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Reaction Mechanisms with low energy RIBs: limits and Reaction Mechanisms with low energy RIBs: limits and perspectivesperspectives

Alessia Di PietroAlessia Di Pietro

INFN-Laboratori Nazionali del SudINFN-Laboratori Nazionali del Sud

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Radioactive Ion Beams: many new problems can beRadioactive Ion Beams: many new problems can be studied…studied…

Using the radioactive beams Using the radioactive beams available today available today

one can study reactions one can study reactions induced by induced by

proton or neutron rich nuclei. proton or neutron rich nuclei.

Some of such nuclei have low break-up Some of such nuclei have low break-up thresholds. In some cases thresholds. In some cases

like like 1111Li,Li,1111Be,Be,66He,He,the last weakly bound nucleon(s) the last weakly bound nucleon(s)

form a large diffuse HALO form a large diffuse HALO around a well bound core. around a well bound core.

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rr

VVpp

Nuclear HaloNuclear Halo

Nuclear Halo can show-up if a Nuclear Halo can show-up if a ss o o pp bound state close to the emission bound state close to the emission particle threshold.particle threshold.

Low binding energy ( < 1 MeV) of Low binding energy ( < 1 MeV) of outer nucleons make possible outer nucleons make possible quantum tunneling of such nucleons quantum tunneling of such nucleons outside nuclear core.outside nuclear core.

Halo states:Halo states:

bound states whose wave function extends to classical forbidden regionbound states whose wave function extends to classical forbidden region

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The study of reactions and in particular fusion at low bombarding energies in collision induced by halo but also weakly bound nuclei is an important issue since it gives a great incentive to better understand the continuum.

Which are the theoretical expectations?Which are the theoretical expectations?Which the experimental methods adopted?Which the experimental methods adopted?Limits of the results obtained with the present facilities.Limits of the results obtained with the present facilities.Have we learned something?Have we learned something?

Reaction mechanisms around the barrierReaction mechanisms around the barrier

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Effect of halo behaviour on reaction mechanism:Effect of halo behaviour on reaction mechanism:

Static effects Static effects due to long tail in density distribution: due to long tail in density distribution:

longer tail in ion-ion potential, lowering of Coulomb barrier, longer tail in ion-ion potential, lowering of Coulomb barrier,

larger sub-barrier fusion probabilities, etclarger sub-barrier fusion probabilities, etc

Dynamical effectsDynamical effects due to coupling to states in the continuum: due to coupling to states in the continuum:

polarization term in optical potential, effect on sub-barrier polarization term in optical potential, effect on sub-barrier

fusion, etc..fusion, etc..

Well established that coupling of colliding nuclei relative motion to Well established that coupling of colliding nuclei relative motion to intrinsic excitations or other open reaction channels causes large intrinsic excitations or other open reaction channels causes large enhancement of fusion cross-section at sub-barrier energies over enhancement of fusion cross-section at sub-barrier energies over prediction of simple penetration models. prediction of simple penetration models.

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EEcmcm(MeV)(MeV)

CF

CF

(mb

) (

mb

)

Fusion excitation function for: Fusion excitation function for: 5858Ni + Ni + 58,6458,64Ni and Ni and 6464Ni + Ni + 6464NiNi

M. Beckerman et al. Phys. Rev. Lett 45 (1980) 1472 , M. Beckerman et al. Phys. Rev. C23 (1981) 1581 M. Beckerman et al. Phys. Rev. C25 (1982) 837 12

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Some examples of different theoretical predictionsSome examples of different theoretical predictions

K.Hagino, et al. PR C 61,037602 (2000) A.Diaz-Torres, et al. PR C 65,024606 (2002) M.Ito et al. PL B637,53,(2006)

CDCC calculations CDCC calculations+continuum-

continuum coupling

Time evolution of a three body system: core, halo, target.

