ppt.ppt

Reaction Mechanisms with low energy RIBs: limits andperspectivesReaction Mechanisms with low energy RIBs: limits andperspectives

Alessia Di PietroAlessia Di Pietro

INFN-Laboratori Nazionali del SudINFN-Laboratori Nazionali del Sud

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

Using the radioactive beamsavailable todayUsing the radioactive beamsavailable today

one can study reactionsinduced byone can study reactionsinduced by

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

Some of such nuclei have low break-upthresholds. In some casesSome of such nuclei have low break-upthresholds. In some cases

like 11Li,11Be,6He,like 11Li,11Be,6He,

the last weakly bound nucleon(s)the last weakly bound nucleon(s)

form a large diffuse HALOform a large diffuse HALO

around a well bound core.around a well bound core.

VpVp

Nuclear HaloNuclear Halo

 Nuclear Halo can show-up if a s op bound state close to the emissionparticle threshold. Nuclear Halo can show-up if a s op bound state close to the emissionparticle threshold.

 Low binding energy ( < 1 MeV) ofouter nucleons make possiblequantum tunneling of such nucleonsoutside nuclear core. Low binding energy ( < 1 MeV) ofouter nucleons make possiblequantum tunneling of such nucleonsoutside 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

The study of reactions and in particular fusion at lowbombarding energies in collision induced by halo but alsoweakly bound nuclei is an important issue since it gives a greatincentive 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

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

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

longer tail in ion-ion potential, lowering of Coulomb barrier,larger sub-barrier fusion probabilities, etc longer tail in ion-ion potential, lowering of Coulomb barrier,larger sub-barrier fusion probabilities, etc

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

polarization term in optical potential, effect on sub-barrierfusion, etc.. polarization term in optical potential, effect on sub-barrierfusion, etc..

Well established that coupling of colliding nuclei relative motion tointrinsic excitations or other open reaction channels causes largeenhancement of fusion cross-section at sub-barrier energies overprediction of simple penetration models.Well established that coupling of colliding nuclei relative motion tointrinsic excitations or other open reaction channels causes largeenhancement of fusion cross-section at sub-barrier energies overprediction of simple penetration models.

Ecm(MeV)Ecm(MeV)

CF (mb)CF (mb)

Fusion excitation function for: 58Ni + 58,64Ni and 64Ni + 64NiFusion excitation function for: 58Ni + 58,64Ni and 64Ni + 64Ni

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

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 threebody system: core, halo,target.

11Be+208Pb

11Be+208Pb

11Be+209Bi

10Be+209Bi

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

b)CDCC calculations  11Be 1stexcited state and continuum-continuum coupling. Continuumconsidered up to 8 MeV.b)CDCC calculations  11Be 1stexcited state and continuum-continuum coupling. Continuumconsidered up to 8 MeV.

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

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

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 6He (no more RIBs available from next summer) Louvain la Neuve 6He (no more RIBs available from next summer)

 REX-ISOLDE 11Be REX-ISOLDE 11Be

 SPIRAL 6He SPIRAL 6He

 Dubna 6He Dubna 6He

Fragmentation beams:Fragmentation beams:

 Riken 11Be (after energy degradation) Riken 11Be (after energy degradation)

In flight separated beams:In flight separated beams:

Notre Dame (6He) Notre Dame (6He)

Available beam intensities 105÷107 ppsAvailable beam intensities 105÷107 pps

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

Experimental methodsExperimental methods

Different techniques have been used for the detection of the reactionproducts: Silicon strip arrays,  detectors, X-ray detectors, n detectors…Different techniques have been used for the detection of the reactionproducts: Silicon strip arrays,  detectors, X-ray detectors, n detectors…

Problems:low beam intensity and small cross-sections low ratehighbackgroundProblems: low beam intensity and small cross-sections low ratehighbackground

Fusion channel identification :Fusion channel identification :

Heavy targets  Fission FragmentsHeavy targets  Fission Fragments

Lighter targets  Evaporation ResiduesLighter targets  Evaporation Residues

but…but…

Direct ER detection difficultDirect ER detection difficult

 

Activation techniques: Detection of  particles, X-rays or following the ER radioactive decay.Activation techniques: Detection of  particles, X-rays or following the ER radioactive decay.

