Nuclear track radiography of “Hot” aerosol particles

Nuclear track radiography was applied to identify aerosol “hot” particles which contain elements of nuclear fuel and fallout after Chernobyl NPP accident. For the determination of the content of transuranium elements in radioactive aerosols the measurement of the -activity of “hot” particles by SSNTD was used in this work, as well as radiography of fission fragments formed as a result of the reactions (n,f) and (γ,f) in the irradiation of aerosol filters by thermal neutrons and high energy gamma quanta. The technique allowed the sizes and alpha-activity of “hot” particles to be determined without extracting them from the filter, as well as the determination of the uranium content and its enrichment by ²³⁵U, ²³⁹Pu and ²⁴¹Pu isotopes. Sensitivity of determination of alpha activity by fission method is 5×10⁻⁶ Bq per particle. The software for the system of image analysis was created. It ensured the identification of track clusters on an optical image of the SSNTD surface obtained through a video camera and the determination of size and activity of “hot” particles.     

Watching at the correlation between the specific track-etch rate and the primary physical parameters of the swift ion interaction with the CR-39

Analysis of etched nuclear tracks in CR-39 solid state nuclear track detectors has recently been made possible by the confocal microscope. This recent innovation has opened up some particularly interesting perspectives. The main originality of this microscopy technique is that it makes it possible to work on numerical, three-dimensional images of chemically etched nuclear tracks in the CR-39. We have studied the performances of this new approach for light ions (H, He, Li and C) with kinetic energies typically of the order of the MeV per nucleus. First, we determined the response functions of the CR-39 for these ions. We were then able to show that aside from decreasing the analysis time of traditional methods, the confocal microscope greatly increases the sensibility of the detection. The aim of this study was to find a correlation between the response functions and the primary physical parameters of the interaction between the ion and the material, such as the linear energy transfer (LET), the restricted energy transfer (REL_ω0), the integrated radial dose or the rate of ionization. Although these parameters presented a strong resemblance to the experimental response functions (they are all “Bragg” curves), none of them were correlated to the response function in an unambiguous way.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.     

Nuclear tracks in solids: registration physics and the compound spike

Observations of GeV heavy ion and MeV cluster-ion tracks in crystalline solids give us new insight into registration physics. Thermal and ion explosion spikes no longer compete; a ‘compound spike’ accounts for both. Ion explosion dominates for surface tracks (electronic sputtering). And there can also be transient plasma stopping in the bulk. For clusters there are ‘vicinage effects’—both electronic and nuclear—which can influence track dimensions and structure. Displacement cascades in large energetic clusters may lead to projectile “fission” and coherent flow into sub-tracks. The absence of tracks in certain targets, and their size/structure in others, leads to a model of projectile assisted prompt anneal (PAPA) in 10⁻¹¹s, either partial or complete, often by swift epitaxy, on elemental lattices (e.g. silicon) or on compound sublattices (e.g. fluorite). Phase transformations are important, but simple target amorphization is rare—the exception, not the rule. For many targets the thermal spike (macroscopic) fails, since ‘point’ defects (atomistic) characteristic of the target, their motion, and the electronic band structure, determine latent track detail. Circumstances in which the Bragg Rule of Additivity fails completely are revealed, and the kinetic threshold for constructive phase transitions in tracks described. This same track physics applies generally also to geothermometry—the opposite time extremum (10⁺¹¹s)—where annealing is due to defect assisted delayed anneal (DADA). Differences between etching rates of induced and spontaneous fission tracks can be explained. The geothermobarometric “Wendt/Vidal effect” (2002)—combined pressure, temperature and stress (with time) influences on fission track annealing (in e.g. apatite)—is briefly discussed.     

The temperature of the track

The energy deposition of swift charged particles penetrating solids is accompanied by such processes as particle (electron, atom, ion, molecular ion, photon …) emission and/or a change of the solid along the particle track. The energy, velocity and mass distribution of such secondary particles obtained from thin solids (such as carbon, polyhydrocarbon, isolators and conductors) penetrated by projectiles (e.g. Ar 1.8 MeV) was measured quantitatively. A number of direct and indirect production mechanisms contribute to the internal source of electrons and secondary ions. The analysis of the ejected radicals give information on the emission processes, the temperature, the charge and the time scale of energy deposition near the surface of the solid which, eventually, become responsible for the track formation.     

Hollow particle tracks in AgCI-detectors

In selected Cd-doped monocristalline silverchloride detectors we observe developed tracks from particles at LET values > 300 keV/μm AgCl and charge state values > 9, without a visible central region: the “hollow track”. The microdensitometric lateral profile shows a density decrease near the track axis, up to a radial extension of some microns from highly charged particles as Fe or U-ions of several MeV/u; the tracks of fast U- or Fe-projectiles, moreover, represent an entanglement of delta-electron tracks only. A recent reanalysis of this “hollow track”-effect is suggested to be caused by recombination effects and structural distortions. They occure in areas of high delta-electron density; together with the characteristic process of track formation in crystalline AgCl/Cd they enable us to vizualise and to measure the radial range of distortion and to evaluate the threshold of the energy density above which the destruction occurs.     

