Production Of Intermediate Mass Fragments Research Articles

Dynamical Fragment Production as a Mode of Energy Dissipation in Heavy-Ion Reactions.

Based on measured correlations between experimental observables in the {sup 209}Bi+{sup 136}Xe reaction at {ital E}/{ital A}=28 MeV, it is shown that multiple intermediate-mass fragment (IMF) production is a dynamical process driven by the energy of relative motion of projectilelike and targetlike fragments. This kinetic energy is converted into thermal energy of the system, until a certain {open_quote}{open_quote}saturation{close_quote}{close_quote} value of approximately 3MeV/nucleon is reached. From this point on, this {open_quote}{open_quote}conventional{close_quote}{close_quote} dissipation mechanism is replaced by dynamical IMF production, constituting a new mode of energy dissipation. {copyright} {ital 1996 The American Physical Society.}

Isospin dependence of intermediate mass fragment production in heavy-ion collisions at E/A=55 MeV.

The production of intermediate mass fragments (IMF's) from the four reactions 55A MeV $^{124,136}\mathrm{Xe}$ + $^{112,124}\mathrm{Sn}$ is studied with an experimental apparatus which is highly efficient for the detection of both charged particles and neutrons. The IMF's are more localized in the midvelocity region than are the light charged particles, and the detected multiplicity of IMF's depends linearly on the charge lost from the projectile and increases with the neutron excess of the system. Remnants of the projectile, with very little velocity reduction, are found for most of the reaction cross section. Isotopic and isobaric fragment yields in the projectile-velocity region indicate that charge-to-mass ratio neutralization is generally not achieved but is approached when little remains of the projectile. For all systems, the fragments found in the midvelocity region are substantially more neutron rich than those found in the velocity region dominated by the emission from the projectile. This observation can be accounted for if the midvelocity source (or sources) is either more neutron rich or smaller, with the same neutron-to-proton ratio, than the source with the velocity of the projectile. Taken together, the observations of this work suggest that the intermediate mass fragments are, to a large extent, formed by multiple neck rupture of the overlap material, a process which might enhance the neutron-to-proton ratio of the primary source material and/or limit the size of the sources. This scenario is reminiscent of low-energy ternary fission and one predicted by Boltzmann-Uehling-Uhlenbeck calculations. However, these calculations predict too much velocity damping of the projectile remnant. The calculations improve, in this regard, when the in-medium nucleon-nucleon cross sections and the cost of creating low density material are reduced. \textcopyright{} 1996 The American Physical Society.

Dependence of intermediate mass fragment production on the reaction mechanism in light heavy-ion collisions at intermediate energy.

The same hot nuclear system (\ensuremath{\Sigma}Z=18) has been studied for two different entrance channels with reaction products detected in a forward array of scintillators: central collisions of $^{24}\mathrm{Mg}$ on a $^{12}\mathrm{C}$ target at 25A and 35A MeV and peripheral pickup reactions of $^{35}\mathrm{Cl}$ on a $^{197}\mathrm{Au}$ target at 43A MeV. The detection-efficiency-corrected charge distributions, multiplicity of charged particles and cross sections as a function of excitation energy are compared. The reaction mechanism is investigated, through comparison to simulations with statistical observables. The central reaction $^{24}\mathrm{Mg}$${+}^{12}$C at 35A MeV is well characterized by a dissipative binary collision scenario. Data at 25A MeV show less evidence of such dynamical characteristics. The intermediate-mass fragments (3\ensuremath{\le}Z\ensuremath{\le}8) production for each reaction is compared to model calculations for different values of excitation energy. The systems formed in the central collision at 25A MeV and the pickup reaction at 43A MeV show similar source characteristics, both statistically and in momentum space. However, the yields of the various exit channels, from evaporation and/or fission to multifragmentation and vaporization, differ for the two reactions. \textcopyright{} 1996 The American Physical Society.

Intermediate mass fragment production in (? +27Al) reaction at low energies

The energy spectra of12C and16O fragments emitted in 50 and 60 MeVα-induced reactions on27Al have been measured using Solid State Nuclear Track Detectors. These data are in good agreement with the predictions of a simple moving source model, indicating statistical emission of the fragments from a nearly equilibrated compound nucleus.

The FOPI Detector at SIS/GSI

The FOPI detector system has been assembled at SIS/GSI to study the properties of hot and dense nuclear matter. The program of FOPI was subdivided to permit the start of a specific program from the beginning of SIS operation. Therefore, the completed system which covers nearly the full 4π solid angle has been built in three stages. (1) The forward plastic wall detector was completed in 1990 and has been successfully employed to measure intermediate mass fragment production in reactions of Au on Au at 150 to 1000 A·MeV. (2) The superconducting solenoid and Central Drift Chamber were completed in 1992 and have also been successfully employed to study charged pion production in Au on Au at 1050 A·MeV. (3) The plastic scintillator/Cherenkov Barrel and forward drift chamber HELITRON are partially installed and the full commissioning is expected at the beginning of 1995. Results from recent test experiments involving all three phases which demonstrate the detector's ability to identify the rare signals from Kaon and A particle production will be presented.

