Track Core Research Articles

Changes in magnetic properties induced by heavy-ion irradiation in magnetic insulators as seen by Mössbauer spectroscopy

The high-energy heavy-ion irradiation of magnetic materials induces not only amorphous matter in the ion tracks but also improvement or even creation of new magnetic properties in a stressed shell surrounding the track core. Two examples are presented in this paper: magnetization creation in the spinel ZnFe2O4 after Kr, Xe and Pb irradiations and trapping of magnetic domains in another spinel, the magnetite Fe3O4, after lead irradiation.

Line shifts in mass spectra of secondary ions ejected from solids by MeV-ion impact

Emission of hydrogen ions has been used to investigate the charging of the nuclear track being produced by fission fragments in organic solids. The question of interest is, if in insulating material the lifetime of ionisation along the nuclear track is sufficiently long to allow atomic motion driven by Coulomb repulsion of neighbouring ionized atoms. It is shown in this article that the hydrogen ions experience the potential produced by the ionized track core, but weaker than Auger electrons (see Schiwietz et al., Phys. Rev. Lett. 69 (1992) 628). In order to determine the mean initial energy of emission, we measured the time-of-flight distributions of secondary ions by means of a linear TOF instrument being equipped with 2 acceleration grids and converted them into axial energy distributions. Monte Carlo simulations were used to simulate the time distributions and to estimate the mean charge along the nuclear track causing the observed line shifts.

Interpretation of Mutation Induction by Accelerated Heavy Ions in Bacteria

In this report, a quantitative interpretation of mutation induction cross sections by heavy charged particles in bacterial cells is presented. The approach is based on the calculation of the fraction of energy deposited by indirect hits in the sensitive structure. In these events the particle does not pass through the sensitive volume, but this region is hit by delta rays. Four track structure models, developed by Katz (in Quantitative Mathematical Models in Radiation Biology, pp. 57-83, Springer-Verlag, 1988). Chatterjee et al. (Radiat. Res. 54, 479-494, 1973), Kiefer and Straaten (Phys. Med. Biol. 31, 1201-1209, 1982) and Kudryashov et al. (Proceedings of the First Soviet Congress on Microdosimetry, Atomizdat, Moscow, 1973), respectively, were used for the calculations. With the latter two models, very good agreement of the calculations with experimental results on mutagenesis in bacteria was obtained. Depending on the linear energy transfer (LET infinity) of the particles, two different modes of mutagenic action of heavy ions are distinguished: "delta-ray mutagenesis," which is related to those radiation qualities that preferentially kill the cells in direct hits (LET infinity > or = 100 keV/microns), and "track core mutagenesis," which arises from direct hits and is observed for lighter ions or ions with high energy (LET infinity < or = 100 keV/microns).

The pore opening process and its influence on conductivity measurements of irradiated polymer films by application of the electrochemical etching method

The technique of the electrochemical etching of irradiated polymer films is an useful method to investigate structures of the track cores. The transversale etching rate as a function of the radius, derived from the time-dependence of the radius of the etching pore, can be interpreted as a corresponding magnitude of the degree of the damage within the track core. In the case of polymer films irradiated by a high track density, the influence of the time distribution of the breakthroughs (pore opening process) of the etching microchannels on the conductivity changes during the etching process is to be discussed. First represented results lead to statements about the statistical nature of the pore opening process.

High sensitive electrolytical conductivity measurements of irradiated polymer films of 1μm thickness measured by a modified experimental set-up

The interaction of heavy ions with solids produces a narrow radial core of primary damage. The actual nature of the damage and the mechanism of its formation is not yet fully understood. The technique of the electrochemical etching of irradiated samples is an useful method to investigate these structures of the track cores. The transversale etching rate as a function of the radius, derived from the time-dependence of the radius of the etching pore, can be interpreted as the corresponding magnitude of the degree of the damage within the track core. To carry through measurements of smallest radii of etching pores, their very high electric resistance respectively, a sophisticated experimental set-up is required. An advanced set-up is performed, which enables, by application of a combined d.c.-a.c.-method, to measure the resistance of the sample during the whole etching process, associated with the origin of pore and its growth up to radii of 10 nm and more.

