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.     

Design of a multifoil electrolytically controlled etching cell

Irradiating polymer films with heavy ions and subsequent electrolytically controlled etching (ECE) produces nuclear track filters with desired pore dimensions and pore density. During ECE, an electric field with low field strength and a frequency in the audio range is applied across the irradiated polymer film, which is immersed in the etching solution. An ECE cell for the simultaneous etching of three irradiated samples has been designed and developed. The average pore diameter and break-through time for tracks due to ²³⁸U(11.4 and 14 MeV/u) and ²⁰⁹Bi(13 MeV/u) ions in polycarbonate and polyethylene terepthalate have been measured.     

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) ^*10³ MeVcm²/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.     

The pore opening process of etching polymer films irradiated by single and multiple heavy ions. The etching process in the range of the track core radius (10 nm)

The technique of electrochemical etching of irradiated polymer films is an useful method to investigate structures of the track cores. In the case of the investigation of multiple track foils, the mean effective radius corresponds to the average of all synchron etching pores. On the other hand, the etching cones of all tracks do not break through to conducting micro channels coincidentally. The statistical character of this pore opening (break through) process is still unexplained, although several effects concerning this topic have been observed in the past. Another computer program simulates by way of the Monte Carlo Method the etching process of an ensemble of tracks within a thin polymer film. The conductivity of the multiple track etching foil can be described by the convolution between the conductivity of a single pore and the time dependent breakthrough rate. By way of the Laplace Transforms the measurements of the multiple and single track etching polymer films can be deconvoluted and yield 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.     

Investigations of heavy ion tracks in polyethylene naphthalate films

The heavy ion beam (with fluence 3x10⁸ ion/cm²) from a cyclotron has been used for irradiation of thin polyethylene naphthalate (PEN) films. Latent tracks in these polymeric films have been sensitized by UV radiation and then chemically etched in NaOH solution. The etching process parameters have been controlled by the electroconductivity method. After etching, parameters of samples have been examined by SEM and bubble point methods (Coulter® Porometer II instrument). Results have shown good quality of PEN track membranes with pore sizes in the range: 0.1 – 0.5 μm. The described procedure is known for thin polythylene terephthalate (PET) films. Taking into consideration that PEN films have got better mechanical, thermal, gas barrier as well as better chemical resistance properties in comparison with PET films, the possibility of application of such membranes is much wider.     

Influence of intensive γ and electron radiation on tracks formation in the PM-355 detectors

Solid-state nuclear track detectors have found wide use in various domains of science and technology, e.g. in plasma experiments. The track detectors used in the fusion-oriented experiments encounter “harsh” conditions (intense X-ray and neutron radiation, heat impact, shock waves and plasma fluxes). Therefore, such detectors should be tested under similar conditions. This paper concerns influence of electron and γ radiation on the track formation in PM-355 detectors. Samples were irradiated with 1.5-MeV protons and ⁴He-ions, and with 5.5 MeV particles. After that they were exposed to γ and electron radiation. Irradiation doses were varied from 100 kGy up to 2000 kGy. The etching was performed in steps, and a bulk etching rate (V_B) was determined versus an absorbed dose of electron and gamma radiation.     

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.     

New results from exposures of CR-39 nuclear track detectors

Six stacks of nuclear track detectors were exposed in December 1994 to ²⁰⁷Pb⁸²⁺ ions of 158 A GeV at the CERN-SPS. Each stack was made of CR39 sheets placed after a target. The CR39 was manufactured by the Intercast Europe Co. of Parma (Italy). The exposures were carried out at normal incidence to an average density of 400 ions/cm². The main purpose of the exposures was the calibration of the CR39 nuclear track detector used in a large area detector to search for magnetic monopoles (experiment MACRO at the Gran Sasso Laboratory). We compare the response of CR39 of different production batches in different etching conditions. The results indicate no relevant aging effect for the CR39 made more than 5 years ago. The first result of a “long period” investigation of possible fading effects for the CR39 is also presented.     

Laser ablated bismuth cuprate thin films preparation and effect of oxygen nonstoichiometry upon superconductivity

Thin films of Bi₂Sr₂CaCu₂O₈ and (Bi, Pb)₂Sr₂Ca₂Cu₃O₁₀ have been prepared on monocrystalline (100) MgO substrates, using a laser ablation method with post annealing treatment. The influence of substrate temperature and oxygen pressure during deposition were investigated. SEM observations, EDS analysis, electric and magnetic measurements have been used to characterize the films. Superconducting “2212” films, with T_c(R = 0) at 80–83 K and J_c (50 K) up to 5 × 10⁵ A/cm², have been currently achieved, while Pb-doped “2223” films exhibit T_c as high as 110 K with J_c = 5 × 10⁴ A/cm² at 77 K. The effect of annealing at low temperature (350°C) in an argon flow has been studied for the 2212 phase, it shows the influence of the oxygen non-stoichiometry, i.e. of the hole carrier density upon T_c's which can be measured up to 89 K (zero resistance).     

