Projectile-like Source Research Articles

Theoretical study of isotope production in the peripheral heavy-ion collision 136Xe + Pb at 1 GeV/nucleon

We have studied the fragment yields emitted from the fragmentation of excited projectile nuclei in peripheral collisions of X136e+Pb at 1 GeV/nucleon, and measured with the high-resolution magnetic spectrometer, the Fragment Separator (FRS) of GSI. The mass, charge and isotope distributions of nuclear fragments formed in the reactions were calculated within a statistical ensemble approach and compared to the experimental data. The ensemble of excited projectilelike source nuclei were created in the framework of a previous analysis of similar reactions performed at 600 MeV/nucleon (ALADIN-experiments, GSI). In addition, the ensemble of the low-excited compound nuclei is involved in the analysis. The overall agreement between theory and experiment was very satisfactory in reproducing the experimental data of isotope yields measured in the heavy-ion collisions. It is confirmed that a broad range of elements can be reproduced within a universal statistical disintegration of the excited projectile residues.

Signals of Bose Einstein condensation and Fermi quenching in the decay of hot nuclear systems

We report experimental signals of Bose-Einstein condensation in the decay of hot Ca projectile-like sources produced in mid-peripheral collisions at sub-Fermi energies. The experimental setup, constituted by the coupling of the INDRA 4$\pi$ detector array to the forward angle VAMOS magnetic spectrometer, allowed us to reconstruct the mass, charge and excitation energy of the decaying hot projectile-like sources. Furthermore, by means of quantum fluctuation analysis techniques, temperatures and mean volumes per particle "as seen by" bosons and fermions separately are correlated to the excitation energy of the reconstructed system. The obtained results are consistent with the production of dilute mixed (bosons/fermions) systems, where bosons experience a smaller volume as compared to the surrounding fermionic gas. Our findings recall similar phenomena observed in the study of boson condensates in atomic traps.

Properties of excited A = 40 nuclear systems with varying matter composition

There exists an intriguing problem when bosonic clusters as bound states of fermions are produced in the reaction, and the Bose character of the composite clusters competes with the fermionic properties of their constituents. In the analysis of 40 Ca + 40 Ca reactions at 35MeV/A we selected classes of projectile-like sources with exit channels consisting of only bosons, only fermions, only even-even nuclei, only odd - odd nuclei, only even - odd nuclei and only alpha-conjugate nuclei, respectively and searched for kinematic characteristics of these systems which might differ depending upon the type of matter selected. The distributions of various observables for the different classes of matter and comparisons between them will be presented and discussed.

An interpretation of staggering effects by correlation observables

The reactions 32 S+ 58,64 Ni are studied at 14.5 A MeV. Evidence is found for odd-even effects in isotopic observables of the decay of a projectile-like source. The influence of secondary decays on the staggering is studied with a correlation function technique, showing that odd-even effects are due to interplay between pairing effects in the nuclear masses and in the level densities.

Towards an understanding of staggering effects in dissipative binary collisions

Abstract The reactions S 32 + 58 , 64 Ni are studied at 14.5 A MeV. Evidence is found for important odd–even effects in isotopic observables of selected peripheral collisions corresponding to the decay of a projectile-like source. The influence of secondary decays on the staggering is studied with a correlation function technique. It is shown that this method is a powerful tool to get experimental information on the evaporation chain, in order to constrain model calculations. Specifically, we show that odd–even effects are due to interplay between pairing effects in the nuclear masses and in the level densities.

