1. No category

Recent results on reactions with
radioactive beams at RIBRAS
Alinka Lépine-Szily,
and
RIBRAS collaboration
ECT* workshop on Low-Energy Reaction
Dynamics of Heavy-Ions and Exotic Nuclei
May 26-30, 2014, Trento, Italy
Outline
1. Quick description of RIBRAS
2. Elastic scattering measurements with 6He beam
3. Optical model and CDCC analysis
4. α-particle production
5. Total reaction cross sections
6. Elastic scattering and reactions on hydrogen target
7. R-matrix analysis and spectroscopic results
Major Facility for Nuclear Physics research in Brazil
Tandem Accelerator – Pelletron 8UD at the
University of São Paulo - Brazil
primary beams:
6Li, 7Li
, 10,11B, 9Be, 12C,
16,17,18O,
3.0 – 5.0 MeV/nucleon
...
RIBRAS - Radioactive Ion Beams in Brazil
First RIB facility in the Southern Hemisphere,installed in 2004
Low energy radioactive ion beam production
with solenoid based system.
University of São Paulo – Brazil
 Max field 6.5 Tesla
 versatile configuration
 persistent mode
 low LHe and LN2 consumption
First scattering chamber
2nd scattering chamber
Selection with the first solenoid
angular acceptance
2 deg - 6 deg
primary beam,
transfer reactions
1- primary target
2- collimator
3- Faraday cup
4- solenoid
5- lollipop blocker
6- collimator
7- scattering chamber,
secondary target and
detectors
30msr
Bρ =
mv
=
q
2mE
q
Maximum Bρ=1.8Tm
ΔE-E Si
telescopes
Beams of interest: 6He, only 16%, 8Li 65%
Double solenoids (cross-over mode)
Second solenoid helps cleaning the secondary beam:
Degrader changes the Br of the particles with different Z (q)
Solenoid -1
Solenoid - 2
Degrader in first
scatt.chamber
ΔE
Target
Detectors
3 new strip-detector
telescopes
Br 
2
E
AE
k 2
q
Present radioactive beams at RIBRAS
secondary ion
reaction
6He
9Be(7Li,6He)
8Li
9Be(7Li,8Li)
7Be
3He(6Li,7Be)
7Be
6Li(7Li,7Be)
10Be
9Be(11B,10Be)
8B
3He(6Li,8B)
18F
12C(17O,18F)
17F
3He(16O,17F)d
intensity / 1A of primary beam
2 x 105 p/s
106 p/s
6x105 p/s
105 p/s
2 x 103 p/s
104 p/s
104 p/s
*
Scientific program at RIBRAS
Elastic scattering:
(only first solenoid)
(two solenoids)
6He
+9Be,27Al,51V,58Ni,120Sn
7Be + 27Al, 51V
8Li + 9Be, 51V
8B + 27Al
8Li, 7Be, 9Be, 10Be on 12C
8Li + p, 6He + p
Transfer reactions: 8Li(p,α)5He,
Future:
Break-up reactions
Inelastic scattering
Fusion – evaporation
12C(8Li,9Li)11C
Elastic scattering measurements with 6He beam
Light, intermediate and heavy targets: 9Be, 27Al, 51V, 56Ni, 120Sn
Static and dynamic effects with 6He halo nucleus
Cluster model
6He = 4He +2n
Weakly bound
B.E.= 0.973 MeV
Neutron Skin and halo: static effects
Correlations and couplings between reaction mechanisms.
binding energy (breakup) effect in elastic scattering: α production
Analysis using Optical Model (São Paulo Potential-SPP), CDCC
Total reaction cross sections.
