M.R. McNeir

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.

Molecular-interference-free accelerator mass spectrometry

Abstract The University of North Texas tandem accelerator (NEC 9SDH) has been configured as an accelerator mass spectrometry (AMS) system for ultrasensitive detection of impurities in materials. This instrument has been used to acquire mass spectra of materials of interest in semiconductor technology. Investigation of the phenomenon of Coulomb dissociation of molecular ions during passage through the stripper canal of the tandem accelerator is reported, particularly for ions of injected mass of approximately 31 u and 56 u arising from the sputtering of clean silicon with cesium. Peaks in the spectra at these masses could be evidence of 31 P or 56 Fe. However, as is well known from secondary ion mass spectrometry (SIMS), silicon substrates produce large currents of molecular ions such as 30 Si 1 H, 29 Si 1 H 2 and 28 Si 1 H 3 at 31 u and 28 Si 2 at 56 u. These molecular ions are shown to dissociate and appear with momenta and energies which are both unique and predictable. The results suggest that AMS will be a useful tool for truly elemental analytical surveys of materials as well as trace-element detection.

L-shell X-ray production cross sections for 29Cu, 31Ga, 32Ge, 35Br, 39Y, 60Nd, 64Gd, 67Ho, 70Yb, 79Au, and 82Pb for 2–25 MeV carbon ions

Abstract Measurements of L-shell X-rays in selected elements produced by carbon ions of 2–25 MeV are presented. The energies of the X-rays under consideration range from about 1 to 15 keV. The Si(Li) detector efficiency at these X-ray energies was determined using multiple normalization techniques. These methods include measuring i) the bremmstrahlung spectral distribution and ii) K-shell X-rays and normalizing to the cross sections of the ECPSSR (energy-loss and Coulomb deflection effects, perturbed stationary state approximation with relativistic correction) theory. Using single-hole fluorescence yields, the measured cross sections are compared to the predictions of first Born and ECPSSR theories. In general results compare favorably to the ECPSSR theory.

X-ray production in fluorine by highly charged boron, carbon, and oxygen ions *

Abstract K-shell X-ray production cross sections of fluorine have been measured as a function of ion charge state for boron and carbon ions at 10 MeV and oxygen ions at energies of 8, 10, and 12 MeV. From the projectile charge-state dependence of the cross sections, both direct-ionization and electron-capture contributions were extracted for comparison to the Coulomb ionization theory, ECPSSR, which accounts for energy loss and Coulomb deflection of the projectile as well as for relativistic and perturbed stationary states of the inner-shell electrons.

Direct ionization in K-shell X-ray production in fluorine by highly-charged lithium, boron, and oxygen ions

Abstract Measurements of fluorine K-shell X-ray production cross sections are reported for thin solid targets of YF 3 for 1.5–4.0 MeV 7 Li + 2–10 MeV 10 B 2,3+ and 3–12 MeV 16 O 3,4,5,+ ions. In order to determine target thicknesses corresponding to near single-collision conditions. X-ray measurements were made using targets of thickness 2–66 μg/cm 2 deposited on 5 μg/cm 2 carbon backings. Experimental procedures as well as comparisons to the ECPSSR theory are discussed.

L-shell X-ray production cross sections in 20Ca, 26Fe, 28Ni, 29Cu, and 32Ge by 0.75 to 4.5 MeV 7Li+ ions

Abstract L-shell X-ray production cross sections are presented for selected elements with X-ray energies between 341 and 1188 eV. X-ray measurements were made using both a windowless Si(Li) and a windowless high purity Ge detector. The efficiency of the Si(Li) detector was determined by comparing bremsstrahlung spectra from electron bombardment of thin foils of aluminum, silver, and gold with theoretically determined bremsstrahlung spectral distributions. The efficiency of the Ge detector was determined by normalizing K-shell X-ray measurements to the Si(Li) detector. The measurements are compared to the first Born and the ECPSSR theories. The ECPSSR theory is shown to be in much closer agreement with the data.

K-shell X-ray production cross sections in 6C, 8O, 11Na, 12Mg and 13Al by 0.75 to 5.0 MeV 7Li+ ions

Abstract K-shell X-ray production cross sections are reported for elements with K-shell X-ray energies between 277 eV (C) and 1487 eV (Al). X-ray measurements were made with a LINK Analytical windowless Si(Li) detector that was calibrated for efficiency by comparing bremsstrahlung spectra from electron bombardment of thin foils of aluminum, silver, and gold with theoretically determined bremsstrahlung spectral distributions. The X-ray measurements are compared to first Born and ECPSSR ionization theories using single-hole fluorescence yields. The ECPSSR theory is shown to be in much closer agreement to experiment than the first Born theory.