Search Results (43 total)

K-shell radiation of fast heavy ions penetrating solid matter was used to analyze the stopping dynamics of ions over more than 80% of the stopping path. The most important advantage of this method is that the data is obtained with a high spatial resolution directly from the interaction volume. In experiments 11.4  MeV∕u Ca projectile were slowed down in solid quartz and low-density SiO₂ aerogel targets. Characteristic projectile and target spectra in the photon energy range of 1.5–4 keV were registered by means of spherically bent crystal spectrometers with high spectral and spatial resolution in the direction of the ion beam propagation. K-shell spectra of heavy ions induced by close collisions with target atoms provided information about the projectile charge state and velocity dynamics. The line intensity distribution of the K-shell transitions arising from ions with different ion charges represents the charge state distribution along the ion beam track. The variation of the line Doppler shift due to the ion deceleration in the target material was used to determine the ion velocity dynamics. The spectroscopic analysis of the stopping process was complemented by measurements of the energy loss and ion charge state distribution after the ion beam emerged from the target using a standard time-of-flight method and magnet spectrometer.

The subject of high-energy density (HED) in matter is of considerable interest to many branches of physics. Intense beams of energetic heavy ions are a promising tool for creating large samples of HED matter which can be used to study the equation-of-state properties of such exotic states of matter experimentally. The Gesellschaft für Schwerionenforschung (GSI), Darmstadt, is a unique laboratory worldwide which has a heavy ion synchrotron facility, SIS18 (with a magnetic rigidity of 18 Tm), that delivers intense heavy ion beams. Using the beams generated at this present facility, interesting experimental work has been carried out in the field of HED matter [D. H. H. Hoffmann , Nucl. Instrum. Methods Phys. Res., Sect. B 161–162, 9 (2000)]. The GSI is planning to significantly expand its accelerator capabilities with construction of a new synchrotron ring, SIS100, which will have a magnetic rigidity of 100 Tm. This new facility will deliver a uranium beam which will have orders of magnitude higher intensity than the existing facility and will also have the possibility of multibeam acceleration. This paper presents two-dimensional hydrodynamic simulations of different target geometries including solid as well as hollow cylinders that are irradiated with beams having different shapes of the focal spot which will be available at the SIS100 facility. These include a circular focal spot, an annular focal spot, and an elliptic focal spot, respectively. The purpose of this study is to determine the region of the physical parameters including density, temperature, and pressure that can be accessed using the SIS100 beam. This information, we hope, will be useful for designing experiments on the studies of thermophysical properties of matter including the designing of appropriate diagnostic tools.

Localized fluctuations in the multiplicity of charged particles and photons produced in central 158A GeV/c Pb+Pb collisions are studied. The charged versus neutral correlations in common η-φ phase space regions of varying azimuthal size are analyzed by two different methods. The analysis provides a model-independent demonstration of nonstatistical fluctuations in both charged particle and photon multiplicities in limited azimuthal regions. However, no correlated charge-neutral fluctuations are observed, contrary to expectations for the production of a disoriented chiral condensate. The result is not explained by the widely used VENUS model.

Employing a two-dimensional simulation model, this paper presents a suitable design for an experiment to study metallization of hydrogen in a heavy-ion beam imploded multilayered cylindrical target that contains a layer of frozen hydrogen. Such an experiment will be carried out at the upgraded heavy-ion synchrotron facility (SIS-18) at the Gesellschaft für Schwerionenforschung, Darmstadt by the end of the year 2001. In these calculations we consider a uranium beam that will be available at the upgraded SIS-18. Our calculations show that it may be possible to achieve theoretically predicted physical conditions necessary to create metallic hydrogen in such experiments. These include a density of about 1 g/cm³, a pressure of 3–5 Mbar, and a temperature of a few 0.1 eV.

A specifically tailored plasma lens could shape a high-energy, heavy-ion beam into the form of a hollow cylinder without loss of beam intensity. It has been experimentally confirmed that both a positive as well as a negative radial gradient of the current density in the active plasma lens can be the underlying principle. Calculations were performed that yield the ideal current density distribution for both cases. A numerical simulation of an experiment with an intense ion beam highlights that the shaping of the beam increases the achievable compression in a lead sample.

A measurement of direct photon production in ²⁰⁸Pb+²⁰⁸Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.5<p_T<4 GeV/c. A significant direct photon signal, compared to statistical and systematical errors, is seen at p_T>1.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.

Three-particle correlations have been measured for identified π^- from central 158A GeV Pb+Pb collisions by the WA98 experiment at CERN. A substantial contribution of the genuine three-body correlation has been found as expected for a mainly chaotic and symmetric source.

