Unified description of angular distributions of target particles in light and heavy ion induced interactions in nuclear emulsions at high energies

Some investigations show that in light ion （LI） induced reactions, such as He, O, Ne, and Ar in nuclear emulsions （Em） at high energies, the angular distributions of the target particles show a wide structure around the polar angle V ≈ 60°. With heavy ions （HI） such as Kr and Au such a wide structure has not been observed. The experimental results on Mg-Em and Si-Em interactions also do not show such a wide structure in the angular distribution of the target particles. Using a multisource ideal gas model we uniformly describe the angular distributions of the target particles produced in LI-Em and HI-Em interactions. The result is not only in agreement with the mean trend, but also with the fluctuations of the experimental data. We conclude that the wide structure observed in LI-Em interactions may be the result of statistical fluctuations.     

Multiplicity and pseudorapidity distributions of charged particles from32S induced heavy ion interactions at 200A GeV

Multiplicity and pseudorapidity distributions have been measured for³²S+Al, Cu, Ag and Au at 200A GeV. The widths of the pseudorapidity distributions increase from central to peripheral collisions. The main contribution is assumed to come from the increasing fraction of charged particles stemming from the fragmentation of the target for peripheral collisions as compared to central collisions. On the average 170–180 charged particles per unit of pseudorapidity are attained for the most central³²S+Au events at the maximum of the distribution. The target dependence of the yield of charged particles for central collisions is investigated. In the target rapidity region the yield is directly proportional to the target mass. The transverse energy per charged particle has been studied as a function of centrality and pseudorapidity. The experimental results are compared to the results from the Monte-Carlo model VENUS 3.11 which includes rescattering among secondaries. A comparison between different centrality triggers is made.     

Polarization measurements of laboratory VUV light: a first comparison between multilayer polarizers and photoelectron angular distributions

The use of photoelectron angular distributions to determine the linear polarization of VUV light over a wide range of photon energies is demonstrated. Light at wavelengths from 256 to 736 Å, partially polarized by large angle reflections in a toroidal grating monochromator and at a refocus mirror, has been analyzed. The results are validated by comparison measurements at spot wavelengths using multilayer polarizers.     

Fusion-fission angular distributions: A new probe of fast fission fractionation in nucleus-nucleus collisions

Fragment angular distributions in heavy ion-induced fission reactions have been analysed in terms of a two component model—fission following compound nucleus formation and fast fission events. It is seen that, contrary to the general assumption, fast fission competes with compound nucleus fission even when the composite system is formed with a spin less than the rotating liquid drop model limit for vanishing fission barrier.     

Fission cross section and fragment angular distribution in gold fission induced by 55 MeV alpha particles using solid state nuclear track detectors

The angular distribution of fission fragments in alpha induced fission has been studied at an incident energy of 55 MeV in¹⁹⁷Au target. The relative differential fission cross sections were measured at different angles between 10° and 170° and the resulting angular distributions fitted by least squares method with Legendre polynomials. In the present work, a correction for the self-scattering and self-absorption of fission fragments in the target itself was applied and a target of 3 mg/cm² was used to get good statistics. The anisotropy for 55 MeV alpha induced fission of gold was 2·83±0·43 and the fission cross section calculated by integrating the measured angular distributions over all the solid angles was 5·2±1.0 mb, confirming the value of 4·0±0·05 mb reported by Burnettet al but contrary to the high value of 10±3 mb reported by Ralarosyet al.     

Proton distributions in the target fragmentation region in proton-nucleus and nucleus-nucleus collisions at high energies

We present measurements of the rapidity and transverse-momentum distributions of the protons emitted in S+W, O+W, andp+W reactions at 200 GeV/A around the target rapidity (y=1). The rapidity density rises linearly with the transverse energy for all three systems, but the slope forp+W is much steeper than for O+W and S+W. The rapidity density forp+W is much higher than predicted by summing single nucleonnucleon collisions without any nuclear effects, indicating substantial rescattering of the produced particles. The predictions of the VENUS 3 model, including rescattering, show reasonable agreement with the data for all three systems. We do not have evidence for a strong collective flow of the outgoing particles.     

Two-dimensional multiplicity fluctuation analysis of target residues in nuclear collisions

Multiplicity fluctuation of the target residues emitted in the interactions in a wide range of projectile energies from 500 A MeV to 60 A GeV is investigated in the framework of two-dimensional scaled factorial moment methodology.The evidence of non-statistical multiplicity fluctuation is found in 16 O-AgBr collisions at 60 A GeV,but not in 56 Fe-AgBr collisions at 500 A MeV,84 Kr-AgBr collisions at 1.7 A GeV,16 O-AgBr collisions at 3.7 A GeV and 197 Au-AgBr collisions at 10.7 A GeV.     

Probabilities for the emission of light particles and their energy and angular distributions for true quaternary nuclear fission

On the basis of quantum-mechanical fission theory, the features of true quaternary nuclear fission are studied by treating this fission process as a sequence of three processes following one another in the course of time. The first two processes are the escape of the first and then the second of the two light particles emitted from the neck of a fissioning nucleus because of a nonadiabatic character of the collective deformation motion of this nucleus. Finally, the third process is the separation of the fissioning nucleus into two rather heavy fission fragments. The differences that arise in the emission probabilities and in the angular and energy distributions upon going over from the first emitted to the second emitted prescission third and fourth particles are analyzed by invoking experimental data on the spontaneous and thermalneutron-induced fission of nuclei, and it is shown that these differences are caused by the changes both in the geometric configuration of the fissioning nucleus and in the shell structure of its neck after the first prescission particle is emitted from it.     