The statistical process of pore growth in multi pore foils

Using the electrolytical etching method the breakthrough-times (i.e. the time when the two etched cones from both sides of the detector contact) and the resulting track etching rates v_t of heavy ion tracks in 8 μm polycarbonate Makrofol KG have been measured. The samples were irradiated at the GSI, Darmstadt (Germany) with gold ions and different fluences at a specific energy of 11.6 MeV/u. All foils were etched in 6 n NaOH at room temperature. Fluctuations of breakthrough-times of single pore foils were analysed. Also the breakthrough-time of multi-pore-foils were measured. The dependence of the mean breakthrough-time on the ion fluence is dicussed. This dependence will be explained by the fluctuations of the breakthrough-time of the pores.     

New detectors on the basis of copolymer of tetrafluoroethylene with ethylene (PTFE-E)

The registration properties of PTFE-E (F-40, Russia; Tefzel, DuPont, USA; Hostaflon ET, Germany) having exceptional chemical and thermal (about 300°C) resistance are considered. The polymer samples were irradiated with ions in a wide region of masses (from O up to Xe) and energies (from 1 to 6 MeV/amu). Alkaline KMnO₄ solutions at 100°C were used for etching. It is shown that the polymer registration properties depend on the hydrogen content in the polymer. For a copolymer with 2–3 wt% of hydrogen the registration threshold is about 8 MeVcm²/mg. The ion tracks were not found after chemical etching for a copolymer with 1 wt% of hydrogen. Track membranes were produced and examined.     

Effect of thermal annealing on pore density,pore size and pore homogeneity of polycarbonate NTFs

When an energetic and electrically charged beam of particles penetrates a nonconducting material, like crystal, polymer, etc., a permanent change in the nature of the chemical matrix along its path occurs giving rise to damaged sites known as latent tracks having diameter of the order of 10 nm. These tracks show an enhanced chemical reactivity as compared to the surrounding material; and, on etching, the damaged sites are enlarged forming nuclear track filters (NTFs). The NTFs so formed find applications in the fields as nuclear physics, geology, radiation dosimetry, material research, medicine and environmental studies. The pore density and pore size homogeneity are important traits of NTFs. These NTFs are also employed for the production of micro/nanostructures through template synthesis which finds various applications for the fabrication of sensors, devices and in the field of opto-electronics. The pore size of the NTF is generally not uniform due to various reasons like material defects in solid state nuclear track detectors (SSNTDs), nonuniform flux density of irradiating beam, variation in the angle of exposure during irradiation, temperature gradient in the etching solution, etc. The present work reports the development of NTFs and study of the effect of pre-etch thermal annealing of irradiated polycarbonate thin films (SSNTDs) on the density and homogeneity of etched pores in developed NTFs. NTFs were developed by irradiating Makrofol (KG) with heavy-ion beam at the UNILAC, Gesellschaft fur Schwerionenforschung (GSI), Darmstadt, Germany and then chemically etching of annealed and unannealed irradiated samples. The pore size, pore density and pore size homogeneity of the NTFs were compared for annealed and unannealed samples. The study indicates decrease in pore size, pore density and increase in homogeneity of pores with post-irradiation and pre-etch thermal annealing. The uniformity of the pores of NTF will lead to uniformity of micro/nanostructures fabricated by using the pores as templates. These uniform micro/nanostructures can act as excellent charge sprayers in the cold cathode emitters.     

Study of latent and etched tracks by a charged particle transmission technique

A recently suggested technique for non-destructive investigation of inhomogeneities in thin objects, which is based on the measurement of the energy spectra of charged particles transmitted through the object, is used for the study of thermal annealing of 10–20 μm thick polyethylene terephtalate, polypropylene and polycarbonate foils irradiated with 1–10 MeV/amu heavy ions. At elevated temperature a foil linear contraction is observed on pristine and irradiated material. Also the foil roughness increases with increasing temperature. On the same foils with etched pores 0.5–1.0 μm in diameter, the thermal annealing results in gradual closing of the pores up to about 30% of their initial diameter at the temperatures of 150–175°C. At higher temperatures the pore diameter increases and achieves its initial value.     

Study of the effect of swift heavy Ni ion irradiation on ruby single crystal by using the XANES and EXAFS techniques

We have made the XAFS measurements at the Cr–K-edge on natural Indian ruby single crystals (corundum) and its two irradiated samples with fluence 1×10¹² Ni⁶⁺ and 5×10¹² Ni⁶⁺ ions/cm². Irradiated samples show interesting changes in their physical appearance. XANES measurements show progressive decrease in Δ_oct value on increase of Ni fluence in irradiated samples. EXAFS measurements on these samples show decrease in Cr–O distance on increase of Ni fluence. Lowering of Δ_oct value is correlated with the increase of Cr–O distance.     