This paper presents two-dimensional numerical simulations of the hydrodynamic response of solid as well as hollow cylindrical targets made of lead that are irradiated by an intense beam of uranium ions which has an annular focal spot. Using a particle tracking computer code, it has been shown that a plasma lens can generate such a beam with parameters used in the calculations presented in this paper. The total number of particles in the beam is 2×10¹¹ and the particle energy is about 200 MeV/u that means a total energy of approximately 1.5 kJ. This energy is delivered in a pulse that is 50 ns long. These beam parameters lead to a specific energy deposition of 50–100 kJ/g and a specific power deposition of 1–2 TW/g in solid matter. These calculations show that in case of the solid lead cylinder, it may be possible to achieve more than 4 times solid lead density along the cylinder axis at the time of maximum compression. The pressure in the compressed region is about 20 Mbar and the temperature is a few eV. In the case of a hollow cylinder, one also achieves the same degree of compression but now the temperature in the compressed region is much higher (over 10 eV). Such samples of highly compressed matter can be used to study the equation-of-state properties of high-energy-density matter. It is expected that by the end of the year 2001, after completion of the upgrade of the existing facilities, the above beam parameters will be available at the Gesellschaft für Schwerionenforschung (GSI), Darmstadt. This will open up the possibility to carry out very interesting experiments on a number of important problems including the investigation of the EOS of high-energy-density matter.

In this paper is presented, with the help of sophisticated two-dimensional hydrodynamic simulations, a suitable design with optimized parameters for a heavy-ion beam-matter interaction experiment that will be carried out at the Gesellschaft für Schwerionenforschung (GSI) Darmstadt by the end of the year 2001 when the upgrade of the existing accelerator facility will be completed. Our simulations show that this upgraded heavy-ion beam is capable of generating strong shocks in solid targets that compress the target material to supersolid densities and generate multi-mbar pressures. This will open up, at the GSI, the possibility of investigation of the equation-of-state properties of matter under such extreme conditions. Numerical simulations can predict the experimental results with reasonable accuracy, which is helpful in designing the diagnostic tools for the experiment.

It is expected that after the completion of a new high current injector, the heavy-ion synchrotron (SIS) at the Gesellschaft für Schwerionforschung (GSI) Darmstadt will accelerate U⁺²⁸ ions to energies of the order of 200 MeV/u. The use of a powerful rf buncher will reduce the pulse length to about 50 ns, and employment of a multiturn injection scheme will provide 2×10¹¹ particles in the beam that correspond to a total energy of the order of 1 kJ. This upgrade of the SIS, hopefully, will be completed by the end of the year 2001. These beam parameters lead to a specific power deposition of the order of 1–2 TW/g in solid matter that will provide temperatures of about 10 eV. Such low specific power deposition will induce hydrodynamic effects in solid materials, and one may design appropriate beam-target interaction experiments that could be used to investigate the equation of state of matter under extreme conditions. The purpose of this paper is to propose suitable target designs with optimized parameters for the future GSI experiments with the help of one and two-dimensional hydrodynamic simulations. Cylindrical geometry is the natural geometry for highly focused ion beams, and therefore cylindrical targets are the most appropriate for this type of interaction experiments. The numerical simulations presented in this paper show that one can experimentally measure the characteristic sound speed in beam heated targets which is an important physical parameter. Moreover, one can study the propagation of ion-beam-induced shock waves in the solid materials. Different values for the specific power deposition, namely, 10, 25, 50, and 100 kJ/g, have been used. In some cases the pulse length is assumed to be 40 ns while in others it is considered to be 50 ns. Various materials including lead, aluminum, and solid neon have been used.

Neutral pion production in central 158A GeV ²⁰⁸Pb+ ²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron. The π⁰ transverse mass spectrum has been analyzed in terms of a thermal model with hydrodynamic expansion. The high accuracy and large kinematic coverage of the measurement allow one to limit previously noted ambiguities in the extracted freeze-out parameters. The results are shown to be sensitive to the shape of the velocity distribution at freeze-out.

Net proton and negative hadron spectra for central Pb+Pb collisions at 158 GeV per nucleon at the CERN Super Proton Synchrotron were measured and compared to spectra from lighter systems. Net baryon distributions were derived from those of net protons. Stopping (rapidity shift with respect to the beam) and mean transverse momentum 〈p_T〉 of net baryons increase with system size. The rapidity density of negative hadrons scales with the number of participant nucleons for nuclear collisions, whereas their 〈p_T〉 is independent of system size. The 〈p_T〉 dependence upon particle mass and system size is consistent with larger transverse flow velocity at midrapidity for Pb+Pb compared to S+S central collisions.

The production of neutral pions in 158A GeV ²⁰⁸Pb+²⁰⁸Pb collisions has been studied in the WA98 experiment at the CERN Super Proton Synchrotron (SPS). Transverse momentum spectra are studied for the range 0.3≤m_T-m₀≤4.0 GeV/c. The results for central collisions are compared to various models. The centrality dependence of the neutral pion spectral shape and yield is investigated. An invariance of the spectral shape and a simple scaling of the yield with the number of participating nucleons is observed for centralities with greater than about 30 participating nucleons. This is most naturally explained by assuming an equilibrated system.

The directed and elliptic flow of protons and charged pions has been observed from the semicentral collisions of a 158 GeV/nucleon Pb beam with a Pb target. The rapidity and transverse momentum dependence of the flow has been measured. The directed flow of the pions is opposite to that of the protons but both exhibit negative flow at low p_t. The elliptic flow of both is fairly independent of rapidity but rises with p_t.

