Thomas A. Tombrello

Ca isotope fractionation on the Earth and other solar system materials

It is demonstrated that differences in the ^(40)Ca/^(44)Ca ratio due to mass dependent isotope fractionation in nature are clearly resolvable to a level of 0.5‰. This precision is obtained (a) by using the double spike technique; (b) by using a mass-dependent law for correction of instrumental nmass fractionation; and (c) by eliminating fractionation effects identified as due to differential elution nof isotopes through ion exchange resins. We have determined the following uniform Ca isotopic composition after removing small natural fractionation effects: ^(40)Ca/^(44)Ca = 47.153 ± 3, ^(42)Ca/^(44)Ca = 0.31221 ± 2, ^(43)Ca/^(44)Ca = 0.06486 ± I, ^(46)Ca/^(44)Ca = 0.00152 ± I, ^(48)Ca/^(44)Ca = 0.08871 ± 2, where the errors correspond to the last figures shown. This yields an atomic weight of 40.076 ± 0.001. The data indicate the absence in the studied samples of detectable, distinct nuclear components in Ca similar to those observed for oxygen. In the samples studied, there is a distinct but small degree of Ca isotope fractionation. Overlapping ranges of fractionation of 2.5‰ for ^(40)Ca/^(44)Ca (four atomic mass units) are observed in meteorites, lunar, and terrestrial samples. Means by which isotope fractionation could arise for Ca are discussed, but the small range of effects and the lack of systematic variations do not permit at present the identification of the mechanisms responsible for the fractionation observed in the suite of samples. Ca in the biological cycle does not show fractionation effects larger than observed for non-biogenic samples. In contrast to these results, we have observed large effects of up to 13‰, for industrially purified Ca.

The fractionation of Ca isotopes by sputtering

Using a double-spike, mass spectrometric technique, we have measured the absolute isotopic composition of Ca sputtered from several Ca minerals by normally incident, low energy, nitrogen beams (130 keV N^+ and 100 keV N^(+)_(2)). The precision in the ^(40)Ca/^(44)Ca ratio is 0.05% for samples as small as 10^(−7) g, corresponding to a resolution of possible mass dependent isotope fractionation effects of ∼0.01% per unit mass difference. In general, the material sputtered first was isotopically light by up to 2% in the isotopic ratio ^(40)Ca/^(44)Ca when compared with the measured isotopic composition of the target; as the bombardment progressed, the isotopic composition became less light and approached the initial composition of the target. We have demonstrated that, after prolonged bombardment, a surface layer develops on the target with a distinctly heavy isotopic composition; the thickness of the layer is a significant fraction of the range of the bombarding ions. Angular distributions of the sputtered material show that, throughout the bombardment, the material ejected back along the beam direction was isotopically light relative to the material ejected at oblique angles; the difference in the isotopic ratio ^(40)Ca/^(44)Ca over the angular range was ∼ 1.5%. This fractionation with angle of ejection persisted even when a quasi-steady state was reached after heavy bombardment, when the isotopic composition of the material averaged over angle had become essentially indistinguishable from that of the pristine target. These results indicate that sputtering produces isotope fractionation and that there is a marked angular dependence in the effects. This suggests that solar wind sputtering of the lunar surface may contribute to the isotopic and elemental enrichments observed for the surface layers of soil grains.

Measurement of hydrogen depth distribution by resonant nuclear reactions

The resonance at E (19F) =6.42 MeV in the reaction 1H(19F,alphagamma)16O has been explored as a potentially useful method for the quantitative determination of hydrogen concentration as a function of depth in a solid substrate. The relative merits of this resonance, the 16.44-MeV resonance in the same reaction, and the 6.39-MeV resonance in the reaction 1H(15N,alphagamma)12C are discussed.

Energy dependence of the trapping of uranium atoms by aluminum oxide surfaces

Abstract We have measured the energy dependence of the trapping probability for sputtered 235U atoms striking an oxidized aluminum collector surface at energies between 1 eV and 184 eV. At the lowest energies approximately 10% of the uranium atoms are not trapped, while above 10 eV essentially all of them stick. In addition, we present trapping probabilities averaged over the sputtered energy distribution for uranium incident on gold and mica.

Radiation‐induced interface phenomena: Decoration of high‐energy density ion tracks

The effect of 20 MeV Cl4 + ions incident on Au-SiO2 and Ag-SiO2 interfaces was investigated using high-resolution transmission electron microscopy. Cross-sectional micrographs expose beam-induced gold interfacial transport and migration into the SiO2. No such migration was observed for silver films. The relevance of this phenomenon to the adhesion improvement found at corresponding irradiation doses is discussed.

Electronic effects in MeV ion tracks affecting thin film adhesion

An ion track model was applied to describe MeV ion induced adhesion improvement of Au thin films on amorphous SiO_2. Good agreement with experimental data was found when assuming that ion track energy densities above and below a certain interval do not contribute to the adhesion enhancement; damage effects detrimental to adhesion may be associated with the high energy densities in the vicinity of the ion path.

Bending MeV proton beams by means of a glass capillary tube

We study in a preliminary and simplified manner the possibility of bending submicron beams by means of glass capillary tubes. We show that protons of 20 MeV can be guided within a 10 nm diameter helical tube, for a distance of 0.55 cm, with the beam bending in the transverse direction by 0.16 mm. Although these distances are not large and the angle of incidence of the beam is very small, applications of great interest warrant further investigation of this study.