Search Results (52 total)

We discuss the longitudinal spin relaxation of ¹²⁹Xe nuclei in frozen xenon. Over a large range of temperatures and magnetic fields, the dominant spin-lattice relaxation mechanism is shown to be nuclear spin-flip Raman scattering of lattice phonons. Two closely related interactions couple the lattice phonons to the spins of ¹²⁹Xe nuclei: (1) the nuclear spin-rotation interaction between nearest-neighbor atoms, and (2) the paramagnetic antishielding of the externally applied field at the site of ¹²⁹Xe nuclei by the electrons of neighboring Xe atoms. We show that relaxation rates can be predicted by using measured chemical shifts of gaseous and condensed xenon. The predicted relaxation rates are in good agreement with measurements. We outline a simple way to estimate the spin-rotation coupling and paramagnetic antishielding in terms of the small perturbations of the outermost electron orbitals of one xenon atom due to a neighboring atom.

The characteristics of the midrapidity and target sources (apparent temperatures, velocities, and neutron multiplicities) extracted from the neutron energy spectra, have been measured for various quasiprojectile (QP) excitation energies, reconstructed from charged particles of well defined peripheral events in the ³⁵Cl+^natTa reaction at 43 MeV/nucleon. The reconstructed excitation energy of the QP is always smaller than the excitation energy calculated from its velocity, assuming pure dissipative binary collision. The latter observation combined with the neutron multiplicity at midrapidity and the apparent temperature suggests important preequilibrium and/or dynamical effects in the entrance channel. The midrapidity source moves at a velocity lower than the nucleon-nucleon center of mass velocity showing the importance of the attractive mean-field potential from the target even at 43 MeV/nucleon. The above picture is confirmed by comparison to Boltzman-Nordheim-Vlasov (BNV) simulations.

We reply to the preceding Comment.

The impact parameter dependence of light charged particle (p,d,t,α) emission has been studied using an impact parameter selection based on coincident detection of residues or fission fragments. The energy spectra at twelve angles between 20° and 150° have been fit by a multiple moving source parametrization. The angle and energy integrated preequilibrium proton multiplicities decrease with increasing impact parameter in qualitative agreement with a Fermi jet calculation. The preequilibrium d/p and t/p multiplicities increase slowly with increasing impact parameter and are nearly identical at the two bombarding energies. The preequilibrium α/p ratio shows a less consistent dependency on impact parameter but decreases significantly with increasing bombardment energy. A calculation of the d/p and t/p multiplicity ratios with a transport model incorporating complex particle emission is quite successful in reproducing the absolute magnitude, impact parameter dependence, and bombarding energy dependence of the experimental total multiplicities.

Charge and transverse energy distributions for intermediate mass fragments have been extracted for central ⁸⁴Kr+¹⁹⁷Au collisions at E/A=35-400 MeV. The slopes of the measured fragment charge distributions decrease monotonically with incident energy, consistent with the expectations for highly charged systems, but not with recent critical exponent analyses. Statistical model calculations, which reproduce the experimental trends, suggest that post-breakup fragment secondary decays alter significantly the observed charge distributions. Radial expansion velocities extracted from these calculations follow the systematics of Au+Au collisions.

The dependence of intermediate-mass-fragment (IMF) element distributions on the multiplicity, N_IMF, of detected fragments has been measured for ⁸⁴Kr+¹⁹⁷Au collisions at E/A=35, 55, 70, 100, 200, and 400 MeV. The observed dependence can be parametrized as P(N_IMF|Z)∝P(Z)exp(-c⋅N_IMF⋅Z), where c is a beam-energy and excitation-energy dependent parameter. Previous work indicated this parameter is zero in the liquid-gas coexistence region and positive in the gaseous phase. In contrast, we observe both negative and positive values for c, revealing the meaning of this parameter to be less straightforward than previously assumed. The magnitude of c appears nonetheless to provide a nontrivial test of multifragmentation models.

Multifragment decays of central collisions in ⁸⁴Kr+¹⁹⁷Au at E/A  =  70 MeV are studied. By utilizing a technique sensitive to the emission order of fragments, it is deduced that carbon fragments are emitted prior to beryllium fragments when these fragments have the same velocity. This observation is consistent with the cooling of a thermally decaying source.

We report the first experimental observation of the pionic fusion of two heavy ions. The ¹²C(¹²C,²⁴Mg)π⁰ and ¹²C(¹²C,²⁴Na)π⁺ cross sections have been measured to be 208±38 and 182±84 pb, respectively, at E_cm  =  137  MeV. This cross section for heavy-ion pion production, at an energy just 6 MeV above the absolute energy-conservation limit, constrains possible production mechanisms to incorporate the kinetic energy of the entire projectile-target system as well as the binding energy gained in fusion.

