J. Kvítek

Neutron depth profiling by large angle coincidence spectrometry

Abstract A measuring device for large angle coincidence neutron depth profiling was installed behind a 5.6 m long neutron guide. Two large area PIN diodes placed at a distance of 2 mm from the sample serve for the detection of particles emitted at different angles. Coincidence event registration and sorting was accomplished using a PC AT computer. A simple but effective technique for two-dimensional data evaluation is proposed. The feasibility of the method was tested on various standards. The detection limits and the depth resolution are determined and possible refinements of the technique are discussed.

Structure and properties of polymers modified by ion implantation

Abstract Thin films of oriented polyethylene (PE), polypropylene (PP), polytetrafluoroethylene (PTFE) and polyamide (PA6) were modified by the implantation of 150 keV Sb+ ions to the fluences 1012–1015 cm−2. The Sb depth profiles were determined by a standard Rutherford back-scattering technique and the range parameters were found to differ from the theoretical TRIM estimate. The thickness of the ion beam modified surface layer is about 150 nm for all polymers examined. Production of conjugate double bonds and polymer surface oxidation were proved by means of i.r. and u.v. spectroscopies. Surface polarity, obtained by goniometric measurements, is an increasing function of the implanted dose with only the exception of PA6 where a polarity decrease is observed. The measured temperature dependence of the sheet resistivity for PE, PP and PA6 do not correspond to the well known variable range hopping mechanism.

Oxidation of polyethylene implanted with As ions to different extents

Polyethylene (PE) samples implanted with 150 keV As+ ions to the extents of 5 × 1013−1 × 1015 cm−2 were studied using a standard RBS (Rutherford Back Scattering) technique. The measured projected range of As+ ions (Rp = 120 ± 20 nm) is significantly lower than theoretical range of 170 nm calculated using TRIM code for pristine PE, the difference probably being due to structural changes in PE resulting from the ion implantation. The measured range straggling ΔRP = 60 ± 10 nm also exceeds the theoretical TRIM value of 33 nm. The profile broadening might be due to the diffusion of the As atoms through the damaged surface layer of PE. Noticeable oxidation of the implanted samples was also observed, with most of the oxygen atoms trapped in the region of maximum radiation defects. The total oxygen content in the sample surface layer and its depth concentration profile depend on the extent of implanted ions. For the highest implanted dose of 1 × 1015 cm−2, the oxygen depth profile exhibits two distinct concentration maxima, the first on the sample surface and the second at a depth close to the expected range of 150 keV As+ ions.

Anomalous diffusion of Pb atoms into polyethylene implanted with F+and As+ ions to different doses

Abstract The diffusion of Pb atoms from a 0.5M aqueous solution of lead acetate into polyethylene (PE) implanted with 150 keV F + and As + ions to different doses was studied at room and boiling temperatures. The standard Rutherford backscattering (RBS) technique was used for the determination of concentration depth profiles of incorporated atoms. Pb diffusion in the pristine polymer proceeds according to the common Fickian law with diffusion coefficients of 1.5 × 10 −16 cm 2 s −1 and D = 5.3 × 10 −14 cm 2 s −1 at room and boiling temperatures, respectively. Anomalous diffusion of Pb atoms was observed into the PE implanted to ion doses below 5 × 10 14 cm −2 . In these cases a pronounced diffusion-capturing mechanism takes place with the Pb atoms being captured on trapping centres produced by electronic energy losses of implanted ions. For implanted doses above 5 × 10 14 cm −2 , nearly Fickian diffusion is again observed with diffusion coefficients close to those of pristine material. Strongly reduced Pb incorporation in specimens implanted to higher doses is explained by structural changes of the polymer surface layer.

Modifications of polypropylene induced by the implantation of iodine ions

The polypropylene (PP) samples implanted with I+ ions at the implantation energies of 50, 100 and 150 keV to the fluence of 1×1015 cm-2 were studied using standard RBS (Rutherford backscattering) technique. The surface of the implanted PP samples is heavily damaged and enriched in carbon. The measured projected ranges of iodine atoms are by 10 to 30% lower than the theoretical ones calculated using Monte-Carlo simulation code TRIM (transport of ions in matter) code for pristine PP. The differences may be due to structural changes of polymer during the ion implantation. The range stragglings exceed the theoretical TRIM estimations about twice. The profile broadening is probably due to the diffusion of the iodine atoms through the damaged PP surface layer. In repeated RBS measurements, the iodine diffusion was confirmed with diffusion coefficient of 2.7×10-18 cm2s-1. Noticeable oxidation of the implanted samples was observed, with majority of the oxygen atoms being trapped in the region of maximum radiation defects. The bulk oxygen concentration profile resembles that of iodine but it is shifted inward by 200-350A. The relative oxygen content in the bulk is an increasing function of the implantation energy and it exceeds considerably the stoichiometric value for common iodine oxides.

