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Vladimir Yurevich Joint Institute for Nuclear Research, Dubna

Vladimir Yurevich Joint Institute for Nuclear Research, Dubna. Study of Neutron Emission and Fission in Relativistic pA - and AA -Collisions.

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Vladimir Yurevich Joint Institute for Nuclear Research, Dubna

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  1. Vladimir Yurevich Joint Institute for Nuclear Research, Dubna Study of Neutron Emission and Fission in Relativistic pA- and AA-Collisions June 2007 V. Yurevich Prague

  2. Motivation Neutron emission and fission reaction are important channels of de-excitation and decay of nuclear system formed in interactions of protons and nuclei with heavy nuclei. Both these processes are investigated for a long time but new experiments and theory development are highly needed as for reaction mechanism understanding as for progress in various applications in science, accelerator-driven systems, and space research. Contents: • Part 2. Fission cross sections • Introduction • NA45 experiment • Fission at SPS energies • Fission in pA-collisions • Discussion of results • Summary • Part 1. Neutron Emission • Introduction • JINR experiment • Moving Source Model • Discussion of results • Summary June 2007 V. Yurevich Prague

  3. Neutron Emission / Introduction • Study of neutron emission gives an unique possibility to observe all stages of nuclear • system evolution and decay. • There are no any distortions in neutron spectra induced by Coulomb forces taking place • in charged particle spectra. • Collisions of light projectiles with heavy nuclei at several AGeV are effective method • to prepare highly-excited nuclear system with small excitation of collective modes. • In such collisions many experiments (EOS, ISiS, FASA et al.) investigated phenomenon • of multifragmentation and “liquid-gas” phase transition during last decade. • We can expect that simultaneously with charged particles and fragments many neutrons • are emitted and neutron measurements could give new information about as space-time • picture of the collision as characteristics of decay modes. • The neutron emission was carefully studied by TOF measurements in LANL, SACLAY, • KEK, ITEP, and JINR with proton beam up to 3 GeV. For light nuclei with energies • above 0.6 AGeV the measurements were carried out in JINR. June 2007 V. Yurevich Prague

  4. Neutron Emission / Introduction Charged fragment emission 1-19 GeV p+Xe (AGS, 1989) 1 AGeV Au+C (GSI, 1998) 2 GeV p,3He+Au (Saturne, 1998) 2-8 GeV p+Au 4,14.6 GeV 4He+Au (Dubna, 1999-2002) 22.4 GeV C+Au 1 GeV p+Au, U (Gatchina, 2001) 6-14 GeV p+Au (AGS, 2004) 1.8,3.6,4.8 GeV 3He+Au (Saturne, 2004) Neutron emission 1-9 GeV p+Pb (ITEP, 1983) 0.1-0.8 GeV p+Pb (LANL, 1989-1993) 0.8-3 GeV p+Pb (KEK, 1995) 0.8-1.6 GeV p+Pb (Saturne, 2002) 0.8-1.6 GeV p+Pb (ITEP, 2003) 2 GeV p,d+Pb Our experiment 4 GeV 4He+Pb Dubna 24 GeV C+Pb 2006 a few AGeV light projectile (p,d,He,C) Heavy nucleus Aim of this work • Search and study of neutrons emitted by hot nuclei • Comparison with results on charged fragment emission June 2007 V. Yurevich Prague

  5. Neutron Emission / JINR Experiment • Particle identification methods: • pulse-shape n/g discrimination for • stylbene detectors D1 and D2, • veto counters VC for n/ch.particle • separation, • TOF-E identification of charged • particles for detectors D3 • Neutron detectors: • D1 – stylbene crystal D41cm, range: 0.3-6 MeV • D2 – stylbene crystal D55 cm, range: 2.5-300 MeV • D3 – plastic scintillator D1220 cm, range: 25-500 MeV June 2007 V. Yurevich Prague

