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In-medium properties of nuclear fragments at the liquid-gas phase coexistence

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International Nuclear Physics Conference

INPC2007

Tokyo, Japan, June 3-8, 2007

A.S. Botvina1,2,3

(In collaboration with W.Trautmann, I.Mishustin, N.Buyukcizmeci, R.Ogul)

1Institute for Nuclear Research, Russian Academy of Sciences,

Moscow, Russia

2Frankfurt Institute for Advanced Studies, J.W.Göthe University,

Frankfurt am Main, Germany

3Gesellschaft für Schwerionenforschung, Darmstadt, Germany

Multifragmentation of nuclei takes place in reactions initiated by all high energy

particles (hadrons, heavy-ions, photons), where high excitation energy of residual

nuclei is reached.

Experimentally established:1) few stages of reactions leading to multifragmentation,

2) short time ~100fm/c for primary fragment production, 3) freeze-out density is

around 0.1ρ0 , 4) high degree of equilibration at the freeze-out.

Thermal multifragmentation of nuclei: Production of hot fragments at temperature T ~ 3---8 MeV and density ρ ~ 0.1 ρ0 (ρ0≈0.15 fm-3) Interpretation: liquid-gas phase transition in finite nuclei. Investigation of properties of fragments surrounded by nuclear species.

J.P. Bondorf, A.S. Botvina, A.S. Iljinov, I.N. Mishustin, K. Sneppen, Phys. Rep. 257 (1995) 133

Ensemble of nucleons and fragments

in thermal equilibrium characterized by

n

IMF

p

neutron number N0

proton number Z0 , N0+Z0=A0

excitation energy E*=E0-ECN

break-up volume V=(1+k)V0

HR

IMF

a

IMF

All break-up channels are enumerated by the sets of

fragment multiplicities or partitions, f={NAZ}

Statistical distribution of probabilities:

Wf ~ exp {Sf (A0, Z0, E*,V)}

under conditions of baryon number (A), electric charge (Z) and energy (E*) conservation

Probability of channel:

mass and charge

conservation

Energy conservation

entropy of channel

Fragments obey Boltzmann statistics, liquid-drop description of individual fragments, Coulomb interaction in the Wigner-Seitz approximation

free energy of channel:

individual fragments:

ALADINdata

GSI

multifragmentation of

relativistic projectiles

A.S.Botvina et al.,

Nucl.Phys. A584(1995)737

H.Xi et al.,

Z.Phys. A359(1997)397

comparison with

SMM (statistical

multifragmentation

model)

Statistical equilibrium

has been reached in

these reactions

The surface (B0) and symmetry (γ) energy coefficients

in the multifragmentation scenario

Fsym = γ·(N-Z)2/A

Fsuf = B0f(T)A2/3

ALADIN: 12C+ 112,124Sn A.Le Fevre et al., Phys.Rev.Lett 94(2005)162701

S(N)=Y(124Sn)/Y(112Sn)=C∙exp(N∙α+Z∙β)

α·T≈ -4γ (Z12/A12-Z22/A22)

25AMeV

Z/A

γ=25

γ=15

1AGeV

A

The symmetry energy coefficient γ and isospin of fragments

A.S.Botvina et al., PRC72(2005)048801

G.Souliotis et al., PRC75(2007)011601

One can distinguish effects of the surface and symmetry energies since

the charge yield of fragments is very sensitive to the surface:

A.S.Botvina et al., PRC74(2006)044609

Fsuf = B0((Tc2-T2)/(Tc2+T2))5/4A2/3

Fsym = γ·(N-Z)2/A

projectiles with different isospin

SMM

ALADIN

A.S.Botvina et al., PRC74(2006)044609

We analyze all previous observables: distributions of IMF , Zmax , T , ...

vs Zbound , and involve additionally new τ - observables for each

projectile (Xe, Au, U)

for single isolated nuclei:

C -- Cameron mass formula (1957)

MS -- Myers-Swiatecki mass formula

(1966)

(include separate volume and surface

contributions to the symmetry energy)

We obtain an evolution of the surface energy of hot

fragments toward region of full multifragmentation

Conclusions

Multifragmentation reactions can be interpreted as a manifestation of the liquid-gas

type phase transition in finite nuclei, and allow for investigating the phase diagram of

nuclear matter. One can investigate properties of hot nuclei/fragments surrounded by

other nuclear species.

By analyzing experimental data it was found:

-- decreasing the symmetry energy of primary hot fragments by ~ 40% when the

systems evolve toward full multifragmentation (with increasing excitation energy

and decreasing the freeze-out density): ALADIN, FRS, MARS;

-- as a result of the same process the surface energy of these fragments becomes

independent on their isospin, this means that the difference between surface and

volume symmetry energies (as adopted in some mass formulas for isolated nuclei)

disappears also: ALADIN.

Important applications in astrophysics:

since mass distributions of fragments in

stellar matter, and electro-weak reactions

are very sensitive to the symmetry energy

A.Botvina and I.Mishustin, PRC72(2005)048801