Nuclear Collision Process Research Articles

Optical spectroscopy study of damage in ion-irradiated 3C-SiC epilayers on a silicon substrate

Epitaxial cubic (100) 3C-SiC films on a (100) silicon wafer were irradiated at room temperature with 2.3-MeV Si+ or 3.0-MeV Kr+ ions up to a fluence of 1 × 1016 cm−2. The evolutions of the epilayer and the substrate were followed as a function of ion fluence by using micro-Raman spectroscopy, optical absorption, and diffuse reflectance spectroscopy in the UV-visible and near infrared range. Raman spectra evidence the amorphization of SiC films at an estimated dose of about 0.1 displacement per atom (dpa) for both ion irradiations. The narrow peaks of the Raman-allowed TO and LO modes of SiC and Si are recorded in the virgin sample, together with few peaks assigned to zone-edge modes of SiC arising from the intrinsic disorder in the strained films. Those crystal phonon peaks broaden or disappear with increasing fluence. The spectra finally exhibit broad extra peaks assigned to the formation of Si–Si and C–C wrong homonuclear bonds in the local order of the amorphous phase. The optical transmission and diffuse reflectance spectra feature interference fringe patterns in the SiC film that are smoothened out with irradiation due to the matching of refractive indices of the amorphous SiC film and Si substrate. The evolution of the refractive index of SiC and optical gap of Si are deduced from those spectra. The respective roles of ballistic effects and electronic excitations in the radiation damage of both SiC and Si are discussed for those two ions with about the same electronic stopping power and about one order-of-magnitude difference in nuclear stopping power. The damage is dominated by the nuclear collision processes and rather well correlated with the estimated irradiation dose in dpa. Optical spectra show that electronic excitations induce damage recovery of the amorphized substrate below the SiC/Si interface. Raman spectra and optical absorption/reflection spectra yield complementary pictures of the radiation damage.

Study on machinability of silicon irradiated by swift ions

Diamond cutting of hard and brittle materials is difficult owing to poor surface quality and severe tool wear. It has been clear that ion implantation can be used to improve the machinability. In previous studies, surface modification is realized by the nuclear collision cascade that induces phase transition during the implantation. Except for the nuclear collision process, electronic interaction between the incident particles and the target material, such as ionization, also occurs particularly during a high-energy ion implantation. However, the influence of ionization on the machinability has not been studied. In this paper, modification on the nanocutting of silicon by MeV oxygen ions is studied. Molecular dynamics simulation is conducted to reveal the nanocutting process on silicon with irradiation damages. Experiments show an enhancement in the critical undeformed chip thickness that is crucial in machining of brittle materials.

Correlation between transverse momenta in the string fusion model

We obtain an explicit analytic expression for the asymptotic behavior of the coefficient of correlation between the average transverse momenta of particles formed in a given event in rapidity-separated intervals. The asymptotic behavior is found for a large density of strings with their fusion taken into account by introducing a lattice in the transverse plane and using the translation invariance in rapidity of the string decay process at high energies. It is established that unlike the correlation between the momenta of individual particles, the correlation between the average transverse momenta of all particles created in a given event in two separated rapidity intervals does not decrease as the total number of formed strings increases, which makes this type of correlation promising for observation in nuclear collision processes at high energies. We also show that in this limit, the asymptotic behavior of the coefficient of correlation between the average transverse momenta is independent of the variance of the number of particles formed during the string fragmentation unlike previously studied correlations between multiplicities or between the average transverse momenta and the multiplicity.

Radiation transport calculations for cosmic radiation

The radiation environment inside and near spacecraft consists of various components of primary radiation in space and secondary radiation produced by the interaction of the primary radiation with the walls and equipment of the spacecraft. Radiation fields inside astronauts are different from those outside them, because of the body’s self-shielding as well as the nuclear fragmentation reactions occurring in the human body. Several computer codes have been developed to simulate the physical processes of the coupled transport of protons, high-charge and high-energy nuclei, and the secondary radiation produced in atomic and nuclear collision processes in matter. These computer codes have been used in various space radiation protection applications: shielding design for spacecraft and planetary habitats, simulation of instrument and detector responses, analysis of absorbed doses and quality factors in organs and tissues, and study of biological effects. This paper focuses on the methods and computer codes used for radiation transport calculations on cosmic radiation, and their application to the analysis of radiation fields inside spacecraft, evaluation of organ doses in the human body, and calculation of dose conversion coefficients using the reference phantoms defined in ICRP Publication 110.

