Violent Collisions Research Articles

Charged particle multiplicities in reactions of Ag with 40Ar of 7, 17, 27, and 34 MeV/nucleon.

Systematic results are presented for multiplicity distributions of $^{1,2,3}\mathrm{H}$ and $^{3,4,6,8}\mathrm{He}$ produced in reactions of $^{\mathrm{nat}}\mathrm{Ag}$ with $^{40}\mathrm{Ar}$ beams of 280, 680, 1080, and 1356 MeV. A variety of gating conditions is explored and classified in connection with the more peripheral or the more central collisions. Correlations between H-He multiplicity and Z or angle of projectilelike fragments follow qualitative expectations for deeply inelastic scattering. Pairs of light charged particles or intermediate-mass fragments at side angles are well correlated with the most violent collisions, leading to incomplete fusion. The cross sections for these more central collisions imply entrance-channel orbital angular momenta up to \ensuremath{\approxeq}300\ensuremath{\Elzxh} in the reactions that form very highly excited composite nuclear systems.

Massive vortex motion in the presence of solid boundaries

In the present paper, the motion of finite-core vortices in the presence of both infinite and semi-infinite bodies is considered. The vortices are massive in the sense that the cores are assumed to be inviscid with constant vorticity after Batchelor [J. Fluid Mech. 1, 177 (1956)]; this requires that the core radius is much greater than the inverse square root of the appropriate Reynolds number. Four geometries are considered: the plane wall, a corner, a thin semi-infinite plate, and a semi-infinite wedge. The flow field is assumed to be inviscid, two-dimensional, and irrotational outside the core of the vortex and the shapes of the vortices are computed as solutions to an initial value problem. Results for the plane wall and the corner flow suggest that the vortices do not rebound due to the effect of the finite core; moreover, the motion of the centroid of the vortex is virtually identical to the motion of the associated point vortex of the same strength. The computed shapes of the vortices are, in general, not elliptical. For the semi-infinite plate and wedge, violent collisions with the leading edge may occur depending on the initial position of the vortex. The results for the wedge are compared with the flow visualization studies of Ziada and Rockwell [J. Fluid Mech. 118, 79 (1982)].

Particle multiplicity dependence of high-energy photon production in a heavy-ion reaction.

The production of high-energy photons in an intermediate energy heavy-ion reaction (65 MeV/nucleon $^{40}\mathrm{Ar}$${+}^{93}$Nb) is studied by characterizing the events, which produce the photons, by the forward and backward, light and heavy, charged particle production. While the absolute yield of high-energy photons increases with increasing charged particle multiplicity, the spectral shape is found to be almost independent of multiplicity. This indicates that the fundamental photon production mechanism is insensitive to the impact parameter but that the production process is more probable for the more violent collisions. This is expected for an incoherent nucleon-nucleon bremsstrahlung mechanism but not for a statistical mechanism. This work also provides a comparison of two observables that have been suggested as impact parameter selectors: the high-energy photon yield and the charged particle multiplicity. We find that the photon yield, binned by particle multiplicity, does not scale as would be expected if both techniques truly measured the impact parameter. This observation can be explained by large fluctuations in the particle multiplicity at fixed impact parameter.

Multifragment disintegration of the 129Xe+197Au system at E/A=50 MeV.

Multifragment disintegrations following $^{129}\mathrm{Xe}$${+}^{197}$Au collisions at E/A=50 MeV have been studied with a multidetector system covering 88% of 4\ensuremath{\pi} in solid angle. The average number of intermediate-mass fragments (Z=3--20) increases strongly as a function of charged-particle multiplicity and reaches values larger than six for the most violent collisions. The results are compared to calculations with both dynamical and statistical models.

Multiplicities in 16O-induced violent heavy-ion collisions from 5A to 2 x 10(5)A MeV.

Target-fragment, shower-particle (meson), interacting-proton (36--400 MeV), and total multiplicities in {sup 16}O+Ag collisions from 5{ital A} MeV to 2{times}10{sup 5}{ital A} MeV are studied. The target disintegration exhibits energy independence above 100{ital A} MeV even in the most central collisions. Above 5{ital A} GeV the energy dependences of the three different categories of particles are similar to those from proton-nucleus interactions. The absolute yield of target fragments is also the same while the yield of protons is twice as large and of shower particles 6--7 times larger in {sup 16}O-induced central collisions as compared to {ital p}-nucleus reactions.

Ion-induced Auger emission from Si, Al and Al xMg 1−x samples

Angular resolved investigations of Auger atomic lines induced by ion bombarment have been conducted on Si, Al and Al x Mg 1− x samples. It is confirmed, by direct experimental evidence, that the atomic-like Auger emission occurs outside the sample and new transitions are also observed. All main Auger atomic lines exibit large Doppler shifts which can be accounted by a simple binary collision model in which only excitation of primary recoils by violent asymmetric collisions are considered. Under normal Ar + ion incidence doubly excited L 2MM Auger peaks are observed for Al x Mg 1−x samples ( x = 0.25, 0.50, 0.75, 1.0), such peaks are absent in pure Al bombarded under the same conditions, indicating that such excitations are generated by asymmetric MgAl collisions. The results also show an enrichment of Al within the first layers for an Al 0.50Mg 0.50 sample as the bombarding energy is increased. This enrichment can be interpreted in terms of preferential sputtering and segregation effects.

