Research on planetary samples

Sputtering yields of solid SO2 by high energy ions were measured in order to study the mechanism for sputtering dielectrics with ions in the electronic stopping power region. The incident ions were helium and fluorine with energies ranging from 1.5 MeV to 25 MeV. Yields as high as 7000 SO2 molecules/incident F ion were measured; the 1.5 MeV He4 beam had a sputtering yield of 50. The data are compared to yield measurements made on UF4 and H2O targets. There is a striking similarity in the yield as a function of the Energy for all three targets. The data compare favorably with theoretical curves based on a model for the sputtering which considers the electronic excitations induced the target by the incident beam. Measurements and calculations of the sort are also useful in understanding processes which occur on the surface of Jupiter's satellite Io, which is covered with SO2 frost and bombarded by energetic ions trapped in the Jovian magnetosphere

Ion-beam analysis of meteoritic and lunar samples

Charged particle-induced nuclear reactions were used in the following problems: the determination of elemental abundances of boron and fluorine in carbonaceous chondritic meteorites; the identification of products of lunar vulcanism; and the study of solar wind-implanted atoms in lunar materials. The technique was seen as an important supplement to other methods of elemental and isotopic analysis. This was especially true for cases involving light elements at very low concentrations or where high resolution depth distribution information was needed in non-destructive analysis

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 Å 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

Method and means for helium/hydrogen ratio measurement by alpha scattering

An apparatus for determining helium to hydrogen ratios in a gaseous sample is presented. The sample is bombarded with alpha particles created by a self contained radioactive source and scattering products falling within a predetermined forward scattering angular range impact a detector assembly. Two detectors are mounted in tandem, the first completely blocking the second with respect to incident scattering products. Alpha particle/hydrogen or alpha particle/helium collisions are identified by whether scattering product impacts occur simultaneously in both detectors or only in the first detector. Relative magnitudes of the two pulses can be used to further discriminate against other effects such as noise and cosmic ray events

Dynamical x-ray diffraction from nonuniform crystalline films: Application to x-ray rocking curve analysis

A dynamical model for the general case of Bragg x-ray diffraction from arbitrarily thick nonuniform crystalline films is presented. The model incorporates depth-dependent strain and a spherically symmetric Gaussian distribution of randomly displaced atoms and can be applied to the rocking curve analysis of ion-damaged single crystals and strained layer superlattices. The analysis of x-ray rocking curves using this model provides detailed strain and damage depth distributions for ion-implanted or MeV-ion-bombarded crystals and layer thickness, and lattice strain distributions for epitaxial layers and superlattices. The computation time using the dynamical model is comparable to that using a kinematical model. We also present detailed strain and damage depth distributions in MeV-ion-bombarded GaAs(100) crystals. The perpendicular strain at the sample surface, measured as a function of ion-beam dose (D), nuclear stopping power (Sn), and electronic stopping power (Se) is shown to vary according to (1–kSe)DSn and saturate at high doses

Surface sticking probabilities for sputtered atoms of Nb-93 and Rh-103

The capture coefficient probabilities for sputtered atoms of Nb-93 and Rh-103 incident on Al2O3 surfaces were measured using the backscattering of MeV heavy ions. In the circumstance where the collecting surface is thickly covered, the sticking probabilities integrated over the energy distribution of sputtered atoms are 0.97 plus or minus 0.01 for Nb-93 and 0.95 plus or minus 0.01 for Rh-103 respectively. In the limit of negligible areal coverage of the collector, the accuracy is less; in this case the sticking probabilities are 0.97 + 0.03 or -0.08 and 0.95 + 0.05 or -0.08

Simulating cluster-ion impacts

Although cluster-ion interactions may superficially appear to resemble those in nuclear physics, there are important differences. First, at the energies of interest the deBroglie wavelength of an atom in the cluster is so short compared to atomic dimensions that the atom’s trajectory in a solid is classical. Thus, one may use the Born-Oppenheimer approximation in which the particles all obey Newton’s equations of motion and only the interaction potentials reflect the quantum mechanical character of the system. Thus, we can treat many atomic interaction processes in the semi-classical limit; in this paper I shall, for example, indicate how in this way one can include the effect of atomic excitation in collisions. Second, the cross sections involved are much larger than those for nuclear interactions, i.e., the mean free paths of the “reaction products” are so short that large collective effects always occur. This means that the calculations must deal with large numbers of interacting particles. For the case of cluster ions at MeV energies this has required us to develop new computational strategies, e.g., the use of massively parallel computers. By using such simulations strategically we are able both to optimize the design of experiments and to interpret their results less ambiguously

Modification of Electronic Materials with MeV Ions

MeV ion beams often allow unique opportunities for the modification of the electronic properties of materials. In this talk I shall discuss three such examples from our recent work. The first is the production of deeply buried SiO2 layers formed in Si by MeV oxygen implantation for a variety of implantation and annealing conditions. The resulting material has been characterized by XTEM. The second involves the modification of InP by N implantation and GaAs by O implantation. In these cases we have used XTEM, x-ray rocking curve analysis, ion channeling, and nuclear reaction profiling to study the structural damage and its behavior under thermal annealing. The third example shows how the resistivity of thin amorphous carbon films can be changed over a range of 10^4 by MeV ion bombardment. The mechanism for this phenomenon is shown to be closely related to that proposed for MeV ion induced desorption

MeV Ion Implantation in Electronic Materials

Using MeV ions for the modification of electronic materials offers a number of advantages: minimizing surface damage; implantation into completed devices – for example, through contacts or photoresist layers; producing controlled radiation damage for flux pinning where the ions pass completely through the sample and thus do not modify its chemical nature; and enhancing the electronic excitation of the target material versus collisional damage, as in adhesion enhancement processes. In all cases, however, one requires a detailed understanding of the new damage mechanisms that occur and how they can be modified in a controlled way by annealing. In this report I shall present examples from a number of our experiments: resistivity and index of refraction modification in semiconductors; adhesion enhancement; mixing of multilayer structures; and modification of the electronic properties of insulators and superconductors

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