Search Results (23 total)

We have measured the production of light nuclei (A<~3) in 11.6 GeV/c Au-Au collisions at the Brookhaven Alternating Gradient Synchrotron (AGS). The transverse mass spectra are analyzed using a thermal fireball model, and the yields for different particle species are discussed assuming coalescence and fragmentation as possible production mechanisms. The wide acceptance range of the ³He measurements permits a broad study of the coalescence parameter B₃ as functions of transverse momentum and rapidity. Comparisons with data obtained previously at AGS energies suggest that the simple models are insufficient to describe fully the production mechanisms of light nuclei.

E896 has measured Λ production in 11.6A GeV/c Au-Au collisions over virtually the whole rapidity phase space. The midrapidity p_t distributions have been measured for the first time at this energy and appear to indicate that the Λ hyperons have different freeze-out conditions than protons. A comparison with the relativistic quantum molecular dynamics model shows that while there is good shape agreement at high rapidity the model predicts significantly different slopes of the m_t spectra at midrapidity. The data, where overlap occurs, are consistent with previously reported measurements.

Neutron production, in coincidence with fragments emitted in the ⁴⁰Ca+H reaction at E_lab=357A and 565A MeV, has been measured using a 3-module version of the multifunctional neutron spectrometer MUFFINS. The mean neutron multiplicities for neutrons detected in the angular range covered by MUFFINS (0°-3.2°) have been estimated from the comparison between the neutron cross sections, in coincidence with the fragments, and the elemental cross sections. We have found evidence for a preequilibrium emission of prompt neutrons in superposition to a “slower” deexcitation of the equilibrated remnant by emission of nucleons and fragments, as already seen in inclusive rapidity distributions. The energy dependence of the inclusive neutron production cross sections, measured in a previous work, is here interpreted as due to the stronger neutron focusing in the forward direction at the higher energy. Comparison with a BNV+phase space coalescence model is discussed.

We have measured the yields of protons and A=2–4 nuclei in collisions between 10.8A GeV/c Au beams and targets of Al, Cu, and Au. The data, which cover a broad rapidity range at low transverse momenta, were measured as a function of collision centrality using a focusing beam line spectrometer and a high-rate centrality detector. We investigate the dependence of coalescence parameters on event geometry. The data are compared with the predictions of an RQMD+coalescence model.

The isotopic production cross sections for ⁴⁰Ca projectiles at 357, 565, and 763 MeV/nucleon interacting in a liquid hydrogen target have been measured by the Transport Collaboration at the LBL HISS facility. The systematics of these cross sections are studied, and the results indicate that nuclear structure effects are present in the isotope production process during the relativistic collisions. The newly measured cross sections are also compared with those predicted by semiempirical and parametric formulas, but the predictions do not fully describe the systematics such as the energy dependence. The consequences of the cross section systematics in galactic cosmic ray studies are also discussed.

Experiment E878 at the BNL-AGS has measured the invariant cross sections of antiprotons produced near p_t=0 in interactions of 10.8 GeV/c Au beams with targets of Al, Cu, and Au. The data were measured for a wide range of centralities and rapidities using a focusing beamline spectrometer and a high-rate centrality detector. We compare our data with the predictions of simple models and sophisticated transport models to explore the physics of antiproton production and annihilation.

Neutrons produced in the ⁴⁰Ca+H reaction at E_lab=357A and 565A MeV have been detected using a three-module version of the multifunctional neutron spectrometer MUFFINS. The detector covered a narrow angular range around the beam in the forward direction (0°-3.2°). Semi-inclusive neutron production cross sections, at the two energies, are reported together with neutron energy spectra, angular, rapidity, and transverse momentum distributions. Comparison with a Boltzmann-Nordheim-Vlasov approach + phase space coalescence model is discussed.

The interactions of ³⁶Ar projectile nuclei with energies of 361, 546, and 765 MeV/nucleon and ⁴⁰Ar nuclei with 352 MeV/nucleon, have been studied in a liquid-hydrogen target as part of a program to study interactions of relevance to the problem of cosmic-ray propagation in the interstellar medium. We have measured the cross sections for the production of isotopic fragments of the projectile nuclei in these interactions. The variations of these cross sections with mass, charge, and energy, are examined for insights into any systematic features of this type of fragmentation reaction that might aid predictions of other, unmeasured cross sections. These cross sections are also compared with the values derived from the most commonly used prediction techniques. It is suggested that these techniques could be improved by taking account of the systematic features identified here.

