Multifragment Events Research Articles

Branching ratios in dissociative recombination of formyl and isoformyl cations

The energy dependence of the branching ratios in dissociative recombination of DCO+ with a known small admixture of DOC+ has been measured for collision energies from 0 to 25eV using an energy- and position-sensitive surface barrier detector which is designed for the analysis of multi-fragment events occurring in a molecular fragmentation study. The measurements are compared with theoretical calculations on the direct mechanism of dissociative recombination of HCO+ including the contribution from HOC+ at the experimental abundance fraction. At low collision energies, dissociative recombination of HCO+ is dominated by dissociation into H + CO. For collision energies above 2eV there is a transition into dissociation to HC+O, which can be explained by electron capture into resonant states. Signatures of DOC+ dissociative recombination are found experimentally and confirmed by the calculations for HOC+. Three-body breakup becomes important for collision energies above 6eV.

Conceptual design of a large area time-of-flight wall for the R3B experiment at FAIR

We present the conceptual design of a large-area time-of-flight wall for relativistic heavy ion identification at the R 3 B experiment at FAIR. The main requirements of this detector are a time resolution better than 50 ps (FWHM), a large surface in order to cover the full angular range and a sufficient granularity to identify multi-fragment events. To achieve these requirements we propose a pioneering solution based on the use of multi-gap resistive plate chambers.

Multifragment events in the 238U+197Au reaction at 15 MeV/nucleon.

The $^{238}\mathrm{U}$+$^{197}\mathrm{Au}$ reaction has been studied at 15 MeV/nucleon bombarding energy. Correlations between two to seven final fragments with $Z>~8$ have been measured using CR-39 nuclear track detectors. An event-by-event kinematic analysis was carried out for three-body exit channel reactions which exhaust about half of the final fragment production cross section. Most of these ternary processes can be associated with intermediate and peripheral impact parameters. The dominant reaction mechanism is shown to be that of a first binary quasielastic or dissipative collision followed by the sequential binary fission of the excited quasi-$U$ (or to a lesser extent of the excited target remnant). We observe also a net drift of nucleons from the target to the projectile prior to its decay.

Violent collisions and multifragment final states in the 40Ca+40Ca reaction at 35 MeV/nucleon.

The 4[pi] multidetector AMPHORA has been used to measure yield distributions and energy spectra for products of the collisions in the reactions of [sup 40]Ca with [sup 40]Ca at 35 MeV/nucleon. Events of high multiplicity ([ge]10) for which [ge]85% of the total entrance channel atomic number is detected have been isolated and found to result from the most violent collisions which lead to excitation energies near 6 MeV/nucleon. A large fraction of these collisions lead to multifragment final states. A detailed comparison of the experimental data with results of various models indicates that statistical models which allow for expansion of the system or treat the multifragmentation process as a simultaneous disassembly are more successful than normal sequential binary models at reproducing the yield data and the event complexity inherent in the multifragment events. Quantum molecular dynamic (QMD) calculations are found to provide generally good agreement with the data but overestimate the proton and neutron emission. The agreement is significantly improved if an appropriate afterburner is used to deexcite the separated primary QMD fragments. The sensitivity of such hybrid calculations to the assumed matching time between the dynamical calculation and the afterburner has been explored. The experimentally filtered QMD calculations whichmore » provide good agreement with the experimental observables suggest that the most complex events observed in this work come not from the most central collisions, which decay more by light particle emission, but from a region of impact parameter [ital b]/[ital b][sub max]=0.5. This suggests that angular momentum effects play an important role in the multifragment decay modes. A comparison of the present results with those for projectile fragmentation in intermediate impact parameter collisions of 600 MeV/nucleon [sup 197]Au with Cu indicates that a similar multifragmenting system is produced in the two very different reaction systems.« less

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.

Studies of intermediate-mass fragment emission in the 3He+natAg, 197Au reactions between 0.48 and 3.6 GeV.

Intermediate-mass fragments formed in reactions of $^{3}\mathrm{He}$ ions with $^{\mathrm{nat}}\mathrm{Ag}$ and $^{197}\mathrm{Au}$ targets have been studied at five energies between 0.48 and 3.6 GeV. Inclusive measurements show that as the bombarding energy increases, there is a strong enhancement in fragment cross sections and a trend toward isotropic angular distributions. Between 0.90 and 1.8 GeV, a change in the emission mechanism is suggested by (1) kinetic energy spectra with high-energy tails that become distinctly flatter, (2) a broadening of the spectral Coulomb peaks toward lower energies, and (3) charge distributions that become constant, exhibiting a power-law exponent \ensuremath{\tau}\ensuremath{\approxeq}2.0. Exclusive studies of the $^{3}\mathrm{He}$${+}^{\mathrm{nat}}$Ag system at 0.90 and 3.6 GeV detected multifragment events with multiplicities up to four. The probability for high-multiplicity events increases about 40-fold between 0.90 and 3.6 GeV. At both energies, the kinetic energy spectra depend on multiplicity, especially when triggering on backward-emitted fragments. For multiplicity three events, a rapidity analysis of the data at 3.6 GeV is consistent with a single, relatively low source velocity, ${\mathit{v}}_{\mathit{S}}$\ensuremath{\approxeq}0.4 cm/ns. The data are compared with predictions of a coplanarity-sphericity calculation, the sequential statistical decay code GEMINI, and a hybrid intranuclear cascade/percolation model.