11Be+208Pb 11Be+208Pb11Be+209Bi

10Be+209Bi

a)CDCC calculations a)CDCC calculations coupling between ground state and continuum up to 2 MeV. coupling between ground state and continuum up to 2 MeV.

b)CDCC calculations b)CDCC calculations 1111Be 1Be 1ststexcited state and continuum-continuum coupling. Continuum excited state and continuum-continuum coupling. Continuum considered up to 8 MeV.considered up to 8 MeV.

c) Time dependent wave packet approach c) Time dependent wave packet approach Interaction: halo-core, core-target, halo-targetInteraction: halo-core, core-target, halo-target

Results depend upon phase space in the continuum (considered range of energies and relative Results depend upon phase space in the continuum (considered range of energies and relative angular momentum). Other approximations considered in the calculations.angular momentum). Other approximations considered in the calculations.

a) b) c)

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Present facilities where low energy beams of halo nuclei have been used.Present facilities where low energy beams of halo nuclei have been used.

ISOL beams:ISOL beams:

Louvain la Neuve Louvain la Neuve 66He (no more RIBs available from next summer)He (no more RIBs available from next summer)

REX-ISOLDE REX-ISOLDE 1111BeBe

SPIRAL SPIRAL 66HeHe

Dubna Dubna 66HeHe

Fragmentation beams:Fragmentation beams:

Riken Riken 1111Be (after energy degradation)Be (after energy degradation)

In flight separated beams: In flight separated beams:

Notre Dame (Notre Dame (66He)He)

Available beam intensities 10Available beam intensities 1055÷10÷107 7 ppspps

The required intensities ..... comparable with stable beam intensities!The required intensities ..... comparable with stable beam intensities!

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Experimental methods Experimental methods

Different techniques have been used for the detection of the reaction Different techniques have been used for the detection of the reaction

products: Silicon strip arrays, products: Silicon strip arrays, detectors, X-ray detectors, n detectors… detectors, X-ray detectors, n detectors…

ProblemsProblems:: low beam intensity and small cross-sections low beam intensity and small cross-sections low ratelow ratehigh high

backgroundbackground

Fusion channel identification Fusion channel identification ::

Heavy targets Heavy targets Fission Fragments Fission Fragments

Lighter targets Lighter targets Evaporation Residues Evaporation Residues

but…but…

Direct ER detection difficultDirect ER detection difficult

Activation techniques: Activation techniques: Detection of Detection of particles, X- particles, X-

rays or rays or following the ER radioactive decay.following the ER radioactive decay.

Alternative technique: characteristic Alternative technique: characteristic rays (but rays (but

very efficient detection systems needed)very efficient detection systems needed)

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66He+He+238238U: fission cross sectionU: fission cross sectionISOL beam ISOL beam ~~101066 pps pps

Experimental set-upExperimental set-up

6He

The strong enhancement of the The strong enhancement of the fission cross-section comes from fission cross-section comes from transfer reactions.transfer reactions.

R.Raabe et al. Nature 431(2004)823

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4,64,6He+He+6464Zn fusion excitation functionZn fusion excitation function

ISOL beam 10ISOL beam 1066 pps pps

Experimental technique: Off-Line X-ray detection.Experimental technique: Off-Line X-ray detection.

44He + He + 6464ZnZn

66He + He + 6464ZnZn

Beam

64Zn targets Nb catcherSi-Strip

Si-strip

Experimental set-Experimental set-upup

A. Di Pietro Europhys. Jour. Special Topics 150, 15 (2007)A. Di Pietro et al. Phys.Rev.C 69(2004)044613

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6He Exp. data from: J.J. Kolata et al: Phys.Rev.Lett.81(1998)4580Comparison from: N.Alamanos et al: Phys.Rev.C65,054606,(2002)

Enhancement of fusion cross-Enhancement of fusion cross-section below the Coulomb barrier section below the Coulomb barrier is observed when compared with is observed when compared with 44He+He+209209Bi cross-section or Bi cross-section or calculations.calculations.

66He+He+209209Bi fusion cross sectionsBi fusion cross sections

In-flight separated beam 10In-flight separated beam 106 6 ppspps

Experimental technique: off-line Experimental technique: off-line detection detection

6He+209Bi4He+209Bi 2n+3n+4n4He+209Bi 1n

Fusion excitation functionFusion excitation function

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66He+He+206206Pb collision: fusion cross sectionsPb collision: fusion cross sections66He ISOL beam degraded in energy.He ISOL beam degraded in energy.