Energy loss dependent transversal etching rates of heavy ion tracks in plastic

By the method of electrolytical etching track etching rates V t and corresponding transversal track etching rates V trans of single heavy ion tracks in thin Makrofol KG foils have been measured at ion energies from 10–480 MeV/u. Makrofol KG foils of 8 μm thickness were irradiated perpendicular to the surface with 79Au and 54Xe ions at specific energies with energy loss values of REL=(10–90) ∗10 3 MeVcm 2/ g at GSI Darmstadt, Germany, and Lawrence Berkeley Lab., Cal., USA. Using the electrolytical etching method by measuring the resistance of the foil during the etching process (etching conditions: 6n NaOH, room temperature and controlled 50° C) the breakthrough time and track etching rates V t, V trans and V m (bulk etching rate) were analysed. Response curves (V t/V m)-1 as a function of Restricted Energy Loss (REL), the maximum extension of the ion induced damage perpendicular to the ion path and the dimension of the ion track core depending on the deposited energy can be estimated.

Survey of Cellular Radiosensitivity Parameters

A model of the formation of particle tracks in emulsion has been extended through the use of biological target theory to formulate a theory of the response of biological cells and molecules of biological importance to irradiation with energetic heavy ions. For this purpose the response to gamma rays is represented by the single-hit, multitarget model with parameters m and D0, while additional parameters kappa (or a0) and sigma 0 are required to represent the size of internal cellular targets and the effective cross-sectional area of the cell nucleus, respectively, for heavy-ion bombardments. For one-or-more-hit detectors, only the first three of these parameters are required and m = 1. For cells m is typically 2 or more. The model is developed from the concept that response to secondary electrons follows the same functional form for gamma rays and for the gamma rays surrounding an ion's path. Originally applied to dry enzymes and viruses in 1967, the model of the one-hit detector has been extended to emulsions, to other physical and chemical detectors, to single- and double-strand breaks in DNA in EO buffer and to three E. coli strains. The two-hit response has been observed for "track core" effects in radiation chemistry, for supralinearity in thermoluminescent dosimeters and for desensitized nuclear emulsions, where hit numbers up to 6 have been observed. In its extension to biological cells, additional concepts are required relating to the character of the track, namely the grain-count and track-width regimes, and to the ability of multitarget systems to acquire damage from intertrack delta rays (called gamma kill) as well as from intratrack delta rays (called ion kill). The model has been applied to some 40 sets of radiobiological data obtained from gamma, track-segment heavy-ion and neutron irradiations. Here we elaborate on the meaning of these concepts, tabulate the cellular parameters, and display their systematic behavior and the relationships among them. In particular the parameter kappa, which serves to determine the location in Z*2/beta 2 of the maximum value of the RBE, shows little variation among cell types, while D0, which describes the response to gamma rays and sigma 0, which appears to indicate the target size, varies over many orders of magnitude.

Radially restricted linear energy transfer for high-energy protons: a new analytical approach.

Radially restricted linear energy transfer (LET) is a basic physical parameter relevant to radiation biology and radiation protection. In this report a convenient method is presented for the analytical computation of this quantity without the need for complicated simulation. The method uses the energy-restricted LET L delta, as recently redefined in a 1993 ICRU draft document and supplements it by a relatively simple term that represents the energy of fast delta rays lost within distance r from the track core. The method provides a better fit than other models and is valid over the entire range of radial distance from track center to the maximum radial distance traveled by the most energetic secondary electrons. Lr computed by this approach differs only a few percent from the values obtained from explicit Monte Carlo simulations. The concept applies equally to heavy ions and to electrons.

