Production of fragments in He+Au reaction at 12.7 GeV

The emission of fragments (A≥16) from reaction of ⁴He with ¹⁹⁷Au has been studied at bombarding energy of 12.7 GeV. Policarbonate Makrofol in sandwich form was used as a detector. Fragments produced in the experiment were identified and an event by event model-free analysis was performed. The decay channels ending with one, two or more (three, four) fragments were detected. A classification scheme based on the multiplicity (M_H) of heavy fragments (A≥40) was used in order to define multifragmentation (M_H = 0), spallation (M_H = 1) and fission (M_H = 2) events. The various experimental features of the fragmentation process are discussed.     

Interaction of 12.7 GeV He ions with U, Pb, Au and Ag targets

The processes of production of fragments Z 8 in the interactions of 12.7 GeV ⁴He with U, Pb, Au and Ag have been analyzed using the polycarbonate track detector Makrofol. A sandwich technique was used which enables direct evidence for multiple fragment emission by a single nucleus. The decay channels ending with one, two or more (three, four) fragments were detected. A classification scheme based on the multiplicity of heavy fragments M_H (Z> 20) was used in order to identify events belonging to the different reaction channels. Cross sections, excitation energies and multiplicities of intermediate mass fragments (8 Z 20) have been evaluated for various reaction mechanisms. The experimental features of the fragmentation process have been analyzed as a function of the target mass.     

Heavy fragment production in He+Th reactions at 0.65–12.7 GeV

The processes of production of heavy fragments in the fission mass region in the interaction of 0.65, 1.74, 5.1, 8.8 and 12.7 GeV ⁴He with ²³²Th have been analyzed using the polycarbonate track detector Makrofol in the form of a sandwich. The decay channels ending with one, two, or more (three, four) fragments were detected. We have analyzed the events in which at least one heavy fragment (Z>20) was registered. Fragments produced in the experiment were identified and an event-by-event model-free analysis was performed in order to separate different production mechanisms. We have identified the events produced in fission, deep spallation and fragmentation processes. The cross-sections and experimental features have been determined for the various reaction mechanisms, and their variation as a function of the incident energy has been investigated.     

Fission of Pb in the He-induced reactions at 0.65–12.7 GeV

The fission processes from the interactions of 0.65, 1.74, 5.1, 8.8 and 12.7 GeV ⁴He with Pb have been analyzed using the polycarbonate track detector Makrofol in the form of a sandwich. The decay channels ending with one, two, or more (three, four) fragments were detected. We analyzed events in which only two heavy fragments (Z>20) are detected, irrespective of the existence of coincident intermediate mass fragments (8Z20). Using the correlation between the common observables which characterize fission events, we identified events originated from thermal (soft) fission and more violent processes. Cross sections, angular distributions and excitation energy have been determined for two fission mechanisms, and their variation as a function of the incident energy has been investigated.     

Production of fragments with Z≥ 8 in interaction of 12.7 GeV 4He+U, Pb, Au and Ag: track detector study

The processes of production of fragments with Z≥ 8 in the interaction of 12.7 GeV ⁴He with U, Pb, Au and Ag have been analyzed using the polycarbonate track detector Makrofol. The sandwich technique was used which enables direct observation of multiply charged fragment emission by a single nucleus. The decay channels ending with one, two, or more (three, four) fragments were detected. A classification scheme based on the multiplicity of heavy fragments with Z > 20 was used in order to identify the events belonging to the different reaction channels. The cross sections, excitation energies and multiplicities of intermediate-mass fragments 8 ≤Z≤ 20 have been determined for the various reaction mechanisms, and their variation as a function of the target mass has been investigated. Received: 28 November 1997 / Revised version: 24 April 1998     

Evaporation residue formation competing with the fission process in the197Au+16O,12C reactions and fission barriers at a specifiedJ window

Evaporation-residue excitation functions for¹⁶O and¹²C+¹⁹⁷Au reactions were measured by means of the activation technique. The competition between evaporation and fission of the compound nuclei was studied by comparing the observed evaporation-residue data with the published fission excitation functions. A newly devised analysis was applied in order to deduce a fission barrier height at a specified angular momentum and determine the relevant fissioning nucleus as well. We found the fission barriers to be 8.2 MeV for the²¹¹Fr nucleus at 16? and 8.2 MeV for the²⁰⁷At nucleus at 27?.     

Nucleon exchange mechanism of mass asymmetry relaxation in fission and other binary nuclear reactions

Mass asymmetry relaxation as manifested in fission and heavy ion-induced binary reactions is reviewed. In fission, the dynamics is characteristic of a fully damped case and is well described by a stochastic theory. In heavy ion deep inelastic collisions and quasi-fission, on the other hand, the relaxation is incomplete giving rise to the possibility of studying its time evolution.     

Correlations between fission fragments and light charged-particles in the reaction20Ne+197Au at 13 MeV/nucleon

rgy of light charged-particles has been measured in coincidence with one or two fission fragments in the reaction²⁰Ne+¹⁹⁷Au at a bombarding energy of 13 MeV/u. The fission cross section was found equal to 1,340±260 mb. Assuming that it represents the totality of the fusion cross section, the critical valuel _c is deduced equal to 93±9?, higher thanl _Bf=0=74 in the rotating liquid drop model. The main emission source for protons and alpha particles seems to be a thermally equilibrated composite system. The competition between fission and charged particle emission is unexpected in the frame of the statistical treatment. A high energy component is observed in the forward direction. These results correspond to the first step of a study programme on the evolution of nucleus-nucleus collisions between 10 and 100 MeV per nucleon.     

