Formation and Decay of Highly Excited Nuclear Matter in Intermediate Energy Heavy-Ion Collisions

S. Hudan , R. Yanez, B. Davin, R. Alfaro, H. Xu, L. Beaulieu, Y. Larochelle, T. Lefort, V. Viola and R.T. de

Souza

Department of Chemistry and Indiana University Cyclotron Facility, Indiana University, Bloomington, Indiana 47405

R. J. Charity and L. G. Sobotka

Department of Chemistry, Washington University, St. Louis, Missouri 63130

T.X. Liu, X.D. Liu, W.G. Lynch, R. Shomin, W.P. Tan,M.B. Tsang, A. Vander Molen, A. Wagner, H.F. Xi,

and C.K. Gelbke

National Superconducting Cyclotron Laboratory and Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824

Nucleosynthesis, Terra Incognita, and the EOS

Radioactive beams (e.g. at RIA) will allow us to probe the N/Z degree-of-freedom

Neutrons

Pro

tons

Stable Nuclei

Known Nuclei

Terra Incognita

N/Z(isospin)

Nuclei are two-component systems (neutrons and protons), the N/Z of the system affects the phase diagram.

Need to know not just ground-state of unstable nuclei (masses,

shapes, etc) but also excited states, level densities, etc.

H. Müller and B.D. SerotPhys. Rev. C 52, 2072 (1995)

Phase transitions for finite systems

Transition from one phase to an other at constant T

Constant PInfinite matterClosed system

“Caloric curve” for nuclear matter

Liquid phase

Gas phase

Liquid-gas coexistenceBOILING

J. Pochodzalla et al., PRL 75, 1040

(1995)1. Why do we observe a caloric curve for a

system which is not infinite, not closed, and not isobaric?

2. If in the plateau region the liquid and the gas are in coexistence, what is the liquid and what is the gas?

Tc = CT(K/m*)ρs-1/3

•Tc = critical temperature• K = nuclear compressibility• m* = effective nucleon mass• ρs = saturation density• CT = constant

Liquid phase

Gas phase?

The system is boiling at a constant T

J. Natowitz et al., PRC65, 034618 (2002)

P. Bonche et al., NP A436, 265 (1985)

J. Natowitz et al., PRC 65, 034618 (2002)PRL 89, 212701 (2002)

Differences in caloric measurements may be related to differences in size of fragmenting system (different finite size, Coulomb energy, isospin ) Limiting temperature

50 100 150 200 250A

Limiting temperature for A=90 system is 6-7 MeV

Caloric curves

Tc

How do we create highly excited nuclear matter?

BUU collision

Method A: Multi-GeV p, - collisions with a nucleus.

C. Mader, Hope CollegeBUU: 5 GeV p + Pb

• , N*•

b=1fm

Stage 1: Excitation of target nucleus by excitation of resonance.

Heating with minimal compression

Stage 2: Disassembly of excited nucleus into light charged particles (LCP:Z ≤2) and intermediate mass fragments (IMF:3 ≤ Z ≤20)

Ejection of fast pre-equilibrium particles

BUU = two-body collisions within a mean fieldNo inherent fluctuations in the field

Interaction stage Equilibrium?

Description of the reaction process

t = 0 t = 100-150 fm/c t =

time//

ThermodynamicsDynamics LINK?

Collision of a nucleus with a light-ion (Z<3) or a heavy-ion (Z>2) converts kinetic energy of relative motion into intrinsic excitation i.e. heats the

nucleus.

From the debris – the fragmentation pattern we need to determine what happened

• identity of all the particles

• number of clusters (Z>2)

• number of light particles Z=1,2

• energy of all the particles

• angles of all the particles V.E. Viola and K. Kwiatkowski, American Scientist 86,449 (1998)

Reconstructing a collision

E detector

Incident particle with

(Z,A,E)dx

E detector

dE = Z2A

dx E

Identifying the reaction products

• 162 individual telescopes covering 74% of 4

• Gas Ionization chamber/500 µm Si(IP)/CsI(Tl(PD)

• Each telescope measures Z,A, E, and

• Identification of Z for 0.6≤E/A≤96 MeV

• Identification of A for E/A ≥ 8 MeV for Z≤4

ISiS: Indiana Silicon Sphere

Probability for emitting one or more IMF exceeds probability for emitting none.

Charge distribution (Z) becomes flat.

Onset of an expansion

IMF Emission time becomes very

short

“If we were at equilibrium we would not only be dead, we would be homogenous”

S. Nagel, FermiNews and Physics Today (September 2002)

L. Beaulieu et al., PRL 84 5971 (2000)

Several quantities tell us that something unusual happens at E*/A=4-6 MeV for a Au nucleus

Liquid-gas Phase transition?