Absence of saturation in energy deposition in [formula omitted] collisions at E = 15–115 AMeV

Fission fragment (FF), intermediate mass fragment (IMF), and light charged particle (LCP) production have been measured in 40 Ar + 232Th collisions at E = 15–115 AMeV with the Michigan State University 4π Array. Trends in IMF and LCP production and in calculated excitation energy indicate there is no saturation in the deposited energy in central collisions of this system in the bombarding energy range studied.

Signature of exotic nuclear shapes from IMF-IMF correlations

Fragmentation of exotic nuclear shapes like toroids and bubbles predicted to be formed in intermediate energy heavy ion central collisions within the framework of BUU type calculations is studied in a statistical model with microcanonical simulation of prompt multifragmentation. The relatively smaller interfragment Coulomb interaction energy in the exotic shapes results in an enhanced production of intermediate mass fragments (IMFs) compared to that in the compact spherical configuration. The differences in the spatial geometry of the fragmenting shapes manifested in the inclusive charge distributions are further projected sharply in the IMF-IMF correlations. We exploit the correlation function to delineate the different fragmenting configurations.

Radial flow and multifragmentation within the framework of molecular dynamics approaches.

Two standard molecular dynamics approaches, differing in their treatment of Pauli blocking effects, are analyzed in terms of their predictions of intermediate mass fragment production and radial flow. Although the time evolution of the system is different when described in the two treatments, they converge to similar asymptotic results. However, both approaches fail to reproduce, in some cases, the experimental radial flow and intermediate mass fragment multiplicities.

Intermediate mass fragment production on Au, Ag, Ni and Al targets induced by 1 GeV protons

The production of intermediate mass fragments (IMF) with the charges Z F ≥ 3 is one of the main channels of the decay of highly excited nuclei produced in proton- and heavy ion-nucleus collisions at intermediate and high energies of projectiles. Despite of considerable experimental and theoretical efforts the mechanism of the IMF production is not clear yet. In our opinion it is caused by the lack of systematical data on the evolution of the IMF production process as a function of the bombarding energy and mass of the decaying system. Such systematical studies could reveal general features of the process evolution and yield some key parameters which govern the IMF production. The main purpose of these experiments was to investigate an evolution of the main characteristics of the IMF production process in dependence upon the size of nuclear system in the wide range of the nuclei from Au to Al at 1 GeV proton energy where the maximum raise of the cross sections for the IMF production is observed. We present here the results on the target mass dependence of the charge distributions, the integral cross sections, the mean excitation energies of nuclei emitting IMF's, the mean values of the slope parameter of the energy spectra and the cross sections of the multifragmentation process.

Dynamical production of intermediate-mass fragments in peripheral 209Bi+ 136Xe collisions at [formula omitted

Intermediate-mass fragments (IMF) from the reaction 209Bi+ 136Xe at E lab A =28 MeV have been measured in coincidence with other charged reaction products, as well as with neutrons. The IMF emission patterns are seen to exhibit signatures of two mechanisms — statistical emission from the fully accelerated massive reaction partners and a fast dynamical emission from a single effective source. The latter mechanism, possibly involving a multiple neck rupture, becomes dominating for least dissipative collisions characterized by low associated light particle multiplicities.

Fragmentation of high energy mass-selected ionic hydrogen clusters

First results on fragmentation of fast ionic hydrogen clusters ( n = 5−31, odd; 60 keV/amu) by single collision with a helium atom have been obtained with the new beams delivered by the variable energy post-accelerator of RFQ type at Institut de Physique Nucléaire de Lyon. The fragmentation apparatus and preliminary results are presented. The inclusive mass distributions of the charged fragments measured are strongly different from those resulting from lower energy collision. An important production of intermediate mass fragments is observed.

Multifragment events in the reaction 58Ni+27Al at 15.8 MeV/nucleon.

Intermediate mass fragment (IMF) production was studied in the inverse kinematics reaction [sup 58]Ni+[sup 27]Al at 15.8 MeV/nucleon bombarding energy using the Heavy Ion Light Ion (HILI) detector. Exit channels with up to 4 IMFs have been observed. The three-body events can be ascribed to different production mechanisms: (i) incomplete momentum transfer with projectile spectator breakup, (ii) sequential fission following a deep inelastic reaction, and (iii) fast sequential fission (i.e., with Coulomb final-state interaction) in competition with a prompt breakup out of a toroidal configuration. The identified ternary reaction mechanisms, covering an extremely wide dynamic range, could be observed simultaneously; a subgroup of events is compatible with an instantaneous breakup of a highly excited nuclear system.