São Paulo Potential (SPP) – optical
potential with non-local interaction
L.C. Chamon, D. Pereira, M.S. Hussein, M.Alvarez, L.Gasques, B.V. Carlson, et al. PRC 66,014610 (2002)
1. Pauli non-locality related with energy dependence
Local-equivalent potential :
VLE (r , E )  V fold (r )e
[ 4 v 2 / c 2 ]
v is the local relative speed
2. Double-folding potential :
V fold (r )   d rp  d ra r p (rp ) r a (ra )v(rpa )
v(rpa): effective zero-range nucleon-nucleon interaction
v(rpa )  Vo (rpa )
3. Imaginary part : W(r,E)= NI VLE (r,E)
limitation:same geometry for W as for V
6He+27Al
elastic scattering
First results of RIBRAS
Optical Model calculation
São Paulo potential (NI~0.7
a=0.56(2)=normal nuclear
diffuseness)
6He+51V
elastic scattering
Optical Model calculation
São Paulo potential (N I~1.4(4)
a=0.67(3) larger than normal nuclear
absorption and diffuseness)
more absorption
6He+9Be
elastic scattering
6He
is 3 body Borromean system
6Healpha+2n
3b-CDCC....
6Healpha +n+n 4b-CDCC
Coupled Channels calculation: includes
low lying excited states of 9Be and 2+ state
of 6He ( is more important)
3 and 4 body CDCC calculations for 6He
Optical Potential: real part: Sao Paulo potential (continuum discretized coupled-channel)
Imaginary part: Wood-Saxon potential used for
6Li+9Be
6He+120Sn
elastic scattering
4bCDCC
 reaction
(mb)  breakup(mb)
6He
+ 120Sn elastic scattering
Details of the coupling to the break-up channel
No-coupling to exited states, equiv to optical model calculation
4b-CDCC only nuclear coupling
4b-CDCC Coulomb + nuclear coupling
Good fit
6He
+ 58Ni elastic
scattering
Comparison with CDCC calc.
3-body and 4-body CDCC
calculations give different cross
Sections at θcm > 40o.
Excellent agreement with
4-body CDCC calculation
Conclusions on angular distribution analyses:
6He
+ 120Sn. Comparison of CDCC calculations with and without coupling
to continuum. Need for Nuclear + Coulomb coupling to
continuum.
6He
+ 58Ni
6He
+ 51V
Optical Model calculations with SPP. NI and aI has to be
increased from 0.78 to 1.4(4) and 0.56 fm to 0.67(3) fm.
Simulates long range absorption due to breakup coupling
6He
+ 27Al
Optical Model calculations with SPP. NI and aI are the
same as normal stable nuclei. No effect of breakup coupling.
6He
+ 9Be
Comparison of CDCC calculations with and without coupling
to continuum. Need for coupling to continuum to get good fit.
Need for 4-body CDCC to fit the data
Production of α-particles
Large amount of alpha particles produced in
6He+120Sn and 6He+9Be reactions
6He+120Sn
6He+9Be
E
6He
α -particles from projectile break-up
+ target break-up + contaminants
Energy spectra and angular distributions of α-particles
from 6He+120Sn collision
120Sn(6He,4He)122Sn
6He+120Sn
4He+120Sn+2n
α-particles resulting from 2n-transfer reaction mostly
Total reaction cross sections
Total reaction cross section can be deduced from
elastic scattering analysis.
This information is useful to investigate the role of
breakup (or other reaction mechanisms) for
weakly-bound / exotic nuclei.
To compare fusion and total reaction cross sections of
systems with different projectiles and targets, including
halo nuclei
two recent reduction methods are available:
First reduction method considered:
reduced energy
E
red
cm
 A1p/ 3  Aa1/ 3  MeV

 Ecm 
 Z Z
 fm
p a


reduced reaction cross section
 Rred 
A
1/ 3
p
R

1/ 3 2
a
A
( mb)
Removes: Geometrical differences arising from sizes and charges
Takes into account: anomalous large radii of weakly bound / halo nuclei
Lowering of Coulomb barrier due to these
Does not take into account: change in width of fusion barrier: important
for fusion, ?? for total reaction cross section,
Second reduction method considered: Canto et al. J. Phys. G36, 015109
(2009)
Based on tunneling concept (Wong model)
Fusion function
RB,VB and hω = radius, height, curvature
Coulomb barrier
Universal Fusion Function (UFF) should
fit F(χ) if tunneling concept holds
Applied to total reaction cross section (Shorto et al. Phys.Lett.B678,77)
However, peripheral reactions (breakup, transfer, inelastic) do not proceed
through tunneling.