Dynamics and thermalization in violent collisions between40Ar and Ag at 27 MeV/nucleon

For the violent collisions of 27 MeV/nucleon40Ar with Ag, coincidence measurements have been made between heavy residues and intermediate mass fragments (3≦Z≦14) or light charged particles. From the analysis of the correlation between heavy residues (mass and velocity) and intermediate mass fragments, the main characteristics of the dominant mechanisms, fusion and partially damped collisions preceded or accompanied by a preequilibrium emission, are presented. Balances concerning mean values of parallel linear momentum, mass and atomic number, are established and confirm that a complete description of violent collisions was obtained. Then thermalization is discussed, first in terms of excitation energies derived from kinematics between heavy residues and intermediate mass fragments, and secondly in terms of initial temperature estimates derived from light charged particle spectra. Very hot nuclei (T⋍5.7–6.6 MeV) are produced over a large impact parameter range from very central collisions to medium peripheral ones. Various experimental results are compared to predictions obtained with semi-classical calculations (Landau-Vlasov equation). From their good agreement one may conclude that, depending on the impact parameter, thermal equilibrium is achieved within 4–10×10−22 s.

Multiply charged carbon ions and their production mechanisms in MeV/amu Arq+ (q = 14-4) + CH4 collisions

Collisions between 1.05 MeV/amu Arq+ (q = 14, 10, 8 and 4) ions and CH4 molecules have been investigated using final charge-selected projectile-recoil ion time-of-flight technique. It is found that, in the overall ionization of target CH4 molecules, pure ionization processes without any change of the projectile charge is by far dominant over any other processes involving electron capture or loss of projectiles. Electron loss or capture processes of projectiles, though weak relative to pure ionization, are found to result in enhancement of the production of multiply charged atomic carbon ions. Another feature, probably not observed earlier, is the fact that in some electron loss and capture processes multiply charged carbon ions become more dominant than any other molecular ions, suggesting that such violent collisions result in nearly complete dissociation of the molecular ions.

Angle-resolved Auger study of 10-keV Ar+-ion-induced Si LMM atomic lines.

We present a detailed, angle-resolved Si L-shell Auger study by bombarding a single-crystalline Si sample with 10-keV ${\mathrm{Ar}}^{+}$ ions. We have observed a new atomic line at kinetic energy of \ensuremath{\sim}99 eV which is tentatively assigned to an Auger transition involving two 2p holes in ${\mathrm{Si}}^{+}$. The existence of two atomic peaks at 61.36 and 91.1 eV has also been clearly confirmed. Our Auger spectra show well-split Doppler peaks for the principal ${\mathrm{Si}}^{0}$ and ${\mathrm{Si}}^{+}$ atomic lines and a strong dependence of the shift amplitude on both incidence and detection angles. Successful computer fitting of the angular dependence of Doppler shift has been achieved by using a simple binary-collision model with the Moli\`ere approximation to the Thomas-Fermi screening potential. These results suggest that the first violent Ar-Si asymmetric collisions contribute remarkably to the Si 2p-vacancy creation process and are responsible for the ejection of energetic ${\mathrm{Si}}^{(\mathrm{*})}$ particles which is highly directional. The critical minimum Ar-Si approach distance for Si 2p-hole excitation is 0.355 \AA{}, in very good agreement with the value predicted by molecular-orbital theory.

Neutralization and inelastic scattering energy loss of low energy He + ions at InP(110) surfaces

The InP(110)-l × 1 surface is investigated with a high-resolution low energy ion scattering system. Attention is paid to the precise measurement of energy spectra of the scattered He + ions from indium atoms on the surface at incident energies ranging from 300 to 1300 eV. The single scattering peaks are found to be composed of two components (peaks P1 and P2) of which the energies are shifted toward the low side with respect to the binary elastic collision model. The peak P2 is concluded to be due to the ions which are neutralized in the incoming path and subsequently re-ionized in the violent collision. The energy shift of the peak P1 has a continuous character, showing a gradual increase with increasing incident ion energy. Apparent neutralization is observed for the ions incident along the channel direction [10] at the (110) surface, and this is discussed in connection with the continuous energy loss.

The zero-degree calorimeter for the relativistic heavy-ion experiment WA80 at CERN

Calibration and performance results are presented for a sampling calorimeter designed for use as a beam calorimeter at zero degrees in the relativistic heavy-ion experiment WA80 at CERN. This uranium-scintillator zero-degree calorimeter (ZDC) was found to have a linear response to heavy ions over the range 60–6400 GeV and an in-beam hadronic resolution ranging from σE = 0.013 + 0.33√E at low intensities to σE = 0.02 + 0.67√E at higher intensities. The eh ratio of the electromagnetic section was measured to be 1.12 at 135 GeV. The ZDC operated reliably with incident beams of 3.2 TeV oxygen and 6.4 TeV sulfur at intensities of over 106 nuclei per spill. It provided a trigger both for minimum bias events and for violent central collisions.