A search for the production of direct photons in S+Au collisions at 200A GeV has been carried out in the CERN-WA80 experiment. For central collisions the measured photon excess at each p_T, averaged over the range 0.5≤p_T≤2.5 GeV/c, corresponded to 5.0% of the total inclusive photon yield with a statistical error of σ_stat  =  0.8% and a systematic error of σ_syst  =  5.8%. Upper limits on the invariant yield for direct photon production at the 90% C.L. are presented. Possible implications for the dynamics of high-energy heavy-ion collisions are discussed.

Measurements of the forward and the transverse energy in 158 GeV per nucleon ²⁰⁸Pb + Pb collisions are presented. A total transverse energy of about 1 TeV is created in central collisions. An energy density of about 3 GeV/fm³ is estimated for near head-on collisions. Only statistical fluctuations are seen in the ratio of electromagnetic to hadronic transverse energy.

The NA35 experiment has collected a high statistics set of momentum analyzed negative hadrons near and forward of midrapidity for central collisions of 200A  GeV/c ³²S+S, Cu, Ag, and Au. Using momentum space correlations to study the size of the source of particle production, the transverse source radii are found to decrease by ∼40% at midrapidity and ∼20% at forward rapidity while the longitudinal radius R_L is found to decrease by ∼50% as p_T increases over the interval 50<p_T<600  MeV/c. Calculations using a microscopic phase space approach (relativistic quantum molecular dynamics) reproduce the observed trends of the data.

We have studied one and two-dimensional scaled factorial moments in ³²S+S and ³²S+Au collisions at 200 GeV/nucleon in a high statistics electronic measurement at the CERN SPS using pad-readout streamer tubes. We observe no intermittency signal beyond that produced by folding the fritiof event generator with a detailed model of our detector. The systematic effects of detector response, two-track separation, and finite statistics in a factorial moment analysis are discussed in detail. Even though the observed signal contains measurable distortions due to these experimental effects, we show that we are sensitive to intermittency. As an alternative method, a two-particle correlation function analysis was applied to the same data to measure correlated particle production at small scales. We show that this method does not suffer as much as the factorial moment analysis does from distortions due to the limited two-track resolution of the detector. The correlation functions also agree with the predictions of fritiof filtered through our detector simulation, down to the limit of the two-track resolution. Since fritiof models nucleus-nucleus collisions by the superposition of nucleon-nucleon collisions, we conclude that there is no evidence in our data of the kinds of collective behavior predicted to give strong intermittency in heavy ion collisions.

With use of numerical simulation we investigate the influence of self-fields on the static properties of one-dimensional parallel arrays of Josephson junctions. We determine the currents and fields around a vortex, and the critical current versus magnetic field when the mutual inductances between all cell pairs are included. We find that for these static properties a good approximation can be obtained by using self-inductances only, but with a larger effective penetration depth which is determined by the ratio of the nearest-neighbor inductance to the self-inductance of the loops. We also find that for small penetration depths there is an energy barrier for vortex motion from one cell to the next but inclusion of all the mutual inductances only has a minor influence on this energy barrier.

The transverse momentum and rapidity distributions of negative hadrons and participant protons have been measured for central ³²S+ ³²S collisions at p_lab=200 GeV/c per nucleon. The proton mean rapidity shift 〈Δy〉∼1.6 and mean transverse momentum 〈p_T〉∼0.6 GeV/c are much higher than in pp or peripheral AA collisions and indicate an increase in the nuclear stopping power. All p_T spectra exhibit similar source temperatures. Including previous results for K_s⁰ Λ, and Λ¯, we account for all important contributions to particle production.

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.

The energy dependence of rapidity distributions and flow effects was studied in central Ar+Pb collisions at 400, 800, and 1800 MeV/nucleon using a streamer chamber. Rapidity distributions for proton and pions are found to have a Gaussian shape whereas those for deuterons exhibit a two-peak structure at the two higher energies. The average in-plane transverse momentum per/nucleon and per/event shows saturation of flow around 800 MeV/nucleon for this asymmetric system. The aspect ratio of the sphericity tensor is closely correlated with the flow angle. This correlation appears to be independent of beam energy. The number of participating nucleons in central collisions varies from 213 at 400 to 135 at 1800 MeV/nucleon indicating that at the lowest energy almost the entire target nucleus participates in the collision.

Distributions of transverse energy, forward energy, and dE_T/dη from 60A GeV and 200A GeV ¹⁶induced and 200A GeV ³²induced nuclear collision with C, Al, Cu, Ag, and Au are presented. The energy, projectile, target, and centrality dependences are shown and discussed within a simple Glauber spectator-participant model. Two universal parametrizations of the dE_T/dη distributions are presented together with various estimates of the nuclear stopping power and attained energy densities. An upper limit for γ_direct/π⁰ in central collisions of 200A GeV ³²S+¹⁹⁷Au is derived.