1. Study of the reactions induced by 6He
K. C. C. Pires
Universidade Tecnológica Federal do Paraná
Brasil

2. Introduction and Purpose
The Experiment 6He+9Be
Total Reaction Cross Section
Alpha Particles Production in the 6He+9Be Collision
Inelastic 6He+9Be* Collision

3. Introduction Atomic nucleus: many-body system
Light stable nuclei: N  Z
Mass increase: N > Z (Coulomb repulsion)
Far from stability line: Exotic Nuclei
Excess of protons or neutrons number
Very different properties in comparison with stable nuclei
 300 stable nuclei
 5000 nuclei far from stability line
A lot of nuclei not observed but foreseen
Large investigation area

4. Purpose
To study the behaviour of the total reaction cross section in exotic light systems.
Why?
6He+51V,58Ni,64Zn,120Sn  large total reaction cross section compared to stable systems
1) Physics Letters B 601 (2004) 20–26 2) Physics Letters B 647 (2007) 30–35 3) The Scientific program with RIBRAS (Radioactive Ion Beams in Brasil)
What happens for very
light systems?
e.g. 6He+9Be

5. The RIBRAS System – 6He+9Be
two superconducting solenoids
able to select light secondary beams of nuclei far from stability line.

6. Two measurements:
Elab(6He)=16.2 MeV
Elab(6He)=21.3 MeV
Production of 6He beam (I104pps)
Production reaction: 9Be(7Li,6He)10B;
7Li  22.2 MeV and 26.1MeV;
The 1st experiment:
Primary beam intensities: 300 nAe;
Secondary beam intensities: 2.4x104 pps.

7. Detection system: Four E-E telescopes formed by silicon detectors.

8. 1st step to do an experiment using the RIBRAS facility is to calculate the solenoid current.
First moment: Monte Carlo simulation
Second moment: Experimental verification
The solenoid current was varied to focus the 6He beam on the telescope.
This procedure mitigates the effect of the beam divergence, on the angular resolution, of the particles scattered by the secondary target.

9. We measured the direct 6He beam placing a telescope at zero degree and reducing the primary beam to < 1nAe.
relative
intensities
70%
18%
10%
Resolution: 1MeV

10. Production of alpha particles in the 6He +9Be collision
A large alpha particles production was observed
Could be produced in several processes:
Breakup, transfer, fusion, etc.
lab=15
Elig(6He) = 0,973 MeV  (n+n+4He)
Elig(9Be) = 1,665 MeV  (n+4He+4He)
We expect a large production of alpha particles!

11. Angular distributions involving these alpha particles were calculated in order to obtain information about the breakup of 6He projectile and 9Be target.
lab=18
The cross section shape is well reproduced
The calculation underestimates the experimental cross section and normalization factors were necessary
Possible explanation: 9Be is unbound  breakup contribution of the inelastic channel could be important but it is not taken into account in CDCC calculations. Fusion-evaporation reactions could also contribute to the alpha yields.

12. Inelastic 6He+9Be* collisions
Events related to the 6He nucleus with lower energy than the elastic scattering peak.
6He nucleus has no bound excited states  hypothesis  events correspond of 6He+9Be collision where 9Be states were excitated. But 9Be states are unbound  interpret these events as due to the breakup of the target.
Coupled Channels Calculations using the Collective Model Formalism (CC)

13. Total Reaction Cross Section
The total reaction cross section for 6He+9Be was obtained from elastic scattering analysis with:
OM, CC, CDCC.
The cross sections for other systems were obtained from the literature
K.C.C. Pires et al – PRC83, (2011).
Method: remove geometric and coulomb barrier effects
[PLB601(2004)20, PRC71(2005)017601]
To compare different systems at different energies we make the transformation:

14. 1st. Strip: nuclei strongly bound
2nd. Strip: systems involving projectiles and targets weakly bound
3rd. Strip: systems involving 6He = projectile weakly bound and halo nucleus.
The total reaction cross section involving the 6He nucleus is probably higher.

15. Enhancements in the Total Reaction Cross Section
reac(6He) = total reaction cross section induced by 6He, obtained from elastic scattering experiments for several systems
 reac(6Li) = obtained from optical model calculations using the standard SPP
Selected only experimental data taken at energies (Ered > 1.1 MeV)
Coulomb barrier for reac(6Li) drops down rapidly and the enhancement effect becomes much more pronounced.
Considerable enhancements in the total reaction cross section are observed when compared to the stable 6Li isobar.

16. Conclusions
Most alpha particles produced in the 6He+9Be collision are probably due to contaminant beams.
Calculation require a high normalization factors.
Relative intensities of the contaminant beams: 71.8% (7Li) ,17.9% (6He) and 10.3% (4He).
This indicate that the reactions induced by 7Li have a more important weight in the alpha particle production cross section.
Total Reaction Cross Section: shows that the systems are grouped into three categories according to the reduced cross section magnitude:
Smaller value: strongly bound stable systems
weakly bound stable systems
Higher value: 6He nucleus
The total reaction cross sections shows a significant relative enhancement for reactions induced by 6He when compared to the stable 6Li isobar.
This enhancement was observed for heavy targets and still persists for light targets where the Coulomb breakup is negligible.

17. Collaboration
R. Lichtenthäler, A. Lépine-Szily, V. Guimarães, M.C. Morais, R. Pampa Condori, E. Leistenschneider
Departamento de Física Nuclear, Instituto de Física,
Universidade de São Paulo, Brazil.
A.M. Moro, M. Rodriguez-Gallardo
Departamento de FAMN, Facultad de Física, Universidad de Sevilla, Spain.
A. Barioni
Departamento de Física da Terra e do Meio Ambiente, Instituto de Física, Universidade Federal da
Bahia, Brazil.
D.R. Mendes Jr, V. Morcelle, P.N. Faria
Instituto de Física, Universidade Federal Fluminense, Niterói, R.J. Brazil.
M. Assunção
Departamento de Ciências Exatas e da Terra, Universidade
Federal de São Paulo, Campus Diadema, Brazil.
J.C. Zamora
TU Darmstadt, Germany.
J.M.B. Shorto
Instituto de Pesquisas Energéticas e Nucleares
(IPEN-CNEN), Comissão Nacional de Energia
Nuclear, São Paulo, Brazil.