J.L. Duggan

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.

Accelerator mass spectrometry at the University of North Texas

An accelerator mass spectrometry system designed for analysis of electronic materials is being developed and installed on the University of North Texas 3 MV tandem accelerator (National Electrostatics Corporation 9-SDH). High-resolution magnetic (40° deflection, MΔM ≈ 350, maximum mass-energy product 69 MeVu) and electro static (45 ° deflection, E/q of 4.8 MeV, EΔE≈ 730) analysis, coupled with a 1.5 m time-of-flight path and total energy detection (surface barrier detector) forms the basis of the detection system. In order to provide stable element detection capability at the parts-per-trillion level in electronic materials (Si, GaAs, HgCdTe), a custom ion source, incorporating mass analysis of the sputtering beam, ultraclean slits, low cross-contamination and UHV capability, is being constructed.

A high-resolution electrostatic analyzer for accelerator mass spectrometry☆

Abstract Accelerator mass spectrometry (AMS) has the potential of unparalleled sensitivities for elemental impurity analysis in materials. To fully exploit the capabilities of AMS, a high-resolution electrostatic analyzer (ESA) is shown to be a necessary part of the detection system. An ESA with a resolution of E ΔE > 700 for ions with energies up to E q = 4.8 MeV has been designed and constructed. The system utilizes a 45° deflection in the ions trajectory with a radius of curvature of 1.2 m. The field is of toroidal geometry, which produces astigmatic focusing of the ion beam, so as to compensate for the lack of vertical focusing in the magnetic spectrometer.

Preparation of Relatively Clean Carbon Backings Used in Charged Particle Induced X-Ray Studies for X Rays below 4 keV

In a recent series of studies of M-shell ionization induced by protons, alpha particles, and fluorine ions, an unmanageable background of low energy contaminant x rays was observed. These K-shell x rays were primarily from Ca, K, Cl, S, P, Si and Na. The energy range of these contaminants is from 3.691 to 1.041 keV. The M-shell x rays being studied were for various elements from U (~ 3.5 keV) down to Eu (1.5 keV). In order to evaluate and reduce the problem, the contaminants for carbon foils from a number of different manufacturers and a wide variety of foil float-off procedures have been studied. Carbon foils have been produced in our laboratory using carbon rods from several different manufacturers. In this paper, techniques will be described that are most appropriate to reduce the above contaminants to a reasonable level. These techniques should be useful in trace element analysis (PIXE) studies and fundamental ionization measurements for low x-ray energies.

Luminescent layers for ion-photon emission microscopy

Abstract Ion beam induced luminescence (IBIL) combined with ion beam induced charge collection (IBICC) is applied in a quantitative study of the IBIL generation yield and detection efficiency for several plastic phosphor materials. The main purpose of this study is to search for strongly luminescence materials that can be used to easily coat samples to be studied with ion-photon emission microscopy (IPEM). A special focus is given to plastic scintillation materials because thin films are easily prepared, and such films have already been used for single event triggering. The emission yield was found to be low for typical Bicron plastic phosphors (only ∼70 photons/ion/μm). The total collection, transmission and photon detection efficiency of the optical microscope used in this study was determined to be only ∼0.00003. For thin film plastic phosphors ∼20 μm thick, the detection efficiency was only 0.04 photons/ion. This means that using these plastics, IPEM would need to be performed with ∼20× more beam fluence to obtain data, such as IBICC, similar to a standard scanned nuclear microprobe. Improvements are discussed.

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.

The high-energy heavy ion nuclear microprobe at the University of North Texas

Abstract The high-energy, heavy ion, microprobe recently installed at the University of North Texas (UNT) has a demagnification factor of ∼60. It has a probe-forming lens system with a new Russian quadruplet configuration. The microprobe is installed on a 3 MV NEC 9SDH-2 Pelletron tandem accelerator, which has ultra stable high energy for heavy ions ( ΔE / E ∼10 −4 ). Sputter and RF sources produce a variety of ions for microprobe applications. A resolution of ∼2 μm has been achieved for 2.0 MeV protons, 4.0 MeV C ions and 9.0 MeV α-particles with a current of about 50–100 pA. Materials characterization and failure analysis of microelectronics are discussed. Current limitations and future improvements to the system are outlined.

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.

Thorium and uranium M-shell x-ray production by 0.4?4.0 MeV protons and 0.4?6.0 MeV helium ions

M-shell x-ray production cross sections for thorium and uranium have been determined for protons of energies 0.4–4.0 MeV and helium ions of energies 0.4–6.0 MeV. The M-shell line and total M-shell x-ray production cross sections are compared to the predictions of the first Born approximation and ECUSAR ionization theory using recently recommended atomic parameters. Both theories are in good agreement with the data for protons and He ions above 1 MeV. The data of others for energies above 1 MeV protons on uranium, however, fall significantly under the present measurements. Below 1 MeV, with decreasing energy of these projectiles, the first Born approximation increasingly overestimates our measurements while the ECUSAR theory underestimates them to a similar degree. The same trends are seen versus the data of others.