Fragment production has been studied as a function of the source mass and excitation energy in peripheral collisions of ³⁵Cl+¹⁹⁷Au at 43 MeV/nucleon and ⁷⁰Ge+^natTi at 35 MeV/nucleon. The results are compared to the Au+Au data at 600 MeV/nucleon obtained by the ALADIN Collaboration. A mass scaling, by A_source∼35 and 190, strongly correlated to excitation energy per nucleon, is presented, suggesting a thermal fragment production mechanism. Comparisons to a standard sequential decay model and the lattice-gas model are made. Fragment emission from a hot, rotating source is unable to reproduce the experimental source size scaling.

The spatial-temporal extent of the emitting system for central collisions in ⁸⁴Kr + ¹⁹⁷Au at E/A = 35, 55, and 70 MeV is probed using fragment-fragment velocity correlation functions. Selection on fragment pairs of high velocity yields a stronger fragment-fragment Coulomb interaction than for inclusive fragment pairs. This result is consistent with fragment emission from a source of decreased spatial-temporal extent. The universality of this association and the effect of different selection criteria are explored. The sensitivity of the spatial-temporal extent deduced by the correlation function technique to measurement uncertainties, assumed source characteristics, and rotational effects is assessed. © 1996 The American Physical Society.

Differential cross sections for evaporation residues and fission fragments for 35A, 100A, 130A and 155A MeV ¹⁴N on targets ranging from ¹⁵⁴Sm to ¹⁹⁷Au have been measured. The angle-integrated cross sections are larger than what might be expected. The fission fragment-fission fragment folding angle correlations for 35A, 100A MeV ¹⁴N and 25A MeV ¹⁶O on similar targets were also measured. The average linear momentum transfer has been deduced from both the fission angle correlation and from the fore-aft asymmetry of the fission angular distributions in the laboratory system. The data are all consistent with a picture where pre-equilibrium particle emission removes an increasing fraction of the orbital angular momentum as the bombarding energy increases. This allows a large range of partial waves to contribute to formation of a composite nucleus with a finite fission barrier. © 1996 The American Physical Society.

Multifragment decays of central collisions in ⁸⁴Kr+¹⁹⁷Au at E/A  =  35, 55, and 70 MeV are studied. The dependence of the extracted emission time on the velocity of the fragment pair is investigated. More energetic pairs manifest a stronger Coulomb interaction, indicating emission from a source of smaller spatial-temporal extent than less energetic pairs. This trend can be understood in the context of a statistical model that allows the source characteristics to evolve as the fragments are emitted.

An investigation of the production of neutron-rich isotopes from the fragmentation of ²⁸Si projectiles at p_lab=14.6 GeV/c per nucleon was performed using the BNL-AGS-E814 spectrometer. We have measured the inclusive production cross sections of neutron-rich fragments (⁶He, ⁸He, ⁸Li, ⁹Li, ¹⁰Be, ¹¹Be, and ¹³B). We have also measured the transverse momentum distributions for ⁶He and ⁹Li, and the forward and transverse energy distributions associated with ⁶He production. The momentum distributions were analyzed in the context of the Goldhaber model. The question of whether the fragments are produced in the decay of the projectile following its electromagnetic excitation was also investigated.

We have studied in detail the electromagnetic dissociation of ²⁸Si projectiles at 14.6 GeV/(c nucleon), interacting with Pb, Sn, Cu, and Al targets. Exclusive cross sections were measured for several decay channels, including final states involving the emission of protons, neutrons, and α particles. Excitation energy distributions for the 1n+²⁷Si and 2p+²⁶Mg decay channels were reconstructed with a resolution of 2 MeV, using a constrained kinematic fit. The energy distributions obtained for 1n+²⁷Si are in good agreement with the σ(γ,n) photoneutron cross sections multiplied by the virtual photon spectrum obtained in the Weizsäcker-Williams approximation. A search for the double photon excitation process, based on the dependence of the cross sections on the target atomic number, was performed.

Fragment-fragment correlations are used to probe the spatial-temporal extent of the emitting source in central ³⁶Ar+¹⁹⁷Au reactions at E/A=35, 50, 80, and 110 MeV. The experimental two particle correlations are compared both with the Koonin-Pratt two-body formalism as well as a three-body Coulomb trajectory calculation. The spatial-temporal extent of the emitting system decreases with increasing incident energy. Within the context of a three-body Coulomb trajectory model the mean fragment emission time rises sharply as a function of the assumed density of the system until ρ/ρ₀≊0.3. If one assumes a fixed density, the extracted mean emission time decreases with increasing assumed charge of the emitting system. Assuming ρ/ρ₀≊0.3 the mean emission time τ according to calculations using a three-body Coulomb trajectory model, is ≊115–135 fm/c at E/A=50 MeV and ≊75–100 fm/c at E/A=110 MeV. Comparisons with a generalized N-body Coulomb trajectory model demonstrate that the effect of interactions with other emitted particles is negligible. The prediction of a microcanonical model which includes pre-emission correlations between the fragments is compared to the measured correlation function at E/A=110 MeV.

The relationship between observed intermediate mass fragment and total charged particle multiplicities has been measured for ⁸⁴Kr + ¹⁹⁷Au collisions at energies between E/A=35 and 400 MeV. Fragment multiplicities are greatest for central or near-central collisions. For these collisions, fragment production increases up to E/A≊100 MeV, and then decreases at higher energies.