Should it apply to total reaction cross section???
Total reaction cross sections
on A~120 targets
First scaling:
σred (6He +120Sn): enhancement
of ~ 50% over σred ( 7Li+138Ba)
Second scaling:
Both scalings yield 3 trends:
Lowest σred -> tightly bound
described by UFF-SPP
Higher σred -> weakly bound
Highest σred -> halo projectile
Total reaction cross sections on A~60 targets
First scaling
σred (6He + 58Ni,51V,64Zn, 8B+60Ni): enhancement
of ~ 40 - 50% over σred ( 6,7,8Li + A~60 targets)
Total reaction cross
sections on 27Al target
First scaling
No enhancement for halo
nuclei over weakly bound
but over tightly bound
Second scaling
No enhancement, UFF
describes all systems
Total reaction cross
sections on 12C target
First scaling
Slight enhancement (15%)
for halo nuclei over weakly bound
Second scaling
UFF describes weakly
bound and halo systems.
Enhancement over tightly
bound (0.6 UFF)
Comparison of total reaction cross section
using first scaling:
A~120 similar results Coupling to Coulomb breakup
and
σred highest for low energy halo nuclei, 6He and 8B
A~60 1.0 < Ered < 1.5, 40-50% enhancement over stable,
weakly bound projectiles
Ered > 1.5 , enhancement reduced
27Al
No enhancement of halo over stable weakly bound at
any energy. Enhancement over tightly bound 16O proj.
12C
No error bars on σred. Slight enhancement (15%) for
halo nuclei over weakly bound at Ered >2.5
Enhancement of 20-30% of 6He over weakly bound
at Ered>5. Breakup of 9Be contributes. Nuclear breakup.
9Be
Comparison of total reaction cross section
using second scaling :
A~120 similar results to first scaling
F(χ)(6He) > F(χ)(6,7Li) > F(χ)(4He)
UFF agrees with F(χ) of 4He +A system (only fusion)
Peripheral reactions are important for 6He and weakly
bound on heavy targets (Coulomb breakup, transfer)
27Al
UFF agrees with F(χ) of stable, tightly bound (16O),
weakly bound and halo projectiles (only fusion ?)
Very little peripheral reactions even for halo and weakly
bound on 27Al target ?
12C
UFF agrees with F(χ) of halo and stable weakly bound
projectiles ????
0.6 UFF agrees with F(χ) of tightly bound 4He and 12C
projectiles ????
Measurements with purified radioactive
beams:
Elastic scattering and transfer reactions
on hydrogen target
Interest of 8Li(p,)5He, 8Li(p,p)8Li and 8Li(p,d)
reactions:
Nuclear Physics:
• Provide spectroscopic information on 9Be states near the p+8Li
threshold (16.88 MeV)
Astrophysics:
• The reaction 8Li(p,)5He destroys the 8Li, preventing the access to
higher mass nuclei.
•Important to measure and compare its strength with the branch
8Li(,n)11B
Previously we have measured the excitation function for
8Li(p,)5He reaction between E =0.2 -2.12 MeV,
cm
2.467 MeV
α+5He
Inelastic scattering
9Be(p,p´) with 180 MeV p
beam.Dixit et al, Phys.Rev.
C43, 1758(1991)
Our results of p(8Li,α)
reaction. Mendes et al,
Phys. Rev. C86, 064321
(2012)
Resonances
with strong α
structure
Results of our previous 8Li(p,)5He measurement:
R-matrix fits:
•Spins
•Energies
•Proton and alpha
widths
Astrophysical reaction rates
The measurement of the 8Li(p,p)8Li elastic scattering
can help to constrain the resonance parameters
We measured simultaneously the 8Li(p,p)8Li, 8Li(p,)5He
and 8Li(p,d)7Li reactions between Ecm = 0.8 – 2.0 MeV.
