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Fusion, transfer and breakup of light weakly bound nuclei at near barrier energies. Paulo R. S. Gomes Univ. Fed. Fluminense (UFF), Niteroi, Brazil. Eurisol Week Lisbon, Portugal – October 2012. Reactions with weakly bound nuclei. - PowerPoint PPT Presentation
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Fusion, transfer and breakup of light weakly bound nuclei at near barrier
energies.
Paulo R. S. GomesUniv. Fed. Fluminense (UFF), Niteroi, Brazil
Eurisol Week
Lisbon, Portugal – October 2012
Reactions with weakly bound nuclei
However, nature is more complicated than that simple picture: Breakup following transfer
RESULTS
measured measured calculated by p conservation
known
beforeafter
np
Courtesy of Luong
Breakup mechanism: Q-valuesRESULTS
n
d
Usual questions
-Does the BU channel enhance or suppress the fusion cross section? Is the effect on σCF or σTF= CF + ICF?
-What are the effects on different energy regimes and on different target mass regions?
- Different BU threshold energies should affect NCBU significantly. Is that true for CF or CF + ICF? And for TR ?
- May prompt and delayed BU affect fusion?
Usual questions• How large is the NCBU compared with CF or CF + ICF ?
How does it depend on the energy region and target mass?
• Is transfer important for every weakly bound projectiles?
• Does direct BU and BU following transfer produce repulsive polarization potentials?
• Is there BTA for the elastic scattering of any weakly bound projectile?
Experimental aspects to be considered
• What is it measured? σCF or σTF= CF + ICF?
• Can ICF be separated from transfer channels leading to the same compound nucleus? So, instead of TF, one measures TF + transfer?
• How to distinguish NCBU from transfer and ICF by measuring single fragments?
Importance of transfer for fusion of 6He. Is it also important for other weakly bound nuclei? If yes,
how to separate it from total fusion?
Raabe – Nature 431, 823(2004) X Trotta – PRL 84, 2342(2000) - same authors
Frequently used procedures to answer “Enhancement or
suppression in relation to what?
a) Comparison of data with theoretical predictions.
b) Comparison of data for weakly and tightly bound systems.
Effects to be considered
• Static effects: longer tail of the optical potential arising from the weakly bound nucleons.
• Dynamical effects: strong coupling between the elastic channel and the continuum states representing the break-up channel.
1. Experiment vs. theory F
F
exp - F
theo 'ingredients' missing in the theory
a) Single channel - standard densities
F arises from all static and dynamic effects
b) Single channel - realistic densities
F arises from couplings to all channels
c) CC calculation with all relevant bound channels
F arises from continuum couplings
d) CDCC
no deviation expected
Theoretical possibilities:
Example: 6He + 209BiSingle channel - no halo
Single channel – with halo
CC with bound channels(schematic calculation)
Shortcomings of the procedure:
• Choice of interaction plays fundamental role• Does not allow comparisons of different systems• Difficult to include continuum – no separate CF and ICF
Example of Model Dependent Conclusions
Kolata et al., PRL 81, 4580 (1998) Gomes et al., PLB 695, 320 (2011)
Raabe et al: Nature 431 (2004) 823
If one uses folding potentials, the halo density is taken into account. So, one does not observe halo effects!!!
Important: Bare Potential deduced from double-folding procedure
Gomes et al, PLB 695, 320 (2011)
Conclusions about static effects of halo nuclei.
• Fusion enhancement when compared with what it should be without halo properties.
• We believe that there is no more discussion left about that.
Differences due to static effects:
2. Compare with F of a similar tightly bound system
1. Gross dependence on size and charge:
ZP , ZT, AP , AT affects VB and RB
VB : ZPZTe2 / RB; geo : RB2 , RB A
P
1/3 AT
1/3)
2. Different barrier parameters due to diffuse densities
(lower and thicker barriers)
Fusion data reduction required !
Fusion functions F(x) (our reduction method)
E x
E VB
hand F
exp Fexp (x) 2E
hRB2
Fexp
Inspired in Wong’s approximation
F
W RB2 h
2Eln 1 exp
2 E VB h
If Fexp F
W F(x) F0(x) ln 1 exp 2 x
F0(x) = Universal Fusion Function (UFF)
system independent !
Direct use of the reduction method
Compare Fexp
(x) with UFF for x values where Fopt
FW
Deviations are due to couplings with bound channels and breakup
Refining the method
Eliminate influence of couplings with bound channels
Renormalized fusion function
Fexp
(x) Fexp
(x) F
exp(x)
R(x), with R(x)
FCC
FW
FCC
Fopt
If CC calculation describes data F
expUFF
Eliminate the failure of the Wong model for light systems at sub-barrier energies
Applications with weakly bound systems
1. Canto, Gomes, Lubian, Chamon, Crema, J.Phys. G36 (2009) 015109; NPA 821(2009)51
2. Gomes, , Lubian, Canto, PRC 79 (2009) 027606
2. Gomes, Canto, Lubian, Hussein, PLB 695 (2011), 320
Use of UFF for investigating the role of BU dynamical effects on the total fusion of heavy
weakly bound systems
No effect above the barrier- large enhancement below the barrier
Use of UFF for investigating the role of BU dynamical effects on the total fusion of very light
weakly bound systems
No effect above the barrier- almost no data below the barrier
Use of UFF for investigating the role of BU dynamical effects on the total fusion of light
weakly bound systems
No effect above the barrier- no data below the barrier
Is TF really total fusion or total fusion + transfer?
Fang – submitted to publication
Use of UFF for investigating the role of BU dynamical effects on the complete fusion of
stable weakly bound heavy systems
We did not include any resonance of the projectiles in CCC.
Suppression above the barrier- enhancement below the barrier
Fusion of neutron halo 6,8He, 11Be weakly bound systems
6He + 206PB (Dubna data) ????
New 6He + 206Pb data (Wolski – EPJA 47, 111 (2011))
Application : transfer coupling effects on medium-heavy systems
Shorto – PRC 81, 044601 (2010)
Zhang – PRC 82, 054609 (2010)
Only inelastic channels were considered
What about proton-halo systems?
So far, there is only one system measured
• Fusion of proton-halo 8B + 58Ni
(Aguilera PRL 107, 092701 (2011))
Fusion of proton-halo 8B + 58Ni (Aguilera PRL 107, 092701 (2011))
New dynamic effect for proton-halo fusion?
New measurements are required
Rangel – submitted to publication
Two independent calculations without any fit parameter
(Rangel et al., submitted to publication)
CCC with SPPCDCC calculations
Conclusion from the systematic (several systems) : CF enhancement at sub-barrier energies and suppression above the barrier, when compared with what it should be without any dynamical effect due to breakup and transfer channels.
Question: Why?
What has to be done?
- Need of additional data at the sub-barrier energy regime. We still do not fully understand this important energy regime, very important for nucleosynthesis and production of SHE.
-Need of more data with radioactive beams. We need better data, with smaller error bars.
-More fusion cross section measurements for proton-halo nuclei.
- Sub-barrier total fusion for light targets. Is there enhancement?- CF and ICF for light systems, below and above the barrier. Is ICF negligible? - CF and ICF for medium mass targets. How does the suppression vary with mass?
- More separated CF and ICF data, specially for light and medium mass systems. Can we separate transfer?
Final Message
• Although we have learned a lot in the last years, we still have many things to learn.
• We need more good data and more calculations and theoretical developments.
Collaborators
J. Lubian, D. R. Otomar, R. Linares (UFF),
L. F. Canto (UFRJ), M.S. Hussein (USP),
M. Dasgupta, D. J. Hinde, D.H. Luong (ANU)
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