Experimental technique: off-line Experimental technique: off-line particle detection. particle detection.

A A largelarge enhancement of enhancement of the fusion cross section for the fusion cross section for the the 66He+He+206206Pb is claimedPb is claimed

Yu. E. Penionzhkevic et al. Phys. Rev. Lett. 96 (2006)162701Yu. E. Penionzhkevic et al. Phys. Rev. Lett. 96 (2006)162701

2n

1n (measured)

(calculated)

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1111Be+Be+209209BiBi

Energy degraded fragmented beam i Energy degraded fragmented beam i ~~101055 pps pps

Experimental technique: off-line Experimental technique: off-line detection+ FF detection detection+ FF detection

9,10,119,10,11Be+Be+209209BiBi

C.Signorini et al. Nucl.Phys.A 735(2004)329

No differences of fusion cross-section No differences of fusion cross-section are observed among the different Be are observed among the different Be isotope induced fusion reactions. Only isotope induced fusion reactions. Only statistical errors considered.statistical errors considered.

Beam profileBeam profile

spectrum from radioactive decayspectrum from radioactive decay

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Other reaction mechanismsOther reaction mechanisms

E.F. Aguilera Phys.Rev.C 63(2001)061603

A strong particle yield due to transfer+B.U. events is observed. The associated cross section saturates 80% of total reaction cross section at the barrier and almost all the

total reaction cross section below the barrier.

-n angular correlation measurements suggest that about 20% of the particle yield is due to 1n transfer events whereas the remaining 80% is shared between 2n transfer

and break-upJ.P.Bychowski et al. Phys. Lett. B 596,26,(2004)

fusfus

reac Ecm=12.4 MeVT+bu =1200150 mb

T+buT+bu//RR 80% 80%

6He+64Zn particle a.d.

A. Di Pietro et al. Phys.Rev.C 69(2004)044613

6He+209Bi

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11Be+209Bi

17F+208Pb

In these two cases R ~ Fus.

R similar to reaction induced

by well bound systems.

No strong direct reaction

process contribution.

For other systems studied….

M.Mazzocco et al. EPJ A28,295(2006)

M.Romoli et al. PRC 69,064614(2004)

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From the data so far collected a completely clear picture of structure effects of halo From the data so far collected a completely clear picture of structure effects of halo bound nuclei on reaction mechanisms is still not available and the role of break-up has bound nuclei on reaction mechanisms is still not available and the role of break-up has still to be understood still to be understood

The experiments so far performed have reached their limit, only little improvements The experiments so far performed have reached their limit, only little improvements can be done.can be done.

New data (possibly taken in exclusive experiments and/or experiments looking at once New data (possibly taken in exclusive experiments and/or experiments looking at once at elastic + all open reaction channels) are needed for a complete understanding of the at elastic + all open reaction channels) are needed for a complete understanding of the reaction dynamics in collision around the Coulomb barrier. reaction dynamics in collision around the Coulomb barrier.

More precise experiments extending to lower energies below the barrier should be More precise experiments extending to lower energies below the barrier should be performed. performed. This is not possible with the existing facilities higher intensities mandatory This is not possible with the existing facilities higher intensities mandatory (EURISOL).(EURISOL).

Summarising

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What could be done with stable beam intensities?What could be done with stable beam intensities?

M.Dasgupta et al. Phys.Rev. C70,024606,(2004) M.Dasgupta et al. Phys.Rev. C70,024606,(2004)

99Be+Be+208208PbPb

1 35 40 45 50 Ec.m. (MeV)

EEcmcm(MeV)(MeV)

CF

CF

(mb

) (

mb

)

M. Beckerman et al. Phys. Rev. Lett 45 (1980) 1472 , M. Beckerman

et al. Phys. Rev. C23 (1981) 1581 M. Beckerman et al. Phys. Rev. C25

(1982) 837 12

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Will this be possible at EURISOL?

Beam currents much higher then the ones currently available (in some cases comparable with stable beam currents). Possibility to detect different reaction products (e.g. neutrons, ) with the new, more efficient detection systems which will be available at EURISOL to discriminate the different reaction mechanisms. More species available.