On the reconstruction of the conduction electron spectrum in metal-oxide superconductors owing to the interaction with coherent atomic displacements

A “pseudospin ferromagnet” model is proposed which describes the interaction of conduction electrons with coherent atomic displacements in metal-oxide high-T_c superconductors (HTSC). Non-quasiparticle states (“pseudospinons”) play an important role in this model. They make an appreciable contribution to thermodynamic and transport properties (e.g., to the linear term in specific heat), although not contributing to the density of states at E_F at T=0. In the superconducting phase, the pseudospinons give rise to gapless superconductivity at finite temperatures. For certain model parameter relations, a new energy scale (“Kondo temperature”) may occur in the electron spectrum near E_F. Using the results obtained, experimental data on the thermopower, nuclear spin-lattice relaxation rate and other properties of HTSC are discussed.     

Search for massive rare particles with the MACRO track-etch detector at Gran Sasso

Searches for massive penetrating particles in the cosmic radiation have been performed with the MACRO nuclear track detector used as a “stand alone” detector. The complete experimental procedure is presented. In absence of candidates, updated estimates of the flux upper limits both for the CR-39 “stand alone” detector and for the global MACRO detector are presented for magnetic monopoles, nuclearites and charged Q-balls.     

Non-radiative transitions as Förster''s energy transfer to solvent vibrations

New experimental evidence is given to the approach suggested by the authors to non-radiative transitions in rare earth and transition metal ions as radiationless energy transfer from ion to ligand and solvent molecule vibrations. Using non-radiative transitions in Yb³⁺ as an example the participation of combinations of vibrations in the electronic energy dissipation was investigated. Dy³⁺ in solutions was used to show the role of anharmonicity in non-radiative transitions. It was proved that the semi-empirical calculations of non-radiative transition rates by Förster's formula gave also good results for Mn²⁺ ions luminescence which is an example of “strong” vibronic interaction. The suggested approach permits to explain the anomalous low probability of the ⁵D₁−⁵D₀ non-radiative transition in the Eu³⁺ ion.     

Itinerant-localized duality description of the spin fluctuations in the normal state of high-Tc cuprates. An explanation of neutron scattering experiments

On the basis of the itinerant-localized duality theory of spin fluctuations, the puzzling aspects of the neutron scattering experiments in the normal state of high-T_c cuprates are clarified from a global point of view. The dynamical spin structure factor exhibits two different aspects depending on the energy transfer ω. At lower energies, ω < ω_c, where ω_c is the fermion coherence energy, the spectrum is coherent so that the characteristic scales for wavevector and energy are temperature dependent, while at higher energies, ω > ω_c, the spectrum is incoherent so that those characteristic scales are temperature independent. The integrated weight of the coherent part of the spectrum exhibits the so-called “spin gap” behavior when the Fermi surface of the itinerant fermion is technically nested, even though there is no excitation gap in the spectrum at all.     

Kinetic model of thin film growth by vapor deposition

A phenomenological kinetic model is proposed for describing the production of a thin film containing two components, A and B, by chemical and physical vapor deposition. The film was created by the “site-to-site” deposition of components A and B. The equations for the densities of components A and B in the surface layers were formed, and analytical and numerical solutions were obtained. The model includes the probabilities of different elementary processes for the interaction of gas phase components (molecules, radicals, atoms and ions) with those of A and B on the film surface. The deposition and erosion rates, the surface and volume densities of components A and B and the relative volume of micro-cavities inside the film were calculated as a function of the probabilities for the elementary processes of gas (plasma)-surface interactions. The experimental characteristics of a-Si: H thin films prepared by SiH₄ plasma deposition and those of carbon nitride thin films deposited from r.f. — magnetron sputtering and ion beam-assisted processes are compared with model calculations.     

Some relations of the diffusion constant of latent tracks in polymers and the ion energy loss

Measurements of the diffusion constant and the permeability of ion irradiated Makrofol KG (polycarbonate) sheets have been done with Argon and Nitrogen as diffusion gas. The polymers were irradiated at the “Gesellschaft für Schwerionenforschung” (GSI, Germany) with Uranium and Gold ions with energies of about 10 MeV/nuc. The fluence varies from 3·10⁹ to 4·10¹¹ ions/cm². For the irradiation the material was put together in stacks with layers of 8 μm thickness. This allows to relate a definite amount of energy loss to each layer and to examine the dependence of the diffusion constant and the permeability on it. For comparison electron irradiated and pristine material (i.e. the untreated material) were also measured.

The general tendency shows an increase of the diffusion constant and the permeability with growing energy loss up to a certain limit. In case of the highest energy loss the material changes its appearance and the diffusion constant and the permeability values are remarkable lower than those of the pristine material. A first trail of explanation might give the three-zone-model presented in this paper.     