39
Experimental method for the measurement:
Inverse kinematics: 8Li beam hitting thick CH2 target
Primary beam 7Li, accelerated by 8UD Pelletron tandem of São Paulo
Radioactive 8Li beam 9Be(7Li,8Li)8Be, selected by the both solenoids
of RIBRAS. Degrader between the solenoids.
Production target: 16 micron 9Be foil
Radioactive beam intensity: 3x105 pps (50% transmission from 1st to
2nd solenoid)
Detection: deltaE(20 microns)-E(1000 microns),
300 mm2 silicon telescopes
Secondary Target – C1H2 – 7.7 mg/cm2
Experimental method: thick secondary target CH2 of 7.7 mg/cm2
Resonances populated in the target.
Energy spectrum of 4He, p, d yields excitation function of resonance
reaction
4He,
8Li
p
Si-telescope
beam
 ( Ei )
dEi
E  E / 2  ( E )
i
Y ( E )  I ( E )
E1
E2
E  E / 2
ε = stopping power
Energy spectra measured on thick CH2 target at Elab=18.5 MeV
Protons hard to measure, due
to low energy (Q=0) and electronic
noise
ΔE=50μm
8Li(p,α)5He
ΔE=20μm
Resonances in 9Be
at Ecm
0,40 0,60
1,10
1,69 1,76 MeV
Contaminant light
particles subtracted
(Au target)
8Li(p,p)8Li
C(8Li,p,d,α) reactions
measured, subtracted
8Li(p,α)5He
8Li(p,d)7Li
Ecm (MeV)
7Li(d,p)8Li
Ecm(MeV)
Resonances at 1.66 and 1.76 MeV
decay to 7Li* (0.477MeV), not to 7Ligs,
not populated in 7Ligs(d,p)8Li. Peak shifted to lower energy.
R-matrix analysis of three excitation functions with AZURE
1.66 and 1.76MeV
R-matrix analysis results
(Masters Thesis of Erich Leistenschneider 04/2014)
Black numbers Tilley et al Nuc. Phys. A745, 155 (2004)
Blue numbers our analysis
Comparison with previous work
With parameters
of the previous
work
With parameters
of the previous
work + width for
(p,d) channel
Conclusions
•
Elastic scattering measurements with 6He beam on light (9Be, 27Al),
medium (51V,58Ni) and heavy (120Sn) targets.
• Optical model and CDCC analysis: for medium and heavy targets,
long range absorption, coupling to Coulomb+ nuclear breakup.
•
Light targets: 27Al, normal OM. 9Be, CDCC fits the data with
coupling to continuum.
•
Total reaction cross sections: strong enhancement with halo
projectiles on medium and heavy targets. Coulomb coupling
. No enhancement on 27Al.
Slight enhancement on 9Be and 12C targets. Nuclear coupling
• The simultaneous measurement of resonant elastic scattering
8Li(p,p)8Li, 8Li(p,α)5He and 8Li(p,d)7Li reactions, allows to
determine the resonance parameters of 9Be.
Thank you
Alinka Lépine-Szily (USP)
and RIBRAS collaboration, as:
USP: Rubens Lichtenthaler, Kelly C.C. Pires, Erich Leistenschneider,
Valdir Guimarães, Valdir Scarduelli
U. Sevilla M. Rodriguez-Gallardo and A. M. Moro
ULB (Belgium) Pierre Descouvemont
UFF (Niteroi) Djalma R. Mendes Jr, Pedro Neto de Faria, Paulo R.S.
Gomes
UNIFEI Viviane Morcelle
UFBa Adriana Barioni
GSI Juan Carlos Zamora
TANDAR (Argentina) Andres Arazi
USC Elisangela A. Benjamim