Dynamical aspects of violent collisions in Ar+Ag reactions at [formula omitted

In a recent study of the Ar+Ag reaction at E A =27 MeV , involving intermediate-mass fragments, am overview of the reaction dynamics of violent collisions was attained. A comparison is made with semi-classical calculations (Landau-Vlasov) which reproduce experimental observations well and sustain the persistence of deeply inelastic processes at high incident relative velocity (∼0.2 c).

Light particle emission in fusion-like 14N + 197Au collisions at E/ A = 35 MeV

Single- and two-particle inclusive cross sections for light nuclei (p, d, t, 3,4He, 6–9Li, 7,8,10Be) were measured for 14N induced reactions on 197Au at E/ A = 35 MeV. At forward angles, θ ≈ 20°, quasi-elastic peripheral reactions and more violent fusion-like collisions were discriminated by measuring the folding angle between two coincident fission fragments resulting from the decay of the heavy reaction residue. More pronounced correlation functions, but very similar emission temperatures are observed for fusion-like collisions as compared to peripheral collisions.

Electromagnetic processes in relativistic heavy ion collisions

Electromagnetic effects in relativistic heavy ion collisions with impact parameter larger than the sum of the nuclear radii are studied using the virtual photon method. With increasing value of the relativistic parameter γ the hardness of the virtual photon spectrum increases. This leads to interesting new effects which will also have to be considered in the design of future relativistic heavy ion machines and experiments. The excitation of high-lying giant E1 and E2 multipole resonances is calculated as well as electromagnetic pion production. Coulomb bremsstrahlung is calculated and compared to the bremsstrahlung emitted in the more violent central nuclear collisions. K-shell ionization and electron-positron pair production is studied. The latter process has a very large cross section for heavy ions and contributes significantly to the stopping power of relativistic heavy ions in a dense medium.

VIOLENT COLLISIONS BETWEEN IONS AS HEAVY AS KRYPTON AT GANIL AND GSI

Data from violent collisions between heavy (A 100) beams and targets of comparable mass are reviewed. The deep-inelastic mechanism dominates up to 20 MeV/u, although some departures are already noticed. Above 20 MeV/u, some data have been interpreted in the frame of a participantspectator model, but taking in account mean field effects which are still important. At 35 and 44 MeV/u, such mechanisms allow to reach temperatures where the multifragmentation becomes possible.

Effect of violent collisions in ion-electron emission

The energy spectra of electrons emitted from the surface of the polycrystalline Pt target due to bombardment with Ne +, Ar + and Kr + ions were examined at large angles of detection. It was found that for emission angles θ = 132° −145° an additional broad peak occurs on the energy spectrum curve. This peak increases rapidly with increase of the angle θ and at θ ≈160° reaches its maximum value. The results obtained are discussed in the framework of the violent collision model. It is assumed that during violent collisions molecular autoionization is possible and as a result electrons with a preferred direction of emission can be generated. The influence of adsorbed layer and target temperature on the yield of autoionization electrons are also discussed.

THE PERCOLATION APPROACH TO NUCLEUS BREAK-UP

Nucleus break up in violent collisions is studied as a critical phenomenon. We show how to determine various critical exponents from experimental data on mass distributions. A comparison is made with the predictions of percolation theory.

Analysis of solid surfaces by Neutral Beam Incidence Ion Scattering Spectroscopy.

Low-energy rare-gas Ion Scattering Spectroscopy (ISS) has been improved by the use of neutral beams as projectiles, which we call Neutral Beam Incidence Ion Scattering Spectroscopy (NBISS).Experiments on ISS and NBISS are compared on the basis of electron exchange between the projectiles and solid surfaces.In ISS, the scattered ions consist of two elements: One is “surviving ions” characterized by the ions which survived neutralization, and the other is “reionized ions” characterized by those which experienced neutralization on the incoming trajectory and subsequent reionization during the violent collision. The shadowing and focusing effects are often weakened in the intensity of surviving ions, whereas these effects appear intensively in the intensity of reionized ions. Since the threshold energy for the reionization is different for the combination of projectiles and targets, these effects are often not observed at all in the ISS spectra. On the other hand, these effects appear explicitly in the NBISS spectra. It is also found that NBISS is valid for analyzing insulator surfaces since charging effects are minimized by the use of neutral beams.

Beam-foil interaction of highly ionized chlorine ions in the high energy region

Distributions of charge states and multiple scattering widths of angular distributions have been simultaneously measured using a two dimensional-position sensitive detector for 150 MeV 37Cl 10+ ions emerging from carbon foils with thickness of 2.5–180 μg cm 2 . For thinner foils, the scattering angle dependence of mean charge states verifies the evidence for the so-called single violent and/or plural collisions

Heavy products from violent collisions in the40Ar+natAg system at 27MeV/u

Heavy products from violent collisions in the40Ar+natAg system at 27MeV/u