Neutron multiplicities in inelastic collisions of132Xe with197Au

The multiplicities of neutrons emitted from the light and heavy fragments, respectively, in the inelastic scattering of 7.5 MeV/amu¹³² Xe ions from¹⁹⁷Au have been studied as a function of total energy loss. The measured multiplicity ratios are close to the Au-Xe mass ratio for all Q-values, consistent with a rapid thermalization of the excitation energy. The absolute multiplicity values are lower than expected and suggest significant pre-equilibrium effects.     

Gamma ray multiplicities in deep inelastic collisions for the Cu + Au 400 MeV system

For the Cu + Au 400 MeV system γ-ray multiplicities have been measured. The mean value of the transferred angular momentum, the variances, and the repartition of angular momentum between the fragments have been deduced. It is shown that, early in the reaction the building up of angular momentum is a very fast process. For longer times, additional angular momentum is induced in the fragments. The excitation of collective modes can account for this effect. The properties of such modes have been analysed.     

Neutron multiplicity in deep inelastic collisions: 400 MeV Cu+Au system

We have detected in nine different positions of space the neutrons associated with the collision of ⁶³Cu on ¹⁹⁷Au at 400 MeV bombarding energy. The deep inelastic products were detected at two different angles: close to the grazing angle and 30° forwards of it. Their masses were measured using a time-of-flight technique. The neutrons were detected in coincidence with the fragments - the efficiency of the neutron detectors was measured relatively to a ²⁵²Cf source during beam time. The neutron threshold was set at 300 keV. Within an accuracy of 10% all the emitted neutrons are evaporated by the fully accellerated deep inelastic fragments. It is shown that the excitation energy is shared between the fragments in proportion to their masses and that the relaxation time for internal equilibration of the composite system is very short (～10^?22 s).     

Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Energy dependence of collective flow of neutrons and protons in197Au+197Au collisions

We investigate the beam energy dependence of neutron and proton squeeze-out in collisions of¹⁹⁷Au+¹⁹⁷Au atE/A=400—800 MeV. The azimuthal anisotropy that describes the enhanced emission of mid-rapidity neutrons perpendicular to the reaction plane rises strongly with the transverse momentum of the neutrons. This dependence of the azimuthal anisotropy follows a universal curve — independent of beam energy — if the neutron momenta are measured in fractions of the projectile momentum per mass unit. Analogously, the kinetic energy spectra of mid-rapidity neutrons exhibit a universal behaviour as a function of the kinetic energy of the projectile. The members of the FOPI-collaboration: J.P. Alard, Z. Basrak, N. Bastid, I.M. Belayev, M. Bini, R. Bock, A. Buta, R. aplar, C. Cerruti, N. Cindro, J.P. Coffin, M. Crouau, P. Dupieux, J. Erö, Z.G. Fan, P. Fintz, Z. Fodor, R. Freifelder, L. Fraysse, S. Frolov, A. Gobbi, Y. Grigorian, G. Guillaume, N. Herrmann, K.D. Hildenbrand, S. Hölbling, O. Houari, S.C. Jeong, M. Jorio, F. Jundt, J. Kecskemeti, P. Koncz, Y. Korchagin, R. Kotte, M. Krämer, C. Kuhn, I. Legrand, A. Lebedev, C. Maguire, V. Manko, T. Matulewicz, G. Mgebrishvili, J. Mösner, D. Moisa, G. Montarou, P. Morel, W. Neubert, A. Olmi, G. Pasquali, D. Pelte, M. Petrovici, G. Poggi, F. Rami, W. Reisdorf, A. Sadchikov, D. Schüll, Z. Seres, B. Sikora, V. Simion, S. Smolyankin, U. Sodan, N. Taccetti, K. Teh, R. Tezkratt, M. Trzaska, M.A. Vasiliev, P. Wagner, J.P. Wessels, T. Wienold, Z. Wilhelmi, D. Wohlfarth, A.V. Zhilin.     