Multi-fragment events as a probe of nuclear disassembly

We review the recent results on the multi-fragment decay of heated nuclear systems that are formed in asymmetric heavy-ion collisions. Particular emphasis is placed on those observables that are sensitive to the fluctuations in the decaying system and their possible role in extracting the physics of phase transitions in nuclear systems.

Complex fragment production and multifragmentation in 139La-induced reactions at 35, 40, 45 and 55 MeV/u

Complex fragment emission ( Z>3) has been studied in the reactions of 35, 40, 45 and 55 MeV/u 139La+X. Charge, angular, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sectins. Multifragment events increase with both bombarding energy and entrance-channel mass asymmetry. The excitation functions for multifragment events rise strongly with excitation energy. These excitation functions are independent of the target-projectile combination and bombarding energy suggesting, the formation of an intermediate nuclear system, whose decay properties depend mainly on its excitation energy and angular momentum.

Correlations in multi-fragment events

Correlations in multi-fragment events

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.

SOS: sequential or simultaneous nuclear multifragmentation

A computer code that simulates the phenomenon of nuclear fragmentation occuring in nuclear collisions at intermediate energies is presented. To underline the observable distinctions between different breakup mechanisms, the code produces multifragment events using a sequential binary decay and a simultaneous breakup resulting in two sets of momenta distributions with the same mass distribution and total energy.

Dynamical and statistical properties of hot nuclei

Properties of medium mass nuclei having excitation energies of 1.5 to 6 MeV/u have been probed in measurements of the multiplicities and energy spectra of emitted particles and fragments. The particle spectra indicate a progressive reduction of both the level density parameter, a, and the emission barriers as the excitation energy is increased. The evolution of the dominant de-excitation modes, observed with increasing excitation energy, is found to be qualitatively consistent with the predictions of statistical models. There is, however, evidence that dynamics may play an important role in fragment emission processes. At energies of 6 MeV/u multifragment emission is an important process. Varying theoretical models predict such multifragment events but differ in their predictions of the probability of such events.

Multifragment emission in reactions induced by 0.90 and 3.6 GeV 3He ions on natAg.

Multifragment events for intermediate-mass fragments (3{le}{ital Z}{le}12) with multiplicity up to four have been observed in the reaction of 0.90 and 3.6 GeV {sup 3}He ions with {sup natAg} nuclei. Fragment energy spectra and angular distributions are found to be dependent on event multiplicity. The data suggest significant modification of the Coulomb field of the emitting source.

Multifragment events from heavy-ion collisions: Sources and excitation functions.

Multifragment events from 35, 40, 45, and 55 MeV/nucleon $^{139}\mathrm{La}$${+}^{12}$C, $^{27}\mathrm{Al}$, $^{40}\mathrm{Ca}$, $^{51}\mathrm{V}$, $^{\mathrm{nat}}\mathrm{Cu}$, and $^{139}\mathrm{La}$ reactions can be assigned to sources characterized by their velocity. At each bombarding energy, the probabilities of threefold, fourfold and fivefold events increase substantially with decreasing source velocity, but are independent of the target mass. To remove the bombarding-energy dependence, a simple transformation has been applied which gives the excitation energy of the fused system in the simple incomplete-fusion picture. These ``excitation functions'' appear to independent of both the system and bombarding energy.

Binary and multifragment decay of very hot nuclei

Compound binary emission of complex fragments is illustrated for a variety of reactions. Complex fragment emission from 35 and 40 MeV/N 139La + 12C, 27Al, 40Ca and 51V reactions has been studied. Multifragment events from these reactions were assigned to sources characterized by their energy and mass through the incomplete-fusion-model kinematics. Excitation functions for the various multifragment channels appear to be nearly independent of the system and bombarding energy. Preliminary comparisons of the data with sequential-statistical-decay calculations are discussed.

Multi-fragment events in Kr + Ag interactions at 17.7 and 35 MeV/n

CR 39 was used as Solid State Nuclear Track Detector (SSNTD) to study the reactions of 17.7 and 35 MeV/n Kr on Ag targets. Cross-sections of each kind of multi-pronged event were calculated and the charge and velocity distributions of the heavy fragments were determined by the calibration of each track from an event by event analysis.

Statistical simulation of complete events in energetic nuclear collisions

A statistical model is developed for the generation of complete multifragment events in medium-energy nuclear collisions. Based on simple geometrical considerations, the collision system is divided into a few (participant/spectator) sources that disassemble independently. The sufficiently excited sources quickly explode into pions, nucleons, and composite, possibly particle unstable, nuclei. The different final states compete according to their microcanonical weight. The less excited sources, and the unstable explosion products, deexcite by sequential light-particle emission. The model has been implemented as a Monte Carlo computer code that is sufficiently efficient to permit generation of large event samples. The analysis of such multifragment events is addressed and some illustrative applications are discussed.

Calculation of complete multi-fragment events in a statistical model for nuclear disassembly

A quantitatively useful approximation method is developed for generating complete multi-fragment events in a statistical model for nuclear disassembly. The method consists in factorizing the exclusive microcanonical probability distribution into successively dependent one-fragment inclusive distributions, each of which is approximated by its respective truncated grand canonical equivalent.