We present a systematic study of transverse energy (E_T) production in collisions of 11.4A GeV/c Au and 14.6A GeV/c Si ions with targets of Al, Au, and Pb. Comparison of data for Au+Au and Si+Al indicates that, for the heavier system, there is an increase in the amount of stopping which is accompanied by a decrease in the width of the dE_T/dη distribution. The ratio of the maximum E_T observed for the two systems is significantly greater than the ratio of the total energy available in the center of mass frame.

We have measured antiproton production cross sections as functions of centrality in collisions of 14.6 GeV/c per nucleon ²⁸Si ions with targets of Al, Cu, and Pb. For all targets, the antiproton yields increase linearly with the number of projectile nucleons that have interacted, and show little target dependence. We discuss the implications of this result on the production and absorption of antiprotons within the nuclear medium.

We studied the very slow nuclear spin-lattice relaxation of solid ¹²⁹Xe as a function of temperature and magnetic field using laser-polarized nuclei. Relaxation times in excess of 500 h were measured. We present evidence for a new relaxation mechanism which results from a Raman spin-phonon scattering process involving the spin-rotation interaction. We also establish the existence of cross relaxation between ¹²⁹Xe and the other magnetic isotope ¹³¹Xe and demonstrate that laser-polarized ¹²⁹Xe can be used to cross polarize other nuclei that are present in the lattice.

The spatial and temporal extent of a system decaying by multifragment emission is deduced. Two-particle intermediate mass fragment correlation functions measured for central ³⁶Ar+¹⁹⁷Au collisions at E/A=50–110 MeV indicate a rapid decay, <75 fm/c, of the highly excited system. Furthermore, the behavior of the correlation function at large relative velocities suggests that considerable charge loss occurs prior to fragment emission.

Collisions of ²⁸Si+Al, Cu, and Pb at E_lab=14.6 GeV/nucleon were studied at the Brookhaven National Laboratory Alternating Gradient Synchrotron. Charged particle multiplicity was measured over the pseudorapidity interval 0.875≤η≤3.86 with a silicon pad detector. A strong correlation is seen between the multiplicity and the transverse energy measured in the interval -0.5≤η≤0.8. Correlation with the energy going forward after the collision and comparison with calculations indicate that rescattering is required to explain the data. The data are compared under the assumption of Koba-Nielson-Olesen scaling. The measured multiplicity scales approximately with the total number of participant nucleons and less well with the available center-of-mass kinetic energy.

We report a direct measurement of the final-state energy spectrum in the electromagnetic dissociation of ²⁸Si into p+ ²⁷Al at an energy of 14.6 GeV/nucleon. The final-state energy is obtained through a calculation of the p-²⁷Al invariant mass in kinematically reconstructed events. The final-state energy spectrum for all targets is peaked near the isovector giant-dipole resonance in ²⁸Si and the dependence of the magnitude of the cross section on target charge confirms that the excitation is largely electromagnetic. By exploiting the expected scaling behavior on target Z and A, the background from nuclear interactions is evaluated and subtracted, leaving a pure electromagnetic dissociation final-state energy distribution. This distribution is well reproduced by simulated events, in which the photon spectrum calculated in the Weiszäcker-Williams approximation is combined with experimental data on the photonuclear reaction ²⁸Si(γ,p) ²⁷Al, and slight differences are observed only at low final-state energy.

We present the rapidity and transverse momentum distributions of protons and neutrons from collisions between 14.6 GeV/nucleon beams of ²⁸Si and targets of Al, Cu, and Pb. The data were measured in the forward spectrometer/target calorimeter detectors of the E814 apparatus. The results indicate the existence of two distinct domains, one of beam rapidity projectilelike nucleons, and the second of participant nucleons. From the former, the in-medium inelastic nucleon-nucleon cross section is deduced. It is found to agree, within 10%, with the ‘‘free’’ value of 30 mb although under present conditions one of the two colliding nucleons has been struck before with a high probability. We compare with the present data the predictions of a fragmentation model as well as of models dealing explicitly with the heavy-ion collision and particle creation and emission.

Projectile breakup processes are probed by studying the emission of α particles in coincidence with projectile-like fragments as a function of the dissipated energy in the collisions of 35 MeV/nucleon ¹⁶O with ⁵⁸Ni. Energy correlations between α particles and projectile-like fragments at small-angle geometries allow the separation of the sources of α emission from projectile-like and target-like fragments. We find that the slope parameters of the decay energy distributions, the average excitation energies, and the α particle multiplicities of the projectile-like fragments increase with increasing dissipation of energy. If the linear dependence, exhibited by the data, of the slope parameter with the dissipated energy is included in model calculations, the majority of the coincidence yield in the forward hemisphere can be explained. However, an excess yield of the data on the opposite side of the beam from the observed projectile-like fragment still remains. Such analysis of the data suggests that the breakup of the projectile is the dominant source of light particles at forward angles. Processes resulting in the breakup of the projectile must be better understood in order to study other processes leading to similar phenomena.