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

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,

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

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  1. 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

  2. Nucleosynthesis, Terra Incognita, and the EOS Need to know not just ground-state of unstable nuclei (masses, shapes, etc) but also excited states, level densities, etc. Stable Nuclei Known Nuclei Terra Incognita Protons Neutrons Nuclei are two-component systems (neutrons and protons), the N/Z of the system affects the phase diagram. N/Z H. Müller and B.D. Serot Phys. Rev. C 52, 2072 (1995) (isospin) Radioactive beams (e.g. at RIA) will allow us to probe the N/Z degree-of-freedom

  3. Phase transitions for finite systems Constant P Infinite matter Closed system  Transition from one phase to an other at constant T “Caloric curve” for nuclear matter J. Pochodzalla et al., PRL 75, 1040 (1995) • Why do we observe a caloric curve for a system which is not infinite, not closed, and not isobaric? • If in the plateau region the liquid and the gas are in coexistence, what is the liquid and what is the gas? Gas phase Tc = CT(K/m*)ρs-1/3 • Tc = critical temperature • K = nuclear compressibility • m* = effective nucleon mass • ρs = saturation density • CT = constant Liquid phase Liquid-gas coexistenceBOILING

  4. Caloric curves The system is boiling at a constant T 50 100 150 200 250 A Gas phase? Liquid phase Differences in caloric measurements may be related to differences in size of fragmenting system (different finite size, Coulomb energy, isospin ) Limiting temperature J. Natowitz et al., PRC65, 034618 (2002) Tc Limiting temperature for A=90 system is 6-7 MeV P. Bonche et al., NP A436, 265 (1985) J. Natowitz et al., PRC 65, 034618 (2002) PRL 89, 212701 (2002)

  5. Method A: Multi-GeV p, - collisions with a nucleus. How do we create highly excited nuclear matter? C. Mader, Hope College BUU: 5 GeV p + Pb BUU = two-body collisions within a mean field BUU collision No inherent fluctuations in the field Stage 1: Excitation of target nucleus by excitation of  resonance. b=1fm Heating with minimal compression • , N* •  Ejection of fast pre-equilibrium particles Stage 2: Disassembly of excited nucleus into light charged particles (LCP:Z ≤2) and intermediate mass fragments (IMF:3 ≤ Z ≤20)

  6. Description of the reaction process time // LINK? t =  t = 0 t = 100-150 fm/c ? Interaction stageEquilibrium Dynamics Thermodynamics

  7. Reconstructing a collision 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)

  8. Identifying the reaction products E detector dE = Z2A Incident particle with (Z,A,E) dx E dx E detector • 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

  9. Liquid-gas Phase transition? Probability for emitting one or more IMF exceeds probability for emitting none. Several quantities tell us that something unusual happens at E*/A=4-6 MeV for a Au nucleus Charge distribution (Z) becomes flat. Onset of an expansion IMF Emission time becomes very short L. Beaulieu et al., PRL 84 5971 (2000) “If we were at equilibrium we would not only be dead, we would be homogenous” S. Nagel, FermiNews and Physics Today (September 2002)

  10. Method B: Intermediate (20 ≤ E/A ≤ 200 MeV) energy heavy-ion collisions How do we create highly excited Nuclear matter? vH>vL vL>vH 2 fragments 1 fragment fragmentation ► Central collisions (head-on collisions) ► Peripheral collisions (glancing collisions) TLF* TLF* ≡ excited target-like fragment PLF* ≡ excited projectile-like fragment PLF* E*, J

  11. Experimental details Beam 48 Projectile 114Cd + 92Mo at 50 A.MeV LASSA : 0.8 Mass resolution up to Z=9 7lab 58 Ring Counter : Si (300 m) – CsI(Tl) (2cm) 2.1lab 4.2 1 unit Z resolution Mass deduced†  Detection of charged particles in 4p † : Modified EPAX K. Sümmerer et al., PRC 42, 2546 (1990)

  12. How do we create highly excited Nuclear matter? Method B: Intermediate (20 ≤ E/A ≤ 200 MeV) energy heavy-ion collisions s b RP+RT TLF TLF PLF PLF Different reaction types as a function of centrality Conventional wisdom P T Central Mid-Peripheral Peripheral Neck fragmentation (shape instability) Binary exit channel + statistical decay (relatively gentle collisions) Multifragmentation (most excited systems) Degrees of freedom : E*, J, density, shape, N/Z : Charged Particles: Z and A; Neutrons; Gammas

  13. Thermometers Kinetic equilibrium: motion of all particles reflects a common temperature Kinetic energy spectra fit  Maxwell-Bolztman distribution: P(E)  exp(-E/Tslope) Angular distribution  emission time as compared to rotation time Chemical equilibrium: different partitions are populated according to their statistical weights. 6Li Emitting system 10B  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 F. Zhu et al., PRC52, 784 (1995) Extract temperature T

  14. Conventional wisdom (participant-spectator model) PLF* Participant (Overlap zone) is highly excited 114Cd spectator 92Mo Shearing mechanism TLF* spectator Select fragments at very forward angles 2.1lab 4.2 • Projectile and target-like nuclei are relatively unexcited • Velocity of PLF* nearly unchanged from beam velocity • Overlap of projectile and target is the key quantity in the reaction

  15. PLF* decay following a peripheral collision r r0 PLF* = good case: (as compared to central collisions) System size (Z,A) is well -defined  Normal density Large cross-section (high probability process) Other emission (mid-rapidity, ...) Circular ridge  PLF* emission “Isotropic” component Projectile velocity

  16. 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.