Track measurements of fast particle streams in pulsed discharge explosive plasma

This paper reports results of track registration for fast particles from the hot spot sparked inside a metal target. CR-39 track detectors placed in the ion pinhole camera, a magnetic analyzer, and Thomson-type mass-spectrometer in order were used to obtain images of the hot spot and to determine spectra of the fast particles. Using track detectors in the pinhole camera, we have obtained specific split images. The “average etch rate ratio versus track length” locus for track sample groups is depicted by means of squared diameters asymptotic method. Our analysis shows the majority of tracks belong to hydrogen nuclei. The track loci for the detectors from the pinhole or analyzers turn out to be split into two parallel streaks that have been interpreted as a proton–deuteron doublet. Data obtained using the Thomson analyzer clearly demonstrate the equal numbers of protons and deuterons for some parallel parabola sections.     

Modification of the Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surface by high-energy bismuth ions

This paper reports on an atomic-force microscopy study of the surface of α-Al₂O₃ single crystals irradiated by Bi ions with energies of 710, 557, 269, and 151 MeV. The shape of the radiation defects produced by single ions was established to depend on the ionization energy loss. The threshold ionization density above which the surface topography is observed to change lies in the 27–35 keV/nm interval. Possible mechanisms of defect formation in the thermal-spike model, namely, a phase transition and the creation of thermoelastic stresses in the high-energy ion track, are considered.     

Mean distance of closest approach of ions: Lithium salts in aqueous solutions

Numerical values for the mean distance of closest approach of ions, “a”, for lithium salts in aqueous solutions are presented and discussed. These values were obtained from both experimental activity and diffusion coefficients, and estimated by using different theoretical approaches.     

Some contributions from SSNTD towards nuclear science: from multifragmentation towards accelerator driven systems (ADS)

Three examples are chosen to show the importance of SSNTD as one of the essential tools in nuclear science:

(1) Multifragmentation into more than two heavy reaction products: Starting with the observation of three heavy reaction products in the interaction of relativistic protons or 414 MeV ⁴⁰Ar with actinides in the early 1960s, up to the observation of five heavy reaction products in the interaction of 2400 MeV ²³⁸U with uranium, SSNTD had a leading role in this research.

(2) In the search for superheavy elements (SHE: Z around 114): Many different techniques are used. However, SSNTD are exclusively decisive in the possible observation of SHE within the heavy element component of galactic cosmic rays.

(3) Accelerator driven systems: They are increasingly important in the discussion of energy producing nuclear power stations and in the corresponding ability to transmute long-lived poisonous radioactive materials (above all plutonium) into shorter lived fission fragments or stable nuclides. SSNTDs play an important role in the determination of the energy dependent neutron fluence in small volumes (≈cm³) or in the exact beam profile determinations of the primary proton beams.

This contribution ends with an outlook into possible future fields of physics research: With the advent of a new generation of relativistic heavy ion accelerators, such as the NUCLOTRON at the JINR in Dubna, Russia, and RHIC in Brookhaven in the United States, one can continue to study (and finally confirm or disprove) all phenomena mentioned already, as well as additional controversial phenomena, such as “enhanced nuclear cross-sections over short distances”, called colloquially “anomalons”. Again SSNDT can be used in at least a twofold manner as an important tool: (a) the enhanced neutron production with ¹²C ions or heavier ions in thick targets at energies above approximately 50 GeV and (b) the reduced “mean-free-path” of secondary fragments produced by the same heavy and energetic ions.     

Energy-dependent Lorentz covariant representation of the NN interaction

A phenomenological Lorentz covariant representation of the NN t-matrix has been obtained with parameters that are explicitly energy dependent. The real and imaginary parts of the t-matrix are separately expanded in a complete set of on-shell Lorentz invariant amplitudes, each of which is represented by the exchange of isoscalar and isovector “mesons” modified by form factors. Direct and exchange contributions are handled explicitly in the model. The model parameters are fit to the summer 1994 Arndt amplitudes over the laboratory kinetic energy range from 200 to 500 MeV. In contrast to the earlier work of Horowitz, a single χ² minimization is carried out over the full energy range considered. Two separate fits are obtained, one with energy dependence confined to the coupling strengths, the other with energy dependence in the form factor masses as well. Although the second fit is somewhat better, both fits reproduce the empirical amplitudes reasonably well. Results are also given for various np and pp observables and compared with the data.     

Dynamic observations of the formation of thin Cu layers on clean and hydrogen-terminated Si(1 1 1) surfaces

The growth of Cu on the clean and hydrogen-terminated Si(1 1 1) surfaces is studied in situ by low-energy electron microscopy (LEEM). The dependence of the growth of the “5×5” layer on the clean Si(1 1 1) 7×7 surface upon the deposition temperature is investigated by combining LEEM with LEED. After completion of the “5×5” layer not only the regular-shaped three-dimensional islands reported before are observed but also irregular shaped more two-dimensional islands. On the hydrogen-terminated Si(1 1 1) surface the formation of the “5×5” structure is suppressed and nano-scale islands form preferentially at the step edges and domain boundaries. This is attributed to the enhancement of the surface migration of Cu atoms by the elimination of the surface dangling bonds.