Nuclear collision processes around the Bragg peak in proton therapy

In the physical processes of proton interaction in bio-materials, most of the proton energy is transferred to electrons. Ionization and excitation occur most frequently around the Bragg peak region, where nuclear reactions also exist. In this study, we investigated the processes of energy deposition by considering interactions including the nuclear reactions between protons and water molecules by a Monte Carlo simulation for proton therapy. We estimated the number of particles produced by a variety of nuclear reactions, and we focused on the interaction in the low-energy region (below 1 MeV). Furthermore, we considered the charge-changing processes in the low-energy region (less than a few hundred keV). Finally, we evaluated the total dose and the contribution of primary protons and secondary particles through nuclear reactions to the absorbed dose. The results showed that the protons generate numerous neutrons via nuclear reactions. Particularly, neutrons with relatively low energies produce recoil protons by elastic collisions with the hydrogen atoms. Around the Bragg peak, low-energy primary protons (slowed-down protons) are prevalent, whereas recoil (secondary) protons gradually become dominant behind the distal falloff region of the Bragg peak. Therefore, around the Bragg peak, the main contribution to the absorbed dose is that of the primary protons (from 80 to 90%), whereas secondary protons created by primary proton-induced reactions contribute to the dose from 20 to 5%. Behind the distal endpoint of the Bragg peak, the absorbed dose is mainly due to the protons produced by (1)H(n, p), and the contribution of these is about 70%.

Yield enhancement of molecular ions with MeV ion-induced electronic excitation

We performed secondary ion mass spectrometry for arginine thin films in the incident energy range from 10 keV to 2 MeV, in which electronic energy loss in the surface region increases with energy and exceeds the nuclear energy loss. The yield of protonated arginine increased with incident energy, and at 2 MeV it was approximately two orders of magnitude larger than at 10 keV, where the nuclear collision process is dominant. It was found that the molecular ion desorption is enhanced by the electronic excitation induced in the near-surface region. In addition, the yield ratios of the fragment ions to the protonated arginine decreased with incident energy, which indicates that the nuclear energy deposition significantly affects molecular fragmentation. These results demonstrate a soft desorption and ionization of biomolecules by the MeV ion-induced electronic excitation.

Importance of correlations and fluctuations on the initial source eccentricity in high-energy nucleus-nucleus collisions

In this paper, we investigate various ways of defining the initial source eccentricity using the Monte Carlo Glauber (MCG) approach. In particular, we examine the participant eccentricity, which quantifies the eccentricity of the initial source shape by the major axes of the ellipse formed by the interaction points of the participating nucleons. We show that reasonable variation of the density parameters in the Glauber calculation, as well as variations in how matter production is modeled, do not significantly modify the already established behavior of the participant eccentricity as a function of collision centrality. Focusing on event-by-event fluctuations and correlations of the distributions of participating nucleons we demonstrate that, depending on the achieved event-plane resolution, fluctuations in the elliptic flow magnitude $v_2$ lead to most measurements being sensitive to the root-mean-square, rather than the mean of the $v_2$ distribution. Neglecting correlations among participants, we derive analytical expressions for the participant eccentricity cumulants as a function of the number of participating nucleons, $\Npart$,keeping non-negligible contributions up to $\ordof{1/\Npart^3}$. We find that the derived expressions yield the same results as obtained from mixed-event MCG calculations which remove the correlations stemming from the nuclear collision process. Most importantly, we conclude from the comparison with MCG calculations that the fourth order participant eccentricity cumulant does not approach the spatial anisotropy obtained assuming a smooth nuclear matter distribution. In particular, for the Cu+Cu system, these quantities deviate from each other by almost a factor of two over a wide range in centrality.

