Secondary electron yield of emissive materials for large-area micro-channel plate detectors: surface composition and film thickness dependencies

The ongoing development of Atomic Layer Deposition (ALD) enables the use of relatively inexpensive and robust borosilicate micro-channel substrates for use as Micro-Channel Plates (MCPs). The surfaces of the channels in these glass plates are functionalized to control the conductivity as well as the Secondary Electron Yield (SEY). The extensive SEY data found in literature show significant variation for a given material depending on the apparatus, the measurement procedure, and the sample preparation and handling. We present systematic studies on the effects of film thickness and surface chemical composition on SEY. We have modified an existing ultra-high vacuum apparatus containing X-ray and Ultraviolet Photoelectron Spectrometers (XPS and UPS, respectively) by adding a modified Low Energy Electron Diffraction (LEED) module for SEY measurements. With these tools, we have characterized the secondary electron emissive properties for MgO, Al2O3, and multilayered MgO/TiO2 structures to serve as electron emissive layers in the channels of the MCPs

Non-additive sputtering of niobium and tantalum as large neutral and ion clusters.

An analysis of available literature data on both the positive ion emission from Nb and Ta bombarded by 6 keV/atom Au{sub m}{sup -} atomic and molecular ions (m=1, 2, 3) and positive ionization probabilities of Nb{sub n} and Ta{sub n} neutral clusters sputtered from the same metals by 5 keV Ar{sup +} ions have been conducted. Dependencies of cluster yields Y{sub n,m} (regardless of a charge state) on number of atoms n in a sputtered particle were found to follow a power law as Y{sub n,m} {approx} n{sup -{sigma}{sub m}} where {sigma}{sub m} decreased with an increase of m. A non-linear enhancement of yields for large Nb{sub n}{sup +} and Ta{sub n}{sup +} cluster ions (n>4) appeared to be due to a non-additive process of sputtering rather than because of a non-additive process of their ionization. A manifestation of the non-additive sputtering in kinetic energy distributions of secondary ions found to be different for atomic and cluster ions

Application of CO2 Snow Jet Cleaning in Conjunction with Laboratory Based Total Reflection X-Ray Fluorescence

The Genesis mission was the first mission returning solar material to Earth since the Apollo program [1,2]. Unfortunately the return of the space craft on September 8, 2004 resulted in a crash landing, which shattered the samples into small fragments and exposed them to desert soil and other debris. Thus only small fragments of the original collectors are available, each having different degrees of surface contamination. Thorough surface cleaning is required to allow for subsequent analysis of solar wind material embedded within. An initial cleaning procedure was developed in coordination with Johnson Space Center which focused on removing larger sized particulates and a thin film organic contamination acquired during collection in space [3]. However, many of the samples have additional residues and more rigorous and/or innovative cleaning steps might be necessary. These cleaning steps must affect only the surface to avoid leaching and re-distribution of solar wind material from the bulk of the collectors. To aid in development and identification of the most appropriate cleaning procedures each sample has to be thoroughly inspected before and after each cleaning step. Laboratory based total reflection X-ray fluorescence (TXRF) spectrometry lends itself to this task as it is a non-destructive and surface sensitive analytical method permitting analysis of elements from aluminum onward present at and near the surface of a flat substrate [4]. The suitability of TXRF has been demonstrated for several Genesis solar wind samples before and after various cleaning methods including acid treatment, gas cluster ion beam, and CO2 snow jet [5 - 7]. The latter one is non-invasive and did show some promise on one sample [5]. To investigate the feasibility of CO2 snow jet cleaning further, several flown Genesis samples were selected to be characterized before and after CO2 snow application with sample 61052 being discussed below

High sensitivity sputter neutral mass spectrometry - sputtering of neutral mixed clusters from gold-aluminum alloys

We have used the surface sensitivity of laser sputter neutral mass spectrometry to make measurements of clusters sputtered from AuAl alloys surfaces with high dynamic range. Polycrystalline AuAl₄ and Au₄Al were bombarded with 15 keV Ar+ at 60° incidence, and the resulting secondary neutral yield distributions were measured using laser postionization mass spectrometry. Neutral clusters containing up to 28 atoms were observed and exhibited an odd–even variation in signal dependent on the stability of the photoion. Clusters sputtered from Au₄Al were gold rich compared to the substrate and the yield of neutral clusters containing <i>n</i> atoms, Y_n, was found to follow a power in <i>n</i>, i.e. Y_n ∝ n^−δ, where the exponent δ was approximately 3.4

Three-dimensional modeling of a time-of-flight mass spectrometer : optimization of SNMS/SIMS transmission using Simion.

A computer program that models secondary ion and post-ionized neutral trajectories through the SARISA instrument has been developed. The program has been tested by determining the transmission of secondary ions for two different ion energies. Results indicate that the model accurately reflects the transmission of the SARISA instrument. The developed algorithm for creating ions and monitoring their parameters is generic and can be used for examining other SIMS/SNMS instruments and ion sources

Sputtering of neutral clusters from silver-gold alloys

Polycrystalline Ag, Ag₂₀Au₈₀, Ag₄₀Au₆₀, Ag₈₀Au₂₀ and Au samples were bombarded with 15 keV Ar⁺ at 60° incidence and the resulting secondary neutral yield distribution was studied by non-resonant laser postionisation mass spectrometry. Neutral clusters containing up to 21 atoms were observed for the targets. The yield of neutral clusters, AgmAun−m, containing n atoms, Yn, was found to follow a power in n, i.e. Ynαn−δ, where the exponent δ varied from 3.2 to 4.0. For a fixed n, the cluster yields showed a variation with number of gold atoms similar to that expected for a binomial distribution. In addition, the cluster compositions from the sputtered alloys were indicative of sputtering from a gold rich surface