J.F. Dias

Charging effects in thick insulating samples

In this paper we analyse the effects observed in X-ray spectra when thick insulating targets are irradiated with beams of light charged particles. Preliminary results show that the background yield due to charge buildup on mylar is larger for protons than for lithium ions, suggesting that particles with low energy loss tend to generate more background. Residual activity has been observed in several thick samples like resin, quartz and mylar. Two methods for suppressing the background due to charging effects have been studied. The first one was based on the use of a transverse magnetic field. The second one made use of a simple electric lamp of 6 V with its glass bulb removed. While the first method proved to be inefficient, the second one eliminated most of the background due to charge buildup in the sample.

PIXE analysis of urease isoenzymes isolated from Canavalia ensiformis (jack bean) seeds

Abstract The main goal of this work was to perform a stoichiometric analysis of metals, in particular Ni and Zn, in two isoforms of urease isolated from jack bean seeds. The urease isoforms were separated by immobilized metal affinity chromatography. The samples were prepared for PIXE by acetone precipitation of the proteins, which were deposited onto regenerated cellulose acetate membranes by filtration. The resulting mass of the samples was of the order of 0.20 mg (dry weight). The results show that urease contains two equivalents of Ni per 90 kDa monomer, in agreement with the results reported in the literature. For canatoxin, the other urease isoform, the presence of one equivalent of Zn per monomer was observed, besides Ni, previously detected by atomic absorption spectrometry as one equivalent per 90 kDa. Problems and further improvements concerning the preparation of the samples are discussed.

Angular dependent energy loss of 0.8–2.0 MeV He ions channeled along the Si〈1 0 0〉 direction

Abstract We present measurements of the electronic stopping power in the 0.8–2.0 MeV energy range of 4 He 2+ ions channeled through the Si〈1 0 0〉 axis as a function of the incident angle. The measurements were carried out using the Rutherford Backscattering (RBS) technique with a SIMOX sample that consists of a single Si crystal on the top of a 500 nm buried layer of SiO 2 built into a Si〈1 0 0〉 wafer. The measured ratios between the angular dependent and random stopping powers have almost the same width near the maximum of the 4 He 2+ stopping power and decrease for higher energies.

ELECTRONIC STOPPING POWER OF (1 0 0) AXIAL CHANNELED 7LI IONS IN SI CRYSTALS

Abstract We report measurements on the electronic stopping power of 7 Li ions along the 〈1 0 0〉 Si direction in the energy region between 1 and 8 MeV. The measurements were carried out using the standard RBS/Channeling technique with SIMOX targets. The results show that the stopping power decreases for increasing energies, from 48 eV/A at 1 MeV down to 19 eV/A at 8 MeV. The general features obtained for the stopping power of 7 Li resemble those obtained for He ions measured previously.

Angular dependence for the energy loss of channeled He ions near the Si〈110〉 and 〈111〉 directions

Abstract In the present work, we report on measurements of angular dependent energy loss for 1.2 and 2.0 MeV He ions incident near the Si 〈1 1 0〉 and 〈1 1 1〉 axes parallel to the {1xa00xa00} and {1xa01xa00} planes, respectively. The measurements were done using the Rutherford backscattering technique (RBS) combined with a SIMOX sample. The experimental results are well reproduced by calculations based on the convolution approximation for the impact-parameter dependence of the energy loss of the core electrons.

Channeling energy loss of O ions in Si: The Barkas effect

In this work we report on measurements of channeling stopping powers of 16 O ions along Sih 100 i axial direction for the energy range between 250 keV/u and 1 MeV/u by using the Rutherford backscattering technique with separated by implanted oxygen targets. In connection with the recent developed unitary convolution approximation, we are able to extract the Barkas contribution to the energy loss with high precision. This effect is clearly separated from other processes and amounts to about 15%. The observed Barkas contribution from the valence-electron gas is in agreement with the Lindhard model for higher energies. However, in contrast to recent investigations for Li ions, the Barkas effect at the lowest energies seems to saturate, indicating other non-perturbative terms in the polarization field induced by the O ions in Si. 2002 Elsevier Science B.V. All rights reserved.

Charge equilibration process for channeled He ions along the Si〈1 0 0〉 direction

Abstract We report on the results of the charge equilibration process of 1.2 MeV He+ and He++ ions along the Si〈1xa00xa00〉 direction. Present and previous results indicate that the charge equilibrium distance increases as a function of the He energy, being about 20 nm for 1.2 MeV.

A PIXE study of elements transport in fluvial waters in the Candiota region, Rio Grande do Sul, Brazil

The transport of elements in fluvial waters within the Candiota region was evaluated by determining the elements associated with suspended particulate matter (SPM). To that end, SPM samples were collected at nine different locations five times from December 1999 to October 2000. The particle-induced X-ray emission technique was used to determine elements associated with SPM. Data interpretation was performed using factor analysis. Among the results explained by factor 1, the highest score was found to be due to the effect of local anthropogenic sources (coal mines) during the dry season. On the other hand, factor 2 was substantially heterogeneous, emphasising the sum of all anthropogenic contributions generated at the basin. Flux ratio and enrichment factor were calculated to elucidate the balance between weathering and solute transport. Results showed that Mn, Ni, Fe, Cu and Zn were carried by Candiota Stream particles, indicating that coal mining is presumably responsible for contamination of these elements in SPM.

Random energy loss and straggling study of Li into Si

In the present work we have measured the random 7Li stopping power and range straggling into Si. With this aim we have employed the Rutherford backscattering (RBS) technique together with a multilayer Sb/Si marker. The stopping power measurements were performed in the 280–8500 keV energy interval, while the range straggling ones were done in the 380–2500 keV energy interval. The present results were compared with the TRIM predictions and the semi-empirical calculations by the Kalbitzer group, and an overall good agreement was found for both cases.

Range parameters of implanted into Si and SiO2

Abstract In the present contribution, we present range parameter results for 18 O implanted into Si and SiO2 targets in a 25–200 keV energy interval. The 18 O depth profiles were determined by using the 18 O ( p , α ) 15 N nuclear resonant reaction at Er=151.2 keV. As a result, we have obtained the 18 O projected range (Rp) and range straggling (ΔRp) for each one of the implantation energies. The experimental data were compared with the TRIM predictions. Concerning 18 O projected ranges in Si, the theoretical–experimental agreement was fairly good, whereas, for the SiO2 case, it was much poorer. However, large differences were found, for both cases, when the ΔRp experimental values were compared with the theoretical ones.