Reactions with weakly bound Nuclei near the Coulomb barrier

1
Reactions with weakly bound Nuclei near the
Coulomb barrier

• A. Navin
• GANIL

2
POA

• Open experimental issues in understanding fusion
• with weakly bound nuclei near the Coulomb
  barrier
• Need for identification of complete fusion
  products in these reactions
• 4,6,8He ? 63,65Cu 188,190,192Os from
  SPIRAL I
• (Illustration of particle-gamma correlations
  as a tool)
• Origin of large cross section for non fusion
• Transfer/ breakup followed by fusion events
• 6,7Li60Ni, 165Ho,
• Implications on effect of weak binding on the
  fusion process
• 1n/2n in borromean nuclei
• 
  transfer to the continuum

4He 6He 8He
Sn(MeV) 20.58 1.863
2.583 S2n(MeV) 0.973
2.138 ltr2gt1/2 (fm) 1.67(01)
2.54(04) 2.49(04)
6Li 7Li Sa/d Sa/t 1.475 2.47
MeV
3

• Fusion with weakly bound nuclei near the
  Coulomb barrier
• Stable and Unstable nuclei

EXPECTATION Suppression in the fusion cross
section - breakup Large
Enhancement -large sizes
(halos,skin) ? Nuclear Potential
Tunneling through a multi-dimensional barrier
V(r,x1,x2,x3 ....)
(Tl (E) 1 exp2p/hw (VBl E) 1)
Quantity to be compared with the model

• Complete fusion (projectiletarget) OR
• Complete fusion (projectile-xtarget)
• Conditions under which this can be done
• (projectile-xtarget) breakup fusion or transfer
• 
  Reaction cross section

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SPIRAL
DE/E 10-3 no measurable impurity
75MeV/A 13C 1013 p/s
SPIRAL
CSS1
CSS2
SISSI
SIRa
Stable beam
LISE
Production Target
SPIRAL
Experimental Area
GANIL
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Experimental Approach
16.5 MeV lt Elab lt 34 MeV a 19.5, 30 MeV
6He 27 MeV
8He
g - ray detector array
?
evaporate
4
Ga
2
compound nucleus
a2n
p2n
3n
0
65Cu
68Ga
68Zn
68Zn
Target 63,65Cu 188,192Os ( 1.1-3.2 mg/cm2)
Projectile 4,6,8He
Intensity of low lying g transitions ? sER
S sE.R sFUSION
6,7Li
60Ni 165Ho
Pelletron, Spiral Intensity(particles/s) 2x1010
a, 6,7Li 1x107
6He 4x104 8He
Annular Si CD detector for charged particle
detection (elastic,transfer breakup)
Particle gamma coincidence for transfer angular
distributions
6
Setup at SPIRAL
Exogam Array CLOVERS
Target
Annular Si CD detector
Plastic scintillator
Faraday Cup
First Singles g-ray measurements with RIB to
obtain fusion and transfer cross sections
s fusion and s transfer from intensity of g-rays
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BUY ONE (FUSION) GET ONE FREE (TRANSFER)
Comparison of evaporation residues
with statistical model calculations
(CASCADE) 4,6He 65Cu
Good agreement for a 65Cu but Large
discrepancy in cross section for an channel
(66Cu) in 6He 65Cu Evidence for non fusion
process? TRANSFER (Same residue as from fusion)
s (6HeCu) (6HeCu)(aCu) (nCu)
Similar results for 4,6 He 63Cu
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More evidence Particle g coincidence 6,8He
Q-value spectra
6He65Cu?4He(67Cu) 8He63Cu?4He(65Cu)

• transfer followed by evaporation

• Observed Q-value spectra consistent with
  transfer, peaking Qopt 0
• Transfer (2n) to the continuum
• ?Measured ratio of residues consistent with
  expectation
• 30 as compared to complete fusion

Large transfer cross sections and greater than
breakup cross sections
Similar effects observed in 6He188,190,192Os Do
not mix with Fusion products (target
like) Bottom line Residues better be only from
CN process PRC 70 044601 (2004)
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Origin of non compound events with weakly bound
nuclei
Transfer - Medium mass targets 6,7Li 60Ni
p,n,g correlations near and away from
grazing Angles below and above the Coulomb
barrier Evidence for Nuclear Breakup Large
Transfer like with He isotopes Contrary to the
understanding for medium mass targets Experimenta
l signatures of Break fusion - Heavy targets
7Li 165Ho 42 MeV Coulomb Breakup? Gamma
rays in coincidence with alpha and tritons to
probe the origin of the residues ( inclusive
measurements PRL 88, 1702) Relevant for a
complete understanding of interaction with weakly
bound nuclei.
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Non compound process breakup or transfer Alpha
Spectra ?? gamma spectra
7Li 60Ni
Qopt
40o grazing
120o grazing
Transfer ?
Nuclear dominated breakup ?
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Particle-g coincidence for 7Li 165Ho
Gamma rays in coincidence with alpha particles?
t fusion
DE

• Beam 7Li, 42 MeV
• Target 165Ho , 2.68 mg/cm2
• Telescopes _at_ 40o, 50o near
• grazing angle
• 4 Clovers

E (channel no)
184.6
a 165 Ho
228
270.8
all lines in 167Tm
297
354
Gamma rays in coincidence with tritons ? a
fusion Similar Spectra with a beam
384
511
454
552
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Coincidence g-spectra for Ea gates? Different Et
fusing !
184.6 166Er
2
4
1
280.4 166Er
184.6 166Er
280.4 166Er
365.6 166Er
205 165Er
306,315 165Er
438.2 166Er
Agreement with stat. model
Breakup followed by fusion ? Coulomb dominated
breakup
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Different Routes ? 166Er from t fusion/transfer
and p transfer
7Li165Ho?4He(168Er) t fusion/transfer ?6He1
66Er proton transfer
184.6 166Er
p transfer
t-transfer/fusion
280.4 166Er
184.6 166Er
365.5 166Er
280.4 166Er

Side feeding pattern Angular momentum
E (keV)
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Implications - non compound events

• 6,7Li,9Be208Pb,209Bi ANU/Brazil
• 18O198Pt7Li209Bi
• sCFsNC sF bound nuclei (same CN)
• Complete fusion break up fusion
• transfer
• Fusion cross section or reaction cross section?