A Reply to the Comment by Y. Pang, D. E. Kahana, and S. H. Kahana.

This paper reports the elemental production cross sections for 17 projectile-energy combinations with energies between 338 and 894 MeV/nucleon interacting in a liquid hydrogen target. These results were obtained from two runs at the LBL Bevalac using projectiles ranging from ²²Ne to ⁵⁸Ni. Cross sections were measured for all fragment elements with charges greater than or equal to half the charge of the projectile. The results show that, over the energy and ion range investigated, the general decrease in cross section with decreasing fragment charge is strongly modified by the isospin of the projectile ion. Significant additional modifications of the cross sections due to the internal structure of the nucleus have also been seen. These include both pairing and shell effects. Differences in the cross sections due to the differing energies of the projectile are also considerable. © 1996 The American Physical Society.

We have measured the yields of antiprotons in Au + Au interactions in the rapidity range 1.2<y<2.8 as a function of centrality using a beam line spectrometer. The shapes of the invariant multiplicity distributions at p_t=0 are used to explore the dynamics of antiproton production and annihilation.

We have investigated particle production using a zero degree two-segment focusing beam-line spectrometer, examining 2×10¹²  Au+Au interactions at a beam momentum of 10.8A  GeV/c. We have measured invariant cross sections for protons, various light nuclei (through A  =  6), antiprotons, and have set upper limits on the production of heavier nuclei, antideuterons, and strangelets. There is no evidence, within the sensitivity of our measurements, for the formation of new particles having lifetimes >100  ns

The azimuthal distributions of light particles (Z=1,2) with respect to the entrance channel reaction plane are investigated for Ar+V collisions in order to characterize the modes of collective motion in intermediate energy heavy-ion reactions. At a beam energy of 35 MeV/nucleon, light charged particles are found to exhibit an enhanced emission in the reaction plane which increases with the mass of the detected particle. As the beam energy is increased to 100 MeV/nucleon, the anisotropy nearly vanishes, providing insight into the dynamics of these reactions in a transitional energy regime. The observed anisotropy exhibits signatures of two distinct modes of collective motion: attractive mean-field deflection and rotation of the fused system. A microscopic calculation based on mean-field mediated interactions plus nucleon-nucleon collisions reproduces both forms of collective motion and their associated azimuthal anisotropies. The calculation also suggests that the anisotropy due to mean-field deflection is established during the initial stages of the collision.

A liquid hydrogen target was used to study the nuclear fragmentation of beams of relativistic heavy ions, ²²Ne to ⁵⁸Ni, over an energy range 400 to 900 MeV/nucleon. The experiments were carried out at the Lawrence Berkeley Laboratory Bevalac HISS facility, using the charge-velocity-rigidity method to identify the charged fragments. Here we describe the general concept of the experiment and present total charge-changing cross sections obtained from 17 separate runs. These new measured cross sections display an energy dependence which follows semiempirical model predictions. The mass dependence of the cross sections behaves as predicted by optical models, but within the experimental energy range, the optical model parameters display a clear energy dependence. The isospin of the projectile nuclei also appears to be an important factor in the interaction process.

The disappearance of collective flow in nucleus-nucleus collisions occurs at an incident energy (E_bal) where the attractive scattering dominant at low energies balances the repulsive scattering dominant at high energies. We have performed the first systematic study of the entrance-channel mass dependence of the disappearance of flow and hence E_bal. The new data presented for the C+C, Ne+Al, Ar+Sc, and Kr+Nb systems show that E_bal scales as A^-1/3 where A is the mass of the combined system. Boltzmann-Uehling-Uehlenbeck model calculations show trends which are in qualitative agreement with these new results.