Observation of diffusion through ion irradiated Makrofol KG

The features of the latent track in polycarbonate can be analysed by measuring the diffusion constant under ultra high vacuum conditions. Stacks of 30 μm Makrofol KG foils were irradiated with uranium ions of 11.4 MeV/u at the GSI Darmstadt, Germany. We used different fluences from 3·10¹⁰ to 5·10¹¹ ions/cm². The diffusion constant was determined by the time-lag-method [1]. A quadrupole mass filter was used to observe the diffusion of the gas. We measured the diffusion of argon through different foils of each stack at room temperature. In all cases also unirradiated foils were measured. The dependence of the permeability and the diffusion constant on the ion fluence and the energy loss of the ions will be given and indications on the size of latent ion tracks concerning gas diffusion will be discussed.     

Nanostructural and photoluminescence features of nanoporous silicon prepared by anodic etching

The nanostructural and photoluminescence (PL) features of nanoporous Si (NPS) were investigated in terms of various process parameters such as current density, etching time and oxidation conditions. The NPS was prepared by electrochemical anodic etching of p-type (0 0 1) Si wafers of 4 Ω cm resistivity in HF solution. The pores are of polygon-type columns with 5, 6 and 7 side walls. The average diameter of the column-shaped pores is critically determined by the current density, while the etching time plays an important role on the pore depth; in particular, when the current densities of 30 and 100 mA/cm² were applied, the pore diameters were 9 nm and 3.3 μm, respectively. The variation in the PL characteristics of the NPS with oxidation condition and etching current density was measured and then related with their structural changes. The aging and thermal treatments produce oxidation and lattice distortion in the NPS. The degree of deviation from the as-prepared NPS during aging or thermal treatment seems to depend on the nanostructure as well as morphology of the NPS. It is found in this study that etching current density plays an important role on such structural features of the NPS.     

Proton tracks and the formation of pores in poly[diethylene glycol bis-(allyl carbonate)

Modern dosimetry needs efficient detectors for registering light ions, especially light ions having energies of up to 10 MeV/amu. That is why this research pays attention to the development of materials for such a task. In this work, a CR-39 detector, which is the most efficient detector, was used. It was irradiated with low-energy protons. Using sensitive electrolytic etching and electron scanning microscopy, a complete analysis was carried out of the process of the formation of a pore starting from its opening to the final stage of its formation. The process of sequential track breakthroughs was observed. The data obtained on the shape of the pore and the parameters of its formation allow simulation of the process of etching. The etch rates and sensitivity of etching are determined. The influence of energy losses on the geometry of the pore is considered.     

Effect of secondary electrons from latent tracks created in YBCO by swift heavy ion irradiation

Swift heavy ions (SHI) with electronic energy loss exceeding a value of 14.4 keVnm⁻¹ create amorphized latent tracks in YBCO type superconductors. In the low fluence regime of an ion beam where tracks do not overlap, a decrease of the superconducting transition temperature as probed through resistivity studies, is not expected due to availability of percolating current paths. The present study however shows T_c decrease by about 1–3 K in thin films of YBCO when irradiated by 250 MeVAg ions at 79 K at a fluence of 5×10¹⁰–1×10¹² ionscm⁻². The highest fluence used in the present study is three times less than the fluence where track overlapping becomes significant. The T_c tends to increase towards the preirradiation value on annealing the films at room temperature. To explain this unusual result, we consider the effect of ion irradiation in inducing materials modification not only through creation of amorphized latent tracks along the ion path, but also through creation of atomic disorder in the oxygen sublattice in the Cu–O chains of YBCO by the secondary electrons. These electrons are emitted radially from the tracks during the passage of the SHI. Considering the correlation between the charge state of copper and its oxygen coordination, we show in particular that the latter process is a consequence of the inelastic interaction of the SHI induced low-energy secondary electrons with the YBCO lattice, which result in chain oxygen disorder and T_c decrease.     

Thermal spike analysis of low energy ion tracks

The theoretical track diameter of low energy ions in organic materials is usually estimated through the model of dose deposition by delta rays, with results remarkably lower than the experimental values obtained via a replica method and electron microscopy. The track detector used here is Makrofol-E and the ions studied have specific energies between 1.4 and 100 keV/n. To evaluate the problem from another point of view, thermal effects for track formation, a modified version of the “liquid drop model” for insulators was applied. The electronic as well as nuclear energy deposition by an individual ion are considered and the thermal spike evolution is studied. The model allows for the formation of ion tracks in a range of energies previously considered as “forbidden”. There still exists a discrepancy between the experimental data and the track diameters predicted by the model, and although this difference is smaller than the obtained with previous calculations, it suggests the necessity of further adjustments.