  6. Neutron Emission / JINR Experiment Studied processes: Not studied processes: Elastic and quasi-elastic interactions and projectile fragmentation • Conception Neutron emission in region of target fragmentation Range of small angles Projectile Pb D1 D2 Low-energy range was studied with single detector D1 placed at 60o or 120o D3 Neutron detectors Angular range: 30o-150o June 2007 V. Yurevich Prague

  7. Neutron Emission / Moving Source Model • Moving Source Model • Traditional interpretation of neutron emission in reactions at intermediate • energies is based on cascade – pre-equilibrium – evaporation approach • (fission is included to evaporation mode). • At the same time new results on charged fragment emission show existence • of new decay mechanisms in central collisions at GeV energies: • hot non-equilibrium stage (fireball decay) • and thermal fragmentation. • It is naturally to expect that many neutron are emitted at these stages. • Motivation • to revise the MSM by involving of these new decay modes June 2007 V. Yurevich Prague

  8. Neutron Emission / Moving Source Model Peripheral Collisions Central Collisions • Modified Moving Source Model Nucleon-nucleon collisions Source 1 Hot non-equilibrium stage (fireball decay) Source 2 (hot) Target spectator decay High E* Medium E* Low E* Low E* Multifragmentation Fragmentation with heavy remnant Source 3 (thermal) Source 4 De-excitation of remnant by evaporation Assumption: pre-equilibrium emission before last evaporation stage is the second order process and gives smaller contribution in comparison with four selected sources Time June 2007 V. Yurevich Prague

  9. Neutron Emission / Moving Source Model Moving Source Model used for data analysis consists of four independent sources of neutrons according to the main decay stages with neutron emission: Source 1 – first nucleon-nucleon collisions Source 2 – hot stage (in central collisions) Source 3 – fragmentation (in central collisions) Source 4 – evaporation (+ fission) The model expression for experimental data fitting is a sum of these contributions • Modified Moving Source Model where E, p – kin. energy and momentum in lab. frame, = V/c – source velocity, m – neutron rest mass, q - angle in lab. frame b Parameters: Ai – amplitude bi – velocity Ti– temperature June 2007 V. Yurevich Prague

  10. Neutron Emission / Moving Source Model p+Pb 2 GeV θ=90o Two step fitting procedure: 1. Sources 1+2 (E>20 MeV) + Source 4 (E<5 MeV) 2. The same + Source 3 (5<E<20 MeV) • Fitting d2σ/dEdΩ, mb MeV-1sr-1 4He+Pb 4 GeV θ=30o 12C+Pb 24 GeV θ=60o E, MeV June 2007 V. Yurevich Prague

  11. Neutron Emission / Moving Source Model • Fitting d2σ/dEdΩ, mb MeV-1sr-1 p+Pb 2 GeV d+Pb 2 GeV 4He+Pb 4 GeV 12C+Pb 24 GeV E, MeV June 2007 V. Yurevich Prague

  12. Neutron Emission / Discussion of results Neutron Emission • Temperature parameter dependence on energy and type of projectile Universal values of temperatures T1=70±10 MeV T2=21±0.4 MeV T3=4.65±0.10 MeV T4=1.6±0.1 MeV June 2007 V. Yurevich Prague

  13. Neutron Emission / Discussion of results Fast stage of decay – fireball decay (Source 2) Neutron production cross section Temperature Velocity Thermal fragmentation (Source 3) June 2007 V. Yurevich Prague

  14. Neutron Emission / Discussion of results • Mean Neutron Multiplicity in decay of nuclear system Contributions Reaction MnHot stageThermal fragmentation [Source 2] [Source 3] (n/interaction) (%) (%) p+Pb 21.8±3.4 16 22 d+Pb 17.1±3.4 14 20 4He+Pb 22.5±3.5 19 23 12C+Pb 29.1±4.5 16 26 The sources 2 and 3 give 14-19 % and 20-26 % contributions respectively. For central collisions (cc ~ ½ R ) these sources give ~ 80 % of all neutrons. June 2007 V. Yurevich Prague