Charged particle multiplicities and inelastic cross sections in high energy nuclear collisions

Inelastic cross sections at 60 and 200 GeV/nucleon are determined in a streamer chamber for ¹⁶O on several nuclear targets. Charged particle multiplicity distributions for inelastic and central collisions are studied and compared with theoretical predictions. The inelastic cross section exhibit a geometrical dependence on nuclear radii. The multiplicity data are governed by the collision geometry. They are consistent with a picture of superposition of independent nucleon-nucleus interactions.     

Dynamical intermediate-mass fragment production in197Au+197Au collisions at energies ≤15 MeV/u

We present radiochemical data on the production of intermediate-mass fragments (IMF, 11≤Z<≈25) in collisions of¹⁹⁷Au+¹⁹⁷Au at 9, 11, 13 and 15 MeV/u. We demonstrate that the IMFs are produced in two-step reactions: Highly excited heavy nuclei are formed in a binary deep-inelastic reaction, and the IMFs originate from fast mass-asymmetric sequential fission of these primary reaction products.     

Breakup temperature of target spectators in 197Au + 197Au collisions at E/A = 1000 MeV

Breakup temperatures were deduced from double ratios of isotope yields for target spectators produced in the reaction ¹⁹⁷Au + ¹⁹⁷Au at 1000 MeV per nucleon. Pairs of ^3,4He and ^6,7Li isotopes and pairs of ^3,4He and H isotopes (p, d and d, t) yield consistent temperatures after feeding corrections, based on the quantum statistical model, are applied. The temperatures rise with decreasing impact parameter from 4 MeV for peripheral to about 10 MeV for the most central collisions. The good agreement with the breakup temperatures measured previously for projectile spectators at an incident energy of 600 MeV per nucleon confirms the universality established for the spectator decayat relativistic bombarding energies. The measured temperatures also agree with the breakup temperatures predicted by the statistical multifragmentation model. For these calculations a relation between the initial excitation energy and mass was derived which gives good simultaneous agreement for the fragment charge correlations. The energy spectra of light charged particles, measured at τ_lab = 150°, exhibit Maxwellian shapes with inverse slope parameters much higher than the breakup temperatures. The statistical multifragmentation model, because Coulomb repulsion and sequential decay processes are included, yields light-particle spectra with inverse slope parameters higher than the breakup temperatures but considerably below the measured values. The systematic behavior of the differences suggests that they are caused by light-charged-particle emission prior to the final breakup stage.     

Charge and angular distributions and secondary fission of deep inelastic products from the reaction 197Au + 979 MeV 136Xe 1443 08

Kinetic energy spectra, charge and angular distributions are presented for over 40 elements (Z = 27?68). The kinetic energy spectra show two distinct components: a deep inelastic one and another from secondary fission of the target-like product. The charge distribution widths are comparable to those observed in Kr bombardments, the angular distributions, however, are more extensively side peaked.     

Fast emission ofα-particles in deep-inelastic36Ar+197Au collisions

Angular and energy correlations betweenα-particles and deep-inelastic projectile-like fragments were measured for the system³⁶Ar+¹⁹⁷Au atE _Lab=380 MeV. At most half of the coincident events can be attributed to statistical emission ofα-particles from the fully accelerated projectile-like fragments. The remainder of the events may be due to direct emission during the first 10^?22 s of the scattering process and to preequilibrium processes taking place within some 10^?21 s.     

Neutron-multiplicity in deep inelastic collisions: 365 MeV Cu+Au system

Neutrons have been detected in coincidence with the two outgoing fragments in deep inelastic reactions of⁶³Cu on¹⁹⁷Au at 365 MeV. Most of these neutrons are evapored from the fully accelerated fragments. An additionnal low energy contribution appears.     

