Search Results (16 total)

Complete distributions of the light and intermediate mass fragments ( Z  =  1–6) produced within the polar angular range 1^∘≤Θ_lab≤30^∘ in highly central collisions of 250 A MeV Au + Au are presented. The results of this measurement and a model analysis are used to study the expansion and clustering of the hot and compressed transient state formed in central collisions of such a heavy system. The influence of the initial conditions on the final observables is discussed.

Velocity correlations of intermediate mass fragments (IMF), produced in collisions of Au+Au at 100, 150, 250, and 400A MeV beam energy, are extracted from measurements with the 4π detector system (FOPI) in construction stage I at Schwerionen-Synchrotron (SIS) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. The IMF correlation functions of peripheral and semicentral events are found to be strongly affected by the collective sideward motion of nuclear matter. The sideflow causes an enhancement of correlations at small relative velocities. This enhancement results from the mixing of differently azimuthally oriented events; it vanishes if the events are rotated into a unique reaction plane. Selecting violent central collisions, the comparison of the data with a Coulomb dominated final-state interaction model points to a radius of the expanding and multifragmenting source of R_s≃13 fm for 100A MeV which appears shrinking by 20% when increasing the projectile energy to 400 MeV per nucleon. The deduced source radii are found to depend on the radial explosion energy used in the model. The inclusion of such a collective expansion is necessary for a reasonable description of the experimental single-particle spectra of the IMF. The unique Coulomb suppression of small relative IMF velocities, found for the given beam energy range, is attributed to rather constant averaged next-neighbor distances 〈d_IMF〉=8.6±0.2 fm of the IMF charge centers within the source at breakup time.

Using the FOPI facility at GSI Darmstadt complete data of Au on Au collisions at 150A MeV were collected for charged products (Z=1–15) at laboratory angles 1°≤Θ_lab≤30°. Central collisions were selected by applying various criteria. The kinetic energy spectra of fragments from an isolated midrapidity source are investigated in detail for center-of-mass angles 25°≤Θ_c.m.≤45°. The heavy products (Z≥3) are used to determine the collective energy which is found to be at least 10A MeV.

The characteristics of the fission step following a binary deep-inelastic interaction have been reconstructed for three-body events detected in the reaction ¹⁰⁰Mo+¹⁰⁰Mo at 18.7A MeV and ^12-Sn+¹²⁰Sn at 18.4A MeV. The observed anisotropy of the in-plane angular distributions points to the fast decay of a rotating (and strongly deformed) nuclear object formed at the end of the deep-inelastic interaction. The derived time scale of the process indicates that asymmetric divisions are faster than symmetric ones.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-20 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

Velocity correlations of intermediate mass fragments (IMFs), produced in central collisions of Au+Au at 150 MeV beam energy, are extracted from measurements with the FOPI (phase I) detector system at SIS in GSI Darmstadt. The IMF correlation function for semicentral events is found to be affected by the directed sideward flow. When rotating the events into a unique reaction plane an enhancement of correlations, resulting from event mixing effects, vanishes. Selecting violent collisions with a high degree of azimuthal symmetry the correlation function appears nearly independent of additional event or single particle gate conditions. The comparison of the data with a Coulomb dominated final-state interaction model points to an expanding and multifragmenting soure with radius R∼14 fm.

The entropy per nucleon (S/A) has been extracted for the Au [(150–800)A MeV] + Au reaction by using the phase I setup of the 4π facility at GSI, Darmstadt. The entropy has been obtained from the comparison of various observables characterizing the dM/dZ fragment multiplicity distributions, extending up to Z∼15, with those calculated with the quantum statistical model. It is the first time that S/A values are determined by considering the full ensemble of charged products detected in the reaction. Consistent values of S/A are found from different methods. These entropy values are shown to be fairly independent of the volume of the ‘‘participant’’ region considered. They are somewhat lower than those extracted in earlier works but are in good agreement with hydrodynamic calculations and suggest a low viscosity for the hot and dense nuclear matter.

Charged particles have been observed in collisions of Au on Au at an incident energy of 150A MeV using a high-granularity detector system covering approximately the forward hemisphere in the center-of-mass system. Highly central collisions have been studied using a double selection criterion which combines large charged-particle multiplicities with small transverse-momentum directivities. In this class of events about one quarter of the total nuclear charge emerges as intermediate-mass fragments with nuclear charges Z>2. These fragments are centered at midrapidity and are produced with large transverse velocities.