On the lithium uptake of MeV ion irradiated polymer foils from a LiCI solution

Abstract Commercial polyimide (PI, ‘kapton') and polycarbonate (PC, ‘makrofol') foils have been irradiated with 135 MeV Ar5+ and 340 MeV Xe7+ ions, respectively, and subsequently boiled in a 5 mole/ltr LiCI solution for different times. The depth distributions of Li diffused into the foils suggest that the doping proceeds via regular diffusion, into both unirradiated bulk and into tracks. Low fluence irradiations appear to lead to a compaction or densification of the polymer matrix. One consequence of this is a decreasing Li uptake with increasing fluence. For higher ion fluences the Li uptakeproportional to track densitybecomes dominant. It is possible to define, to first order, a mean ‘effective track radius’ for the average amount of lithium taken up per track, and this can be shown to be of the order of only an Å. The diffusional behaviour of lithium suggests that tracks in 340 MeV Xe ion irradiated polyimide act as open micropores for diffusing dopants. Conversely, in 135 MeV Ar irradiated PC, the dopant may only access the primary track core by passage through nearby undisturbed polymer target. A characteristic surface effect is the eventual appearance of crater-like profiles associated with the cores of ion tracks at projectile points of entry. These may be partly due to reverse ‘blow-out’ (backwards sputtering) in plumes of dissociated polymeric matter.

Principle of light ions micromapping and dosimetry using a CR-39 polymeric detector: Modelized and experimental uncertainties

In this paper we propose a model for light ion track etching in CR-39. This model is rigorously applicable to the case of light ions for which the mean latent track core diameter can be considered small compared with one micron. The relations describing the opening of an etch-pit, which can be simulated on a computer screen, are given their mathematical derivation. The comparison of simulated tracks with experimental tracks has presented good qualitative agreement. It is thus possible to use the theoretical model in order to evaluate the capabilities of CR-39 SSNTD in the field of micromapping or autoradiography. This simulation showed that a single measurement of the minor and major axis of a given etched track, repeated for several etching steps, is sufficient for calculating both energy and incidence angle of the particle. The case of reverse tracks (particle generated in the detector material (n, p) conversion…) is not treated in this paper. The study of critical angle will be treated in another paper. An evaluation of the uncertainty associated with this method is necessary for interpretation of the experimental measurements. We find that both experimental and simulated data contribute to the uncertainties of the used method. In particular, the necessary use of the optical microscopy is one of the most important sources of uncertainty if we consider that CR-39 SSNTD is an ideal detector (isotropic physical and chemical properties of the bulk material). Considering the mean track diameter distributions and standard deviations for normal incident ions of calibrated energy, such an assumption is realistic at the scale of the micrometer. On the other hand, the non-linear behaviour of the geometrical parameters (diameter, minor/major axis, area,…) which were obtained as a function of energy impose the variation of uncertainty with energy. Moreover, this uncertainty also varies with a sequential etching. This can be seen on the experimental plots (energy, diameter) for normal incident alphas or protons. Finally, the average uncertainty for micromapping and autoradiography using CR-39 SSNTD depends principally on the optical measurement device and on the range/energy relation, which is experimentally determined using our method.

Principle of light ions micromapping and dosimetry using a CR-39 polymeric detector: Modelized and experimental uncertainties

In this paper we propose a model for light ion track etching in CR-39. This model is rigorously applicable to the case of light ions for which the mean latent track core diameter can be considered small compared with one micron. The relations describing the opening of an etch-pit, which can be simulated on a computer screen, are given their mathematical derivation. The comparison of simulated tracks with experimental tracks has presented good qualitative agreement. It is thus possible to use the theoretical model in order to evaluate the capabilities of CR-39 SSNTD in the field of micromapping or autoradiography. This simulation showed that a single measurement of the minor and major axis of a given etched track, repeated for several etching steps, is sufficient for calculating both energy and incidence angle of the particle. The case of reverse tracks (particle generated in the detector material ( n, p) conversion…) is not treated in this paper. The study of critical angle will be treated in another paper. An evaluation of the uncertainty associated with this method is necessary for interpretation of the experimental measurements. We find that both experimental and simulated data contribute to the uncertainties of the used method. In particular, the necessary use of the optical microscopy is one of the most important sources of uncertainty if we consider that CR-39 SSNTD is an ideal detector (isotropic physical and chemical properties of the bulk material). Considering the mean track diameter distributions and standard deviations for normal incident ions of calibrated energy, such an assumption is realistic at the scale of the micrometer. On the other hand, the non-linear behaviour of the geometrical parameters (diameter, minor/major axis, area,…) which were obtained as a function of energy impose the variation of uncertainty with energy. Moreover, this uncertainty also varies with a sequential etching. This can be seen on the experimental plots (energy, diameter) for normal incident alphas or protons. Finally, the average uncertainty for micromapping and autoradiography using CR-39 SSNTD depends principally on the optical measurement device and on the range/energy relation, which is experimentally determined using our method.