Intermediate mass fragment production in central collisions of intermediate energy heavy ions.

We present Z distributions for fragments with 1\ensuremath{\le}Z\ensuremath{\le}12 from central collisions of $^{40}$ Ar${+}^{45}$Sc at incident energies ranging from 35 to 115 MeV/nucleon. We find that the Z distributions can be described by a power law or an exponential and steepen with increasing incident energy. Over the range of incident energies studied, the average number of intermediate mass fragments decreases while the average number of particles increases. When combined with previous results for the charge distributions, a minimum is observed in the extracted power-law parameter.

Multifragmentation of thermalized residual nuclei in intermediate-energy heavy-ion collisions

Nucleus-nucleau reactions leading to the copious production of intermediate mass fragments are considere as a two-stage process. The first nonequilibrium stage of the reactions is simulated by the intranuclear cascade model and (preequilibrium emission) exciton model. At the equilibrium stage of the reaction thermalized residual nuclei (TRN) undergo a multifragment break-up which is described within the framework of the statistical multifragmentation model. The comparison to experimental correlation characteristics of intermediate mass fragments allows us to extract the important information concerning the actual masses and excitation energies of TRN.

Isotopic effects in the disassembly of hot nuclear systems.

The analysis of isotopic effects in the production of intermediate mass fragments from light-ion-induced reactions on heavy nuclei reveals a high degree of equilibration in the isotopic degree of freedom. The observed effects are quantitatively reproduced by the quantum statistical model based on chemical equilibrium. The nearly identical isotopic yield ratios for reactions induced by light ions (p,d,\ensuremath{\alpha}) of several GeV and by heavy ions ${(}^{12}$C${,}^{14}$N${,}^{18}$O) of 30--100 MeV/nucleon suggest that the statistical parameters of the hot nuclear systems produced in both types of reactions are very similar.

Production of three nearly equal mass fragments in the Xe + Cu reaction at 45 MeV/u

An experimental study of three intermediate mass fragment production in the reaction Xe + Cu at 45 MeV/u is presented. Results are compared with the predictions of a conventional statistical binary decay model starting from equilibrated sources obtained both from incomplete fusion systematics and from dynamical calculations at several impact parameters. The inclusive experimental data agree with the dynamical predictions, whereas a disagreement between the exclusive data and the calculations has been found.

An event generator for the study of nuclear collisions in the Fermi energy domain (I). Formalism and first applications

An event generator is proposed to simulate the response of complex detectors used for the study of heavy-ion collisions in the Fermi energy domain. The scenario of the generator is based on some well-established experimental findings characterizing the considered energy range. The physics used for the description of the decay of the various remnants produced in the collision is based on a version of the statistical model taking into account the different time scales involved in the decay processes. As examples of the first applications of the event generator, the intermediate-mass fragment production and the question of the saturation of the thermal energy deposited in heavy targets are studied. Multifragment events studied with complex multidetectors will be analysed in a forthcoming paper.

Intermediate mass fragment production in interaction 40Ar+208Pb at E/A=19.6 MeV.

The intermediate mass fragment production in the interaction {sup 40}Ar+{sup 208}Pb at {ital E}/{ital A}=19.6 MeV was studied using a CR-39 plastic track detector. Exit channels with up to three intermediate mass fragments were observed. It seems that events having at least one intermediate mass fragment originate from central collisions. The relative velocity correlation functions were determined for these events and discussed.

Heavy fragment emission in alpha-induced reactions on aluminum at 60 MeV

The production of intermediate-mass fragments ranging from Z=3 to 8 emitted in the reaction of 60 MeV alpha on an aluminum target has been experimentally studied. The energy spectra of the emitted fragments in the angular range of 20\ifmmode^\circ\else\textdegree\fi{}--130\ifmmode^\circ\else\textdegree\fi{} have been measured and the total production cross sections of each fragment have been estimated from the data. The double-differential cross-section data have been compared with the predictions made using (a) a phenomenological moving source model and (b) a binary fragmentation model. The absolute total emission cross sections for various fragments have been calculated assuming the statistical decay of a fully equilibrated compound nucleus and have been compared with the corresponding experimental estimates. It is found that the exit-channel deformation plays a significant role in the estimation of the Coulomb barrier of the separating nuclei. A theoretical procedure for the estimation of the exit-channel Coulomb barrier for the deformed system has been incorporated in the present calculation.

On the determination of reaction product parameters using polycarbonate track detector Makrofol

The improved Makrofol detection technique was applied in the study of intermediate mass and heavy fragment production in the interaction of 8.8 GeV alpha particles with gold nuclei. The limitations of the applied technique were discussed and the uncertainties of results obtained by the standard version of this technique were put forward. Fragments produced in the experiment were identified and an event by event model-free analysis was performed in order to separate different production mechanisms.