Observation of defect formation process in silica glasses under ion irradiation

We have performed in situ measurements of luminescence from silica glass induced by H+ beam at various energies to investigate the dynamic defect formation process in a silica glass. The luminescence spectra showed a broad emission band centered at around 2.7eV assigned to B2α oxygen deficient centers. The intensity of the 2.7eV band rapidly increased to H+ fluence at first, and then gradually increased to a steady value. We found that the change in the intensity of the 2.7eV band relates to two processes, fast and slow processes with different reaction rates for producing luminescence centers. The dependences of calculated reaction rates for faster and slower transformation versus H+ energy correspond well to those of electronic and nuclear stopping power, respectively. Consequently, the production of the luminescence centers under sub-MeV H+ irradiation is very likely dominated by electron excitation process at first, and subsequently, by nuclear collision process.

Boundary conditions of the hydrocascade model and relativistic kinetic equations for finite domains

A detailed analysis of the coupled relativistic kinetic equations for two domains separated by a hypersurface having both space- and time-like parts is presented. Integrating the derived set of transport equations, we obtain the correct system of the hydro+cascade equations to model the relativistic nuclear collision process. Remarkably, the conservation laws on the boundary between domains conserve separately both the incoming and outgoing components of energy, momentum and baryonic charge. Thus, the relativistic kinetic theory generates twice the number of conservation laws compared to traditional hydrodynamics. Our analysis shows that these boundary conditions between domains, the three flux discontinuity, can be satisfied only by a special superposition of two cutoff distribution functions for the ``out'' domain. All these results are applied to the case of the phase transition between quark gluon plasma and hadronic matter. The possible consequences for an improved $\text{hydro}+\text{cascade}$ description of the relativistic nuclear collisions are discussed. The unique properties of the three flux discontinuity and their effect on the space-time evolution of the transverse expansion are also analyzed. The possible modifications of both transversal radii from pion correlations generated by a correct hydro+cascade approach are discussed.

Comparison of Secondary Ion Emissions from Carbon Nanotubesunder Bombardments of MeV Si and Si2 Clusters

The emission yields of H, H2, H3 and heavy ions from carbon nanotubesunder bombardments of Si and Si2 clusters in an energy range of0.3–3 MeV per atom are measured by using the time-of-flight technique(TOF). The emission yields of the secondary ions increase withincreasing energy of Si and the electronic stopping processes play animportant role. The enhanced emission yields of secondary ions inducedby Si2 clusters at the low energies are clearly seen and attributed tothe vicinage effect of the nuclear collision processes of clusterconstituents and the secondary ion emissions are still dominated byelectronic stopping processes at high energies.

Metastable defects in SI-GaAs: Effect of high energy ion-irradiation

The intrinsic deep donor level, known as EL2, is responsible for semi-insulating behavior of undoped GaAs. EL2 is the most extensively studied point defect in GaAs. The most important property related to EL2 is the metastability under sub-bandgap light exposure at low temperature. The microscopic origin of EL2 and its photo-induced metastability is still under debate. We present an experimental investigation on unirradiated and irradiated semi-insulating GaAs using thermally stimulated current spectroscopy and photo-current quenching. In addition to EL2, we have discovered some other metastable defects in semi-insulating GaAs. Li ions of 48 MeV were used to irradiate semi-insulating GaAs with fluences between 10 11 and 10 13 ions/cm 2. The energy loss of 48 MeV Li ions in GaAs is dominantly due to electronic processes because the energy loss due to nuclear collision processes is much less than the lattice binding energy of GaAs. Extended range of 48 MeV Li ions in GaAs has been used to modify the defect levels by electronic energy loss processes. Several modifications of defect levels, including the metastable levels in semi-insulating GaAs observed after irradiation, are discussed.

