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

Production of proton-rich nuclei - complete fusion + ISOL technique

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 ?

Fragment separator VAMOS ( GANIL Caen ) - angular acceptance 9 deg for beam energies 5-100 AMeV.

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:

Participant-spectator reactions at relativistic energies ( above 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

PE+DIT/ICF+GEMINI PE+DIT/ICF+SMM

124Sn+27Al at 20 AMeV

A=65-75

Single source: 144Nd, E*=310 MeV, v=0.17c

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, carbon isotopes

Experiment (COMBAS) PE+DIT/ICF+SMM

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

Phase diagram of nuclear matter

Multifragmentation- demonstration of phase transition in dilute nuclear matter ?

Sauer, Chandra, Mosel, NPA 264(1976)221

First order phase transition ?

Pochodzalla et al. PRL 75(1995)1040

GANIL data

Isospin-asymmetric liquid-gas phase transition

( figures from Muller, Serot, PRC 52(1995)2072 ).

Yield ratios and temperature

T(2,3H/3,4He)

T(3H/3He)

M. Veselsky et al., PLB 497 (2001) 1.

Distribution of isospin between phases ( M. Veselský et al., PRC 62(2000)41605 )

IMF

LCP

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

Isoscaling, chemical potencials, phase separation

M. Veselský et al., PRC 69(2004)31602.

Correlated signals of phase transition -

M. Veselsky et al., NPA 749 (2005) 114c.

Kinematic temperatures - signal of equilibration with nucleon gas

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

Extras

D. Henzlova et al, nucl-ex/0507003