  15. Neutron Emission SUMMARY • Developed MSM gives very good description of neutron spectra and • adequate space-time picture of light nucleus - heavy nucleus collisions • at intermediate energies. • There is good agreement between results obtained for neutron • emission and charged fragment emission that supports a conclusion • about common nature of the sources. • Slope temperature for hot source (fireball decay) is 21±0.40 MeV. • “Neutron thermometer” gives estimation of freeze-out temperature for • thermal fragmentation as Tf = 4.65±0.10 MeV. • In central collisions fireball decay and thermal fragmentation give about • 80% of emitted neutrons. • Temperatures at any stage of decay do not depend on energy and • type of projectile. June 2007 V. Yurevich Prague

  16. Fission / Introduction Fission reaction is important mode of heavy nucleus decay in high-energy collisions but it is rather poorly studied in high-energy region and especially in nucleus-nucleus collisions. Fission cross section measurements were mainly carried out with proton beam below 30 GeV using SSNTD. With appearance of ultrarelativistic lead ion beam at SPS CERN some attempts to measure fission cross sections were undertaken. Target Fission fragments SPS beam Pb • The most considerable results at 40 and 158 AGeV have been recently reported by • NA50 experiment [Phys.Rev.C69 (2004)] • NA45 collaboration [Winter Meeting on Nuclear Physics, Bormio, Italy, 2007] June 2007 V. Yurevich Prague

  17. Fission / NA45 experiment CERES spectrometer CERES/NA45 experiment was dedicated to study of direct electron pair emission in ultrarelativistic nucleus-nucleus collisions. Special target area was designed with extra low material budget for study of the Pb-Au collisions. Target Area 1 – vacuum beam pipe 2 – BC1 3 – Veto-Wall 4 – Veto-counter 5 – Target area 6 – SDD1&SDD2 7 – RICH1 8 – RICH2 9 – TPC Only information from beam detector system placed in target area was used in data analysis for fission cross section estimation. June 2007 V. Yurevich Prague

  18. Fission / NA45 experiment Target Area 1 2 3 4 5 1 – carbon vacuum pipe 2 – PMT housing (BC2) 3 – BC2 (mirror) 4 – Au target (0.338 mm) 5 – BC3 (mirror) 6 – MC scintillator 6 SDD1 SDD2 PMT(BC2) 2 PMT(MC) 4 6 5 PMT(BC3) Au target:13 disks (600diam.26 m each) June 2007 V. Yurevich Prague

  19. Fission / NA45 experiment CERES trigger detectors June 2007 V. Yurevich Prague

  20. Fission / NA45 experiment Material Budget of Target Area Contribution to peak of fission events H– thickness, n– number of nuclei per cm2, R – reaction cross section, P – nuclear interaction probability June 2007 V. Yurevich Prague

  21. Fission / NA45 experiment BC3-MC Correlation measured with Beam Trigger Run 1423 Pb-Au MC pulse height, ADC chan. Region of fission events Pb nucleus BC3 pulse height, ADC chan. Fission corresponds to events with small charged particle multiplicity Fission occurs only in peripheral collisions June 2007 V. Yurevich Prague

  22. Fission / NA45 experiment Estimation of fraction of Pb ions hitting Au target Run 1244 SDD2-SDD1 correlation 952 % of Pb ions pass through Au target SDD1 hits Run 1423 SDD2 hits June 2007 V. Yurevich Prague

  23. Fission / Pb fission at SPS energies Collisions of 158-AGeV Pb with various targets MUSICs detectors (ionization chambers) CR39 (SSNTD method) S. Cecchini et al. Nucl. Phys. A707, 513 (2002) C. Scheidenberger et al. Phys. Rev. C70, 014902 (2004) Pb-Pb fission Pb-Au fission June 2007 V. Yurevich Prague

