The use of pXRF for light element geochemical analysis: a review of hardware design limitations and an empirical investigation of air, vacuum, helium flush and detector window technologies

Light element data are required for robust and accurate lithogeochemical interpretations and are important components in the study of hydrothermal alteration and mineralization processes. In this contribution we review the latest available portable energy dispersive X-Ray Fluorescence (pXRF) technologies exclusively in the context of light element analysis, with focus on the acquisition of data for Na, Mg, Al and Si. We discuss pXRF hardware design limitations, quantify variables that attenuate X-ray energies through numerical modelling, including common pXRF configurations, and empirically investigate modern pXRF technologies used to mitigate X-ray attenuation and improve light element analysis.The void between the sample and detector is a key issue regarding the success of pXRF light element analysis. Dry-air (normal conditions), vacuum purge and helium flush systems are evaluated. Modelled data that use a nominal sample-detector void of 10 mm show that using helium in lieu of air improves X-ray transmission effectiveness from ≈2% to ≈99% for Na and ≈10% to ≈100% for Mg. Modelled detector window data show that using a graphene detector window in lieu of a traditional beryllium detector window improves X-ray transmission effectiveness for Na from ≈38% to ≈64% and ≈57% to ≈77% for Mg. Progressive X-ray transmission effectiveness equates to ≈63% Na and ≈76% Mg when using a helium-graphene pXRF configuration v. ≈1% for Na and ≈6% Mg when using a traditional in-air beryllium pXRF arrangement (i.e. without sample or X-ray entrance window media).Empirically determined improvements of the resolved signal are more modest than those of modelled X-ray transmission effectiveness data. Instrument noise, spectral overlaps and random counting errors are unavoidable and inherent with the limitations of modern detector technologies. However, the employment of helium with graphene detector window technology allows very precise data to be obtained at significantly shorter scan times (i.e. 20 s, instead of the traditional 60–180 s, i.e. 3–9 times faster): a scan time of 20 s can achieve a precision of ≈18% @ ≈0.4% Na and ≈8% @ ≈0.3% Mg for elemental interference-free samples. Precision will improve with increasing analyte concentration.

PULSE HEIGHT TALLY RESPONSE EXPANSION METHOD FOR APPLICATION IN DETECTOR PROBLEMS

A new incident flux response expansion method (IFLEX) is developed to perform on-the-fly detector pulse height spectra calculations with Monte Carlo accuracy. Given the flux incident on the detector window, the method uses pre-computed continuous energy Monte Carlo based response functions to generate the pulse height tallies. B-spline functions are selected for the expansion of the incident flux in the energy phase space. Response functions are generated for a CsI(Na) crystal using an energy dependent B-spline form of the IFLEX method. The method is verified for two incident flux spectra on the crystal. Results of the energy dependent B-spline form of the method are compared to the solutions generated using the direct Monte Carlo code MCNP5. The method is shown to be several orders faster than MCNP5 while maintaining paralleled accuracy.

LOW ENERGY NEUTRINO AND DARK MATTER PHYSICS WITH SUB-keV GERMANIUM DETECTORS

The theme of the TEXONO-CDEX research program is on the studies of low energy neutrino and dark matter physics at Kuo-Sheng Reactor Neutrino Laboratory and China Jin-Ping Underground Laboratory. The current goal is to open the "sub-keV" detector window with germanium detectors. The three main scientific subjects are neutrino magnetic moments, neutrino-nucleus coherent scattering, and dark matter searches. We highlight the status, results and plans in this article.

K-Factor Standards for Low-Z Quantification

Although elements of low atomic number (Z<11) are detectable in TEM specimens using a modem energy-dispersive (EDX) detector, quantification remains a challenge. Problems include the large absorption corrections needed for specimens of typical thickness and the need for calibration specimens which provide k-factors appropriate to a particular detector/window arrangement. We have addressed this second problem by fabricating thin films containing several light elements in combination with silicon, which typically acts as the reference element when specifying k-factors.Because the EDX peaks of adjacent light elements overlap considerably, it is preferable to have two specimens: one containing C, O and Si, the other B, N, F and Si. We have used both e-beam evaporation and sputtering techniques to deposit thin films on a substrate which can later be removed to leave a freestanding film mounted on a TEM grid.

Standardless Quantification of Conductive Oxides In The Sem

In the last 10 years the development of new polymer type detector window materials has dramatically increased the opportunities for light element analysis with Energy Dispersive Spectrometers (EDS). With the introduction of light-element analysis the need also arises to accurately quantify X-ray spectra. Traditional quantification techniques, using a probe current measuring device and pure elemental standards, have been introduced into the field of EDS, but these techniques prevented much of the conveniences of the EDS techniques with respect to speed and ease of use. Many users are therefore willing to sacrifice part of the maximum achievable accuracy in return for a method that is more convenient: standardless analysis.With EDS analysis the widely published ZAF and φ(ρz) models can be used to convert relative intensities into weight percentages, but for standardless analysis the inaccuracy of the result is mainly caused by the reference intensities.

Window Area Effects in the Detector Efficiency for Source Excited EDXRF Geometries

The efficiency dependence on detector window area for three different geometries of source excited energy dispersive X-ray fluorescence (EDXRF) analysis systems are calculated using the Monte Carlo method. Two typical samples (coal and oil) are considered for discussion. The results show that the efficiency is affected mainly by geometric factors: source-to-sample distance and absorption of the exciting X-rays; fluorescent photon self-absorption has a much smaller effect.