Experimental studies ofN/Zequilibration in peripheral collisions using fragment yield ratios

Peripheral collisions of $^{40}\mathrm{Ca}$ and $^{48}\mathrm{Ca}$ projectiles at 32 MeV/nucleon on $^{112}\mathrm{Sn}$ and $^{124}\mathrm{Sn}$ targets were studied in this work. The fragments of the projectile-like source (quasiprojectile) were collected with a charged-particle multidetector array. The average value of the neutron-to-proton ratio $N/Z$ of the quasiprojectiles formed in the reactions was determined with two approaches. The first is a direct reconstruction approach using isotopically resolved fragments and is hindered by undetected neutrons leading to lower $N/Z$ values. The second approach, based on the assumption of early fragment formation, employs yield ratios of fragment isobars and is not hindered by undetected neutrons. Using this approach, the amount of $N/Z$ mixing that occurred in the quasiprojectiles (compared to a fully $N/Z$ equilibrated system) was found to be approximately $53%$. The experimental results were compared with model calculations. First, the phenomenological DIT (deep inelastic transfer) model was used, followed by the statistical multifragmentation model (SMM). The results of these calculations are in close agreement with the data and indicate that the mean number of undetected neutrons increases with the $N/Z$ of the composite system, accounting for the difference observed between the two approaches of quasiprojectile $N/Z$ determination. Second, the microscopic transport model IBUU (isospin-dependent Boltzmann-Uehling-Uhlenbeck) was employed, providing preliminary results in reasonable agreement with the data. The determination of the degree of $N/Z$ equilibration employing the present fragment yield ratio approach may provide a valuable probe to study the isospin part of the nuclear equation of state in conjunction with detailed microscopic models of the collisions in the Fermi energy regime.

Sources and emission time scales in E/A=50 MeV 129Xe+natCu reactions.

Velocity distributions and fragment-fragment correlation functions have been measured as a function of charged particle multiplicity for $^{129}\mathrm{Xe}$${+}^{\mathrm{nat}}$Cu collisions at E/A=50 MeV. With increasing multiplicity, the velocity distributions evolve from a pattern exhibiting targetlike and projectilelike sources to an anisotropic distribution centered near the projectile-target center-of-mass velocity. Emission times extracted from the correlation functions are consistent with the decay of a projectilelike source in low multiplicity collisions, but the time scale for fragment emission in the high multiplicity collisions is too short for a separation of sources to occur.

Mechanisms of intermediate mass-fragment formation from threshold to E/A = 100 MeV

Excitation functions have been measured for the formation of intermediate-mass-fragments in reactions of E/A = 20–100 MeV 14N + natAg and 197Au. The data have been analyzed in terms of a phenomenological moving-source model based on equilibrium, nonequilibrium and projectile-like sources. Systematic features of the data and the moving-source fit parameters are examined.

Intermediate mass fragment emission in 36Ar+197Au collisions at E/A=35 MeV.

Intermediate mass fragment (IMF: 3\ensuremath{\le}Z\ensuremath{\le}20) emission for the $^{36}\mathrm{Ar}$${+}^{197}$Au reaction at E/A=35 MeV has been studied with a low-threshold 4\ensuremath{\pi} charged-particle detector array covering the angular range of 16\ifmmode^\circ\else\textdegree\fi{}\ensuremath{\le}${\mathrm{\ensuremath{\theta}}}_{\mathrm{lab}}$\ensuremath{\le}160\ifmmode^\circ\else\textdegree\fi{}. While most IMF's are emitted in central collisions characterized by large charged-particle multiplicities, contributions from peripheral collisions exist at forward angles which are reminiscent of damped collisions or emission from a projectile-like source. Energy spectra, angular distributions, element distributions, and two-fragment correlation functions are presented for various gates on the charged-particle multiplicity, and the time scale of fragment emission is deduced.

Neutron inclusive measurements of 36Ar+Ag reactions at 35 MeV/nucleon.