Statistical and systematical errors in RBS depth profiling of implanted atoms

Simple expressions for estimation of statistical uncertainties in depth-profile parameters determined using standard RBS technique are derived. Possible sources of systematic uncertainties are also discussed along with some simple procedures for their elimination. For the simple case of RBS depth profiling of heavy atoms implanted into light matrices at low implantation energies statistical uncertainties as low as < 1% (Rp), < 5% (ΔRp), < 20% (profile skewness) and < 10% (profile kurtosis) may be achieved providing the measuring statistics is sufficiently high.

PIXE-INP computer code for PIXE analyses of thin and thick samples

Abstract The Pascal 6.0 program package PIXE-INP was developed at Institute of Nuclear Physics (Academy of Sciences of Czech Republic) for quantitative evaluation of PIXE spectra from thin and thick samples. The K-, L- and M-sensitivities are calculated using semi-empirical and theoretical ionization cross sections, tabulated relative X-ray emission probabilities, Coster-Kronig and fluorescence yields. The X-ray detector efficiency is determined by measuring a set of thin standards with known element content. The package is supplemented with the data bases on X-ray attenuation coefficients and proton stopping powers which are needed for the calculation of thick sample matrix corrections. The X-ray spectrum evaluation comprises an iterative background removal and non-linear least squares fitting of X-ray multiplets with Gaussian peak shape. The escape and summation peaks are taken into account. The performance of the program package is documented on measurements of reference materials.

RBS study of oxidation processes in polypropylene and polyethylene implanted with fluorine ions

Abstract The oxidation on polypropylene (PP) and polyethylene (PE) samples implanted with 50–200 keV F+ ions to the ion fluences from 1 × 1011–1 ×1015 cm−2 was studied using standard Rutherford backscattering (RBS) technique. The implanted samples were stored in the air and in darkness, and the oxidation process was followed by repeated RBS measurements in the time interval 1–150 days after the ion implantation. In PP, the oxidation, initially limited to the radiation damaged surface layer, proceeds steadily inward and a homogeneous oxidized layer is built up. The well defined sharp interface between the oxidized layer and the bulk PP moves inside with a mean velocity of 4.0 × 10−5 nms−1. The ion implanted PE samples are homogeneously oxidized from the sample surface up to the depths accessible to RBS technique. The surface ratio of oxygen and carbon atomic densities in ion implanted PP and PE samples increases from about 0.06 for the ion fluences below 1 × 1014 cm−2 to about 0.17 for higher ones.

STUDY OF POLYCARBONATE DEGRADATION INDUCED BY IRRADIATION WITH HE+ IONS

A stack of five polycarbonate foils, each 1.4 μm thick, was irradiated with 1.3 MeV 4 He + ions to the dose of 1.1 × 10 14 cm −2 . Ion beam induced polymer degradation, as a function of the particle energy, was studied by UV-VIS and IR spectroscopy of individual foils. In the irradiated foils, a significant reduction of characteristic absorption bands is observed, indicating polymer degradation. Significant increase of the surface polarity, characterized by a polar component of the surface free energy, is also found. Both the degree of the polymer degradation and the surface polarity correlate with the total energy deposited by 4 He + ions in the foils.

Electrical resistivity of carbon black loaded polyethylene modified by ion implantation

Different properties of the mixtures of polyethylene with carbon black modified by the implantation of Sb + ions were studied. Chemical changes of polymer were examined by IR- and UV-visible spectroscopy. Sheet resistivity as a function of sample temperature was studied. Depth profiles of implanted Sb atoms and incorporated oxygen were determined by the Rutherford backscattering technique. The percolation threshold of unimplanted mixtures is found at 4.5 and 5 wt. % of carbon black. As a result of ion implantation, the polymer is oxidized and conjugated double bonds are produced. The mixtures with carbon black concentration above percolation threshold exhibit metal-like conductivity. For the mixtures below percolation threshold, the measurements of resistivity versus temperature dependence indicate semiconductor type conductivity and charge transport via a variable range-hopping mechanism.