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 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 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 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, ) 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) GeV p+Pb (LANL, ) GeV p+Pb (KEK, 1995) GeV p+Pb (Saturne, 2002) 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 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: MeV D 2 stylbene crystal D5 5 cm, range: MeV D 3 plastic scintillator D12 20 cm, range: MeV June 2007 V. Yurevich Prague Neutron Emission / JINR Experiment 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 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 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 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 Fitting p+Pb 2 GeV =90 o 4 He+Pb 4 GeV =30 o 12 C+Pb 24 GeV =60 o d 2 /dEd , mb MeV -1 sr -1 E, MeV Two step fitting procedure: 1. Sources 1+2 (E>20 MeV) + Source 4 (E