Carrier generation in highly oriented WO3 films by proton or helium implantation

Implantation of H+ or He+ ions into highly oriented WO3 films, which were deposited on sapphire R-plane substrates by the pulsed laser deposition method, was carried out to generate carrier electrons. X-ray diffraction measurements revealed that the films consisted of only the two types of grains that were epitaxially grown with their c axes perpendicular to the surface of the substrate. After implantation to a dose of 1×1017 cm−2, a remarkable increase in conductivity at 300 K was observed to 200 Scm−1 for H+ implantation or to 3 Scm−1 for He+ implantation. The efficiency of carrier generation was ∼60% for H+ implantation or ∼10−3% for He+ implantation. This striking difference demonstrates that the chemical effect of bronze formation is much more effective for carrier generation in WO3 than is the physical effect of oxygen vacancy formation by nuclear collision processes. The resulting conductivity of the oriented films was higher by 1–3 orders of magnitude than that of the corresponding polycrystalline film prepared by the rf sputtering method. Hall mobility in the oriented films (∼1 cm2 V−1 s−1) was as high as that (1–2 cm2 V−1 s−1) in single-crystal Na0.40WO3 or WO2.99 and was higher by several orders of magnitude than that in polycrystalline films. The results of Hall voltage and x-ray diffraction measurements led to the conclusion that the higher conductivity in the highly oriented films mainly originates from an increase in carrier mobility, which results from decreases in grain boundary effects as scattering centers.

Fragmentation in dissipative collisions: a computer model study

Qualitative aspects of multifragmentation in atomic, molecular or nuclear collision processes are studied by means of a simple chain model, i.e., the collision of two chains of particles is analyzed and the possible reaction channels are determined. It is shown that the introduction of dissipative in addition to conservative forces increases the spectrum of possible reactions considerably. It leads to a large variety of fragmentation channels as they are observed experimentally. In this way, dissipative forces may give rise in mechanical systems to structure formation processes which are absent if exclusively conservative forces act between the particles. Moreover, it is shown that reaction patterns obey quite different features depending on whether symmetric collisions (similar masses of target and colliding chain of particles) or asymmetric collisions are considered (for the same initial values of the energy per particle taking part in the collision process). With increasing symmetry, the variety of possible reaction channels increases, in general. In addition, by introducing and increasing the strength of repulsive long-range (Coulomb) forces, the variety of possible reaction channels increases further. The possible fragment size distributions are compared with results obtained by statistical (probabilistic) fragmentation approaches. As it seems, at least for the simple model considered here, the dynamics and not the statistical properties determine the outcome of the fragmentation process.

Ion irradiation effects on critical temperature of Bi2Sr2Ca2Cu3O10- and Bi2Sr2Ca1Cu2O8-

We have measured resistivity modification of Bi2Sr2Ca2Cu3O10- (Bi-2223) and Bi2Sr2Ca1Cu2O8- (Bi-2212) films H+, 100 keV He+ and 1 MeV O+ irradiation at normal incidence and at room temperature. It appears that degradation of critical temperature (Tc) follows the nuclear collision process or scales with the displacement per atom as in other high critical temperature superconductor (HTSC) oxides such as YBa2Cu3O7- (YBCO), assuming a displacement threshold energy of 20 eV. No significant difference of Tc degradation is observed between Bi-2223 and Bi-2212 phases. It is also found that Tc degradation of Bi-based HTSC oxide films is quite similar to that of Tl2Ca2Ba2Cu3O10- (Tl-2223), indicating that Bi-based and Tl-based HTSC oxides are more sensitive to ion irradiation than YBCO.

Dilepton production by bremsstrahlung of meson fields in nuclear collisions

We study the bremsstrahlung of virtual \ensuremath{\omega} mesons due to the collective deceleration of nuclei at the initial stage of an ultrarelativistic heavy-ion collision. It is shown that electromagnetic decays of these mesons may give an important contribution to the observed yields of dileptons. Mass spectra of ${e}^{+}{e}^{\ensuremath{-}}$ and ${\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}$ pairs produced in central $\mathrm{A}\mathrm{u}+\mathrm{A}\mathrm{u}$ collisions are calculated under some simplifying assumptions on the space-time variation of the baryonic current in a nuclear collision process. Comparison with the CERES data for $160A\mathrm{GeV}$ $\mathrm{P}\mathrm{b}+\mathrm{A}\mathrm{u}$ collisions shows that the proposed mechanism gives a noticeable fraction of the observed ${e}^{+}{e}^{\ensuremath{-}}$ pairs in the intermediate region of invariant masses. Sensitivity of the dilepton yield to the in-medium modification of masses and widths of vector mesons is demonstrated.