Recoil energies of fragments photoproduced in Cu,Ag and Au

The recoil energies of²⁴Na produced in Cu, Ag and Au and of a series of fragments produced in Cu irradiated with 800 MeV bremsstrahlung have been measured in directions forward, backward and perpendicular to the beam using the thick target-thick catcher method. Comparison of the measured values with theoretical calculations indicates that²⁴Na is produced through fission in Ag and Au and that fission is competing with spallation even in Cu.     

Production of neutral mesons in the reaction12C+12C at 2 GeV/u

A systematic study of neutral meson production in heavy-ion reactions at 2 GeV/u was started with a¹²C+¹²C experiment using the photon spectrometer TAPS. Special emphasis is put on the possible observation of the-meson in a heavy-ion reaction exploiting the decay channel ⁰.Presented at the International School-Workshop Relativistic Heavy-Ion Physics, Prague (Czech Republic), 19–23 September 1994.     

Intermediate mass fragments emitted in the reaction Ag+14N at 100, 160 and 250 MeV bombarding energy

The fragments produced in the reaction between a ¹⁴N beam of various energies and a natural Ag target have been studied. The atomic numbers of the fragments have been identified up to Z = 17 by means of a E-ΔE counter telescope. The cross sections, the kinetic energy distributions as well as the angular distributions have been measured for each atomic number. The kinetic energy distributions show two components: a high-energy component (quasi-elastic), prevailing at angles close to the grazing angles and for atomic numbers close to Z = 7, and a low-energy component (relaxed), at energies close to the Coulomb repulsion energy, present at all angles and for all the Z. A detailed study of the relaxed components of the kinetic energy seems to account for both them means and the widths of these distributions on a purely statistical basis. The cross sections of the relaxed components appear to be quite large at low Z and to decrease rapidly to a fairly constant value in the region of 10 ≦ Z ≦ 17. A marked even-odd alternation in the cross sections is observed. The angular distributions are strongly forward peaked for Z < 7. For Z > 7 the forward peaking decreases rapidly until, for Z > 13, the limiting form 1/sinθ is attained. Evidence for the existence of a diffusion process along the mass asymmetry coordinate is discussed.     

Production of helium fragments in197Au-emulsion collisions at 10.6 A GeV

We present a systematic analysis on the helium projectile fragments produced from 10.6 A GeV¹⁹⁷Auemulsion interactions in an experiment conducted at the Brookhaven AGS. Total charge changing and partial production cross-sections are measured experimentally on the basis of helium multiplicity. The multiplicity distribution of helium fragments which are produced collectively obey a KNO scaling. The transverse momentum distribution of these particles indicates that they are produced from two different independent sources.We are thankful to the BNL technical staff and specially to Dr. D. Beavis for his help in the exposure of emulsion stacks. Thanks are due to Profs. Y. Takahashi and G. Romano for their help in the preparation and development of the emulsion stacks. We appreciate the discussions with Prof. H. Stocker and J. Aichelin. This work was supported by D.O.E. under Grant No. DE-FGO2-90ER 40566.     

Production of charged pions, kaons and antikaons in relativistic C + C and C + Au collisions

Production cross-sections of charged pions, kaons and antikaons have been measured in C+C and C+Au collisions at beam energies of 1.0 and 1.8 AGeV for different polar emission angles. The kaon and antikaon energy spectra can be described by Boltzmann distributions whereas the pion spectra exhibit an additional enhancement at low energies. The pion multiplicity per participating nucleon M(π⁺)/<A _part> is a factor of about 3 smaller in C+Au than in C+C collisions at 1.0 AGeV whereas it differs only little for the C and the Au target at a beam energy of 1.8 AGeV. The K⁺ multiplicities per participating nucleon M(K⁺)/ <A _part> are independent of the target size at 1 AGeV and at 1.8 AGeV. The K^- multiplicity per participating nucleon M(K^-)/ <A _part> is reduced by a factor of about 2 in C+Au as compared to C+C collisions at 1.8 AGeV. This effect might be caused by the absorption of antikaons in the heavy target nucleus. Transport model calculations underestimate the K^-/K⁺ ratio for C+C collisions at 1.8 AGeV by a factor of about 4 if in-medium modifications of K-mesons are neglected. Received: 10 December 1999 / Accepted: 14 November 2000     

Thermal multifragmentation in p + Au interactions at 2.16, 3.6 and 8.1 GeV incident energies

Multiple emission of intermediate-mass fragments has been studied for the collisions p + Au at 2.16, 3.6 and 8.1 GeV with the FASA setup. The mean IMF multiplicities for events with at least one IMF are equal to 1.7, 1.9 and 2.1 (±0.2) respectively. The multiplicity, charge distributions and kinetic energy spectra of IMF are described in the framework of a intranuclear cascade model followed by the statistical multifragmentation model. However, between the two parts of the calculation the excitation energies and the residual masses and charges are modified to take into account the losses during expansion. The results support a scenario of true thermal multifragmentation of a hot and expanded target spectator. Received: 15 December 1997 / Revised version: 24 April 1998     

Free electron-positron pair production in collisions of Au (10.8 GeV/u) with Cu , Ag and Au

Total and differential cross-sections for the production of free electron-positron pairs are calculated for the reactions Au⁷⁹⁺ (10.8 GeV/u) on Cu²⁹⁺, Ag⁴⁷⁺ and Au⁷⁹⁺. The methods used are the semiclassical approach and the solution of the time-dependent Dirac equation by a coupled channel method with free wavepakets describing the created fermions. The obtained total cross-sections are in good agreement with the experimental data. The differential cross-sections give informations about the correlation between the electron and positron. Received 21 December 1999 and Received in final form 6 March 2000