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N-wall EXOGAMParticles
Coming Soon in a beamhall near you
6He65Cu?4He(67Cu) 66Cu(g)4Heneutron To
decompose 1n /2n neutron transfer Transfer to
the continuum (Followed by evaporation)
8He63Cu fusion transfer
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Perspectives

• Need for
• a) for exclusive experiments to disentangle
  reaction mechanism before comparing with models
  especially for medium mass nuclei
• b) Identify source of the non compound
  process transfer- breakup fusion
• c) Improved calculations for the
  understanding of reactions near Vc
• Higher intensity low energy 8He beams more
  from SPIRAL
• and of course
• SPIRAL2

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SPIRAL 2 Workshops

• Neutrons for science (on the web)
• Nuclear Astrophysics
• Low energy physics
• Direct reactions
• Gamma spectroscopy

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The Collaboration .
D.Bazin, Y. Blumenfeld,, J.M. Casandjian,
A.Chatterjee, M Dasgupta, G. de France S. Kailas,
R. Lemmon, K. Mahata, T. Nakamura, V. Nanal, R.G
Pillay, E.C.Pollacco, R. Raabe , K. Ramachandran,
M. Rejmund, A. Shrivastava, J.L Sida,
V.Tripathi, E.Tryggestad
IPN, IN2P3-CNRS
BHABHA ATOMIC RESEARCH CENTER
GANIL
Tata Institute of Fundamental Research
TIT, Japan
Australian National Univ.
Daresbury Lab
NSCL, MSU
CEA/ SACLAY
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Investigations in 7Li60Ni system

n p t d a gamma coinc
Near and away from grazing angle Near and away
from the barrier
40o,50o 30 MeV 120o,140o 15 MeV
Presence of two components in alpha Spectra
snc430mb and scn350mb
a angular distbn
(Vb 14 MeV)
Ea
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triton gate
alpha Fusion/ transfer
62Cu, 62Ni,61Cu,61Ni,58Co
Alpha gate Triton Fusion/ transfer
63Zn, 60Ni
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Transfer followed by breakup
7Li 65Cu _at_25 MeV particle coincidences
1n stripping 6Li (gnd) 66Cu 6Li(3) 66Cu
Break up cross-sections
at and ad
And more A. Shrivastava et al (work in progress)
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1n transfer or 2n transfer
6He 65Cu Elab 19.5 MeV
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Analogous for Ea gates? n fusing !
186.01 66Cu
Similar E mostly 66Cu
315.71
66Cu
65Cu
30 MeV
186.01
19.5 MeV
1115.5 65Cu
315.71
CN
186.01
higher E more 65Cu
1115.5 65Cu
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Transfer Q-values and Sns

• a63Cu?3He64Cu -12.66
• a63Cu?2He65Cu -
• 6He63Cu? 5He64Cu 6.052 7.91,
  18.768
• 6He63Cu? 4He65Cu 16.852
  9.91, 17.826,7.45(Sp)
• 6He65Cu? 5He66Cu 5.20
  7.07, 16.98
• 6He65Cu? 4He67Cu 15.21
• 8He63Cu? 7He64Cu 5.332
• 8He63Cu? 6He65Cu 15.687
• 8He63Cu? 4He67Cu 30.897
  9.12,16.18

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elastic angular distbn
s/sR
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Direct Reactions
Elastic scattering angular distributions
Reaction cross section sR? relative contribution
of transfer and breakup
Angular distribution and Q value spectra for
alpha particles gated with 66Cu consistent with
transfer Direct evidence for
transfer, Raabe et al Nature 2004 sR -
(sfusstrans) sbreakup strans 1.5 sbreakup
Large transfer cross sections and greater
than breakup cross sections
Also for 8He63Cu at 27 MeV
a elastic scattering NPA 147 107 (1970) NPA 388
573 (1982)
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Fusion with weakly bound nuclei
?
E.R.
In beam ?-rays
Fission
Offline x rays
n What do I do now
Alpha decay fission 6He209Bi, 9Be208Pb,
11Be208Pb Fission coincidence 6He 238U,
nU Characteristic X-ray, Gamma activity
measurements 6He64Zn Online gamma-ray
measurements 6,7Li59Co,165Ho Combined with
particle coincidence 6,8HeCu,Os
Experimental limitations?
Broken up a populates the same channels (as pt)
difficult to distinguish experimentally Differenti
ate transfer and fusion products (Especially
for Acn lt100)
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Os isotopes
6He Heavier targets.
6He190Os? 4He(192Os) Features Gate on 2? 0
of 190Os Shows inelastic and transfer
contributions Target like products so not mixed
up with residues unlike for lower ACN!
Comparison with a induced reaction
sdir_6Hegt sa sf sa
like Cu
Bottom line Residues better be only from CN
process PRC 70 044601 (2004)