K.K. Dwivedi

Fullerene formation in the tracks of energetic ions

Chromatographic analyses of solvent extracts from graphite bombarded with 130 MeV/amu 161 Dy 22+ ions, and polyimide (kapton) bombarded with 5 MeV 6 Li 1+ ions, give clear evidence for the in situ synthesis of buckminsterfullerene −C 60 . The role played by the production of highly confined, dense assemblies of energetic electrons in this phase change is discussed

The Emergence of New Ion Tract Applications

Abstract The recent years have brought a renaissance of interest in ion tracks, for the sake of novel applications. This paper summarizes some of the newly emerging possibilities, and the strategies that have been initiated. Only a few applications that are based on latent tracks have emerged since then, such as the exploitation of phase transitions, chemical changes, the enhanced free volume along latent tracks, or their capability to trap diffusing penetrants. For contrast, etched tracks in both polymer foils and SiO 2 layers appear to have a much greater application potential. Compact rods and tubules as well as dispersed nanosized matter can be embedded within the etched tracks to form the base of various applications. Some of them are summarized, and a few examples are described in detail.

Transport processes during the incubation time of ion track etching in polymers

Abstract The processes which take place in polymers before visible etch track formation sets in after exposure to alkaline solutions, i.e. during the so-called “incubation time”, are thought to consist of the penetration of the liquid into the polymer and the polymer’s subsequent glassy-to-rubbery phase transformation. We have studied systematically the first of these steps – the penetration process – by measuring the depth distributions of tracer ions dissolved in the penetrant solutions in different polymers as a function of pH value, temperature, and penetration time. Determination of the free volume of pristine and ion-irradiated polymers by positron annihilation spectroscopy served as an additional important source of information. Our findings show that the solutions’ penetration is often a highly complex process which eventually dominates the overall incubation time.

NEW WAYS OF POLYMERIC ION TRACK CHARACTERIZATION

Abstract New ways have been applied for characterization of ion tracks in polymers in the last few years, which are essentially related to depth profile determinations of ions, molecules, or positrons penetrating into these tracks. In combination with tomography, the first three-dimensional results have been obtained. Extensive diffusion simulations accompanying the measurements have enabled us to obtain a better understanding of the transport processes going on in ion tracks. This paper gives an overview about the range of new possibilities accessible by these techniques, and summarizes the presently obtained understanding of ion tracks in polymers.

Range and energy-loss of heavy ions by a nuclear track technique

Abstract A simple and fairly accurate nuclear track technique has been described to measure the range and energy-loss rate of any heavy ion in any elemental or complex media. The experimental values of range and energy loss of 16.34 MeV/u 238 U ions in nickel and tantalum have been measured by the track technique using CR-39 as track detector. The error in measurement varies from 5–10%. Measured ranges are compared with the corresponding theoretical values obtained from some commonly used theories and data tables.

On the penetration of aqueous solutions into pristine and radiation damaged polyimide

Abstract The penetration of aqueous solutions at different pH values into pristine and ion-irradiated commercial polyimide (PI, “Kapton”) foils was examined, by using Li ions as tracers to probe the corresponding penetrant depth distributions. These distributions were recorded for different polymer exposure times and temperatures. For comparison, examinations have also been made on the time dependence of the polyimide conductivity during water uptake. The results derived here for PI are compared with those of other polymers. For both pristine and low-fluence irradiated PI, an initially rapid solution penetration was observed up to some maximum penetrant uptake value. The penetrant depth distributions point at nanocapillaric penetration mechanisms in this initial stage. At longer exposure times of the polymer, the penetrant concentration diminishes transiently, suggesting that polymer swelling in the confined track zone leads to a reduction of the available free volume. In the subsequent stage of penetrant uptake this process is reversed again which we explain by crazing and overall swelling. Whereas thereafter saturation is reached for non-alkaline penetrants, alkaline solutions in irradiated PI show a pronounced continuous uptake, which is correlated with the ion track etching process. Whereas low-fluence ion irradiation of PI leads to a reduction in penetrant uptake compared with the pristine material, high-fluence irradiation results in a pronounced increase of the uptake capability.

On the origin of anomalous ranges of 1.65 GeV 132Xe ions in Muscovite Mica

Abstract Ranges of 1.65 GeV 132 Xe ions in muscovite mica, determined using solid state nuclear track detector (SSNTD) techniques, are shown to vary strongly with angle of incidence to the basal plane. This is explained within the framework of the Chadderton-Koul-Biersack (CKB) model for latent track formation in real crystals. Not only are there clear bursts of electronic energy loss at individual molecular layers, as seen in intermittent tracks in the electron microscope, but the range variation can also be ascribed to a basic refractive effect of individual molecular layers on the particle trajectories. This, in turn. provides a strong nonlinear electronic stopping. The strong uniaxial anisotropy of muscovite mica imposes a new orbital motion, referred to as “ridging” (or “bridging” in the upper angular limit) on the swift xenon ions. which are almost completely stripped of electrons. Neither “channelling” nor “quasichannelling“. which are determined by smaller critical angles, play any part in this specific aspect of anomalous range determination, which is entirely due to the smooth angular dependence of the “ridging” orbits on refracted pathways, and on the integrated stopping power sampled. The fact that incorporation of “ridging” and “bridging” into the broader picture now produces a complete angular set of classical positively charged particle orbits, encompassing all of real space in crystals. leads naturally to the construction of a new classical range ( R ) surface, quite analogous to the familiar Fermi surface of constant energy, described by conduction electron momentum vectors ( k ) in reciprocal space.

Energy-loss measurement of 132Xe in lead by a nuclear track technique

Abstract A simple nuclear track technique has been described to measure energy-loss rate of any heavy ion in any medium. Here we present the results obtained from our measurements of energy-loss rate of 17.0 MeV/u 132 Xe ions in lead using special zinc phosphate (ZnP) glass detector. The errors in measurement range from 5–10%. Experimental energy-loss data has been compared with calculated values. The significance of the results and scope of the track technique is discussed.

Calibration of CR-39 and ZnP-glass detectors with 350 MeV 90Zr ions

The response of sensitive detectors, CR-39 and ZnP-glass, for 350 MeV 90Zr ions, has been studied. Bulk and track-etch parameters have been evaluated by successive etching. Calibration of the two detectors has been done by correlating the measured track-etch rate (VT) with the residual range and total energy-loss rate. A linear correlation has been observed in both cases. Track registration threshold values of 4.45 MeV mg-1 cm2 for CR-39 and 12.50 MeV mg-1 cm2 for ZnP-glass have been obtained.

Effects of fullerene destruction

We discuss the destruction of fullerene, and we summarize at this occasion a number of peculiar and yet poorly understood effects which are related to fullerene destruction, such as fullerite sputtering, SiC formation, and enhanced etchability of ion tracks formed in presence of fullerite. We give a tentative explanation for these effects.