T. A. Tombrello

Ionizing beam-induced adhesion enhancement and interface chemistry for Au-GaAs

MeV ion beam-induced adhesion enhancement of Au-films (∼500 A thick) on p-type and n-type GaAs substrates has been studied by the scratch test, ESCA, and nuclear reaction hydrogen profiling. For films resistively deposited in a diffusion pumped chamber at 2−5×10^(−6)torr, the data indicate that the adhesion enhancement is associated with oxide layers on the substrate surface adsorbed before the film deposition. The ESCA data suggest that water vapour dissociates and forms Ga(OH)_3 at the interface layers under ionizing radiation. The oxide concentration at the interface varies with substrate electronic properties and gives a large difference in the adhesion enhancement. However, the data obtained so far on the hydrogen concentration at the interface indicate that within our range of sensitivity it is about the same for substrates with different electronic properties. Our data demonstrate the importance of a thin absorbed (impurity) layer for the interface chemistry and adhesion enhancement by ionizing radiation.

Molecular dynamics simulations of inner shell electronic energy losses in cluster-surface collisions

Previous molecular dynamics simulations have predicted that in energetic cluster-surface collisions atoms can be core-excited leading to Auger transitions outside the target [5]. However, in these previous simulations the inelastic energy losses accompanying core-excitations were not taken into account. In our simulations a simple critical distance of approach to detect inner-shell excitations is used. Energy is removed from atoms in a single time step by moving the atoms apart from each other, taking away potential energy. Molecular dynamics simulations are carried out for 63-atom Al clusters and composite Al_(38)Au_(25)clusters incident on Au(100) targets and Au(111) targets. Each event is simulated with and without inelastic losses. Incident cluster energies ranged from 30 to 100 keV. Our results indicate a threshold for significant inelastic loss effects at approximately 70 keV/cluster, i.e., about 1 keV/atom. Above the threshold energy region, sputtered atom and ejected-cluster atom energy distributions were affected substantially.

VAX/LSI-11/CAMAC Nuclear Data Acquisition System under Development at the W.k. Kellogg Radiation Laboratory, Caltech

A CAMAC data acquisition system is currently under development at the W.K. Kellogg Radiation Laboratory which utilizes LSI-11 crate controllers and a 250 Kbyte/sec 7-computer Q-bus DMA network. The central network computer is a VAX 11/750 with Unibus and Q-bus adaptors. Processing is distributed between the LSI-11s and the VAX as required to meet input-output and event processing speeds needed by the experiment. Use of the Q-bus on all machines allows CAMAC controllers and tape drives to be moved as needed between the LSI-11s and the VAX.

Experimental investigations of the critical vortex dynamics in extreme type-II superconductors with controlled static disorder

Experimental investigations of the vortex dynamics in high-temperature and conventional amorphous superconductors reveal universal critical phenomena which are associated with various phase transitions in the vortex systems with different types of static disorder. Evidence for second-order vortex-glass, Bose-glass and splayed-glass transitions are manifested by the universal critical exponents and scaling functions via measurements of ac and dc transport properties, ac magnetic susceptibility, ac third-harmonic transmissivity, and microwave surface impedance. In contrast, current-induced effects in the weak pinning limit result in novel non-equilibrium vortex dynamics which conceal the direct detection of the order of the thermodynamic vortex-solid melting transition with electrical transport measurements.

Numerical calculation of the vortex-columnar-defect interaction and critical currents in extreme type-II superconductors - a two-dimensional model based on the Ginzburg-Landau approximation

We extend our previous one-dimensional Ginzburg-Landau calculations of the pinning energy of vortices to two dimensions, in order to achieve an understanding of the pinning forces exerted on vortices by defects. By minimizing the free energy using a relaxation scheme, we obtain the spatial variation of the order parameter and supercurrents for a vortex in the vicinity of a cylindrical defect in an extreme type-II superconductor. The resulting two-dimensional field distributions provide a direct mapping of the spatial dependence of the vortex-defect pinning potential, thereby yielding the pinning force and depinning current as a function of the defect size and magnetic field. We also use periodic boundary conditions in the two-dimensional Ginzburg-Landau equations to solve for the known vortex-vortex interaction, in order to verify the resolution and accuracy of our approach for extreme type-II superconductors. Our direct numerical derivation of the pinning force per vortex is shown to be applicable to a wide range of magnetic fields and columnar-defect densities, and the calculated results are consistent with experimental observation.

