Cheng-Ming Fou

The Bartol-University of Delaware Proton Microprobe

The Bartol Research Foundation and the Physics Department of the University of Delaware have jointly set up a proton microprobe for the observation of proton induced x rays and ¿ rays. The protons with energies up to 2.5 MeV are accelerated in a Van de Graaff and pass out into the atmosphere through holes as small as 10 ¿m. The x rays, for elements above sodium, are detected in a 6 mm diameter by 3 mm deep Si(Li) diode; whereas the gamma rays, for the lighter elements, are detected in a 55 cc Ge(Li) diode. Examples are given of preliminary studies of amorphous alloys, trace elements in microtektites metamorphosed marbles, skarns and pegmatites, chemical composition of the shell of oysters cultured in controlled systems and archaeological artifacts.

Low energy sputtering of eutectic Ag/Cu alloys

Abstract Experiments were carried out on the sputtering of a eutectic Ag/Cu (NAg/NCu = 1.5) alloy by normally incident 110 eV Ar+ ions. The collected sputtered flux was analyzed by Rutherford backscattering (RBS). Material sputtered normal to the surface at a fluence of 7 × 1019 Ar/cm2 was stoichiometric (NAg/NCu ≈ 1.5), but became increasingly Cu rich at larger angles of ejection up to 75° at which NAg/NCu was about 1.09. The overall ejection was slightly Cu rich (NAg/NCu ≈ 1.3). RBS analysis of the sputtered alloy surfaces indicated they were Cu rich to a depth of about 1500 A, with the greatest Cu content at about 450 A depth (up to 56% Cu), the effect depending on the fluence of the bombarding Ar+ ions. Scanning electron micrographs of the sputtered alloy surfaces showed dense microtopographic formations.

Water purity investigation using proton induced X-ray emission

Abstract An investigation into the possibility of using the highly sensitive proton induced X-ray emission analysis technique (PIXE) to carry out large scale monitoring of the trace elements in household tap water is presented. Quantitative result of lead contamination from samples examined is also presented as an example.

Evaluation of the Effectiveness of Tap-Water Filter Using in-Air PIXE

Large scale drinking water monitoring is a formidable task no matter what analysis technique is used. The commercially available tap-water filter that can be easily adapted to the faucet offers a unique possibility to collect and accumulate contaminations in the drinking water supply from a wide spread area at relatively low cost. It has been demonstrated that such filters can be examined without preparation using in-air PIXE. In the present investigation, the effectiveness of such filters is examined by comparing the water before and after filtration. This information is important if such a large scale monitoring is to be carried out. The present work also shows the merit of accumulation.

Difference in the anisotropy of the L X-rays emitted from gold atoms aligned by impact ionization using H+ or H+2 of the same energy/amu

Abstract The L X-ray emitted from gold atoms, impact-ionizated by 120° scattered 1 MeV/amu protons, and singly charged molecular hydrogen ions were measured at 0° and 90° relative to the reaction plane using a coincidence technique. Unambiguous difference under identical experimental conditions was observed. An attempt to understand this “molecular effect” using a simple model of randomly oriented incident molecular ion is made.

Anisotropy of L X-rays emitted from gold atoms aligned by proton impact ionization

Abstract The L-X rays emitted from gold atoms impact-ionized by 65° scattered 1 MeV protons were measured at 0° and 90° relative to the normal of the reaction plane using a coincidence technique. The anisotropy determined is used to deduce roughly the degree of alignment due to the difference in the differential ionization cross section of |M j | = 1 2 and 3 2 magnetic substrate

Characterization of surface profiles of LPCVD silicon nitride films by RBS-profile

Abstract A Rutherford backscattering spectrometry (RBS)‐profile was applied to the characterization of the surface profile of silicon nitride films prepared by low pressure chemical vapor deposition (LPCVD). This method detected inhomogeneity in the silicon nitride films and demonstrated Si‐richness near the interface. This method was also used to study the silicon nitride profiles associated with birds beak formation in VLSI devices. This paper presents a scheme for an auto‐search routine for an RBS‐profile program. The potential of the RBS‐profile method for the characterization of LPCVD silicon nitride films are demonstrated.

Application of in-Air PIXE for Corrosion-Erosion Studies of Metal Surfaces

Two methods are presented to demonstrate the usefullness of In-air PIXE for studying enviromental corrosion and erosion of metal surfaces. (1) Surface adsorption of corrosive vapors on gases in the ppm range is detected with the In-air PIXE technique. The adsorbed chlorine and Argon can be pumped away with moderate (200°C) heating of the metal surface. Therefore such investigation can only be done with the In-air PIXE. (2) Airborne particulates with high velocity, such as sands in the desert wind, can erode metal surface. They can also imbed foreign materials into the metal surface. Those foreign materials may lead to surface corrosion. Metal surface and the glass beads of a sand-blaster, before and after sandblasting, were examined with In-air PIXE to show its sensitivity and the ease with which it can detect minute quantities taken off or imbedded into the metal surface after just a short blasting time.

Molecular effect in ion-atom collisions — a summary review

Abstract A survey of the experimental studies of ion-atom collisions in the MeV/amu range using molecular-ion beams (such as H + 2 or H + 3 ) will be presented. Particular attention will be paid to those studies where comparisons were made with experimental results using isotachic (same velocity or same energy/amu) atomic-ion beams (H + ). A semiclassical discussion and/or an explanation of the observed differences — the so called molecular effect — based on the spatial and temporal correlations of the atoms belonging to the same molecular ion will be given. The role played by the binding electron(s) which accompanies the atoms of the same molecular ion in the process of collision with the target atom will be assessed. I shall point out some of the unanswered questions and doubts of some of the published experimental studies. I shall also attempt to make some suggestions for further studies.

Quantitative analysis of metal traces in blood serum using in-air PIXE

Abstract Using a regulated pipette dropper, a droplet of blood serum can be hung at the tip for a period of time long enough for PIXE analysis in air. The external proton beam of the University of Delaware proton microprobe was used to bombard the droplet in room atmosphere. Since no sample preparation is needed, the sample was analyzed in five to ten minutes when only the relative concentrations were needed. To obtain absolute concentration, known concentration of a certain element has to be mixed in the serum to be used as a normalizing factor. This is necessary because charge collection in room atmosphere is not reliable, and no PIXE detectable element in serum is known to be constant enough in concentration that can serve this purpose of normalization. NH 4 Br has been tried and found to be very suitable and satisfactory for this purpose.