How do we create highly excited Nuclear matter?

Method B: Intermediate (20 ≤ E/A ≤ 200 MeV) energy heavy-ion collisions

1 fragment

vH>vL vL>vH

2 fragments fragmentation

E*, J

PLF*

TLF*

► Central collisions (head-on collisions)► Peripheral collisions (glancing collisions)

PLF* ≡ excited projectile-like fragment

TLF* ≡ excited target-like fragment

Ring Counter :Si (300 m) – CsI(Tl) (2cm)2.1 lab 4.21 unit Z resolutionMass deduced†

Beam

LASSA : 0.8Mass resolution up to Z=97 lab 58

114Cd + 92Mo at 50 A.MeV

Detection of charged particles in 4

† : Modified EPAX K. Sümmerer et al., PRC 42, 2546 (1990) Projectile

48

Experimental details

How do we create highly excited Nuclear matter?

Method B: Intermediate (20 ≤ E/A ≤ 200 MeV) energy heavy-ion collisions

Different reaction types as a function of centrality

: Charged Particles: Z and A; Neutrons; Gammas

P

T

bRP+RT

PLF

TLF

PeripheralCentral Mid-Peripheral

PLF

TLF

Degrees of freedom : E*, J, density, shape, N/Z

Multifragmentation (most excited systems)

Binary exit channel + statistical decay (relatively

gentle collisions)

Neck fragmentation (shape instability)

Conventional wisdom

Chemical equilibrium: different partitions are

populated according to their statistical weights.

Kinetic energy spectra fit Maxwell-Bolztman distribution: P(E) exp(-E/Tslope)

Angular distribution emission time as compared to rotation time

Kinetic equilibrium: motion of all

particles reflects a common temperature

Thermometers

F. Zhu et al., PRC52, 784 (1995)

Emitting system

10B

6Li

Relative energy spectrum of daughters reflects internal quantum levels of parent

Pm = (2Jm+1)e-(E*-Em/T)

Pm/Pn = (2Jm+1)/(2Jn+1)e-(En-Em)/T

Extract temperature T

114Cd

92Mo

Participant (Overlap zone) is highly

excited

1. Projectile and target-like nuclei are relatively unexcited

2. Velocity of PLF* nearly unchanged from beam velocity

3. Overlap of projectile and target is the key quantity in the reaction

Conventional wisdom (participant-spectator model)PLF*

TLF*Shearing

mechanism

Select fragments at very forward angles 2.1 lab 4.2

spectator

spectator

PLF* decay following a peripheral collision

PLF* = good case: (as compared to central collisions)System size (Z,A) is well -defined Normal densityLarge cross-section (high probability process) 0

Circular ridge PLF* emission“Isotropic” component

Projectile velocity

Other emission(mid-rapidity, ...)

KE spectra in PLF* frame selected on VPLF*

Decreasing VPLF*,

increasing dissipation, increasing excitation

• Decay of PLF* dominated by a single exponential (statistical evaporation).

• Pre-equilibrium emissions comprise at most 2% of the yield.

• Systematic increase of exponential slope with decreasing VPLF*

• 6He exhibit systematically higher

slope parameters (temperatures) emission from hotter sources possibly earlier in the de-excitation cascade.

Multiplicities increase with velocity damping

Tslope increases with velocity damping “Linear” trend for both observables

Evaporation and velocity damping

# emitted from the PLF* in a given

collision

(Linear) dependence of E* with velocity damping

High E* is reached (6 MeV/n), consistent with the beginning of the plateau

in the caloric curve.

Velocity damping and excitation energy

Reconstruct excitation of PLF* by doing calorimetry: particle multiplicity, kinetic energies, and binding energies.

Good agreement with GEMINI Some sensitivity of M to J, level density

“Statistical model code” supports E*/A scaleR.J. Charity et al., PRC63, 024611 (2001)

• Select PLF* size by selecting residue Z.

• Select excitation by selecting VPLF*

• Vary N/Z by changing (N/Z)proj.,tgt.

To study N/Z dependence of EOS:

Total excitation of PLF* depends on velocity damping and is relatively independent of PLF size.

Results are consistent with following scenario:

1. For each impact parameter a distribution of contact times exists.

2. While impact parameter determines the size of the PLF*, it is contact time that determines the velocity dissipation and excitation of the PLF*.

What causes the distribution of contact times? Mean field fluctuations?

Thermodynamic SummaryWe can create highly excited nuclear systems by:

High energy p, + A collisions

Central collisions of two heavy-ions at intermediate energies

Peripheral collisions of two heavy-ions at intermediate

energies (Excitation connected with velocity dissipation not

overlap!)