High-energy γ rays have been measured in coincidence with heavy fragents in deeply inelastic reactions of ¹³⁶Xe+⁴⁸Ti at 18.5 MeV/nucleon. The giant dipole resonance (GDR) strength function is deduced from an analysis of the photon spectra within the statistical model. The GDR width Γ is studied as a function of the fragment excitation energy E^*. A saturation at about Γ=10 MeV is observed for E^*/A≥1.0 MeV/nucleon.

Exclusive measurements of two- and three-body events were performed for the system ¹²⁰Sn+¹⁰⁰Mo at 19.1 MeV/nucleon. Most ternary events are consistent with sequential processes in which one of the two deep-inelastic fragments fissions. For such events large differences are found between the fission probabilities of projectilelike and targetlike fragments of a given mass, this probability being larger for the nucleus which gained nucleons. This behavior demonstrates that there is a lack of equilibrium at the end of the deep-inelastic collision.

Photons with energies from 2 to 60 MeV have been measured in coincidence with binary fragments in the reaction ⁹²Mo+⁹²Mo at an incident energy of 19.5A MeV. The rapid change of the γ-ray spectrum and multiplicity with the fragment total kinetic energy in the exit channel indicates that the γ rays are emitted statistically by the highly excited fragments. Temperatures as high as 6 MeV are inferred.

The total capture cross section for the system ³²S+²³⁸U has been measured at energies from 0.93 to 1.08 times the s-wave interaction barrier by detecting coincident fission fragments following full momentum transfer reactions. The subbarrier cross section cannot be reproduced by a one-dimensional barrier penetration model. Using a quantum mechanical coupled channels model, good agreement is obtained. The measured angular distributions of fission fragments were compared to the predictions of saddle and scission point transition state theory. Saddle point transition state model calculations fail to reproduce the data, while scission point transition state calculations are in agreement with their qualitative trend. Evidence for nonequilibrium processes is presented.

The production of neutral pions has been studied in reactions of 35 MeV/nucleon ¹⁴N+ ²⁷Al,Ni,W and 25 MeV/nucleon ¹⁶O+ ²⁷Al,Ni. Inclusive pion differential distributions dσ/dT_π, dσ/dΩ, dσ/dy, dσ/dp_⊥, and d²σ/dy dp_⊥ have been measured by detecting the two pion-decay γ rays in a setup of 20 lead glass Čerenkov detector telescopes. Special care was taken to understand and suppress background events. Effects of pion reabsorption are discussed and it is found that the cross sections presented here are substantially affected by such final state interactions. The comparatively large experimental cross sections and the shape of the spectral distributions cannot be accounted for in single nucleon-nucleon collision or statistical models; they rather call for a coherent pion production mechanism.

Inclusive production of neutral pions was observed at E_lab/A=25 MeV in the reactions ¹⁶O+Al,Ni→π⁰+X. Neutral pions were detected by observing their two decay γ rays in coincidence in an array of 20 Pb-glass Cerenkov-detector telescopes. An array of plastic Cerenkov detectors was added to the above array to tag and reject a significant cosmic-ray background. The observed cross sections are much larger than predicted by nucleon-nucleon collision or statistical models, even if cooperative action of target and projectile nucleons and cluster formation in the final channel are taken into account. This indicates the presence of a collective production mechanism.

Excitation functions are reported for total fusion near and below the Coulomb barrier of the systems ^32,34S+^24,25,26Mg and ²⁷Al. The data cannot be reproduced by one-dimensional barrier penetration calculations. The enhancement of the cross sections at low energies is compared to predictions of models taking into acount the static deformation or zero point vibration of the reaction partners. Calculations including zero point motion do not reproduce the observed variations of the measured cross sections with respect to the neutron number of target and projectile. Reasonable agreement is obtained when calculating fusion between statically deformed nuclei. Finally, the fusion process is described in a quantum mechanical coupled channels model, indicating the importance of dynamical effect on sub-barrier fusion.

NUCLEAR REACTIONS ^24,25,26Mg, ²⁷Al(^32,34S, Fusion) 0.9<E_c.m. / E_Coul<1.1; measured evaporation residues, deduced barrier parameters; discussed importance of zero point motion, static deformation, coupled channels effects on fusion process.

Excitation functions for ¹²C(¹⁶O, ¹⁶O)¹²C elastic and inelastic scattering have been measured in the energy range 23≤E_c.m.≤32 MeV. Two strong structures at E_c.m.=25.5 and 29.6 MeV are observed in the ¹²C + ¹⁶O(3^-,6.13 MeV) exit channel; angular correlation measurements at these energies suggest spin assignments of 15^- and 16⁺, respectively.

NUCLEAR REACTIONS ¹²C(¹⁶O, ¹⁶O*)¹²C*; E_c.m.=23-32 MeV, θ_c.m.(¹⁶O)=130°-155°; measured σ(E) angular correlations.