Production of Kr and Xe isotopes by interaction of197Au with 0.15–24 GeV protons

Cross sections and thick-target recoil properties of several Kr and Xe isotopes formed in the interaction of¹⁹⁷Au with 0.15–24 GeV protons have been determined in order to study their variations with mass product and incident energy. Noble gas isotopic composition and concentration have been obtained by using high sensitivity mass-spectrometry. The measurement of the experimental range, 2W(F+B), and of the forward-to-backward ratio,F/B, permits (with the help of a mathematical formalism based on the two-step model) the determination of some characteristics of nuclear reactions. The results are discussed in terms of fission and spallation mechanisms.     

17N+197Au反应中碎片与中子的符合测量

在3?3?.4MeV/u ¹⁷N束轰击¹⁹⁷Au靶产生的反应中,利用放置于不同角度组合的17个中子探测器(4°—83°)和14个半导体望远镜(2.3°—9.0°)对反应产物碎片与中子进行了符合测量.经对所得实验角分布积分得到Z=3—6元素的同位素产额分布.在参加者-旁观者模型框架下,采用¹⁷N原子核内部的不同密度分布计算了同位素产额分布并与实验数据做比较.     

Fusion and deep inelastic collisions studied on the Ar + Au system

We investigated the Ar + Au system at two different bombarding energies 201 and 248 MeV. The mass and the energy of the products have been measured at different angles. Mass distributions exhibits two components separated at the low bombarding energy and partly merging into each other at the high bombarding energy. One of them can be attributed to fission following complete fusion, the other one is centered around mass 40 and corresponds to deep inelastic products. These two different mechanisms correspond to different time scales. Angular distributionsd ² σ/dθdM are peaked a little bit forward the grazing angle for products close to the projectile and, when the mass transfer increases, becomes constant. For deep inelastic collisions the mass transfer occurs in the way predicted using potential energy considerations, but the small FWHM and the slight shift of the position of the maximum of these distributions indicates a short contact time. Due to the increase of the temperature, the FWHM of the mass distribution of deep inelastic products increases with the bombarding energy. The mean total kinetic energy studied as a function of the detection angle shows the influence of statistical fluctuations at backward angles. One also observes for this system that the relaxation time connected with the mass asymmetry degree of freedom is larger than the one associated to the energy damping. Complete fusion cross sections measurements were also done at 183, 189 and 195 MeV which allowed to draw the excitation function for this process. Calculations of the fusion cross section using the concept of critical distance are in agreement with the data.     

Multiscaling in inelastic collisions

We study relaxation properties of two-body collisions on the mean-field level. We show that this process exhibits multiscaling asymptotic behavior as the underlying distribution is characterized by an infinite set of nontrivial exponents. These nonequilibrium relaxation characteristics are found to be closely related to the steady state properties of the system.     

Temperatures in heavy-ion collisions from pion multiplicities

A thermodynamically consistent treatment of the nuclear interaction is employed to study the dependence of pion production on the nuclear equation of state in heavy-ion collisions. Massive baryon resonances, heavy mesons and the Bose condensation of pions are incorporated into a macrocanonical relativistic quantum-statistical treatment of the highly excited system. The measured pion multiplicities, which vary over eight orders of magnitude in the bombarding energy range from 30 MeV/nucleon to 4 GeV/nucleon, are reproduced within a simple one-dimensional fluiddynamical model if it is assumed that nuclear matter is rather incompressible. The pion yields are in this model directly related to the compression energy, which amounts to one-half of the total center-of-mass energy at all BEVALAC energies. The maximum compression derived is uncertain by about 10% and 30% at E_lab = 0.4 and 1.8 GeV/nucleon, respectively. The temperatures of the system in the moment of the chemical freeze-out of the pion/delta degree of freedom are determined from the measured pion yields and range from 10 MeV to 100 MeV. An extrapolation to CERN/BNL energies, i.e. E_lab > 10 GeV/nucleon, yields T = 150–200 MeV. A strong energy dependence of the cross sections and the slopes of hard γ's is predicted by this model. The calculated photon yields are in surprising agreement with the data on γ- production at intermediate energies.