Projectile-like Fragments Research Articles

Origin of slow, heavy residues observed in dissipative 197Au+86Kr collisions at E/A=35 MeV.

An exclusive measurement of slow, massive residues from the $^{197}\mathrm{Au}$+$^{86}\mathrm{Kr}$ reaction at E/A=35 MeV has been performed in coincidence with projectile-like fragments, neutrons, as well as light- and intermediate-mass charged products. The highly efficient (double $4\ensuremath{\pi}$) detector setup used included the University of Rochester SuperBall neutron detector and the Washington University Microball. The observed large yield of slow, massive residues shows characteristics consistent with a production scenario similar to that of binary dissipative collisions. The residues result from the statistical decay of primary targetlike fragments, produced even in the most dissipative collisions identified in the present experiment.

Systematics of angular momentum transfer in intermediate energy nuclear collisions

We have measured the fission fragment angular correlations, both in- and out-of-the-reaction-plane, in coincidence with projectile-like fragments, to infer the linear and angular momentum transfer and its alignment in the reaction of 16O with 197Au at 22 and 30 MeV/nucleon. A general correlation between transferred angular and linear momentum is observed for fissioning nuclei that is different from that observed in reactions where the intermediate complex survives fission.

Momentum distributions in stripping reactions of single-nucleon halo nuclei.

The widths of the longitudinal momentum distributions of projectilelike fragments produced in ${(}^{11}$Be${,}^{10}$Be+n) and in ${(}^{8}$B${,}^{7}$Be+p) breakup reactions are analyzed using a single-particle description. The strong absorption limit of the Serber model reproduces both data sets quite accurately. The transparent limit, on the other hand, can reproduce the width of the $^{11}\mathrm{Be}$ data reasonably well but it fails dramatically for the $^{8}\mathrm{B}$ data. The reason is that the absorption of the projectilelike fragment has a much larger effect on the width when the orbital angular momentum of the valence nucleon is nonzero. \textcopyright{} 1996 The American Physical Society.

Fusionlike reactions of 40Ar up to 1.36 GeV: Prethermalization and postthermalization particles and fragments.

Heavy residual nuclei are shown to result from the most violent (i.e., central) collisions for {sup 40}Ar + Ag reactions of up to 1.36 GeV; their average velocities are {approx_gt}80{percent} of the c.m. velocity. Angular and energy distributions for {sup 1,2,3}H, {sup 3,4}He, and Li are measured in coincidence with these heavy nuclei. The dominant light particle components are nearly isotropic in a frame of reference having the velocity of the heavy residues. In addition there are forward-peaked high-energy components of the H, He, and Li emission attributable to prethermalization emission. Fractional abundances of these prethermalization components increase markedly with increasing incident energy. Mass and momentum balance preclude the presence of a projectilelike fragment and thus indicate fusionlike reactions with large but incomplete linear momentum transfer. The remainder of the momentum is carried away by the spray of forward-peaked ejectiles. For 1.36 GeV {sup 40}Ar {approximately}1/2 of the 900 MeV available is completely thermalized, and {approximately}1/2 goes into prethermalization emission after strong collisional mixing. {copyright} {ital 1996 The American Physical Society.}

Importance of one- and two-body dissipation at intermediate energies studied by hard photons.

Hard photons were measured in coincidence with projectilelike fragments in peripheral reactions of Ar-36 + Tb-159 at E/A = 44 MeV. The probability for hard photon production was found to scale linearly with the transferred mass and to depend on the direction of the transfer, thus indicating the relative importance of one- and two-body dissipation in peripheral reactions.

Time scale for the binary disintegration of the projectile in 48Ti+ 93Nb collisions at 19.1 MeV/u

The time scale for the binary disintegration of projectile-like fragments was investigated in 48Ti+ 93Nb collisions at 19.1 MeV/ nucleon. The influence of the target proximity to the decaying projectile-like fragment was utilized to determine the time interval between the primary re-separation of the projectile and target and the secondary projectile disintegration. This time interval for near symmetric decay of the projectile, is less than ≈ 200 fm/ c. Taking into account the time interval required to separate the two fragments of the projectile to their scission configuration, the projectile decay process is one which quite likely, or quite often, is initiated before the projectile and target-like fragments are fully separated.

Multifragmentation study on 30 AMeV32S+58Ni

In a multidetector experiment on 26 or 30 AMeV32S+58Ni, up to four coincident heavier or intermediate-mass fragments were observed. One of these occasionally has the characteristics of a projectile-like fragment, up to three may be attributed to the decay of the heavy reaction product. Taking the velocity of the fragments as a measure of the heavy-product excitation energy, one finds evaporation, fission and multifragmentation to follow one another with rising excitation. Model simulations of sequential decay with up to two binary fissions and, alternatively, of simultaneous statistical multifragmentation were performed for comparison with experimental distributions of mass, velocity and (for events with three slow intermediate-mass fragments) relative azimuthal angle. Though in the three-fragment events indications of simultaneous multifragmentation are present, the sequential binary decay predominates. Evaporated protons and α particles detected in coincidence have a mean multiplicity growing with excitation energy, while the temperature governing the spectra has a plateau with a value of about 5.5 MeV.

