D.L. Weathers

Triply-ionized B2 molecules from a tandem accelerator

Abstract Beams of 10B3+2, 11B23+, and 10B11B3+ ions have been observed to emerge from a tandem accelerator. B2− mo into the accelerator, and positive ions emerging from the machine were analyzed for mass per charge and total energy. For 10B11B, intensities of singly-, doubly-, and triply-charged molecules were measured as a function of N2 gas pressure in the accelerator terminal stripper canal. These intensities were found to exhibit the same qualitative behavior for all charge states, with the number of triply-charged molecules a factor of ~ 2 × 10−4 less than for the doubly-charged molecules. No quadruply-ionized molecules were seen. The observation of particles corresponding to the breakup products of the multiply-ionized molecules indicates that these species were decaying in flight, and are apparently metastable with lifetimes of ~ μs. Comparisons are made between these observations and molecular orbital calculations.

An ultra-clean microprobe ion source for atomic mass spectrometry

Abstract The design and initial implementation of an ultra-clean microprobe ion source (Chimera), which is under construction at the University of North Texas in collaboration with Texas Instruments, is presented. The source, which is intended for trace impurity analysis of stable isotopes, combines the features of existing secondary ion mass spectrometry (SIMS) microprobe instruments with the molecular discrimination and single-ion detection capabilities of accelerator mass spectrometry (AMS). Issues of cesium ion microbeam formation and cleanliness, secondary ion collection and transmission efficiency are discussed. Novel features of the Chimera include high-resolution magnetic analysis of the primary Cs+ ion beam ( m Δm > 200) , high-purity silicon slits and an ultra-high vacuum analysis chamber with unique silicon-based extraction optics and sample holder.

Efficiency determination for a windowless Si(Li) X-ray detector for photon energies below 5 keV using atomic-field bremsstrahlung

Abstract The efficiency of a windowless Si(Li) X-ray detector has been determined experimentally for photon energies down to 600 eV. Thin foil targets of Au, Ag, and Al were bombarded with 66.5 keV electrons, and the resulting atomic-field bremsstrahlung was measured with the detector. The shape of the detectors efficiency function was determined by comparing these measured distributions (away from characteristic X-ray lines) with calculated spectral distributions for the bremsstrahlung, which are accurate to within 11%. The efficiency determined with a calibrated radioactive source at 5.4 keV was used for absolute normalization of this curve. The overall uncertainty in final efficiency was 12–13%. The general applicability of the calibration technique at low photon energies, its limitations, and proposals for further refinement are discussed.

An overview of the facilities, activities, and developments at the University of North Texas Ion Beam Modification and Analysis Laboratory (IBMAL)

The Ion Beam Modification and Analysis Laboratory (IBMAL) at the University of North Texas includes several accelerator facilities with capabilities of producing a variety of ion beams from tens of keV to several MeV in energy. The four accelerators are used for research, graduate and undergraduate education, and industrial applications. The NEC 3MV Pelletron tandem accelerator has three ion sources for negative ions: He Alphatross and two different SNICS-type sputter ion sources. Presently, the tandem accelerator has four high-energy beam transport lines and one low-energy beam transport line directly taken from the negative ion sources for different research experiments. For the low-energy beam line, the ion energy can be varied from ∼20 to 80 keV for ion implantation/modification of materials. The four post-acceleration beam lines include a heavy-ion nuclear microprobe; multi-purpose PIXE, RBS, ERD, NRA, and broad-beam single-event upset; high-energy ion implantation line; and trace-element accelerator...

Atomic mass spectrometry of materials

Abstract Texas Instruments and the University of North Texas (UNT) are collaborating on the design of an accelerator mass spectrometry (AMS) system dedicated primarily to the analysis of impurities in electronic materials and metals. An AMS beamline consisting of high-resolution magnetic ( M d M > 350) and electrostatic ( E d E > 700) analysis followed by a surface barrier detector has been installed on the NEC 9SDH pelletron at UNT, and a “clean” ion source is under development. An existing ion source (NEC Cs sputter source) has been used in conjunction with the AMS beamline to generate computer controlled molecule-free mass analyses of solid samples. Through a careful choice of isotopes and charge states a robust algorithm can be developed for removing molecular interferences from the mass analysis for essentially all materials. Examples using graphite, Si and CdZnTe are discussed.

Fabrication of silicon‐based optical components for an ultraclean accelerator mass spectrometry negative ion source

Article discussing the fabrication of silicon-based optical components for an ultraclean accelerator mass spectonomy negative ion source.

Model of the contamination effect in ion-induced electron emission

Abstract Ion-induced electron emission yields from contaminated surfaces are well known to be enhanced relative to the yields from atomically clean surfaces. Under the bombardment of energetic ions, the surfaces become sputter-cleaned with time and the yields from the samples are reduced accordingly. The time dependent reduction of yields observed are shown to be due to various effects such as desorption of contaminant atoms and molecules by incident ions and adsorption of residual gas onto previously clean sites. Experimental results obtained in the present work show the lower, saturated yield (γs) to be a function of residual gas pressure (P) and the fluence (oi) of the ion. We present a dynamic equilibrium model which explains the increase in yields for surface gas contamination, the decrease in yields for contaminant desorption, and the pressure/fluence dependence in the time required to reach γs. The predictions of the model agree well with the observations of γs as a function of the ratio of gas flux to ion flux, and the electron yields of clean and gas covered surfaces.

Analysis and reduction of low-Z contaminants on thin carbon films

Abstract Thin, self-supporting foils prepared from arc-evaporated carbon films have been examined for low- Z contaminants ranging from oxygen to calcium. The foils were analyzed with a windowless Si(Li) X-ray detector using the PIXE technique. Foils from different commercial vendors and foils made in-house using different parting agents have been compared, as well as several different solutions for floating the foils from their glass substrates. A technique of treating foils in ultrasonic baths of weak acetic acid has been developed, and is shown to be at least modestly successful at reducing contaminant levels. Evidence of particulate contamination was observed for some of the foils. For additional strength, most of the foils analyzed were coated with collodion, the most dramatic influence of which was to increase the amount of oxygen seen on the foils.

The University of North Texas atomic mass spectrometry facility for detection of impurities in electronic materials and metals

Abstract An accelerator mass spectrometry (AMS) facility is being developed at the University of North Texas through a collaboration between UNT and Texas Instruments Inc. The computer controlled AMS instrument will presently allow automatic mass scans of stable isotopes in solid materials using a conventional NEC SNICS ion source. Even though the SNICS ion source contaminates the sample, the AMS instrument allows molecular interference-free mass scans to be obtained with a higher sensitivity than SIMS for some elements, A new low sample contamination microbeam ion source under construction should allow sensitivities of ppt (1 part in 1012 or 1010 atoms/cm3) for any element in the periodic table, as well as sputter depth profiling and secondary electron imaging of a sample.

Materials characterization using accelerator mass spectrometry

Abstract Several materials, including TiH, stainless steel (No. 304), CaCO 3 and a boron nitride/graphite mix have been analysed using accelerator mass spectrometry. Elemental and molecular negative ion injection have been examined to enhance the sensitivity for elements with low electron affinities. Ca and Sr negative ions, as well as metastable molecules of charge state 3 + are demonstrated. Conditions for high sensitivity analysis in unknown samples are also discussed.