Isospin dependence of nucleus-nucleus collisions
Martin Veselský
Institute of Physics of Slovak Academy of Sciences, Bratislava, Slovakia
Variation of isospin asymmetry ( N/Z - ratio ) in nucleus-nucleus collisions
- access to the limits of nuclear stability and production of exotic nuclei
- influence of isospin asymmetry on reaction scenarios
- isospin dependence of nuclear equation of state
Exotic nuclei
- only 2500 out of approx 6000 possible nuclei known - large region of very neutron-rich nuclei still unknown- region of superheavy nuclei
Physics questions
- properties of nuclear surface ( neutron skin )- nuclear astrophysics ( r-process ) - hyperheavy nuclei, nuclear molecules
Superheavy nuclei - up to Z=112 produced in cold fusion ( Pb, Bi-target, 1n-channel ), heavier nuclei produced in hot fusion ( 3-4n-channel )
Production of exotic nuclei in relativistic fragmentation Separator FRS ( GSI Darmstadt ), beam energy 1 GeVA, Separátor LISE ( GANIL ), beam energy 70-100 MeVA.
FRS
Observed excess of neutron-rich nuclei in reactions 86Kr+64Ni at 25 AMeV ( right ) and 124Sn+124Sn pri 20 AMeV ( left ). Experiments at fragment separator MARS ( Cyclotron Lab, Texas A&M University ).
EURISOL
- 6FP Project "EURISOL Design Study" started in Feb 2005 ( MV/Bratislava involved ) http://www.eurisol-ds.lnl.infn.it/
- theoretical and experimental studies of production mechanisms are planned
- open question - Heavy Ion capability for driver accelerator - yes or no ?
- selection of the key experiment(s)
How to reach the extremely neutron-rich nuclei:
- What is the optimum beam energy ? Lower than 20 AMeV ?
- Two-step scenario ?
Production of extremely neutron-rich nuclei - two-step process. Experiment approved at GSI ( a part of EURISOL effort ).
Primary target
Separation of secondary beam
Secondary target Identification of final products
- peripheral elastic and quasi-elastic ( QE ) collisions - semi-peripheral deep-inelastic collisions ( DIT ) collisions - incomplete ( ICF ) and complete ( CF ) fusion in central collisions - pre-equilibrium emision typically preceding ICF/CF and DIT
( for details see M. Veselský, Nuclear Physics A 705(2002)191 )
Nucleus-nucleus collisions at beam energies below 100 AMeV:
Central collisions
reaction: 124Sn+27Al at 20 AMeV ( M. Veselsky et al., NPA 724 (2003) 431 )
test of the model description of central collisions( M. Veselsky, NPA 705 (2002) 193 )
Model description ( M. Veselsky, NPA 705 (2001) 191 ) :
- classical Coulomb trajectories assumed- pre-eq. emission multiplicity proportional to maximum overlap- participant-spectator scenario assumed at maximum overlap- incomplete fusion of the participant zone with one of the spectators
Conclusions - 124Sn+27Al at 20 AMeV
- description of central collision as PE+ICF is consistent, including kinematics and isospin asymmetry of heavy residues
- SMM ( multifragmentation ) describes the de-excitation of hot nuclei better than GEMINI ( sequential binary decay )
Peripheral collisions ( M. Veselsky, G. Souliotis, nucl-th/0507026 ) :
reactions 86Kr + 64Ni, 112,124Sn at 25 AMeV
effect of nuclear periphery ( neutron skin ? )
Standard DIT ( Tassan-Got and Stefan, NPA 524 (1991) 121 )
86Kr + 64Ni at 25 AMeV
solid - GEMINI, dash -SMM
Correlation of skin thickness to isovector chemical potential(V. Kolomietz et al, PRC 64(2001)024315)
Modified DIT (nucl-th/0507026):86Kr + 64Ni at 25 AMeV
Pn PŒT Œe 0.5À º SnP º S pP º S nT ƒ º S pT Pn PŒT
S... = S
...exp - S
...mac , = 0.53, s > 0
solid - GEMINI, dash -SMM
86Kr + 124Sn at 25 AMeV
Standard DIT ( Tassan-Got )
Modified DIT (nucl-th/0507026):
solid - GEMINI, dash -SMM
86Kr + 112Sn at 25 AMeV
Standard DIT ( Tassan-Got )
Modified DIT (nucl-th/0507026):
solid - GEMINI, dash -SMM
dash-dotted - SMM, s > 0.8 fm
Conclusions 86Kr + 64Ni, 112,124Sn at 25 AMeV
- a correction to DIT describes the effect of isospin asymmetry at nuclear periphery
- inversion of the bulk isospin flow due to microscopic structure at nuclear periphery
- consistent parameters for all reactions
- SMM reproduces the yields of n-rich species well while overestimating the yields of -stable isotopes close to the projectile
- GEMINI typically overestimates the width of mass distributions
- for p-rich target 112Sn stronger Coulomb interaction supresses the effect of isospin asymmetry at nuclear periphery at s < 1 fm
18O + 181Ta at 35 AMeV, beryllium isotopes
7Be - fast component, intense pre-eq emission, ICF kinematically impossible, motion along classical Coulomb trajectory ? Transparency ?
Experiment (COMBAS) PE+DIT/ICF+SMM
7Be
7Be
Multifragmentation- demonstration of phase transition in dilute nuclear matter ?
Sauer, Chandra, Mosel, NPA 264(1976)221
Isoscaling in nuclear processes
M.B. Tsang et al., PRL 86(2001)5023
G. Souliotis et al., PRC 68(2003)24605
M. Veselský et al., PRC 69(2004)44607
Conclusions:
- way to extremely neutron-rich nuclei has still to be found
- interesting effects can be observed when comparing high resolution experimental data to precise simulations
- tracing of isospin evolution is complicated by de-excitation
- microscopic structure at nuclear periphery leads to isospin evolution away from isospin equilibrium
- transparent mean field ?
- isospin-asymmetric liquid-gas phase transition indicated by correlated signals such as evolution of chemical potentials, caloric curve and kinematic temperatures of fragments
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