J.A. Leavitt

Isoform-specific complementation of the yeast sac6 null mutation by human fimbrin.

The actin cytoskeleton is a fundamental component of eukaryotic cells, with both structural and motile roles. Actin and many of the actin-binding proteins found in different cell types are highly conserved, showing considerable similarity in both primary structure and biochemical properties. To make detailed comparisons between homologous proteins, it is necessary to know whether the various proteins are functionally, as well as structurally, conserved. Fimbrin is an example of a cytoskeletal component that, as shown by sequence determinations and biochemical characterizations, is conserved between organisms as diverse as Saccharomyces cerevisiae and humans. In this study, we examined whether the human homolog can substitute for the yeast protein in vivo. We report here that two isoforms of human fimbrin, also referred to as T- and L-plastin, can both substitute in vivo for yeast fimbrin, also known as Sac6p, whereas a third isoform, I-fimbrin (or I-plastin), cannot. We demonstrate that the human T- and L-fimbrins, in addition to complementing the temperature-sensitive growth defect of the sac6 null mutant, restore both normal cytoskeletal organization and cell shape to the mutant cells. In addition, we show that human T- and L-fimbrins can complement a sporulation defect caused by the sac6 null mutation. These findings indicate that there is a high degree of functional conservation in the cytoskeleton, even between organisms as diverse as S. cerevisiae and humans.

REDUCTION OF DOPPLER BROADENING OF SPECTRAL LINES IN FAST‐BEAM SPECTROSCOPY

We show that the Doppler broadening, which arises in a fast‐beam light source because of the high speed of the emitters and the nonzero acceptance angle of the spectrometer, can often be practically eliminated by proper adjustment of the spectrometer.

Ion-assisted deposition of lanthanum fluoride thin films

Ion-assisted deposition has been used to deposit lanthanum fluoride thin films with near-unity film packing densities and no significant increase in absorption. Rutherford backscattering analysis has determined the effect of ion bombardment on the film stoichiometries including the degree of fluorine deficiency. Oxygen atoms or compounds appear to occupy most of the available anion vacancies if sufficient oxygen is available in the ion beam or the residual atmosphere.

Non-Rutherford 4He cross sections for ion beam analysis

Abstract Increasing use of 4 He analysis beams with energies between 2 and 10 MeV for depth profiling and backscattering analysis of thin films and near-surface materials requires accurate measured values of non-Rutherford cross sections for scattering of 4 He by the lighter elements. Cross sections for scattering of 4 He through large angles deviate from Rutherford at 4 He laboratory energy ∼ 2.2 MeV for target nuclei 12 C and 16 O; deviations for target atomic numbers Z = 20 and 40 occur at 4 He energies of about 5 and 10 MeV, respectively. We review the experimental cross section data currently available for large angle scattering of 4 He from the light elements (4 ≤ Z ≤ 20) for incident 4 He laboratory energies 1.5–10 MeV. For each target element, we indicate energy regions with smooth cross section variation suitable for use for simple backscattering analysis as well as strong narrow resonances that may be used for depth profiling light elements in heavy-element matrices.

Ion beam analysis with a 6 MV van de Graaff

Rutherford backscattering and channeling techniques using 4He analysis beams with energies less than 2.5 MeV have proven to be powerful methods for determination of stoichiometry, structure, thickness and impurity concentrations in thin films. We discuss examples illustrating the advantages of routine use of analysis ion beams (from a single-ended 6 MV Van de Graaff) with energies greater than 2.5 MeV. We compare 170° 4He backscattering spectra obtained at higher energies (3.8 and 4.7 MeV) with similar spectra obtained at 1.9 MeV to show that use of the higher energies leads to simplified data analysis, greater probing depth and more accurate determination of film stoichiometry. We briefly report progress in attempts to 1) use backscattering of 12C analysis beams to obtain improved mass and depth resolution, 2) use 4He analysis beams to depth profile P in Si using (α, p) nuclear reactions at incident 4He energies of 3.655 and 4.343 MeV and 3) use 15N2+ analysis beams with energies ≥ 6.3 MeV to depth profile 1H in films using the (15N, αγ) nuclear reaction at 6.385 MeV.

Determination of nitrogen using the 14N(α, p)17O nuclear reaction

Abstract A technique for quantifying nitrogen in thin films using the 14N(α, p)17O reaction is reported. An incident alpha particle beam with energy near 3.9 MeV is used and emitted protons are detected at a lab angle of 135°. A 15 μm Kapton foil is placed over the detector to stop elastically scattered alpha particles. A thin film containing a known areal density of nitrogen is used for calibration. We present relative yield data on protons from the 14N(α, p0)17)O reaction at 135° lab angle from 3.4 MeV to 4.0 MeV as well as an example of the application of this technique in determining nitrogen content in thin films. Possible interfering reactions, particularly 28Si(α, p)31, are also discussed.

MeV ion beam analysis of high-Tc superconducting films

Areal densities [at./cm2], average stoichiometric ratios, impurity concentrations and elemental depth profiles for high-Ec thin films may be determined by high-energy backscattering using 3–5 MeV 4He analysis beams. Accuracies of about ± 3% and ± 1% are obtained for areal densities and average stoichiometric ratios, respectively. We present results of the analysis for several YBaCuO films in the thickness range 2500–9000 A. We indicate the advantages of using the 3–5 MeV 4He analysis beams rather than the usual 1–2 MeV 4He beams used for Rutherford backscattering spectrometry.

CANCELLATION OF SLIT‐WIDTH AND DOPPLER BROADENING EFFECTS IN THE SPECTROSCOPY OF FAST ION BEAMS

We use the Doppler shift of light emitted by fast ion beams to cancel the broadening introduced by opening the spectrometer entrance slit. Hence, wide slits are used to increase collected flux with minimal increase in recorded linewidth.

Amorphization implants and low temperature rapid thermal processing to form low sheet resistance, shallow junction, boron implanted layers

The effects on Si wafers of (1) Ge or Si amorphization implants, (2) B or BF2 dopant implants at low energies (15 keV through 16 nm of screen oxide) and (3) rapid thermal processing (RTP) at 800–1050° have been systematically studied. Sheet resistance (Rs), spreading resistance probe (SRP), Rutherford backscattering (RBS) and ultrahigh vacuum secondary ion mass spectrometry (SIMS) have been used to study the resulting electrical and materials properties of these wafers. Values for Rs versus junction depth are below any results reported in the literature. The Rs maps show lower Rs and better uniformity in the wafers that were amorphized with Ge or Si and received a B rather than a BF2 implant. The effects of channeling, molecular breakup of BF2, B solid solubility, and annealer uniformity are discussed.

Cross sections for 170.5° backscattering of 4He by the isotopes of boron for 4He energies between 1.0 and 3.3 MeV

Elastic scattering cross sections for 4He on both isotopes of boron have been measured at a laboratory angle of 170.5° in the energy range 1–3.3 MeV to an accuracy of about 7%. Self-supporting targets of natural boron (18μgcm2) and 99% enriched 10B (6μgcm2) were used. The scattering was observed to be Rutherford between 1 and 1.3 MeV for both isotopes. Previously unreported elastic scattering anomalies were observed in 10B near 1.5 and 1.65 MeV and in 11B near 1.5 and 2.05 MeV. Results above 2.1 MeV are compared to previous measurements; discrepancies of up to 50% are noted. The major features in the cross sections above 2.1 MeV are broad resonances near 2.4 MeV in 10B and near 2.6 MeV in 11B. The cross section is found to be smooth and approximately Rutherford for 11B in the energy range 2.2 to 2.5 MeV.