H. Oeschler

Central collisions in the argon induced fission of thorium in the transition energy regime

Abstract Angular correlations between fission fragments induced by an 40Ar bombardment of 232Th targets at four energies, 31, 35, 39 and 44 MeV/u, have been measured as well as the energies, velocities and masses of both fragments. The characteristics of peripheral collisions remain about constant in the range studied, whereas the component attributed to the central collisions which shows a saturation in linear momentum transfer at 180 MeV/c per incident nucleon, has a cross section decreasing with bombarding energy which is correlated to an increase of the excitation energy of the fissioning nuclei.

High momentum and energy transfer induced by 1760 MeV 40Ar ON 197Au and 232Th targets

Abstract Angular correlations between fission fragments induced by 1760 MeV 40Ar bombarding 197Au and 232Th targets have been measured. Energies, velocities and masses have been obtained for both fragments. The momentum transfer and energy deposited were deduced from an event-by-event analysis. The recoil velocity and total mass spectra show that mass transfer is still a dominating process. Momentum transfer of at least 6.5 GeV/c and 900 MeV excitation energy are deduced.

FASA - A 4π detector setup for the investigation of target multifragmentation in nucleus-nucleus collisions

Abstract The FASA setup, installed at the JINR synchrophasotron providing light ion beams with energies up to 3.65 GeV/nucleon, is a fragment multiplicity detector, consisting of 55 scintillation counters made of thin CsI(TI) films, five time-of-flight telescopes and a large-area position-sensitive parallel-plate avalanche chamber. The basic aim of the device is to determine with high precision the energy, mass, and velocity of the fragments detected in the time-of-flight telescopes (TOF) while for the other fragments global multiplicity information is obtained. Therefore, the TOF telescopes serve as a trigger. In addition, angular correlations and distributions and relative velocity correlations for coincident fragments can be measured with the FASA setup.

Evidence for simultaneous breakup in reactions with relativistic α-projectiles☆

Abstract The relative-angle distributions of IMFs, studied for the system α + 197 Au at 3.6 GeV/nucleon, show a strong suppression of small relative angles. By multi-body Coulomb trajectory calculations it is shown that the magnitude of this effect is very sensitive to the time scale of the IMF emission. The experimental angular distributions are well described assuming a fast breakup of the system with time less than 100 fm/c between IMF emissions.

The Tori spectrometer, a magnetic transport system separating positrons and electrons emitted in heavy ion collisions ☆

Abstract A new type of magnetic transport system, the Tori spectrometer, was constructed to measure positrons and electrons simultaneously. Via inhomogeneous toroidal magnetic fields the two charges are spatially separated and transported to different energy-analysing Si(Li) detectors. By means of a magnetic mirror a high positron transmission of about 70% is achieved while electrons are suppressed by about 10 4 . In the particular application for in-beam operation two gas counters for kinematical coincidences are included.

Transition from fission to multifragmentation

Abstract Fragment-fragment correlations have been studied in α -induced reactions at incident energies from 70 to 800 MeV/u on Ag, Ho and Au targets. Even at the highest energy binary thermal fission is observed with excitation energies up to 800–900 MeV. In addition, events with mass number around 30 and relative velocities up to twice the values expected for thermal fission occur and are attributed to multifragmentation. The energy deposit rather than the projectile velocity seems to be parameter governing the appearance of the multifragmentation process which competes strongly with fission for imparted energies beyond 800 MeV.

On the systematics of positron production in elastic heavy ion collisions

Abstract The integral production probability of atomic positrons associated with elastic heavy ion collisions at beam energies from 5.9 to 10 MeV/u has been investigated as a function of the characteristic collision time t and with respect to its dependence on the combined nuclear charge Z united . Recent data from the systems Pb+Pb at 8.6 MeV/u and Pb+U at 8.4 MeV/u exhibit deviations from coupled-channels calculations: The experimental yield increases stronger towards shorter collision times than predicted.

Deep inelastic collisions in the system [sup 40]Ar+[sup 232]Th at 31 MeV/nucleon

The charge, velocity, and angular distributions of three coincident fragments measured for the system [sup 40]Ar on [sup 232]Th at an incident energy of 31 MeV/nucleon evidence a deep inleastic collision process followed by fission. The estimated total kinetic energy loss is about 600--800 MeV which represents roughly 60--80% damping of the initial kinetic energy.

Positrons and Electrons Emitted in Elastic and Dissipative Heavy Ion Collisions

The main research line of the Tori group is the study of the reaction dynamics of dissipative collisions between heavy ions ia positron and electron spectroscopy. The last five years since the Lahnstein-Conference1 are marked for our group by the installation of a new experimental device for detecting positrons and electrons emitted in these collisions, the so-called Tori spectrometer2. The first part of this report is devoted therefore to describe the main characteristics of this apparatus.

Positron-Electron Angular Correlations in Heavy Ion Collisions

Recent measurements of positrons in coincidence with electrons emitted in U+Th collisions near the Coulomb barrier show sharp line structures in the sum and difference energy spectra of the two leptons (ref. 1). A possible method to clarify the origin of these lines is to study the angular correlation of the leptons. In this short contribution we want to report on a positron-electron coincidence test measurement performed with the TORI spectrometer. This apparatus offers the possibility to measure positrons in coincidence with electrons emitted into the same and opposite hemisphere as well.