We have measured the inclusive neutron cross section as a function of neutron energy and angle at 15\ifmmode^\circ\else\textdegree\fi{}, 30\ifmmode^\circ\else\textdegree\fi{}, 45\ifmmode^\circ\else\textdegree\fi{}, 60\ifmmode^\circ\else\textdegree\fi{}, 120\ifmmode^\circ\else\textdegree\fi{}, and 160\ifmmode^\circ\else\textdegree\fi{} for the system Ag${(}^{36}$Ar,n) at 35 MeV/nucleon. All spectra can be fitted with a moving-source model consisting of a projectilelike source, an intermediate-rapidity source, and a targetlike source. Temperatures for the targetlike and intermediate-rapidity sources were found to be about 4 and 11 MeV, respectively. Neutron multiplicities for each were 5.6 and 1.0. These parameters are compared with those from the neutron inclusive measurement of Ag${(}^{14}$N,n) at 35 MeV/nucleon and with proton exclusive data from the reaction $^{32}\mathrm{S}$+Ag at 30 MeV/nucleon.

Neutron inclusive measurements in 14N+Ag reactions at 35 MeV/nucleon.

Neutron inclusive (singles) spectra were measured by the time-of-flight method at laboratory angles from 15\ifmmode^\circ\else\textdegree\fi{} to 160\ifmmode^\circ\else\textdegree\fi{}. When parametrized in the thermal model, a targetlike source was sufficient to fit the data at the largest angle, but in addition a half-beam-velocity source was required at the other angles, and at the smallest angles a projectilelike source was also needed. The model parameters are in general agreement with values obtained previously in fitting proton inclusive data. Comparison is also made with neutron coincidence data. Less phenomenological models---Fermi jet, Boltzmann-Uehling-Uhlenbeck, and Boltzmann master equation---are also able to account for the main features of the data.

Target mass dependence of neutron emission in collisions with 35 MeV/nucleon 14N ions.

Energy and angular distributions of neutrons emitted from collisions of 35 MeV/nucleon /sup 14/N ions with carbon, nickel, and holmium nuclei were measured. The neutrons were in coincidence with Li, Be, B, or C fragments at angles from 7/sup 0/ to 23/sup 0/ in the plane of the neutron detectors and at 15/sup 0/ out of this plane. Using fragment velocity bins of width corresponding to E/A = 7 MeV, we find the shapes of the neutron spectra above 15 MeV to be similar for the different targets for a given coincident fragment species, velocity bin, and angle. The cross sections are discussed in terms of moving thermal sources. In all cases the velocity and temperature parameters of the intermediate rapidity source are consistent with E/A = 8.5 +- 2.5 MeV and T = 9 +- 2.5 MeV, respectively. In agreement with a simple stripping-pickup model, the associated neutron multiplicities of this source decrease approximately linearly with the velocity of the coincident light fragments for fragment angles less than or equal to15/sup 0/. Using the model to compute the mass of the source, we find that, with some fluctuations, these multiplicities are proportional to the mass. Also, the linear relationshipmore » is approximately the same for the three targets. The temperature parameters of the target-like source are between 1 and 3.5 MeV for all three targets, while the associated neutron multiplicities increase considerably with target mass. For colinear neutron-fragment coincidences for a given projectile-like isotope, the neutron multiplicities associated with the projectile-like source are about the same for all three targets, indicating that the average excitations of the parent fragments are similar.« less

Evaporation recoil effects: Asymmetries in neutron, projectile-fragment coincidences for 35 MeV/nucleon 14N

Simulations of neutron evaporation from light heavy ions are presented to illustrate experimental biases present in coincidence experiments. In particular, the case presented involves neutrons observed in coincidence with quasi-elastic boron fragments from collisions of $^{14}\mathrm{N}$ with $^{165}\mathrm{Ho}$ at a bombarding energy of 35 MeV/nucleon. The left-right asymmetry observed experimentally in the high energy neutron spectra at angles of \ifmmode\pm\else\textpm\fi{}60\ifmmode^\circ\else\textdegree\fi{} and \ifmmode\pm\else\textpm\fi{}90\ifmmode^\circ\else\textdegree\fi{} cannot be reproduced together with the correct angular distribution if only a projectile-like source is assumed to create the observed spectra, and this suggests that some other process is required to explain the data. Some general features of the simulations will be compared with results of moving-source analyses.