Scintillation response of liquid argon for 3–20 MeV He ions

The scintillation yields from liquid Ar produced by 3–20 MeV He ions have been measured. The yields per unit energy deposited are observed to be about 10% smaller for 15–20 MeV He ions than with 5.305 MeV α particles. Scintillation yields are found to initially increase in the presence of external electric fields (≲ 6kV/cm) by 4–15% with no concurrent decrease in charge yields. This increase in scintillation may be attributed to a decrease in quenching processes in the track core of He ions.

Assessment of overetching factor for confined fission-track length measurement in zircon

The effect of prolonged etching provides a fundamental bias in observed confined track lengths in minerals, particularly those having highly anisotropic etching characteristics, such as zircon. In this paper, the anisotropic prolonged-etching factor around the ends of horizontal confined tracks (HCT's), which are practically used for track length analysis, was assessed quantitatively by step-etch of spontaneous tracks in zircon. The observed etching characteristics are two-fold: (1) the track etch rate for the direction perpendicular to the crystallographic c-axis is significantly smaller than that parallel to it: and (2) for a particular crystallographic direction, the growth rate of track length is consistently greater than the bulk etch rate, measured as the growth rate of track width for the direction perpendicular to that of interest. To further constrain the etching characteristics of HCT's, the shape of their ends were compared and contrasted with that of track etch pits on a prismatic surface. In contrast to the diamond shape of surface tracks, the ends of HCT's are generally convex outward with the core of etched track longest, regardless of track orientation. This suggests that at the ends of HCT's, the core of track has a growth rate greater than the surrounding bulk region, probably with decreasing rate outward. Hence the traditional model of track etching, dividing a latent track into the “etchable” and “unetchable” ranges, should be refined: etch rate of a latent track decreases rather continuously toward both ends. This model is concordant with the ionization pattern in a mineral detector produced by nuclear fission, and explains reasonably the experimental results. Because the correction of such a complicated overetching factor for an individual sample is extremely laborious for routine analysis, it is recommended that HCT's nearly perpendicular to the crystallographic c-axis (e.g., ⩾ 70°) should be used for track length measurement. Nevertheless, the reduction in number of tracks to be measured may be rather small since HCT's with smaller angles to the c-axis are less detectable due to the etching anisotropy.

Effect of an electric field on luminescence quenching in liquid argon.

Scintillation yields in liquid argon are found to increase in the presence of external electric fields (\ensuremath{\le}6 kV/cm) by 4%, 9%, 12%, and 15% for He ions with energies of 5.5, 10, 15, and 20 MeV, respectively. Comparative studies with \ensuremath{\alpha} particles of 5.305 MeV found very little effect on the magnitude of the scintillation increase with orientation of the incident particle. The increase in scintillation was not associated with an observable decrease in charge collected and it may be due to a decrease of quenching in the track core of the incident particle.

Electron-microscopic investigation of 132Xe ion tracks in a single crystal volume of gypsum

A surface cleavage in an irradiated single crystal of gypsum, crossed by volume tracks of 132 Xe ions in the cleaved plane, has been investigated by transmission electron-microscopy (TEM). It has been found that the most defective region (the core) of a volume track is a hollow channel of asymmetric cross-section found along almost the entire ion range.

