L. D. Hulett

Mass spectrometry studies of the ionization of organic molecules by low-energy positrons

Abstract Positrons having energies in the range of 2.0–10 eV have been used to ionize large organic molecules. For positrons having energies above 2–3 eV ionization occurs by positronium formation. In this paper new results are reported that describe the ionization of molecules at energy below the positronium formation threshold. Contrary to expectations, ionization by positrons having energies below the positronium threshold has been found to induce extensive fragmentation in sigma-bonded molecules. The necessary energy for ionizing the molecules and inducing fragmentation is supplied through the positron-electron annihilation process.

A quadratic potential time-of-flight mass spectrometer for use with slow positron ionization sources and other large-volume ion sources

A time‐of‐flight spectrometer has been specially designed for measuring the masses of ions produced by low‐energy positrons interacting with organic molecules in a Penning trap. To make the flight times insensitive to the starting positions of the ions in the trap, acceleration was done using a potential that varied as the square of the distance of the ion from the detector. The containment of the positrons in the Penning trap for extended time periods effected very long collision paths between the positrons and the molecules. The entire length of the Penning trap source could be sampled. These features produced high sensitivity, enabling the measurement of ionization processes with small cross sections. The spectrometer is useful for mass spectrometry of ions produced by processes other than positron interaction. Both large and small volume sources can be accommodated.

A design for a high intensity slow positron facility using forward scattered radiation from an electron linear accelerator

A tungsten moderator will be placed behind the target of the Oak Ridge Electron Linear Accelerator (ORELA) to convert gamma radiation to slow positrons. These will be extracted and led through evacuated solenoids to an experiment room. A Penning trap will be used to extend the slow positron pulses to achieve duty factors of 10% or greater. The facility will be used for atomic and molecular physics studies, positron microscopy, and materials research. Operations will be inexpensive and will not interfere with the normal function of ORELA, the measurement of neutron cross sections by flight-time spectrometry.

QUANTITATIVE ANALYSIS BY X-RAY FLUORESCENCE USING FIRST PRINCIPLES FOR MATRIX CORRECTION

The quantitative interpretation of X-ray fluorescence (XRF) data is often difficult because of matrix effects. The intensity of fluorescence measured for a given element is not only dependent on the elements concentration, but also on the mass absorption coefficients of the sample for the excitation and fluorescence radiation. Also, there are interelement effects in which high-energy fluorescence from heavier elements is absorbed by ligher elements, with a resulting enhancement of their fluorescence. Recent theoretical treatments of this problem have shown that X-ray fluorescence data can be corrected for these matrix effects by calculations based on first principles. Fundamental constants, available in atomic physics data tables, are the only parameters needed. It is not necessary to make empirical calibrations. In this paper we report the application of this correctional procedure to alloys and alumina-supported catalysts. We also discuss how it may be applied to other matrices. A description is given of a low-background spectrometer which uses monochromatic AgKα radiation for excitation. Matrix corrections by first principles can be easily applied to data from instruments of this type because fluorescence excitation cross-sections and mass absorption coefficients can be accurately defined for monochromatic radiation.

Fragmentation and ionization of CH3F by positron and electron Impact

Abstract New data for the fragmentation of CH3F by low energy e+ using an improved spectrometer are presented. The positively charged fragment ions are detected and mass analyzed using a time-of-flight mass spectrometer and the ion yields are measured as a function of impact energy. For comparison, corresponding data have been obtained for e− scattering, using a similar apparatus.

Microdefects in Al2O3 films and interfaces revealed by positron lifetime spectroscopy

We have studied microdefects and interfaces of Al2O3 films on iron and nickel aluminide substrates using variable-energy positron lifetime spectroscopy. Di-vacancies, vacancy clusters, and microvoids were observed in the oxide scales. Their sizes and distributions were determined by the nature of the process used to synthesize the alumina film, and influenced by the composition of the alloy substrates. For oxide–iron aluminide interfaces, positron lifetimes are longer than those for the alumina layer itself, suggesting a greater defect concentration at such sites.

Analysis of a Nd: YAG laser crystal by X-ray fluorescence, scanning electron microscopy and neutron activation analysis

X-ray fluorescence, using the fundamental parameters method for matrix correction, is used routinely for quantitative analysis in our group. The accuracy of this method has been demonstrated with a Nd:YAG (Nd:Y3Al5O12) laser crystal. The neodymium concentration was determined relatively to the yttrium concentration by X-ray fluorescence using241Am for excitation. The yttrium-aluminum ratio was confirmed by scanning electron microscopy (SEM-EDX). Although there was a 10% discrepancy for yttrium due to the uncertainty in the neutron capture cross-section, the fluorescence results for aluminum and neodymium were in very good agreement with neutron activation analysis (NAA) results. Based on this agreement, the Nd:YAG sample will be used in future NAA calibration measurements.

Positrons at CEBAF

Abstract It is estimated that about 2.5 × 1010 slow positrons per second might be produced if 1.0 mA of 400 MeV electrons were incident on target. This estimate and some problems of target design are discussed. The electron beam structure is 1–2 ps pulses with a repitition rate of 7.5 MHz which is appropriate for a free electron laser (FEL) which is being proposed for CEBAF. The electron beam would exit the FEL and would otherwise be transported to a beam dump.

Positron ionization mass spectrometry: Ionization of organic molecules by positronium formation

Low-energy (<10 eV) positrons have been used to ionize organic molecules in the gas phase followed by mass spectrometric analysis. High-energy positrons from a linear electron accelerator-based facility are re-moderated to less than 3 eV and trapped in a miniature Penning trap where they interact with the target molecules. Ions thus formed are mass analyzed in a time-of-flight mass spectrometer. The predominant ionization mechanism in this energy regime involves positronium formation and the spectra are comparable to electron impact spectra at energies a few eV above the ionization energy of the molecules.

The characterizations of solid specimens: Approaching the whole problem

The purpose of this paper is to demonstrate that complicated mixtures of solids can be characterized to a rather high degree if a coordinated examination by non-destructive methods is used. The techniques discussed are X-ray fluorescence, scanning electron microscopy, photoelectron spectroscopy, transmission electron microscopy, electron diffraction and X-ray diffraction. The application of these methods to the characterization of corrosion scale on an inconel coupon is illustrated. The types of information accumulated were elemental composition, chemical forms of elements, special distributions of elements and compounds in the scale, sizes of particles that made up the scale, variations in composition of particle surfaces from that of their interiors, and composition of scale-alloy interface.