  17. Evaporation and velocity damping # emitted from the PLF* in a given collision • Multiplicities increase with velocity damping • Tslope increases with velocity damping • “Linear” trend for both observables

  18. Velocity damping and excitation energy • Good agreement with GEMINI • Some sensitivity of M to J, level density Reconstruct excitation of PLF* by doing calorimetry: particle multiplicity, kinetic energies, and binding energies.  (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. “Statistical model code” supports E*/A scale R.J. Charity et al., PRC63, 024611 (2001)

  19. Total excitation of PLF* depends on velocity damping and is relatively independent of PLF size. Results are consistent with following scenario: • For each impact parameter a distribution of contact times exists. • 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? To study N/Z dependence of EOS: • Select PLF* size by selecting residue Z. • Select excitation by selecting VPLF* • Vary N/Z by changing (N/Z)proj.,tgt.

  20. Thermodynamic Summary We 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)

  21. vH>vL vL>vH 2 fragments 1 fragment fragmentation Dynamics: The two fragment case TLF* PLF* E*, J

  22. Binary breakup: PLF* reconstruction PLF* ZL ZH vL > vH ZH vH > vL ZL If the PLF*, subsequent to the collision process, decays statistically we expect both cases to be the same.

  23. Process characterization a 6  NC 10 B. Davin et al., PRC 65, 064614 (2002) Different charge correlation  Different relative velocities Different alignments

  24. Process probability : channel opening 1 fragment (x 0.1) statistical dynamical  Dynamical process appears at higher velocity lower damping  lower excitation  Up to 10% of the cross-section in binary breakup

  25. Energy transferred to the fragments dynamical statistical • More kinetic energy in the fragments for the dynamical case • For a given velocity damping, difference of  20-30 MeV

  26. A picture of the process Initial kinetic energy? Q Coulomb Collective TKE “Extra” energy Saddle-point Scission-point Time Time scale? Deviation of TKE from (Q+Coulomb)

  27. Dynamics : a new process? • Process with a large cross-section • 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?

  28. E432@GANIL Caen, France 124,136Xe + 112,124Sn at E/A=50 MeV LASSA FIRST and LASSA are highly segmented  600 Si channels together with ISiS 1000 channels FIRST Measure Z,A,E, ISiS 50 neutron TOF detectors (DEMON) to measure neutrons (KE spectra, multiplicities, free n/p at mid-rapidity)`

  29. FIRST :Forward Indiana Ring Silicon Telescopes Large number of channels use of ASIC Design : P.H. Sprunger Device dedicated to measure the decay of the PLF* : Limiting temperature Dynamical process PLF* fragmentation  ... 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

  30. HiRA Telescope Design 4x CsI(Tl) 4cm Si-E 1.5 mm Si-DE 65mm pixel 32 strips v (front) 32 strips h. (back) 32 strips v. (front) Target Beam (High Resolution Array) • 20 Telescopes • 62.3 x 62.3 mm2 Active Area • Pitch 1.8 mm • 1024 Pixels per telescope Designed to study transfer reactions, resonance decay spectroscopy, etc with radioactive beams

  31. Silicon detectors Developed at IU/IUCF Design characteristics • dq =± 0.15° at 35cm • dE/E=40 keV for 5 MeV a’s Si(IP) specifics • Bulk material is n type • Interstrip on junction side is 25 mm • Interstrip on ohmic side is 40 mm • 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

  32. Electronic Readout developed at Washington University (St. Louis) And Southern Illinois University, Edwardsville With 2000 channels to readout, cost of “traditional” readout is prohibitive. Application Specific Integrated Circuit • Design Includes: • Multiple Preamps (100 MeV, 250 MeV, external) • Slow Shaper and Timing Filter Amplifier • Discriminator (5 bit) • Time to amplitude converters • Design Characteristics • Excellent energy resolution ( 25-40 keV) • 2. Dynamically switchable range • 3. Excellent time resolution (~500 pS) • 4. Sparsified readout of both energy and time information.

  33. Electronic Readout ASIC 32 channels in 6mm x 6mm format (presently 16) ADC module, used for ALL 20 telescopes ASIC Chip ULM for control of ASIC

  34. Summary • 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.

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