Mass and charge distributions for the reaction 40Ca + 209Bi at 600 MeV.

The charge and mass of the projectile-like fragments produced in the 15-MeV per nucleon $^{40}\mathrm{Ca}$${+}^{209}$Bi reaction were determined for products detected near the grazing angle. Neutron number-charge (N-Z) distributions were generated as a function of the total kinetic energy loss and parametrized by their centroids, variances, and correlation coefficients. Although the initial system is very asymmetric, after the interaction, a drift of the charge and mass centroids toward further asymmetry is observed. The production of projectile-like fragments is consistent with a tendency of the projectile-like fragments to retain the projectile neutron-to-proton ratio 〈N〉/〈Z〉\ensuremath{\simeq}1. The correlation coefficient remains well below 1.0 for the entire range of total kinetic energy lost. Predictions of two nucleon exchange models, Randrup's and Tassan-Got's, are compared to the experimental results. The models are not able to reproduce the evolution of the experimental distributions, especially the fact that the variances reach a maximum and then decrease as function of the energy loss. This behavior supports the hypothesis that some form of projectile-like fragmentation or cluster emission is perturbing the product distribution from that expected from a damped mechanism.

Mass Dependence of Cross Section Fluctuation in Dissipative Reaction 19F+51V

Excitation functions for projectile-like fragments from 19F+51V reaction have been measured at 102.25 to 109.50 MeV in 250 keV steps. A TOF+(ΔE - E) telescope at θL = ∠1.7° is used to identify atomic number Z and mass number A of ejectiles simultaneously. Fluctuations of the dissipative cross section for a series of isotopes are observed. Their coherence energy widths are extracted by generalized statistical fluctuation analysis and the dependence on mass number and on N/Z values are observed for the first time.

Dissipative aspects in 200 MeV/u Kr-induced reactions

Energy dissipation is studied in 200 MeV/u 84Kr-induced reactions. Two experiments were performed at the SATURNE facility with the SPES4 spectrometer. Projectile-like fragments were measured for both the 84Kr+197Au and 84Kr+59Co systems. In the latter case light charged particles emitted in the forward direction were detected in coincidence with PLF. From the projectile-like fragment isotopic distributions one can estimate that the mean excitation energy value per abraded nucleon deposited in primary fragments is quite large (20–30 MeV). However, the excitation energy distribution is very broad: the neutron-rich nuclei come from weakly excited neutron-rich primary fragments. The mean velocity loss of projectile-like fragments with respect to the projectile velocity as well as the correlation between the proton coincidence rate and the projectile-like fragments momentum also claim for a dissipative mechanism.

Temperature and excitation energy determination in 208Pb + 197Au reactions at 29 MeV/u

The violence of collisions induced by 208 Pb projectiles bombarding a 197 Au target at 29 MeV/u has been deduced by detecting the accompanying neutrons. With this filter, the temperature of the projectile-like fragments formed in very dissipative collisions has been extracted from the slope of the alpha-particle energy spectra. Temperatures of the order of 7 MeV are found, in very good agreement with the energy dissipations deduced from an energy balance assuming binary kinematics. The associated neutron multiplicities are well reproduced by a standard evaporation model, despite the dissociation into intermediate mass fragments observed in previous experiments. However, the measured temperatures exceed by a significant amount the limits predicted by theoretical calculations.

Mass and charge distributions in Fe-induced reactions.

The charge and mass of the projectilelike fragments produced in the 12-MeV/nucleon $^{56}\mathrm{Fe}$${+}^{165}$Ho reaction were measured at a laboratory scattering angle of 16\ifmmode^\circ\else\textdegree\fi{}. The mass and charge distributions of the projectilelike fragments were generated as a function of total kinetic energy loss (TKEL), and characterized by their neutron and proton centroids and variances, and correlation factors. A weak drift of the system towards mass asymmetry, opposite to the direction which minimizes the potential energy of the composite system, was observed. The increase in the variances with energy loss is consistent with a nucleon exchange mechanism as a means for energy dissipation. Predictions of two nucleon exchange models, Randrup's and Tassan-Got's models, are compared to the experimental results of the 672-MeV $^{56}\mathrm{Fe}$${+}^{165}$Ho reaction and to other Fe-induced reactions. The proton and neutron centroids were found to be generally better reproduced by Tassan-Got's model than by Randrup's model. The variances and correlation factor are well reproduced for asymmetric systems by both models.

Dissipative orbiting in 209Bi+136Xe collisions at Elab/A=28 MeV.

Massive, projectilelike fragments (PLF) from the [sup 209]Bi+[sup 136]Xe reaction at [ital E][sub lab]/[ital A]=28MeV were measured in coincidence with lighter charged products and neutrons, detected with 4[pi] angular coverage. Strong correlations are observed between the deflection angle of the massive fragment, on the one hand, and various experimental measures of energy dissipation, on the other hand. These correlations, as well as the observed broadening of the primary PLF atomic-number distribution with increasing energy dissipation, are strikingly reminiscent of damped reaction features that are well understood in terms of dissipative orbiting.