Emission of helium nuclei in heavy ion interactions at energies ≧ 100 MeV/nucleon

The energy and angular distributions of helium particles emitted in interactions between nuclei in the cosmic radiation and nuclei in photoemulsions at energies ≧ 100 MeV/nucleon have been studied. The data obtained is impossible to interpret on the basis of a statistical decay of excited nuclei. For example, it is found that more than 28% of the helium nuclei are emitted in processes different from simple evaporation. The differential energy distribution of the helium nuclei in the energy interval (40–200) MeV can be represented by the relationN(E)dE=constE ^?a dE, wherea≈1.2. The large spread in angles and energies of the fast helium particles emitted in heavy ion interactions can to a certain degree be understood, if it is assumed that interactions between nucleons and clusters of nucleons occur.     

Hydrodynamic description of spectra at high transverse mass in ultrarelativistic heavy ion collisions

Transverse mass spectra of pions and protons measured in central collisions of heavy ions at the SPS and at RHIC are compared to a hydrodynamic parameterization. While the chemical temperature needed at RHIC is significantly higher compared to SPS, the spectra may be described using kinetic freeze-out parameters which are similar for both beam energies. At RHIC either the temperature or the flow velocity is higher, but the data provide no unambiguous proof for much stronger transverse flow. The contribution of such hydrodynamic emission at high transverse momenta is investigated in detail. It is shown that hydrodynamics may be relevant up to relatively high transverse momenta. The importance of the velocity profile used in this context is highlighted. Received: 4 July 2002 / Revised version: 6 November 2002 / Published online: 20 December 2002     

A combined model for pseudorapidity distributions in p-p collisions at center-of-mass energies from 23.6 to 7000 GeV

In p-p collisions, the charged particles produced consist of two leading particles and those frozen out from the hot and dense matter created in the collisions. The two leading particles are in the projectile and target fragmentation regions, respectively, which, in this paper, are conventionally supposed to have Gaussian rapidity distributions. The hot and dense matter is assumed to expand according to unified hydrodynamics, a hydrodynamic model which unifies the features of the Landau and Hwa-Bjorken models, and freeze out into charged particles from a space-like hypersurface with a fixed proper time of τFO. The rapidity distribution of these charged particles can be derived analytically. The combined contribution from both leading particles and unified hydrodynamics is then compared against experimental data from a now available center-of-mass energy region from 23.6 to 7000 GeV. The model predictions are consistent with experimental measurements.     

Correlations and fluctuations in high energy heavy ion collision experiments

An overview of research status of soft physics in high energy heavy-ion collision experiments and recent experimental results are presented. The experimental status on fluctuations and correlations has been reviewed and the outlook for research status of soft physics in LHC/ALICE has been introduced in this paper.     

3.7 A GeV 16O与乳胶作用相对论性α粒子的双源发射

对3.7 A GeV 16O与原子核乳胶作用α射弹碎片的发射进行了研究, 发现α射弹碎片的角分布不能用单一的旁观体-反应体模型来解释，所以假设α射弹碎片来源于两个温度不同的发射源。     

Coulomb breakup of light nuclei in the field of a heavy ion at relativistic collision energies

A simple method for calculating the amplitude and the cross section for the Coulomb breakup of a light nucleus into two fragments in the field of a heavy ion at relativistic collision energies is proposed on the basis of time-dependent perturbation theory. It is shown that the resulting amplitude for the process in question has a correct nonrelativistic limit. The contribution of the longitudinal component of the Coulomb field of a heavy ion tends to zero in the ultrarelativistic limit. A specific implementation of the method is demonstrated by taking the example of the Coulomb breakup reaction ²⁰⁸Pb(⁸B, ⁷Bep)²⁰⁸Pb at various collision energies. The results are found to be in agreement with experimental data.     

A model for multiplicity distributions in hadron-nucleon and α-α interactions at very high energies

We construct a simple stochastic model for particle production in hadron-nucleon collisions, which agrees with experimentally observed KNO scaling. We then use this model to calculate the multiplicity distribution of negative particles produced in α-α collisions at the CERN-ISR, based on a model of multiple collisions. The agreement of the measured spectra with the theory supports this hypothesis.     

Dynamics of high energy heavy ion collisions — nuclear matter at high density and temperature

Nuclear collisions at E_LAB > 50 MeV/n are studied using the nuclear fluid dynamical and the cascade model. Evidence for the predicted compression effects has been found in recent experimental data: The preferential sidewards emission of light fragments as well as the azimuthal p-p correlations indicate the presence of collective flow phenomena. The analysis of future 4π exclusive experiments in terms of the kinetic flow tensor can yield information about the equation of state of the compressed nuclear matter. We also discuss the role of pions and strange particles as probes for the properties of nuclear matter at high excitation energy.