experiment 6He+64Zn
Fusion excitation function on a medium mass system.
Activation technique used.
Elastic scattering angular distribution.
Transfer and break-up cross section measurement.
Comparison of the results with 4He+64Zn reaction.
Experimental set-up
Si-Strip
Beam
64Zn
Si-Strip
targets Nb catcher
4,6He+64Zn
Heavy residue Excitation Function
6He+64Zn
1000
A strong enhancement of
the fusion cross-section
seems to be present!
s (mb)
4He+64Zn
100
CASCADE predictions compared with
experimental results
10
5
7
9
11
13
EXPERIMENTAL
CASCADE
15
350
Ecm(MeV)
s(mb)
300
Ecm =10.55 MeV
250
200
150
2
2
s/p R (mb/fm )
100
6He+64Zn
10
50
0
68
Ga
68
Ge
167Ga
65
Zn
4He+64Zn
1
0.1
0
0.5
1
Ec.m./Vc
1.5
2
The strong enhancement comes
only from one residue, 65Zn.
65Zn can be produced also by
1n and 2n transfer reactions.
A. Di Pietro et al. Phys.Rev.C 69(2004)044613
6He+238U
Experimental set-up
The strong enhancement of the
fission
cross-section
comes
from transfer reactions.
R.Raabe et al. Nature 431(2004)823
Elastic scattering
For halo projectiles break-up responsible for a damping in the
elastic angular distribution at large angles.
New features such as the disappearance of threshold anomaly
effect of the Optical Potential observed in reactions induced by
weakly bound and halo nuclei.
The Optical Potentials basic ingredient for the description of elastic
scattering but also important for break-up, transfer and fusion.
Calculations should be performed using models which take into
account explicitly or by Polarisation Potentials the coupling with
transfer and break-up channels.
Optical Model analysis at low energies performed using very large
imaginary diffuseness parameter in order to reproduce data.
4,6He+64Zn
Elastic scattering
6He+64Zn exp. Ecm=12.4MeV
10
4He+64Zn exp. Ecm=12.4MeV
1
srea0.65 b
0.1
srea1.45 b
0.01
0
50
100
q cm(deg)
aI=0.85fm for
150
Transfer+break-up
200
6He+64Zn@E
6He+64Zn
Most of the reaction
cross-section corresponds
to transfer and break-up
events rather than fusion.
cm=12.4
1000
ds /dW (mb/sr)
s ela/s R
Opt. Mod. Fit
Large reaction cross-section is
6He+64Zn
found
in
when
compared with 4He+64Zn at
the same Ecm
MeV
s ~1.2b
100
10
A.Di Pietro et al. Phys.Rev.C 69(2004)044613
A.Di Pietro et al. Europhys.Lett. 64(2003)309
0
50
100
qLAB(deg)
150
200
Other elastic scattering results
Very large imaginary diffuseness must be used to best-fit the data
6He+209Bi
6He+208Pb
aI=1.22 fm
aI=3.02 fm
E.F. Aguilera et al. Phys.Rev.C 63(2001)061603R
Energy dependent aI
aI=1.964-0.045xEc.m.
parameter:
O.R. Kakuee et al. Nucl.Phys.A 728(2003)339
Effects on reaction and/or fusion cross-section induced
by weakly bound nuclei above the Coulomb barrier?
Results on medium mass targets
9Be+64Zn
9Be+64Zn
shows no effects
on fusion and reaction
cross-section.
6,7
6,7Li+64Zn
shows no effects on
fusion but large reaction crosssection
P.R.S.Gomes et al. Phys.Lett.B 601(2004)20
Li+64Zn
Results on heavy targets
9Be+208Pb
6,7
Li+209Bi
M.Dasgupta et al. PRL82(1999)1395
M.Dasgupta et al. Phys.Rev.C 66(2002)041602R
Suppression of fusion cross-section above the barrier of about 70%. This
suppression is attributed to break-up which leads to incomplete fusion.
Fusion cross-section for the reaction 13N+9Be
(weakly bound projectile on weakly bound target)
Experimental set-up
Detection system based on Si-strip detectors and Monolithic
E-E telescopes
Si-strip detector
Monolithic detection module
Monolithic Si detectors
E thickness 1mm
Identification threshold  300-500keV/A
for Z  6-20
E [channels]
N
b
Energy [channels]
G.Cardella et al. NIMA378(1996)262
A.Musumarra et al. NIMA409(1998)414
13N+9Be
sfus (mb)
10B+12C
The results are in agreement with
CASCADE predictions. No evident
suppression of fusion crosssection is present.
1/Ecm (1/MeV)
10B+12C
E*(22Na)  40 MeV
40
CASCADE
Experiment
30
20
50
30
20
10
10
0
0
6
7
8
Z
9
10
=45 MeV
40
s/sfus
50
s/sfus
13N+9Be
=42 MeV
6
7
8
Z
9
10
Summary and conclusions
From the data so far collected a clear picture of structure effects
of halo and weakly bound nulcei on reaction mechanisms is still not
available.
The role of the break-up has still to be understood.
More theoretical and experimental efforts are needed.
The experiments with radioactive beams are quite difficult due to
the low intensity of such beams. Our results show that X-ray offline detection seems to be a good tool to obtain fusion excitation
functions in reactions induced by light Halo nuclei on intermediate
mass targets.
EXCYT installation at LNS
MAGNEX
Facility scheme
RIBs intensity table
Beam
8
Li
11
C
13
N
14
O
15
O
19
O
22
O
17
F
18
F
20
Na
22
Na
24
Na
33
Cl
34
Cl
Projectile
15
N
18
O
14
N
16
O
16
O
22
Ne
26
Mg
20
Ne
19
F
24
Mg
19
F
19
F
35
Cl
35
Cl
Primary
beam
energy
(MeV/amu)
50
50
50
50
50
50
50
50
50
50
50
50
50
50
Target
Pre-accelerated
(pps/pmA)
C
C
C
C
C
C
C
C
C
C
C
C
C
C
6.7*107
8.0*107
2.1*108
2.6*107
1.5*108
1.3*107
8.0*103
4.6*105
1.2*107
1.0*107
6.0*107
1.6*107
1.4*106
1.1*108
Intensity
Post-accelerated Total 500W
(pps)
(pps/pmA)
2.4*106
2.2*107
5.0*107
1.1*106
6.0*107
4.7*106
2.6*103
1.3*105
4.7*106
1.4*104
7.6*104
2.0*104
3.1*105
2.5*107
1.6*106
1.2*107
3.6*107
7.0*105
3.8*107
2.2*106
1.0*103
6.5*104
2.5*106
6.0*103
4.0*104
1.1*104
9.0*104
3.2*106
Effects on reaction and/or fusion cross-section induced by
weakly bound nuclei above the Coulomb barrier?
Example :
A. Szanto de Toledo et al. Nucl.Phys. A 679(2000)175
According to this systematic study there is a hindrance of fusion
cross-section in reaction between light weakly bound nuclei above the
Coulomb barrier.
Fusion excitation function measured with an
activation technique
Evaporation Residues produced in fusion reaction are radioactive
and decay by Electron Capture.
Discrimination of E.R. by X-ray energies and half-lives.
X-ray spectrum
67,68Ga67,68Zn
65Zn65Cu
Activity curve
T1/2= 67.6 m
T1/2=3.26 d