We have measured two-fragment reduced-velocity correlation functions of the intermediate mass fragments (IMF: 3≤Z≤7) produced in multifragment final states for the Kr+Nb system (E/A=35, 45, 55, 65, and 75 MeV). From the measured correlation functions we extract mean IMF emission lifetimes (τ) which are observed to decrease from τ≊400 fm/c at E/A=35 MeV to τ≊125 fm/c at E/A=55 MeV. For beam energies in excess of E/A=55 MeV, no further decrease in τ is observed, indicating a possible saturation of the mean emission lifetime for IMF produced in multifragment exit channels.

We present Z distributions for fragments with 1≤Z≤12 from central collisions of ⁴⁰ Ar+⁴⁵Sc at incident energies ranging from 35 to 115 MeV/nucleon. We find that the Z distributions can be described by a power law or an exponential and steepen with increasing incident energy. Over the range of incident energies studied, the average number of intermediate mass fragments decreases while the average number of particles increases. When combined with previous results for the charge distributions, a minimum is observed in the extracted power-law parameter.

Multifragment azimuthal correlation functions have been measured as a function of beam energy and impact parameter for the Ar+Sc system (E/A=35 to 115 MeV). The observed azimuthal correlation functions—which do not require corrections for dispersion of the reaction plane—exhibit strong asymmetries which are dependent on impact parameter and beam energy. Rotational collective motion and flow seem to dominate the correlation functions at low beam energies. It is proposed that multifragment azimuthal correlation functions can provide a useful probe for intermediate energy heavy ion reaction dynamics.

Two proton correlation functions have been measured at 30 MeV/nucleon for various systems (²⁰Ne+¹²C,²⁷Al,⁵⁹Co,¹⁹⁷Au) in the forward hemisphere. Source radii extracted by comparisons to the final state interaction model of Koonin turn out to be independent of the mean emission angle of the protons as well as of the size of the studied system, and little affected by energy cuts on the detected proton pairs. Typical size (∼5.5–6 fm) exceeds the radius of the compound nucleus for almost all studied systems, suggesting significant time delays between proton emissions. However, similarity of transverse and longitudinal correlation functions seems to eliminate the existence of a long lifetime. Due to the poor sensitivity of the proton-proton correlation function, this method seems not well suited for source size measurements in this energy domain.

Energy spectra and angular distributions have been measured for high energy gamma rays (E_γ≥20 MeV) from proton-nucleus reactions at 104, 145, and 195 MeV on targets of C, Zn, and Pb. Gamma rays were observed with energies up to 170 MeV. The spectra showed differences from the typical exponential shape that is observed in gamma ray production from heavy-ion reactions. The angular distribution of the gamma ray is forward peaked in the laboratory, which is consistent with emission from a moving source. A comparison is made with previous measurements at 72, 140, 168, and 200 MeV. The experimental evidence indicates that first-chance incoherent proton-neutron bremsstrahlung is the main production mechanism.

Energy spectra and angular distributions have been measured for high energy gamma rays (E_γ≥20 MeV) from the p+d reaction at 145 and 195 MeV. Gamma rays were observed up to the maximum energy allowed by kinematics. A comparison is made with previous measurements for the p+d system at 140, 197, and 200 MeV. Below the free pnγ threshold the general shape of the energy spectra and angular distributions are in reasonable agreement with a recent calculation of the free pnγ elementary process. However, the magnitude of the predicted cross section is not in good agreement with the present data.

Two-neutron correlations at small relative momentum in ²⁰Ne+C and ²⁰Ne+Co reactions at 30A MeV exhibit correlation functions dominated by the attractive final-state interaction. The results are consistent with two-proton data. The data suggest large apparent emission sources for both reactions with contributions from both pre-equilibrium emission and evaporation.

We have added a new measurement at 100 MeV/nucleon to our previous excitation function for collective flow of light fragments from ⁴⁰Ar+⁵¹V collisions. In the earlier work, flow decreased as the beam energy was raised from 45 to 85 MeV/nucleon. This provided hints to the disappearance of flow, but the lack of measurements at higher beam energies precluded the observation of the reappearance of flow. At a beam energy of 100 MeV/nucleon the flow has reappeared and this allows an experimental determination of the region where attractive scattering balances with repulsive scattering in these collisions. A simultaneous least-squares fit of the flow data for light fragments yields (85±10) MeV/nucleon for the balance energy.