New fluorescence detection system for x-ray absorption spectroscopy

Abstract

The design and performance of a 24-channel fluorescence detection system for use in x-ray absorption spectroscopy are discussed, and comparisons are drawn with other detection systems currently in use. Each channel of the current system consists of a NaI scintillation detector with a filter and collimator arrangement designed to reduce the scatter and filter fluorescence background. Given a sufficiently intense and stable x-ray source, this apparatus can record a useful EXAFS spectrum for an element (Z ≥ 23) at 100 ppm concentration in approximately 30 minutes. The new design permits the study of plate-like samples (such as molybdenum enzyme solutions) unsuitable for previously described slit assemblies. Breaking the detection system up into discrete elements also allows (1) optimization of filter thickness for each direction, (2) use of a weighting scheme to optimize the total S/N, and (3) operation at high total count rates, while retaining the advantages of pulse counting electronics. The relative merits of filters and crystal monchromators are discussed quantitatively, using the two criteria of efficiency and selectivity. It is shown that practical filters and barrel monochromators give similar performance over a 100-fold concentration range, for a given solid angle, and that the 20-fold solid angle advantage of the multielement approach makes it the design of choice for many cases. Finally, the construction of an array of solid-state detectors with the appropriate filters and collimating elements is proposed as a device capable of yielding unprecedented performance in x-ray absorption spectroscopy.