Fast heavy ion induced desorption of insulators

During the last three years the research of fast heavy ion induced desorption was focussed again on secondary ion emission from organic material. New experiments on total sputter yields have not been performed. The measurements with ordered surface structures — i.e. Langmuir-Blodgett films — gave further evidence for the formation of craters by the impacting particle. This picture is supported by molecular-dynamics calculations. Concerning energy and angular distributions of secondary ions a particular progress was achieved by means of position sensitive stop detectors installed in time-of-flight spectrometers. Corresponding results show that desorption from organic samples occurs via an expansion mechanism in the solid, but also via a gas flow out off the track crater. Comparing secondary ion yields with other radiation effects in bioorganic material, in the high energy regime electronic sputtering is obviously a track core effect.

Atomic-force-microscopy observations of tracks induced by swift Kr ions in mica.

For the first time, latent tracks induced by swift Kr ions have been directly observed in mica. These tracks are imaged by atomic-force microscopy as hollows which are associated with softer areas in the mica surface. The track core is formed by disordered mica. The mean diameter of the observed hollows increases with the electronic stopping power of the ions. The track shape along the ion path is deduced from the analysis of both the number of the tracks per unit area and their diameter distribution. These observations are the first images of nanometric changes of elastic properties.

Effects of high-energy heavy ion bombardment on the electrical conductivity of yttrium iron garnet

Epilayers of yttrium iron garnet Y 3Fe 5O 12 on a (111)-Gd 3Ga 5O 12 substrate were irradiated with 50 MeV 32S, 50 MeV 63Cu a 235 MeV 84Kr ion beams. Film thicknesses were always smaller than the mean projected ranges of the incoming ions to avoid implantation effects in the layers. Dc-resistivity and saturation magnetization of ion-bombarded films were then measured at room temperature as a function of ion fluence ranging between 10 11 and 10 16 ions cm −2. Resistivity was found to decrease when fluence is increased above a threshold ( ≤10 13 ions cm −2). The threshold fluence increases as the ion energy loss is reduced. 84Kr resistivity data level off above 3 × 10 12 ions cm −2 after a two-order-of-magnitude decrease, whereas 63Cu and 32S data decrease by seven orders of magnitude according to a nearly parabolic law versus fluence without any saturation up to 10 16 ions cm −2. The saturation of damage observed with 84Kr bombardment is interpreted by the overlap of paramagnetic disordered track cores, while in the case of 63Cu and 32S bombardment, such a picture is not valid. These two different behaviours might be interpreted by a change in the morphology of the extended defects produced by electronic excitations above a threshold of electronic stopping power.

Proton and alpha track profiles in CR39 during etching and their implications on track etching models

Abstract A calculation expressing a bijective relation between a track's opening geometric parameter, the incidence angle and the energy of the charged projectile is required to perform an alpha cartography and/or a proton dosimetry. Track profiles during chemical etching must be studied before carrying out such a calculation. An experimental technique which is easily set up, displaying microphotographs of proton and alpha track profiles is presented. We also present a simulation of the track core evolution. This simulation takes into account the etching conditions, incidence angle and energy of the charged projectile. Experimental profiles enable us to deduce the value of the parameters appearing in the expression of Vt, track etch velocity, along the damaged trail. The same simulation model, applied to overetched tracks, gives the etchable range or the time needed to develop the damaged zones.

Conductometric studies of multiply charged ion track structure in various polymers

Abstract The radial structure of the etchable tracks of multiply charged ions in polethylene terephthalate, polycarbonate, polyarylate and polypropylene is investigated using the conductometric method. It is found that the track of a highly ionizing particle in a polymer consists of a core in which intensive destruction occurs and a halo where the cross linking of macromolecules may predominate. A hypothesis is proposed that the cross linking of a polymer in the track halo is conditioned by the effect of the hydrogen ions and atoms which are generated in the track core due to the ion explosion spike mechanism and diffuse to the surrounding space.