  24. Fission / Pb fission at SPS energies NA50 experiment 40 & 158 AGeV Pb-Pb collisions B. Alessandro et al. Phys. Rev. C69, 034904 (2004) fission fission fission June 2007 V. Yurevich Prague

  25. Fission / Pb fission at SPS energies CERES/NA45 158-AGeV Pb-Aucollisions BC3 Runs 1419+1423 Pb fission Counts/bin fission ADC chan. • Experimental errors: R1419+R1423 • Background subtraction 11 % • Target thickness 3 % • Number of Pb ions 2 % • Fission in other materials 0.7 % Run Fission events 75087 1211 30234 1419+1423 Total error for f : 12 % June 2007 V. Yurevich Prague

  26. Fission / Pb fission at SPS energies Calculation of Coulomb fission cross section Maximum virtual photon energy in peripheralPb-Au collision at 158 AGeV , Emax ћc/bmin  2.2 GeV where bmin– min. impact parameter of collision calculated by formula , where Ap, At - mass numbers of projectile and target nuclei, r0=1.34 f, x=0.75 According to the Weizsacker-Williams (WW) method, projectile fission may be induced by virtual photon emitted by the target nucleus with spectrum, integrated over the impact parameter, , where a - the fine structure constant, K0 and K1 – the modified Bessel functions of order 0 and 1, ξ=(Eγbmin)/(γћc). The Coulomb fission cross section of the projectile nucleus is calculated as , where σγf – cross section of fission induced by photon. June 2007 V. Yurevich Prague

  27. Fission / Pb fission at SPS energies Calculation of Coulomb fission cross section of 208Pb in158-AGeV Pb-Au σfC = 233 mb Valuesσγf,Nγandσγf Nγ as functions of photon energy 208Pb:□ - L.G. Moretto et al., 1969, ○ - J.D.T. Arrruda-Neto et al., 1990; natPb:▲- J.B. Martins et al., 1991, ▼- Yu.N. Ranyuk et al.., 1967,  - A.V. Mitrofanova et al.,1968, ● - M.L. Terranova et al., 1996&1998, ■ - C. Cetina et al.,2002;curve – fit for208Pb data June 2007 V. Yurevich Prague

  28. Fission / Pb fission at SPS energies Fission of 208Pb at 158 AGeV Exp. Year Collision σf (mb) EMU13 1998 Pb-Pb ~340 NA50 2004 Pb-Pb 33219 NA45 2007 Pb-Au 30522 σf , mb June 2007 V. Yurevich Prague

  29. Fission / Fission in pA-collisions pA-collisions Study of dependence of heavy nucleus fissility on proton energy ~ 40 % Appearance of new decay mode – fragmentation f /in Fissility changes in energy range between 0.5 and 5 GeV ~ 10 % Proton energy, GeV June 2007 V. Yurevich Prague

  30. Fission / Discussion of results AA-collisions Katkoff et al. (1976) N+Au f p= const? NA50 (2004) Pb+C f , mb   Pb+Pb • Pb+Au CERES/NA45 (2007) f Cfalls down f C<<f p ? Katkoff et al. (1976) N+Bi f A = f(E,A)? June 2007 V. Yurevich Prague

  31. Fission SUMMARY • For pA-collisions • Fission probability changes in interval from 0.5 to 5 GeV. It decreases with • energy because of appearance of new decay modes. • Above 5 GeV fission probability has weak energy dependence. But experimental data • set is poor and new electronic experiments are required to confirm this conclusion. • For AA-collisions • Fission cross section dramatically changes, decreases with energy in region • from 2 to 40 AGeV for light nucleus - heavy nucleus collisions. • Fission reaction takes place only in peripheral collisions and for collisions of • heavy nuclei Coulomb interactiongives main contribution to the fission cross section • that increases with charge Z and energy. • Future research • New electronic experiments are needed for understanding of fission dependence on • energy and type of colliding nuclei above 1 AGeV. June 2007 V. Yurevich Prague

  32. THANK YOU for YOUR ATTENTION! June 2007 V. Yurevich Prague

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