Nuclear disassembly of the Pb+Au system atElab=29 MeV per nucleon

Abstract

Nuclei with Z up to 82 emitted in the 208 Pb+ 197 Au reaction at isiab s== 29 MeV per nucleon have been measured as a function of the associated neutron multiplicity. The data reveal the presence of strong correlations between character of a collision and neutron multiplicity. The trends suggest a disassembly of the nuclear system into a large number of nucleons and small fragments in the events with the highest neutron multiplicity. In such events, approximately one-third of the neutrons are released from the system and fragment yields decrease in an exponential fashion with increasing mass.PACS numbers: 25.70.Np, 25.70.Lm Recent experiments 1 studying interactions of intermediate-energy Ar projectiles with massive targets (Au,Th) have shown that, above a bombarding energy of 30 MeV per nucleon, the measured most probable multiplicity of neutrons emitted from the reaction in dissipative collisions remains almost independent of the bombarding energy. Since for heavy systems the neutron multiplicity is a good measure of the total excitation energy introduced into the system, such a "saturation" effect could have represented a manifestation of limits to the temperature attainable by a heavy nucleus. 2 However, in subsequent experiments it has been observed 3 that a further increase in excitation energy per nucleon is possible when heavier projectiles such as Kr are used at similar velocities (E\ 3L \y= s 32 MeV per nucleon). Such a dependence of the limiting temperature on the projectile-target combination suggests that the limits reached in the above experiments are not yet those characteristic of nuclear systems in general but, rather, are a reflection of the influence of the collision dynamics on the efficiency of heat-generation mechanisms.The purpose of the present work is to extend the above systematics to very-heavy-projectile-target combinations and to search for evidence for changes in reaction mechanisms, at the higher temperatures that can presumably be generated in such heavier systems. Therefore, an experiment was carried out at GANIL, using the 29-MeV-per-nucleon beam of Pb, the heaviest projectile available, to bombard a Au target. Charged reaction products, ranging from protons to projectilelike fragments, were measured with an array of different types of detector telescopes, ensuring a wide dynamic range and a broad angular coverage. In the following, charged-fragment data are discussed as obtained with a forward telescope consisting of 200-^m (SE) and 500-jum (E) Sistrip detectors. The 24x24-mm 2 strip detectors (with six strips each) were aligned so as to ensure X-Y position sensitivity of the telescope. The detector covered the angular range from 6.1° to 20°, including the grazing angle of 6.2°. The 200-jum detector imposed a detection threshold of 16-18 MeV per nucleon for fragments with atomic numbers from Z =30 to 82.The sensitive An neutron-multiplicity detector ORION was employed to provide a measure of the number of neutrons produced in a collision. A more detailed description of this d...

Multifragment production in Au+Au at 35 MeV/u

“…In 100 MeV /u Au + Au [3] central collisions, dynamical and statistical analyses [7] suggest that the large multiplicities, observed for light particles and Intermediate Mass Fragments, are compatible with the prompt multifragmentation of a heavy, thermalized composite system with freeze-out density ≈ 1 3 ÷ 1 6 of the normal nuclear density (ρ 0 = 0.15 f m −3 ), even if the fragment probability emission resulted strongly influenced by the radial flow [3,8]. In the nearly symmetric P b + Au reaction at 29MeV /u [4] the charged products emission, studied for increasing neutron multiplicity, shows that the emission of Intermediate Mass Fragments becomes the largest component of the cross section at the expense of Projectile Like Fragments and Fission Fragments emission. On the other hand an analysis, mainly based on the event shape, of the same reaction P b + Au [5,9] at the same incident energy seems to reveal that, even selecting the most central collisions, the largest part of the total reaction cross section is due to strongly damped binary collisions.…”

“…Several recent experimental studies of central collisions, performed with very heavy nuclei at different incident energies, give different indications on this point [3,4,5,6]. In 100 MeV /u Au + Au [3] central collisions, dynamical and statistical analyses [7] suggest that the large multiplicities, observed for light particles and Intermediate Mass Fragments, are compatible with the prompt multifragmentation of a heavy, thermalized composite system with freeze-out density ≈ 1 3 ÷ 1 6 of the normal nuclear density (ρ 0 = 0.15 f m −3 ), even if the fragment probability emission resulted strongly influenced by the radial flow [3,8].…”

Inclusive fragment production in84Kr +197Au atE/A=35 MeV

“…It was made possible by the extension of earlier research with heavy ion beams of intermediate energies (E/A __< 100 MeV) to relativistic energies [2,3] and by employing very heavy projectiles of intermediate energy such as xenon or lead [4,5]. In both cases the complete breakup of the formed excited nuclear systems into intermediate mass fragments (IMF's, fragments with atomic number Z> 3) and light particles was observed.…”

Excitation and decay of projectilelike fragments formed in dissipative peripheral collisions at intermediate energies

“…To date, a variety of means has been used to excite finite nuclear matter to the limits of stability. These approaches range from multi-GeV hadronic probes [5,6] or projectile fragmentation at relativistic energies [7,8], to central collisions of intermediate-energy heavy-ions [9,10,11]. Recent advances in the availability of high-intensity radioactive beams in the intermediate energy domain raises the question of how the N/Z dependence of the EOS is best probed.…”