Current-driven vortex dynamics in untwinned superconducting single crystals

Current-driven vortex dynamics of type-II superconductors in the weak-pinning limit is investigated by quantitatively studying the current-dependent vortex dissipation of an untwinned YBa2Cu3O7 single crystal. For applied current densities (J) substantially larger than the critical current density (Jc), non-linear resistive peaks appear below the thermodynamic first-order vortex-lattice melting transition temperature (Tm), in contrast to the resistive hysteresis in the low-current limit (J < Jc). These resistive peaks are quantitatively analysed in terms of the current-driven coherent and plastic motion of vortex bundles in the vortex-solid phase, and the non-linear current - voltage characteristics are found to be consistent with the collective flux-creep model. The effects of high-density random point defects on the vortex dynamics are also investigated via proton irradiation of the same single crystal. Neither resistive hysteresis at low currents nor peak effects at high currents are found after the irradiation. Furthermore, the current-voltage characteristics within the instrumental resolution become completely ohmic over a wide range of currents and temperatures, despite theoretical predictions of much larger Jc-values for the given experimental variables. This finding suggests that the vortex-glass phase, a theoretically proposed low-temperature vortex state which is stabilized by point disorder and has a vanishing resistivity, may become unstable under applied currents significantly smaller than the theoretically predicted Jc. More investigation appears necessary in order to resolve this puzzling issue.

Charge state of C10 and C5 energetic cluster ions in amorphous carbon targets: simulations

We present here detailed simulations of the interaction of energetic C10 and C5 clusters at the energies of 1, 2, and 4 MeV per carbon atom with an amorphous carbon target. The spatial evolution of the cluster components is simulated accounting for both scattering and Coulomb explosion. The former is calculated by means of the Monte Carlo method while the latter is computed by means of molecular dynamics. The charge state of the individual cluster components is calculated as a function of penetration depth, and is determined by the competition between electron ionization and recombination. The results of calculations of the effect of the neighbouring cluster components on the suppression of the values of the charge state are presented and compared to the experimental values of Brunelle et al. Charge state suppression calculations for the 2 MeV/C clusters for both C10 and C5 agree well with the experimental results for penetration depths of less than about 500 and 250 A respectively, assuming the intracluster Coulomb potential is screened by four target valence electrons. At 4 MeV/C the results are similar although less screening is required; a possible explanation is the inability of the plasma to completely screen the higher velocity projectiles. The 1 MeV/C calculated results however differ in their behaviour from the 2 and 4 MeV/C cases.

RF Losses in Superconducting Lead Cavities

At the present time the only materials seriously considered for superconducting accelerators are lead and niobium. In the electron linacs under construction at Stanford and the University of Illinois cylindrical niobium cavities are being employed. The relatively uncomplicated geometry of such cavities permits the fabrication and annealing procedure to be approached in a straightforward (though expensive and time-consuming) manner and offers the optimum final energy gradient for these accelerators. However, the complicated geometrical structures (e.g., drift tubes or helically loaded wave guides) required for low phase velocity heavy-ion accelerators make the use of niobium less appealing. In contrast, lead may be easily plated on even very complicated structures with a minimum degree of difficulty and cost.

Formation mechanism and yield of molecules ejected from ZnS, CdS, and FeS{sub 2} during ion bombardment

Neutral species ejected from single crystals of ZnS, CdS, and FeS{sub 2} during ion bombardment by 3 keV Ar{sup +} were detected by laser post-ionization followed by time-of-flight mass spectrometry. While metal atoms (Fe, Zn, Cd) and S{sub 2} were the dominant species observed, substantial amounts of S, FeS, Zn{sub 2}, ZnS, Cd{sub 2}, and CdS were also detected. The experimental results demonstrate that molecules represent a larger fraction of the sputtered yield than was previously believed from secondary ion mass spectrometry experiments. In addition, the data suggest that the molecules are not necessarily formed from adjacent atoms in the solid and that a modified form of the recombination model could provide a mechanism for their formation.

Accelerator Simulation of Astrophysical Processes

The interaction of energetic ions with matter is responsible for many of the processes by which the elements were synthesized, energy is generated in stars, interstellar grains are destroyed, and molecules are created in space. All of these processes are amenable to simulation in the laboratory using accelerated ion beams, which allows us a more comprehensive understanding of Nature than we could obtain by observation alone. In addition, ion beam techniques are extremely useful in the determination of the elemental and isotopic abundances that arise from astrophysical nuclear synthesis.