Alternative technique: characteristic  rays (butvery efficient detection systems needed)Alternative technique: characteristic  rays (butvery efficient detection systems needed)

6He+238U: fission cross section6He+238U: fission cross section

ISOL beam ~106 ppsISOL beam ~106 pps

Experimental set-upExperimental set-up

6He

The strong enhancement of thefission cross-section comes fromtransfer reactions.The strong enhancement of thefission cross-section comes fromtransfer reactions.

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

4,6He+64Zn fusion excitation function4,6He+64Zn fusion excitation function

ISOL beam 106 ppsISOL beam 106 pps

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

4He + 64Zn4He + 64Zn

6He + 64Zn6He + 64Zn

Beam

64Zn targets

Nb catcher

Si-Strip

Si-strip

Experimental set-upExperimental set-up

A. Di Pietro Europhys. Jour. Special Topics 150, 15 (2007)

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

6He Exp. data from: J.J. Kolata et al: Phys.Rev.Lett.81(1998)4580

Comparison from: N.Alamanos et al: Phys.Rev.C65,054606,(2002)

Enhancement of fusion cross-section below the Coulomb barrieris observed when compared with4He+209Bi cross-section orcalculations.Enhancement of fusion cross-section below the Coulomb barrieris observed when compared with4He+209Bi cross-section orcalculations.

6He+209Bi fusion cross sections6He+209Bi fusion cross sections

In-flight separated beam 106 ppsIn-flight separated beam 106 pps

Experimental technique: off-line  detectionExperimental technique: off-line  detection

6He+209Bi

4He+209Bi 2n+3n+4n

4He+209Bi 1n

Fusion excitation functionFusion excitation function

6He+206Pb collision: fusion cross sections6He+206Pb collision: fusion cross sections

6He ISOL beam degraded in energy.6He ISOL beam degraded in energy.

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

A large enhancement of thefusion cross section for the6He+206Pb is claimedA large enhancement of thefusion cross section for the6He+206Pb is claimed

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

1n (measured)

(calculated)

11Be+209Bi11Be+209Bi

Energy degraded fragmented beam i ~105 ppsEnergy degraded fragmented beam i ~105 pps

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

9,10,11Be+209Bi9,10,11Be+209Bi

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

No differences of fusion cross-sectionare observed among the different Beisotope induced fusion reactions. Onlystatistical errors considered.No differences of fusion cross-sectionare observed among the different Beisotope induced fusion reactions. Onlystatistical errors considered.

Beam profileBeam profile

 spectrum from radioactive decay spectrum from radioactive decay

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 crosssection saturates 80% of total reaction cross section at the barrier and almost all thetotal reaction cross section below the barrier.

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

J.P.Bychowski et al. Phys. Lett. B 596,26,(2004)

fusfus

reac

Ecm=12.4 MeV

T+bu =1200150 mb

T+bu/R  80%T+bu/R  80%

6He+64Zn  particle a.d.

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

6He+209Bi

11Be+209Bi

17F+208Pb

In these two cases R ~ Fus.

R similar to reaction inducedby well bound systems.

No strong direct reactionprocess contribution.

For other systems studied….

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

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

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

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

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

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

Summarising

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)

9Be+208Pb9Be+208Pb

1 35 40 45 50

Ec.m. (MeV)

Ecm(MeV)Ecm(MeV)

CF (mb)CF (mb)

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

Will this be possible at EURISOL?

 Beam currents much higher then the ones currently available (in somecases comparable with stable beam currents).

 Possibility to detect different reaction products (e.g. neutrons, ) with thenew, more efficient detection systems which will be available at EURISOLto discriminate the different reaction mechanisms.

 More species available.