PLF* decay Access to highly excited well-defined system Explore same E* for different system size Radioactive beams

Exploration of EOS (mass and N/Z)

Dynamics: The two fragment case

1 fragment

vH>vL vL>vH

2 fragments fragmentation

E*, J

PLF*

TLF*

Binary breakup: PLF* reconstruction

ZH

ZL

ZHZL

PLF*

vL > vH

vH > vL

LH*PLF ZZZ )f(ZAA *PLFL*PLF HA

*PLF

LLHH*PLF

A

vAvAv

If the PLF*, subsequent to the collision process, decays statistically we expect both cases to be the same.

6 NC 10

Different charge correlation

Different alignments

Different relative velocities

B. Davin et al., PRC 65, 064614 (2002)

*PLFvrelv

Process characterization

Dynamical process appears at higher velocity lower damping lower excitation Up to 10% of the cross-section in binary breakup

1 fragment (x 0.1)

dynamical

statistical

Process probability : channel opening

More kinetic energy in the fragments for the dynamical caseFor a given velocity damping, difference of 20-30 MeV

statistical

dynamical

(L)E(H)ETKE PLF*k

PLF*k

Energy transferred to the fragments

A picture of the process

TimeSaddle-point Scission-

point

TKE

Q

Coulomb

Collective

Initial kinetic energy?

Deviation of TKE from (Q+Coulomb)

“Extra” energy

Time scale?

Dynamics : a new process?

As compared to standard fission, the dynamical process has :

Large asymmetry

Strong alignment

Lower E* threshold

Large kinetic energy in the 2 fragments, for all E*

Same dependence of TKE with E*

Do we have a new process?

Process with a large cross-section

E432@GANIL Caen, France

50 neutron TOF detectors (DEMON) to measure neutrons (KE spectra, multiplicities, free n/p at mid-rapidity)`

FIRST and LASSA are highly segmented 600 Si channels together with ISiS 1000 channels

Measure Z,A,E,

ISiS

LASSA

FIRST

124,136Xe + 112,124Sn at E/A=50 MeV

FIRST :Forward Indiana Ring Silicon

Telescopes

T1 : 200 m Si(IP), S2/ 1mm Si(IP), S2/ 2-3cm CsI(Tl)At 28 cm, = 2.25-7.05 with = 0.1

T2 : 300 m Si(IP), S1/ 2-3cm CsI(Tl) At 19 cm, = 7.37-14.5 with = 0.4

T3 : 300 m Si(IP), S1/ 2-3cm CsI(Tl) At 9 cm, = 15.2-28.5 with = 0.7

Device dedicated to measure the decay of the PLF* :

Limiting temperature Dynamical process PLF* fragmentation ...

Large number of channels use of ASIC

Design : P.H. Sprunger

HiRA Telescope Design

• 20 Telescopes • 62.3 x 62.3 mm2 Active Area• Pitch 1.8 mm• 1024 Pixels per telescope

4x CsI(Tl) 4cm

32 strips v. (front)Target Beam

Si-E 65 m

32 strips v (front)

Si-E 1.5 mm

pixel

32 strips h. (back)

(High Resolution Array)

Designed to study transfer reactions, resonance decay spectroscopy, etc with radioactive beams

Design characteristics

• =± 0.15° at 35cm• E/E=40 keV for 5 MeV ’s

Si(IP) specifics

•Bulk material is n type•Interstrip on junction side is 25 m•Interstrip on ohmic side is 40 m

•P+ implant for better interstrip isolation

•Depletion voltage for 1.5 mm detector < 500 V•10 guard ring structure on periphery (2mm dead area region)

Detectors are mounted on (G10) frames with a flexible polyimide cable for readout in tight packing geometry

Developed at IU/IUCF

Silicon detectors

Electronic Readout developed at Washington University (St. Louis) And Southern Illinois University, Edwardsville

With 2000 channels to readout, cost of “traditional” readout is prohibitive.

Design Includes:• Multiple Preamps (100 MeV, 250 MeV, external)• Slow Shaper and Timing Filter Amplifier• Discriminator (5 bit)• Time to amplitude converters

Design Characteristics1. Excellent energy resolution ( 25-40 keV) 2. Dynamically switchable range3. Excellent time resolution (~500 pS) 4. Sparsified readout of both energy and time information.

Application Specific Integrated Circuit

ASIC Chip

ULM for control of ASIC

ADC module, used for ALL 20 telescopes

Electronic ReadoutASIC 32 channels in 6mm x 6mm format (presently 16)

• Mid-peripheral collisions of two heavy-ions at intermediate energies (via PLF* decay) provides the opportunity to study phase diagram of nuclear matter as a function of isospin (with radioactive beams)

• It also allows one to study the dynamics of the collision process (equilibration of charge, mass, and energy) and dynamical decay.

Summary