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

2. 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. June 2007 V. Yurevich Prague Part 1. Neutron Emission Introduction JINR experiment Moving Source Model Discussion of results Summary Part 2. Fission cross sections Introduction NA45 experiment Fission at SPS energies Fission in pA-collisions Discussion of results Summary Contents: Motivation

3. 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. Neutron Emission / Introduction June 2007 V. Yurevich Prague

4. a few AGeV light projectile (p,d,He,C) Heavy nucleus Charged fragment emission 1-19 GeV p+Xe (AGS, 1989) 1 AGeV Au+C (GSI, 1998) 2 GeV p, 3 He+Au (Saturne, 1998) 2-8 GeV p+Au 4,14.6 GeV 4 He+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 3 He+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 4 He+Pb Dubna 24 GeV C+Pb 2006 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 Neutron Emission / Introduction

5. Particle identification methods: pulse-shape n/  discrimination for stylbene detectors D 1 and D 2, veto counters VC for n/ch.particle separation, TOF-E identification of charged particles for detectors D 3 Neutron detectors: D 1 – stylbene crystal D4  1cm, range: 0.3-6 MeV D 2 – stylbene crystal D5  5 cm, range: 2.5-300 MeV D 3 – plastic scintillator D12  20 cm, range: 25-500 MeV June 2007 V. Yurevich Prague Neutron Emission / JINR Experiment

6. Conception Studied processes: Neutron emission in region of target fragmentation Not studied processes: Elastic and quasi-elastic interactions and projectile fragmentation Projectile Pb Angular range: 30 o -150 o Range of small angles D1D1 D2D2 D3D3 Low-energy range was studied with single detector D 1 placed at 60 o or 120 o Neutron detectors June 2007 V. Yurevich Prague

7. 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 Neutron Emission / Moving Source Model June 2007 V. Yurevich Prague

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

9. 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,  angle in lab. frame Parameters: A i – amplitude  i – velocity  T i – temperature  June 2007 V. Yurevich Prague Neutron Emission / Moving Source Model

10. Fitting p+Pb 2 GeV θ=90 o 4 He+Pb 4 GeV θ=30 o 12 C+Pb 24 GeV θ=60 o d 2 σ /dEd Ω, mb MeV -1 sr -1 E, MeV 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) June 2007 V. Yurevich Prague Neutron Emission / Moving Source Model

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

12. Temperature parameter dependence on energy and type of projectile T 1 =70±10 MeV T 2 =21±0.4 MeV T 3 =4.65±0.10 MeV T 4 =1.6±0.1 MeV Universal values of temperatures Neutron Emission Neutron Emission / Discussion of results June 2007 V. Yurevich Prague

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

14. Contributions Reaction M n Hot stage Thermal fragmentation [ Source 2] [Source 3] (n/interaction) (%) (%) p+Pb 21.8±3.4 16 22 d+Pb 17.1±3.4 14 20 4 He+Pb 22.5±3.5 19 23 12 C+Pb 29.1±4.5 16 26 Mean Neutron Multiplicity in decay of nuclear system 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 Neutron Emission / Discussion of results

15. 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 T f = 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 Neutron Emission SUMMARY

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. 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] SPS beam Pb Target Fission fragments June 2007 V. Yurevich Prague

17. CERES spectrometer 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 Fission / NA45 experiment CERES/NA45 experiment was dedicated to study of direct electron pair emission in ultrarelativistic nucleus-nucleus collisions. Only information from beam detector system placed in target area was used in data analysis for fission cross section estimation. Special target area was designed with extra low material budget for study of the Pb-Au collisions. June 2007 V. Yurevich Prague

18. 1 2 3 4 5 6 2 4 5 PMT(BC2) PMT(MC) PMT(BC3) SDD1SDD2 1 – carbon vacuum pipe 2 – PMT housing (BC2) 3 – BC2 (mirror) 4 – Au target (0.338 mm) 5 – BC3 (mirror) 6 – MC scintillator Target Area 6 Au target: 13 disks (600diam.  26  m each) June 2007 V. Yurevich Prague Fission / NA45 experiment

19. CERES trigger detectors DetectorRadiator/ScintillatorMirror/Acceptance PMT Pulse height resolution BC1 BC2 BC3 MC Air at normal conditions, 170-mm length Air at normal conditions, 60-mm length Air at normal conditions, 54-mm length 1-mm plastic scintillator, 14.7-mm outer diam., 4.9-mm diam. hole 12-μm Al-mylar at angle of 45 о to the beam axis 12-μm Al-mylar at angle of 16 о to the beam axis 12-μm Al-mylar at angle of 18 о to the beam axis 1.8 o <  <5.4 o Philips XP2020 5.6 % HAMAMATSU 3.4 % R2496 HAMAMATSU 5.5 % R2496 HAMAMATSU R1635 June 2007 V. Yurevich Prague Fission / NA45 experiment

20. H – thickness, n – number of nuclei per cm 2,  R – reaction cross section, P – nuclear interaction probability Material H (mm) n (nucl./cm 2 )  R (b) P (%) Al (entrance window) Air (BC2 radiator) Mylar (BC2 mirror): H C, O Air (BC2 mirror - last Au disk) Au (target) Mylar (target foils): H C, O Air (BC3 radiator) 0.100 60 0.0125 49 0.338 0.039 54 6.025 10 20 3.16 10 20 4.2 10 19 7.5 10 19 2.58 10 20 1.995 10 21 1.31 10 20 2.34 10 20 2.84 10 20 3.84 3.29 1.87 3.26 3.29 6.96 1.87 3.26 3.29 0.234 0.104 0.0078 0.0245 0.0844 1.389 0.0243 0.0764 0.0943 Material Budget of Target Area Contribution to peak of fission events June 2007 V. Yurevich Prague Fission / NA45 experiment

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

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

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

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

25. ADC chan. Counts/bin fission Pb BC3 Runs 1419 +1423 158-AGeV Pb-Au collisions CERES/NA45 Run Fission events 1211 1419+1423 750  87 302  34 Experimental errors: R1419+R1423 1.Background subtraction 11 % 2.Target thickness 3 % 3.Number of Pb ions 2 % 4.Fission in other materials 0.7 % Total error for  f : 12 % fission June 2007 V. Yurevich Prague Fission / Pb fission at SPS energies

26. Maximum virtual photon energy in peripheral Pb-Au collision at 158 AGeV E  max   ћ  c/b min  2.2 GeV where b min – min. impact parameter of collision calculated by formula where A p, A t - mass numbers of projectile and target nuclei, r 0 =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  - the fine structure constant, K 0 and K 1 – the modified Bessel functions of order 0 and 1, ξ=(E γ b min )/(γћ  c).,, The Coulomb fission cross section of the projectile nucleus is calculated as where σ γf – cross section of fission induced by photon. Calculation of Coulomb fission cross section June 2007 V. Yurevich Prague Fission / Pb fission at SPS energies

27. Values σ γf, N γ and σ γf N γ as functions of photon energy 208 Pb: □ - L.G. Moretto et al., 1969, ○ - J.D.T. Arrruda-Neto et al., 1990 ; nat Pb: ▲- 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 for 208 Pb data σ f C = 233 mb Calculation of Coulomb fission cross section of 208 Pb in 158-AGeV Pb-Au June 2007 V. Yurevich Prague Fission / Pb fission at SPS energies

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

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

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