Some aspects of reaction mechanism study in collisions induced by Radioactive BeamsAlessia 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 cross-sections.

6He+209Bi

The experiment 6He+64ZnFusion 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.

4,6He+64ZnHeavy residue Excitation FunctionCASCADE predictions compared with experimental resultsA strong enhancement of the fusion cross-section seems to be present!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

Grafico1

104104072870173402773323231414223333

68Ga

68Ge

67Ga

65Zn

s(mb)

Ecm=10.55 MeV

EXPERIMENTAL

CASCADE

MISURATA

CASCADE

Foglio1

Ecm68Ga68Ge67Ga65Zn68Ga68Ge67Ga65Zn

mismismismiscascadecascadecascadecascade

11.83137117362361461149554

10.551047217277104873433

9.140453.51736565319

7.530100905155014

Foglio1

0000000000061611616552929

68Ga

68Ge

67Ga

65Zn

Ecm=11.83 MeV

EXPERIMENTAL

CASCADE

0000000000023231414223333

68Ga

68Ge

67Ga

65Zn

s(mb)

Ecm=10.55 MeV

EXPERIMENTAL

CASCADE

MISURATA

CASCADE

s(mb)

The strong enhancement of the fission cross-section comes from transfer reactions.6He+238U

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.

Elastic scattering

Large reaction cross-section is found in 6He+64Zn when compared with 4He+64Zn at the same EcmMost of the reaction cross-section corresponds to transfer and break-up events rather than fusion.4,6He+64ZnaI=0.85fm for 6He+64Zn

Other elastic scattering results6He+209Bi6He+208PbVery large imaginary diffuseness must be used to best-fit the data Energy dependent aI parameter: aI=1.964-0.045xEc.m.O.R. Kakuee et al. Nucl.Phys.A 728(2003)339E.F. Aguilera et al. Phys.Rev.C 63(2001)061603R

Results on medium mass targets9Be+64Zn shows no effects on fusion and reaction cross-section. 6,7Li+64Zn shows no effects on fusion but large reaction cross-sectionEffects on reaction and/or fusion cross-section induced by weakly bound nuclei above the Coulomb barrier?

Results on heavy targetsSuppression of fusion cross-section above the barrier of about 70%. This suppression is attributed to break-up which leads to incomplete fusion.

Experimental set-upDetection system based on Si-strip detectors and Monolithic E-E telescopesFusion cross-section for the reaction 13N+9Be(weakly bound projectile on weakly bound target)

DE thickness 1mmIdentification threshold 300-500keV/Afor Z 6-20

Monolithic Si detectorsG.Cardella et al. NIMA378(1996)262A.Musumarra et al. NIMA409(1998)414

The results are in agreement with CASCADE predictions. No evident suppression of fusion cross-section is present./fusZE*(22Na) 40 MeV

Summary and conclusionsFrom 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 off-line 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

Projectile

Primary beam energy

(MeV/amu)

Target

Intensity

Pre-accelerated

(pps/p(A)

Post-accelerated

(pps/p(A)

Total 500W

(pps)

8Li

15N

50

C

6.7*107

2.4*106

1.6*106

11C

18O

50

C

8.0*107

2.2*107

1.2*107

13N

14N

50

C

2.1*108

5.0*107

3.6*107

14O

16O

50

C

2.6*107

1.1*106

7.0*105

15O

16O

50

C

1.5*108

6.0*107

3.8*107

19O

22Ne

50

C

1.3*107

4.7*106

2.2*106

22O

26Mg

50

C

8.0*103

2.6*103

1.0*103

17F

20Ne

50

C

4.6*105

1.3*105

6.5*104

18F

19F

50

C

1.2*107

4.7*106

2.5*106

20Na

24Mg

50

C

1.0*107

1.4*104

6.0*103

22Na

19F

50

C

6.0*107

7.6*104

4.0*104

24Na

19F

50

C

1.6*107

2.0*104

1.1*104

33Cl

35Cl

50

C

1.4*106

3.1*105

9.0*104

34Cl

35Cl

50

C

1.1*108

2.5*107

3.2*106

Effects on reaction and/or fusion cross-section induced by weakly bound nuclei above the Coulomb barrier?According to this systematic study there is a hindrance of fusion cross-section in reaction between light weakly bound nuclei above the Coulomb barrier.Example :

Fusion excitation function measured with an activation techniqueEvaporation Residues produced in fusion reaction are radioactive and decay by Electron Capture.Discrimination of E.R. by X-ray energies and half-lives.

Titolo