Missing momentum vector in the 197Au(20Ne;F1,F2PLF) reaction at 15 MeV/u

Abstract Projectile-like fragments (PLF's) emitted in damped collisions of 15 MeV/u 20Ne ions with 197Au were observed in coincidence with two fission fragments. For given detection angles of the PLF's differential and energy-integrated fission probabilities were obtained and the average missing momentum vectors were deduced by using improved analysis methods of the fission fragment angular correlation. The average missing momentum vectors were found to point in the direction of the observed PLF, with a systematic deviation towards beam direction for larger mass transfers. The missing momentum thus appears to mainly originate from the sequential decay of the PLF's.

Hard photons as a probe to study dissipation mechanisms

The probability for Bremsstrahlung has been measured as function of the mass of the projectile-like fragment in peripheral reactions of 36Ar + 159Tb at 44 MeV/nucleon. The Bremsstrahlung probability is found to depend on the amount of mass transferred and the direction of the transfer.

Experimental isotopic effects in comparison to statistical prescriptions for fragment production in 32A MeV and 14A MeV 14N + 112,124Sn reactions

Results on isotopic effects in the production of light fragments in reactions of 14A and 32A MeV 14N ions with 112Sn and 124Sn are presented. The data are discussed in the frameworks of classical Weisskopf evaporation and an alternative Fermi gas prescription. The source isospin dependence of the production rates is consistent only with the latter prescription. It is emphasized that similar data from p-nucleus reactions and from projectile-like fragments in 40Ar induced reactions behave in the same way.

Systematic study of neutron production cross sections for heavy ions by intranuclear cascade-evaporation calculations

A systematic study was made on the equilibrium and nonequilibrium neutron production cross sections for heavy ions based on intranuclear cascade evaporation model calculations. The cross sections were estimated by the HIC code at 337 MeV/u for projectiles of α, 12C, 20Ne and 40Ar and targets of 12C, 56Fe and 207Pb. Using the calculated cross sections, simple expressions for the production cross sections of both the equilibrium and nonequilibrium neutrons, that were previously proposed at a lower energy range through the analysis of experimental data, were extended to higher energies. The extended formulae well reproduced the systematic behaviors of the cross sections with varying the projectile and the target nuclei. The extended expression of the equilibrium neutron cross section is further modified so that a separate component of the cross section for each of the target-like and projectile-like fragments is described. The obtained expression is tested by HIC calculated data, showing a good reproducibility of the cross sections for both the fragments.

Deexcitation of primary projectile-like fragments in the reaction 40Ca+natCu at 35 MeV/nucleon: Comparison with sequential binary decay and percolation models.

This article presents a new analysis of primary projectile-like fragments produced in peripheral and mid-peripheral collisions in the reaction $^{40}\mathrm{Ca}$${+}^{\mathrm{nat}}$Cu at 35 MeV/nucleon. The data were obtained using the charged particle amphora array. The distributions of multiplicity, inclusive charge, and heaviest fragment charge of the deexcitation products as well as the moments characterizing the single event charge partitions are remarkably well reproduced by a statistical sequential binary decay model even up to excitation energies of more than 8 MeV/nucleon. The results produced by the sequential binary decay model and a simple bond percolation model are qualitatively very similar. However the sequential binary decay mechanism leads to charge fluctuations which are in better agreement with the data than those predicted by the percolation model.

Gamma-ray multiplicity measurements in the28Si+64Ni reaction at 163.8 MeV

The28Si+64Ni reaction at 163.8 MeV incident energy is studied by measuring in coincidenceγ-rays and charged particles identified from Z=2 to Z=16. The transition from quasi-elastic to more damped reactions is observed when the difference between the detected charge and the projectile one is increased. The strong influence of the particle decay on the measuredγ-ray multiplicity is evidenced with the help of the statistical model computer code CASCADE. Dissipative events are well described in the rolling limit with excitation energy equally shared between the fragments. The overall agreement is lost for the fragments with the projectile charge which show a small value of theγ-multiplicity even for dissipative events. This is probably connected with the previously observed non statistical behavior of gamma rays emitted in coincidence with projectile-like fragments. In the alpha-spectrum measured in coincidence with gamma-rays, the deexcitation of fused systems is clearly separated from in flight emission of deep inelastic fragments. The low measured gamma-ray multiplicity for fusion events is qualitatively explained taking into account the effect of alpha-emission in the statistical decay.

Equilibrium versus non-equilibrium emission in projectile fragmentation for the 40Ar + natAg system at 58.7 A MeV

Coincidences between light-charged particles and projectile-like fragments were measured at forward angles. An analysis of the data based on a kinematical reconstruction of the emitter evidences a thermalized emission for protons and a large non-equilibrium α emission. From the slope of the equilibrated component of the particle energy spectra in the emitter frame, high temperatures in the primary projectile, up to ∼4.5 MeV, have been extracted and are found to increase as the atomic number of the projectile-like fragment decreases.