KeV ion irradiation of solid glycine: an EPR study

The radicals formed in keV ion irradiation of solid glycine were studied using electron parameter resonance (EPR). Spectra were observed at different temperature from 100 to 310 K after irradiation at room temperature. Different radicals are formed in different radiation conditions. The structure and concentration of the main radicals formed under 30 keV ion irradiation remain stable in different measurement temperature and radicals formed under 20 keV ion irradiation vary. The results indicate that more kinds of radicals are formed under keV ion irradiation which is attribute to the nuclear collision process. It is in agreement with previous study with mass spectrometer.

Repair by radiation itself or not? A study of threonine irradiated by keV ion

A saddle type dose-survival rate curve of threonine was confirmed in low energy ion irradiation. Mass spectrometry has been used as an analytical tool in determining the species and quantities of volatile reaction products generated during the decomposition of threonine by fast atom bombardment. The radiation chemistry was related to the radiation physics of fundamental interaction between the projectile and target molecule. The nuclear collision process was proposed as the main reason of the particular dose effects curve, based on the analysis of the interaction process between the projectile and target molecule. A new repair model was assumed.

Tracks of high energy heavy ions in solids

The observation of tracks of high energy heavy ions produced in some materials was made with electron microscope in order to clarify the governing factors and mechanisms for track formation. Irradiation experiments were performed with specified and well collimated ion beams of 100 MeV class obtained from the tandem accelerator. Target specimens were evaporated thin films of germanium, silicon and titanium, and crystals of MoO3 and MoS2. The present experiments showed that there were threshold values in the energy deposition of ion to specimens for the track formations and that the size of the tracks depended on the energies deposited into the specimens. The correlations between the sizes of the tracks and the energy depositions of incident ions were examined for the tracks produced in the evaporated films of germanium, and the results were interpreted in terms of a time dependent line source model of thermal spike. Even in such a high energy region, tracks produced through nuclear collision process were observed under some conditions.

Baryon-antibaryon pair production in time-dependent meson fields.

Strong mean meson fields, which are known to exist in normal nuclei, experience a violent deformation in the course of a heavy-ion collision at relativistic energies. This may give rise to a new collective mechanism of the particle production, not reducible to the superposition of elementary nucleon-nucleon collisions. We study the baryon-antibaryon (NN\ifmmode\bar\else\textasciimacron\fi{},\ensuremath{\Lambda}\ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{},...) pair production under some simplifying assumptions about the space and time variation of meson fields in the nuclear collision process. The mutual deceleration of nuclei is described schematically by introducing the time-dependent relative velocity. For comparison the yield of NN\ifmmode\bar\else\textasciimacron\fi{} pairs is also calculated within a convolution model assuming the same deceleration scenario. Due to the specific Lorentz structure, the vector meson field turns out to be more efficient for producing pairs than the scalar field. Within the perturbative approach we study two processes: the bremsstrahlung of a virtual meson and the fusion of two virtual mesons, both leading to the baryon-antibaryon final states. The calculated multiplicities of NN\ifmmode\bar\else\textasciimacron\fi{} and \ensuremath{\Lambda}\ensuremath{\Lambda}\ifmmode\bar\else\textasciimacron\fi{} pairs grow fast with the bombarding energy, reaching a saturation above the RHIC energy (\ensuremath{\surd}s=200A GeV). At lower energies the coherent production mechanism gives higher pair yields than predicted by microscopic cascade-like models. The rapidity spectra of antibaryons exhibit a characteristic two-hump structure which may serve as an observable signature of the bremsstrahlung mechanism. A strong sensitivity of the predicted yield to the baryon effective mass and the parameters characterizing the stopping power of nuclear matter is demonstrated.

Incorporation of quantum statistical features in molecular dynamics.

We formulate a method for incorporating quantum fluctuations into molecular- dynamics simulations of many-body systems, such as those employed for energetic nuclear collision processes. Based on Fermi's Golden Rule, we allow spontaneous transitions to occur between the wave packets which are not energy eigenstates. The ensuing diffusive evolution in the space of the wave packet parameters exhibits appealing physical properties, including relaxation towards quantum- statistical equilibrium.